Chapter 9: The role of the carbon cycle in climate change

Chapter description

This chapter summarizes the role of the natural carbon cycle in regulating the atmospheric concentration of carbon dioxide. It explains how anthropogenic carbon dioxide emissions have caused global warming by disturbing the natural carbon cycle, assesses where carbon is currently stored in Canada’s land and ocean ecosystems and how those reservoirs are changing, and briefly discusses the role of nature-based approaches in climate change mitigation.

Chapter key messages

Plain language summary^{Footnote 1}

This chapter presents the background information needed to understand the role of the natural carbon cycle in controlling the response of the climate system to anthropogenic carbon dioxide (CO₂) emissions. The purpose is to illustrate how anthropogenic CO₂ emissions drive climate warming at the global scale, leading to the changes described in the other chapters. The chapter includes an assessment of the current state of the carbon cycle on land and in the oceans in Canada and how it is changing, as well as a brief assessment of the potential of nature-based approaches for mitigating climate change.

The concentration of CO₂ in the atmosphere is regulated by the exchange of carbon between the atmosphere and the underlying land and ocean. For 800,000 years before the industrial era, the atmospheric CO₂ concentration varied between about 170 ppm and 300 ppm (parts per million). Since the start of the industrial era, anthropogenic emissions of CO₂ associated with fossil fuel combustion and land-use change (for example, deforestation) have increased the atmospheric CO₂ concentration to around 423 ppm (in the year 2024). Current global anthropogenic emissions of carbon are about 11 Gt C/yr (which is equivalent to about 41 Gt CO₂/yr; Gt = gigatonne = 10¹⁵ grams).

Of all the anthropogenic CO₂ emitted into the atmosphere, less than half has remained there. The remainder has been absorbed by the land^{Footnote 2} and ocean. The excess CO₂ in the atmosphere is causing global warming^{Footnote 3} and associated changes in the global and regional climate. Although anthropogenic CO₂ emissions vary geographically, the increased CO₂ in the atmosphere affects the climate of the entire world, regardless of where the CO₂ is emitted. However, climate change affects each region differently, and the uptake of carbon by the land and ocean in response to the increased CO₂ in the atmosphere also varies by location. As a greenhouse gas (GHG), CO₂ has caused most of the warming to date, but changes in emissions of aerosols and other GHGs (for example, methane [CH₄], nitrous oxide [N₂O], and halocarbons,^{Footnote 4} referred to collectively as non-CO₂ GHGs) have also contributed. GHGs make up only a very small fraction of the atmosphere (< 1%), which is mainly composed of nitrogen (78%) and oxygen (21%). However, GHGs have a major impact on climate, because they trap heat in the atmosphere. Emissions of non-CO₂ GHGs are often expressed in CO₂-equivalent (CO₂-eq) units; each unit represents the amount of CO₂ that would yield the same cumulative radiative forcing (the net change in energy entering and leaving the Earth’s atmosphere) over a specified period.

There is a roughly linear relationship between global warming attributable to CO₂ and cumulative CO₂ emissions. This means that, the more human-caused emissions that enter the Earth’s atmosphere, the more the climate will warm. If net CO₂ emissions reach zero (that is, when CO₂ emissions are balanced by CO₂ drawdown and removals from the atmosphere) and emissions of other GHGs are strongly reduced, the global temperature will stabilize. The relationship between cumulative CO₂ emissions and global warming also allows us to calculate how much CO₂ can be emitted before global warming reaches a specific level, for example, 2°C warmer than the pre-industrial climate.

In Canada, national anthropogenic GHG emissions in 2023 (the most recent year of reporting) totalled 694 Mt CO₂-eq (1 Mt [megatonne] = 10¹² grams). Canada’s anthropogenic GHG emissions peaked in 2007 at 774 Mt CO₂-eq/yr and have declined since then. Canada is currently the 10th highest GHG-emitting country, responsible for approximately 1.3% of global emissions.

Since the last Ice Age, Canada’s land mass has acted as a net carbon sink. At present, land ecosystems in Canada continue to remove carbon from the atmosphere, but this sink appears to be weakening as the area affected by disturbances such as wildfires has increased. In the future, both carbon uptake from enhanced vegetation growth and carbon emissions from permafrost thaw and ecosystem disturbances are expected to rise, making the net effect on Canada’s land carbon balance uncertain. The area burned by wildfires in Canada is projected to increase further as the climate continues to warm.

Changes to the ocean carbon cycle are assessed inside Canada’s Exclusive Economic Zone (EEZ), which is the area of ocean that extends 200 nautical miles (370 km) offshore from the coast and islands. On average, Canada’s oceans act as a net sink of CO₂ from the atmosphere, although some nearshore waters are a net source of atmospheric CO₂. Canada’s oceans are expected to continue to take up carbon in the future, as long as the atmospheric CO₂ concentration continues to increase. However, the rate of uptake by the ocean will slow as the ocean becomes warmer and more acidic (Chapter 7, section 7.7).

Carbon dioxide removal (CDR) methods aim to mitigate climate change by removing CO₂ from the atmosphere, using geological, chemical, and nature-based methods. Geological and chemical CDR methods are not assessed in this report. Nature-based climate solutions (NBCS) have received a great deal of public attention. NBCS address climate change by protecting existing carbon stocks or by increasing CO₂ uptake from the atmosphere. They provide environmental, economic, and social co-benefits. However, the potential of NBCS to offset anthropogenic CO₂ emissions is limited. NBCS only provide short-term carbon storage (lasting from a few seasons to a century), which is vulnerable to disturbances, development, and climate change. Reducing fossil fuel emissions to reach net-zero CO₂ emissions remains central to achieving global climate change mitigation targets.

9.1: Introduction

The purpose of this chapter is to provide background information on the natural carbon cycle and its role in regulating how much of the anthropogenic carbon dioxide (CO₂) that is emitted remains in the atmosphere. Excess atmospheric CO₂ causes global warming (Archer et al., 2009; Eyring et al., 2021) and leads to the changes that are discussed in the other chapters of this report. In addition, this chapter assesses the current and future state of the carbon cycle on land and in the oceans in Canada, as well as the potential of nature-based solutions for mitigating climate change in Canada.

Earth’s long-term climate and average temperature are regulated by the balance between the energy arriving from the sun and the energy leaving the Earth (see CCCR 2019, Chapter 2, Section 2.3.1) (Bush et al., 2019). The Earth’s atmosphere allows sunlight to pass through and reach the surface of the Earth, where some of this energy from the sun is re-radiated as heat. Atmospheric CO₂ and other greenhouse gases (GHGs) absorb some of the re-radiated heat (that is, long-wave radiation), thus raising the temperature of the atmosphere. A wide variety of natural processes that operate on different timescales regulate the concentration of CO₂ in the atmosphere and therefore this energy balance. The natural global carbon cycle is described in section 9.2. In this chapter, the fluxes between the various components of the Earth system (atmosphere, land, ocean, sediments, etc.) are expressed in mass units of carbon (C) or CO₂ per unit of time. Emissions of non-CO₂ GHGs are expressed in gas-specific units (for example, mass of methane [CH₄] per unit of time), as well as in CO₂-equivalent (CO₂-eq) units by multiplying by each GHG’s 100-year global warming potential. This yields the amount of CO₂ that would result in the same amount of warming, making it easier to compare the effects of different GHGs.

Anthropogenic (that is, human-caused) emissions of CO₂ associated with the burning of fossil fuels and land-use changes (for example, deforestation) have disturbed the natural global carbon cycle. Global and Canadian anthropogenic GHG emissions are assessed in section 9.3, which also describes how Earth’s natural carbon cycle has responded to increased atmospheric CO₂ in the atmosphere. At the global scale, over half of anthropogenic CO₂ has been absorbed by the land and oceans, leaving less than half in the atmosphere.

Global warming is caused by anthropogenic emissions of CO₂ that stay in the atmosphere. Section 9.4 presents the roughly linear relationship between cumulative anthropogenic CO₂ emissions and global warming. It also introduces the concept of net-zero CO₂ emissions. Global warming is also caused by non-CO₂ GHGs. When emissions of long-lived GHGs, such as CO₂, reach net zero and emissions of short-lived GHGs decline strongly, surface air temperatures will then stop increasing.

Section 9.5 assesses Canada’s current land-based carbon sources and sinks (which include inland lakes, rivers, and wetlands), as well as those in Canada’s oceans (up to 200 nautical miles off the coast), and how they are changing. Finally, section 9.6 discusses the potential of nature-based climate solutions for removing CO₂ and helping Canada to mitigate climate change.

Figure take-away: A visual snapshot of the contents of this chapter and of important cross-chapter linkages.

Figure title: Visual guide to the content of Chapter 9 and key cross-chapter linkages.

Figure 9.1: Visual guide to Chapter 9 content and cross-chapter linkages

9.2: Natural global carbon cycle

Key Message 9.1: Before the industrial era, carbon stored in fossil fuels had remained isolated for millions of years, playing no discernable role in the exchange of carbon between the land, ocean, and atmosphere (very high confidence^{Footnote 5}).

Key Message 9.2: During the last 800,000 years before the industrial era, the atmospheric carbon dioxide concentration varied between 170 and 300 parts per million (very high confidence). For comparison, the atmospheric carbon dioxide concentration for the year 2024 was 423 parts per million.

Carbon is exchanged continuously among the components of the Earth’s climate system—atmosphere, land, biosphere, ocean, and the crust and upper mantle—on timescales that vary from minutes to millions of years. Natural carbon cycle processes that operate on timescales of up to thousands of years (also called “fast” processes) in or on the land, ocean, and atmosphere are described in sections 9.2.1.1, 9.2.1.2, and 9.2.1.3, respectively. Geological carbon cycle processes involving the Earth’s crust and upper mantle that operate on timescales from thousands to millions of years are described in section 9.2.1.5. The natural exchange of carbon between the climate system components regulates the atmospheric concentrations of carbon dioxide (CO₂) and methane (CH₄). This section summarizes how the natural carbon cycle works, with a primary focus on the “fast” components of the carbon cycle.

Carbon exchanges occur readily between the atmosphere, ocean, vegetation, soil, and surface sediment (Figure 9.2). By far, the ocean is the largest store of carbon. However, owing to the impact of human activities, present-day exchanges of carbon are not in a steady state, as discussed in section 9.3.

Figure take-away: Anthropogenic carbon dioxide emissions have perturbed the natural carbon cycle, causing atmospheric carbon dioxide to increase.

Figure title: Magnitude of natural carbon pools and fluxes, and resulting changes in fluxes due to anthropogenic activities

Figure 9.2: Illustration showing the magnitude of global carbon pools (that is, their storage capacity) in gigatonnes of carbon (Gt C) (circles) and the fluxes (that is, exchanges) between them in Gt C/yr (arrows) during the decade of 2012 to 2023. The thinner, curved arrows represent balanced, natural fluxes. The arrows in the “Anthropogenic fluxes” box represent unbalanced fluxes between the pools. The thickness of the arrows is scaled to the magnitude of each flux, with thicker arrows representing larger fluxes. The direction of an arrow corresponds to the direction of the flux: upward-pointing arrows indicate emissions into the atmosphere, and downward-pointing arrows, drawdown and removals from the atmosphere. Combined fossil fuel and land-use change emissions drive the increase in the atmospheric carbon dioxide (CO₂) concentration, which leads to carbon uptake by the land and ocean, as explained in the text. The net change in atmospheric CO₂ burden is thus 9.6+  1.3 – 3.3 – 2.8 = 4.8 Gt C/year. This is 0.4 Gt C/yr lower than the observed increase of 5.2 Gt C/yr, hence the imbalance of ‒0.4 Gt C/yr. This imbalance of around 8% in the present-day fluxes generates uncertainty in reconstructing the change in the atmospheric CO₂ burden using the individual terms of the budget. Note that the sizes of the fossil fuel pools represent known reserves; more might be discovered. The “Soils” category represents unfrozen soil, and the frozen soils in the “Permafrost” category. Adapted from: Candell et al. (2021) and Friedlingstein et al. (2023).

9.2.1: Natural surface-atmosphere carbon exchange

This subsection explains the roles played by the land, ocean, and atmosphere in the cycling of carbon. It shows that the ocean is by far the largest reservoir of carbon. Both land and ocean play important active roles in the exchange of carbon with the atmosphere, while the atmosphere plays a mainly passive role. This subsection also shows that the fossil fuel reservoir has been disconnected from the modern-day carbon cycle. It focuses on “fast” carbon cycle processes, which occur on timescales ranging from minutes to thousands of years; to complete the picture, processes that occur over geological timescales of millions of years are outlined briefly in section 9.2.1.5.

Carbon cycle processes operate somewhat differently on land and in the ocean, with two main distinctions. First, on land, biological processes are the main drivers of the carbon cycle, as photosynthesis converts CO₂ from the atmosphere to organic carbon compounds, which then decompose and release CO₂ back into the atmosphere (for the terrestrial carbon cycle, the term “vegetation productivity” is generally used synonymously with the term “photosynthesis,” both referring to the process of carbon uptake by vegetation). In contrast, in the ocean, the physical process of CO₂ dissolution in seawater drives the uptake of carbon from the atmosphere. Dissolved CO₂ in seawater occurs in the form of dissolved inorganic carbon. This dissolved inorganic carbon is the largest reservoir of carbon in the ocean, although the ocean also contains dissolved organic carbon, derived mainly from marine phytoplankton and land plants.

Second, on land, carbon uptake and storage occur at the same physical location because plants and soils are stationary. In contrast, in the ocean, where the water is constantly in motion, the uptake of carbon may occur in one region, while its long-term storage may take place elsewhere, often at greater depths or in distant parts of the ocean.

9.2.1.1: Land

Carbon enters the land biosphere through plant photosynthesis (Figure 9.2), which occurs on a timescale of minutes (Sellers et al., 1997). Plants take up CO₂ and convert it to organic carbon, which forms the basis of plant biomass. As plants grow and die, organic carbon is stored, both above ground, in the leaves and stems, and below ground, in the roots and soil (this carbon storage capacity is referred to as a carbon pool). In turn, carbon is returned to the atmosphere in the form of CO₂ through a number of processes, including the respiration of living vegetation; the decomposition of dead organic matter in plant litter (the slightly decayed, uppermost layer of organic matter on the ground), coarse woody debris, and soil carbon; and disturbances such as wildfires (Figure 9.2). Surface leaf litter decomposes fairly quickly, taking from less than a year to about two years to break down. Coarse woody debris, especially larger logs, decomposes at a much slower rate and can persist on the forest floor from a few decades to over a century, depending on the type of wood, its diameter, and most importantly, the climate (Harmon et al., 1986). The organic carbon stored in the soil carbon pool decomposes on a timescale of years to centuries. These processes result in the build-up of organic carbon in the soil, particularly in wet and cold regions, where organic matter decomposes more slowly (Schmidt et al., 2011). This is the reason why large amounts of carbon are stored below ground in wetlands, peatlands, and permafrost. When organic carbon decomposes in places where oxygen is not available (for example, in waterlogged conditions), the carbon is primarily released as methane rather than as CO₂ (Bridgham et al., 2013). Wetlands and peatlands are among the largest natural sources of methane.

The balance between carbon taken up through photosynthesis and carbon lost through plant and microbial respiration and combustion determines whether land is gaining or losing carbon overall. Typically, both plants and soils gain carbon during the growing season, since the amount of carbon added via its conversion from CO₂ during photosynthesis is greater than the amount of carbon lost through plant and microbial respiration. Plants lose carbon during the dormant season, when photosynthesis is absent (or much lower), but respiration continues. When the terrestrial biosphere is in a steady state, the carbon gained during the growing season is roughly balanced by the carbon lost during the dormant season. When the land biosphere gains or loses carbon over a long period of time, it is not in a steady state.

A small fraction of organic matter on land also enters rivers and lakes. This carbon is transported downstream toward the ocean, with some of it decomposing along the way. Global CO₂ emissions from rivers and lakes are estimated to total around 1.8 and 0.3 gigatonnes of carbon per year (Gt C/yr), respectively, with an estimated 1 Gt C/yr discharged into the ocean (that is, riverine discharge) (M. Liu et al., 2024; Raymond et al., 2013). These fluxes are much smaller in magnitude than the natural land-atmosphere (130 Gt C/yr) and ocean-atmosphere (80 Gt C/yr) exchanges noted in Figure 9.2.

On land, at the global scale, soil organic matter represents the largest carbon pool (1500 to 1700 Gt C; 1 Gt = 10¹⁵ g) (Friedlingstein et al., 2022; Scharlemann et al., 2014), followed by vegetation (around 400 to 500 Gt C) (Friedlingstein et al., 2022; He et al., 2021; Santoro et al., 2021), and plant litter (around 80 to 100 Gt C). The estimates for the soil carbon pool are generally assumed to include a small fraction of living microbes (around 1.5%) (Xu et al., 2013). In addition to the carbon in unfrozen soils, an estimated 1100 to 2000 Gt C is locked in permafrost, where it has been since the last Ice Age (Hugelius et al., 2014; Lindgren et al., 2018).

9.2.1.2: Ocean

Globally, the ocean is by far the largest carbon reservoir in the active carbon cycle. It contains about 37,000 Gt C, mainly in the form of dissolved inorganic carbon in the seawater (Figure 9.2), with a small additional fraction stored in the form of organic carbon, mainly in sediment. Ten times more carbon is stored in the ocean than in soils and permafrost combined.

Because the ocean holds vast amounts of carbon, it plays a dominant role in controlling atmospheric CO₂ levels, especially over long timescales (10,000 to 100,000 years). The concentration of dissolved inorganic carbon in seawater varies geographically. In areas where the CO₂ concentration in the surface ocean is lower than that in the atmosphere, the surface ocean will absorb CO₂ from the atmosphere. Conversely, in areas where the CO₂ concentration in surface waters is higher than in the atmosphere, the ocean will emit CO₂ into the atmosphere. As the concentration of CO₂ in seawater increases due to the increasing atmospheric concentration, the ocean’s capacity to absorb more CO₂ decreases.

Some of the dissolved inorganic carbon is taken up by phytoplankton, which photosynthesize it to produce organic carbon. Since phytoplankton form the basis of the ocean’s food web (the interconnected feeding patterns among all organisms), this organic carbon cycles through the ocean ecosystem. A very small fraction (a few percent) settles to the sea floor, where it is buried in sediment and potentially isolated from ocean–atmosphere exchanges for hundreds or thousands of years. The remainder is recycled in the water column (that is, at various depths in the water), and some of it is emitted into the atmosphere as CO₂. There is no evidence that the rate of transfer of carbon into the deep ocean via phytoplankton has increased as a result of rising CO₂ levels in seawater, as phytoplankton growth is limited by the availability of nutrients and light, rather than by the availability of carbon.

Dissolved inorganic carbon is also carried into some regions of the deep ocean by ocean currents and mixing. Some of the dissolved carbon makes its way back to the sea surface on a timescale of hundreds to thousands of years. When the total flux of CO₂ from the atmosphere to the ocean is the same as the flux of carbon from the ocean to the atmosphere, the ocean is at a steady state, in terms of its carbon exchange with the atmosphere.

Carbon also enters the ocean from land, via runoff and coastal erosion (section 9.2.1.1). This carbon may be organic or inorganic, and it enters the ocean in both dissolved and particulate (consisting of solid particles such as plant debris or soil) forms.

9.2.1.3: Atmosphere

The concentration of CO₂ in the atmosphere is primarily regulated by the exchange of carbon between the land and the atmosphere, and between the ocean and the atmosphere. When the total flux of CO₂ from the atmosphere to the land and ocean is the same as the flux leaving these pools, the system is at a steady state, and the atmospheric CO₂ concentration remains constant on a timescale of centuries. However, when CO₂ is added to the atmosphere (for example, in the form of anthropogenic emissions) faster than it is taken up by the land or ocean, the atmospheric CO₂ concentration increases. Owing to anthropogenic emissions, this has been the case since the end of the pre-industrial era. Once emitted, CO₂ is drawn down from the atmosphere at a range of timescales varying from minutes to millennia (Inman, 2008; Torres Mendonça et al., 2024). Therefore, it is difficult to assign a fixed atmospheric lifetime to CO₂. Even 100 years after CO₂ is emitted into the atmosphere, about 35% remains there (Torres Mendonça et al., 2024). In fact, a fraction of the CO₂ that is emitted into the atmosphere remains there for tens of thousands of years (Archer et al., 2009).

Measurements in air bubbles trapped in Antarctic ice cores show that, during the 800,000 years immediately preceding the industrial era, the atmospheric CO₂ concentration ranged between approximately 170 and 300 parts per million (ppm) (Canadell et al., 2021; Lüthi et al., 2008; Uemura et al., 2018). During glacial periods, this concentration was around 170 to 200 ppm and, during interglacials, around 250 to 300 ppm. The current atmospheric CO₂ concentration of roughly 423 ppm (for the year 2024) (Forster et al., 2025) is unprecedented in at least the last 800,000 years, and there is high confidence that this concentration has not been experienced for at least 2 million years, as noted in the Technical Summary (TS) of the Working Group I contribution to the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (IPCC AR6 WGI TS) (Arias et al., 2021).

According to the analysis of Antarctic ice cores, as well as records of past sea-level changes, at least eight glacial periods occurred during the last 800,000 years, separated by interglacials. Each glacial period typically lasts 70,000 to 90,000 years, followed by a shorter interglacial period of about 10,000 to 30,000 years (Augustin et al., 2004; Past Interglacials Working Group of PAGES, 2016). These cycles are driven by changes in the Earth’s orbit and its tilt. The natural change in atmospheric CO₂ concentrations between low glacial values and high interglacial values is about 100 ppm. The increase from glacial lows to interglacial highs has generally occurred faster, while the subsequent decline back to glacial levels has taken place more gradually, with both phases spanning tens of thousands of years. The recent rate of increase in atmospheric CO₂ from about 285 ppm in the year 1850 to 423 ppm in the year 2024, an increase of 139 ppm over the last 175 years, is therefore unprecedented in the last 800,000 years (Masson-Delmotte et al., 2013).

Methane has a much shorter lifetime in the atmosphere than CO₂. Methane breaks down into CO₂ and water (H₂O) in the atmosphere on a timescale of about 10 to 12 years. Ice core records show that the atmospheric concentration of methane ranged roughly between 0.35 ppm and 0.80 ppm from 800,000 years ago until the end of the pre-industrial era (Canadell et al., 2021; Loulergue et al., 2008). The current concentration of methane in the atmosphere, about 1.9 ppm, is determined by the balance between methane emissions from the land and ocean and the destruction of methane in the atmosphere. Although methane breaks down into CO₂, its contribution to the overall atmospheric CO₂ concentration is negligible, because the current atmospheric concentration of methane is much smaller than that of CO₂ (1.9 ppm for methane compared to 423 ppm for CO₂ (Forster et al., 2025).

However, methane is a much stronger GHG than CO₂. Emissions of methane or other greenhouse gases aside from CO₂ (referred to collectively as non-CO₂ GHGs) can be expressed as carbon dioxide equivalent units (CO₂-eq), or the amount of CO₂ that would result in the same amount of cumulative radiative forcing over a specific period. Radiative forcing is the net change in energy entering and leaving the Earth’s atmosphere in response to an increase in the atmospheric concentration of a given GHG.

The factor used to calculate CO₂-eq emissions for a specific non-CO₂ GHG is referred to as its global warming potential (GWP), with the GWP of CO₂ set at one. Since the GWP is calculated over a specified period, its value for a non-CO₂ GHG depends on its atmospheric lifetime and its ability to cause warming. Short-lived non-CO₂ GHGs like methane cause most of their warming soon after they are emitted, while long-lived non-CO₂ GHGs like nitrous oxide (N₂O) (Prather et al., 2023) remain in the atmosphere for centuries, causing long-term warming. The GWP of methane at 20- and 100-year timescales is estimated to be 84 and 28, respectively (Appendix 8.1 in Myhre et al., 2013). This implies that, averaged over a 20-year period, one unit of methane causes the same amount of warming as 84 units of CO₂.

9.2.1.4: Fossil carbon

The fossil reserves of carbon, which we have been using for energy in the form of fossil fuels (coal, oil, and natural gas) since the start of the industrial era, originated from organic material that settled hundreds of millions of years ago on the bed of ancient seas and oceans. Over millions of years, the high temperature and pressure deep in the Earth’s crust compressed and transformed this organic material into hydrocarbon fossil fuels. Different combinations of heat and pressure yielded different types of hydrocarbons (coal, oil, or natural gas).

Until the industrial era, the fossil reservoir of carbon was disconnected from the active, modern carbon cycle (Figure 9.3). Here, the terms “modern carbon cycle,” and “fast carbon cycle,” are used synonymously to reflect timescales of up to thousands of years, as opposed to the millions of years over which organic carbon is transformed into hydrocarbon fossil fuels. Fossil fuels have a distinct isotopic signature, which refers to the specific ratio of carbon isotopes present in their composition. Studies based on radiocarbon dating (carbon-14) and stable carbon isotopes (carbon-12 and carbon-13) provide evidence that fossil fuels have ancient origins (Friedli et al., 1986; Keeling, 1979; Stuiver and Polach, 1977; Tans et al., 1979).

The continuous stable carbon isotope record (the ratio of carbon-13 to carbon-12) of atmospheric CO₂ from ice cores extends back only 24,000 years (Schmitt et al., 2012). However, broader evidence from geological studies, stable isotope records of non-carbon elements, and radiocarbon data supports the conclusion that fossil fuels were isolated from the modern carbon cycle for hundreds of millions of years (Ogbesejana et al., 2022; Peters et al., 2004).

9.2.1.5: The geological carbon cycle

Carbon cycle components operating at timescales of millions of years cycle carbon between the land, ocean, and the atmosphere through chemical reactions and plate tectonics (shifts in the planet’s crust). These long-term processes begin with rain, which, when combined with CO₂ in the atmosphere, forms a weak carbonic acid. When this mildly acidic rain falls on land, it reacts with rocks in a process called chemical weathering, and releases calcium ions. These ions are then transported via rivers to the ocean, where shell-building marine organisms combine them with CO₂ to form solid calcium carbonate. As these organisms die and sink to the sea floor, the calcium carbonate is buried and eventually turns into rock (for example, limestone), locking the carbon in place.

Plate tectonics governs the movement of the continental and oceanic crusts. When two tectonic plates collide, one of them is subducted (pulled down) into the Earth’s interior, where the rocks melt under the extreme heat and pressure, releasing CO₂. Over millions of years, this CO₂ is vented back into the atmosphere by volcanoes, and the cycle begins again (Tamburello et al., 2018). These carbon cycle processes that operate on geological timescales of millions of years act as a natural thermostat, regulating atmospheric CO₂ levels. Without volcanic CO₂ emissions, atmospheric CO₂ levels would slowly fall, due to carbon burial on the ocean floor. Conversely, in the absence of rain to drive rock weathering, atmospheric CO₂ levels would rise unchecked. Volcanic and other geological activity releases about 0.08 to 0.1 Gt C/yr (0.3 to 0.4 Gt CO₂/yr) in the form of CO₂ into the atmosphere and ocean (Werner et al., 2019), about one hundred times less than present-day anthropogenic emissions (11 Gt C/yr or 41 Gt CO₂/yr) (Friedlingstein et al., 2022). These carbon cycle processes that operate at geological timescales are not discussed further in this chapter, because they are not relevant at the timescales at which climate change is currently occurring and affecting humans, which range from a few decades to a few centuries.

Figure take-away: Fossil fuels have been isolated from the modern carbon cycle for hundreds of millions of years.

Figure title: Illustration of the modern carbon cycle, in the context of the carbon cycle processes occurring at geological timescales and the fossil fuel reservoir

Figure 9.3: Diagram illustrating the exchange of carbon between the components of the carbon cycle on many different timescales. Carbon exchanges between the atmosphere (where carbon occurs in the form of carbon dioxide, or CO₂), surface ocean, vegetation, and surface soils occur on a timescale of minutes to about a hundred years. It takes decades to roughly a millennium for water circulation to carry dissolved carbon from the surface ocean to the deep ocean. Fossil fuels have been isolated from the modern carbon cycle for hundreds of millions of years. Fossil fuel combustion adds carbon to the modern carbon cycle. The only natural processes that draw down excess atmospheric carbon from the modern cycle on a long-term basis are deep burial in marine sediment and silicate rock weathering, which take thousands to hundreds of thousands of years. The time ranges (for example, “years–decades” and “10,000 to 100,000 years”) are the timescales associated with the exchange between each pair of compartments. Note that the pool sizes in the figure are not to scale. Relative reservoir sizes are shown in Figure 9.2. Adapted from: Johannessen and Christian (2023).

9.2.2: Confidence terms in key messages: summary of evidence

Key Message 9.1: Before the industrial era, carbon stored in fossil fuels had remained isolated for millions of years, playing no discernable role in the exchange of carbon between the land, ocean, and atmosphere (very high confidence).

Key Message 9.2: During the last 800,000 years before the industrial era, the atmospheric carbon dioxide concentration varied between 170 and 300 parts per million (very high confidence). For comparison, the atmospheric carbon dioxide concentration for the year 2024 was 423 parts per million.

Key messages 9.1 and 9.2 are interrelated, and there is very high confidence in Key Message 9.1 and the first sentence of Key Message 9.2. The evidence for both key messages is based on strong agreement among many high-quality studies that have estimated the origin and atmospheric concentrations of ancient CO₂ using measurements of CO₂ trapped in the air of ice cores from Antarctica. The atmospheric CO₂ concentration for the year 2024 is based on the third annual update of global climate change indicators (Forster et al., 2025). A large number of peer-reviewed studies report measurements of CO₂ from Antarctic ice core data (for example, Augustin et al., 2004; Jouzel et al., 1987; Lorius et al., 1985; Lüthi et al., 2008; Petit et al., 1999; Uemura et al., 2018). The measurements of the isotopic composition of CO₂ trapped in Antarctic ice cores and from present-day atmospheric samples (Etheridge et al., 1996; Friedli et al., 1986; Keeling, 1979; Schmitt et al., 2012), as well as geological and radiocarbon data, confirm the ancient origins of fossil fuels and their isolation from the modern carbon cycle for millions of years. These studies provide the basis for the scientific understanding of glacial-interglacial cycles and the associated variations in atmospheric CO₂ over the past 800,000 years.

9.3: Human-caused changes to the global carbon cycle

Key Message 9.3: The increase in atmospheric carbon dioxide concentrations since the pre-industrial era has unequivocally been caused by emissions from the burning of fossil fuels and human-caused land-use change. In 2023, annual human-caused global CO₂ emissions reached 41 Gt CO₂/yr (± 3 Gt CO₂/yr) (gigatonnes of carbon dioxide per year); including contributions from other greenhouse gases, total greenhouse gas emissions were 57 Gt CO₂-eq/yr (carbon dioxide equivalent units per year).

Key Message 9.4: Based on Canada’s national inventory report, total anthropogenic greenhouse gas emissions for Canada peaked in 2007 at 774 Mt CO₂-eq (megatonnes of carbon dioxide equivalent units). Notable reductions occurred in 2008 to 2009 during the global recession (714 Mt CO₂-eq/yr) and in 2020 during the COVID-19 pandemic (682 Mt CO₂-eq/yr). Emissions for 2023 (694 Mt CO₂-eq/yr; most recent year reported) are approximately 10% lower than their 2007 peak, despite continued growth in Canada’s population and economy.

Key Message 9.5: Land and ocean together have taken up more than half of the global anthropogenic carbon dioxide emissions since the pre-industrial era (very high confidence). The ocean is expected to continue taking up carbon dioxide in the long term (beyond a century), but the land carbon sink may weaken or become a carbon source (high confidence).

This section presents an overview of global and Canadian anthropogenic greenhouse gas (GHG) emissions: their sources, magnitudes, and trends. It also summarizes the response of the natural carbon cycle to global anthropogenic emissions.

9.3.1: Anthropogenic emissions (global)

Since the pre-industrial era, anthropogenic carbon dioxide (CO₂) emissions have resulted from the production and burning of fossil fuels, industrial activities such as cement production, and land-use change. Since 1850, the combined emissions from fossil fuels and cement production have represented about two thirds of total anthropogenic CO₂ emissions, with the rest coming from land-use change (Friedlingstein et al., 2025). Although CO₂ emissions from cement and fossil fuel production and burning are routinely combined in estimates (for example, Friedlingstein et al., 2025), cement production only contributes approximately 4% of these emissions (Andrew, 2019). For the 2014−2023 period, emissions from fossil fuels and cement production accounted for about 90% of all human-caused emissions (Friedlingstein et al., 2025).

Fossil carbon reserves were isolated from the natural carbon cycle for hundreds of millions of years (section 9.2.1.4). Their combustion adds extra CO₂ to the atmosphere, and the resulting increase in the atmospheric CO₂ concentration will last for millennia. Fossil fuels (such as coal, oil, and natural gas) are forms of organic carbon. They are burned for electricity generation, heating, transportation, and industrial activities. The extraction and burning of fossil fuels convert the carbon from these otherwise stable, long-term hydrocarbon reservoirs and release it into the atmosphere as CO₂.

On the basis of the distinct isotopic signature of fossil fuels, it has been confirmed that the increase in the atmospheric concentration of CO₂ has resulted primarily from the burning of fossil fuels and secondarily from deforestation (Francey et al., 1995; Friedli et al., 1986; Keeling, 1979). The Summary for Policymakers (SPM) in the IPCC AR6 WGI (IPCC AR6 WGI SPM) concluded unequivocally that the increases in atmospheric concentrations of CO₂ and other GHGs (for example, methane and nitrous oxide) in the atmosphere during the industrial era are the result of human activities (IPCC, 2021).

At the global scale, total annual anthropogenic CO₂ emissions from fossil fuels (and cement production) have continued to increase year after year since 1850 or earlier, with a few exceptions, such as the emission reductions in 1945 at the end of the Second World War, in 2020 during the COVID-19 pandemic, and during major recessions (IEA, 2024). Global annual CO₂ emissions from fossil fuel burning (and cement) were less than 0.2 Gt CO₂ in 1850 (1 gigatonne, or Gt, equals 1 billion tonnes), but surpassed 10 Gt CO₂ in 1963, and reached their highest level (37.4 Gt CO₂) in 2023, the most recent year of reporting (IEA, 2024). When the radiative forcing of other GHGs, such as methane (CH₄) and nitrous oxide (N₂O) is accounted for, global GHG emissions in 2023 totalled 53 Gt carbon dioxide equivalent units per year (CO₂-eq/yr), excluding emissions from land use, land-use change, and forestry (IEA et al., 2024).

Historically, the majority of CO₂ emissions have come from developed countries. Over the 1850 to 2023 period, cumulative emissions from the United States amounted to 24% of the world total, compared with 16% from the European Union, 15% from China, and just 3% from India (Friedlingstein et al., 2025). In the past two decades, however, the increase in global CO₂ emissions has mostly been driven by the growth in emerging economies and less developed countries (for example, China and India). In 2023, the five highest-emitting countries and their share of global CO₂ emissions from fossil fuels and cement manufacturing were China (30.1%), the United States (11.2%), India (7.8%), Russia (5.1%), and Brazil (2.5%) (Crippa et al., 2024). Canada was the 10th highest emitter in 2023, producing 1.3% of global GHG emissions.

The quantity of CO₂ emitted per unit of energy obtained is highest for coal, followed by oil and natural gas. For example, 101 kg CO₂ is emitted per gigajoule (GJ; one billion joules) of electricity generated from lignite (brown coal), 73.3 kg CO₂/GJ for crude oil, and 56.1 kg CO₂/GJ for natural gas (Chapter 2, Table 2.2, in IPCC, 2006).

Methane is the primary component of natural gas and burning methane results in CO₂ emissions. In addition, unintended methane leakage also occurs from industrial and residential infrastructure. A complete accounting of GHG emissions from anthropogenic sources requires the inclusion of these “fugitive” emissions. As mentioned in section 9.2, over a 100-year time frame, methane has a global warming potential (GWP) about 28 times that of CO₂, so 1 kg of methane emitted is equal to 28 kg CO₂‑eq. It is therefore important to account for fugitive methane emissions when quantifying the reduction in emissions obtained when switching from coal or oil to natural gas.

The uncertainty in global annual CO₂ emissions from fossil fuel combustion is relatively low, at roughly 5 to 6% (Olivier and Peters, 2020), since most carbon in fossil fuel is converted to CO₂ in a predictable way. However, with the increase in the proportion of emissions from emerging economies, which frequently have challenges in their emissions reporting capacity, the overall uncertainty in global CO₂ emissions values has risen in recent years (Olivier and Peters, 2020). The uncertainty in annual global methane emissions is much greater (around 25%) (Olivier and Peters, 2020), due to the difficulty in quantifying fugitive emissions. Fugitive emissions have been under-reported in developed countries like the United States and Canada (Plant et al., 2019) and, in many emerging economies, are surrounded by very substantial uncertainties. Therefore, reducing fugitive methane emissions is widely regarded as a good opportunity for short-term climate change mitigation.

Land-use change due to human modification of the Earth’s land surface can result in CO₂ emissions or uptake. Typical land-use changes, such as deforestation, logging, urbanization, agricultural expansion, and shifting cultivation (a cycle of cutting forest for agriculture, followed by agricultural abandonment), release CO₂ from land; removing or degrading the natural vegetation reduces carbon storage by increasing the decomposition and burning of organic matter. Wood harvested for paper and wood products releases CO₂ at the end of these products’ life when they decompose. In contrast, other land-use changes—such as the regrowth of forests (following wood harvest or agricultural abandonment), afforestation (the establishment of a forest or tree stand where there was none recently), and soil carbon conservation practices—enhance carbon storage on land.

A substantial proportion of the emissions from anthropogenic land-use change result from the permanent removal of natural vegetation to increase agricultural area. The global cropland area has expanded from roughly 3 million km² in the 1700s to roughly 16 million km² at present (Hurtt et al., 2020). If the removal of vegetation is not permanent, then it will not cause a long-term increase in atmospheric CO₂, since the carbon temporarily lost to the atmosphere is regained as the vegetation grows back over a period of a few decades. However, croplands behave differently from natural vegetation, especially forests, because the regular harvesting of crops prevents organic matter from building up in the soil. Natural vegetation stores more carbon per unit area than crops because of the lack of harvesting. Since 1850, the human-caused carbon loss from land (for example, due to activities such as deforestation) has been greater than the carbon gain (for example, due to activities such as afforestation and cropland abandonment). As a result, human-caused land-use change represented a net cumulative carbon source of about 225 Gt C (gigatonnes of carbon) (825 Gt CO₂) during the 1850 to 2023 period, and, on average, an annual source of 1.1 Gt C/yr (4.0 Gt CO₂/yr) over the 2014 to 2023 period (Friedlingstein et al., 2025). Land-use change emissions for the year 2023 (the latest year reported) are estimated to be 1 Gt C/yr (3.6 Gt CO₂/yr) (Friedlingstein et al., 2025), resulting in total anthropogenic emissions of around 11 Gt C/yr (41 Gt CO₂/yr). When the global warming potential of non-CO₂ GHGs is taken into account, this is equivalent to around 57 Gt CO₂-eq/yr (obtained by adding 3.6 Gt CO₂/yr to the 53 Gt CO₂-eq/yr number from IEA et al., 2024).

9.3.2: Anthropogenic emissions in Canada

According to Canada’s national inventory report (ECCC, 2025), the country’s annual anthropogenic GHG emissions increased steadily from 606 Mt CO₂-eq/yr in 1990 to 774 Mt CO₂-eq/yr in 2007. Emissions then fell over the years 2008 to 2009 to 714 Mt/yr CO₂-eq/yr, due to the global recession, and subsequently stabilized, averaging 740 Mt CO₂-eq/yr during the 2010 to 2019 period. In 2020, Canada’s emissions dropped to 682 Mt CO₂-eq/yr, due to the reduced economic activity related to the COVID-19 pandemic. Average emissions from 2021 to 2023 (the most recent year of reporting) were 696 Mt CO₂-eq/yr. Figure 9.4 illustrates these changes. It should be noted that the changes in Canada’s GHG emissions have all occurred despite the continued increase in Canada’s population. This implies that Canada’s per capita emissions are decreasing (see Figure 9.4).

Figure take-away: In 2023, Canada’s anthropogenic greenhouse gas emissions were about 10% lower than their 2007 peak, and per capita emissions were 29% lower than their year-2000 peak.

Figure title: Canada’s national and per capita greenhouse gas emissions, excluding those from land-use change and forestry (1990 to 2023)

Figure 9.4: Graph showing annual anthropogenic greenhouse gas (GHG) emissions in Canada (blue line) (in megatonnes of carbon dioxide equivalent units per year, or Mt CO₂-eq/year), and annual per capita anthropogenic GHG emissions (orange line) (in tonnes of CO₂-eq perperson per year) from 1990 to 2023 based on Canada’s national inventory report (ECCC, 2025). Per capita emissions were calculated using population estimates from Statistics Canada for July 1 of each year. Emissions from land-use change and forestry are excluded. Note that the minimum values on the vertical axes are not zero.

Under the Paris Agreement of the United Nations Framework Convention on Climate Change (UNFCCC), each country may select the base year against which emission reductions or Nationally Determined Contributions (NDCs) will be measured. Canada’s base year is 2005. Canada has made a commitment to reduce its emissions by 45 to 50% below the 2005 level by 2035. Using the value for 2005 emissions of 759 MtCO2-eq (ECCC, 2025)^{Footnote 6}, this emission reduction target is 380 to 417 Mt CO₂-eq/yr. The Canadian Net-Zero Emissions Accountability Act, passed in June 2021, enshrines in legislation Canada’s commitment to achieve net-zero emissions by 2050 (see Chapter 1, Box 1.2, for more information on Canada’s climate change mitigation commitments).

Canada currently reports emissions annually to the UNFCCC. Every year, improvements to the reporting process are implemented, which can result in refinements in values from the previous years. From 2005 to 2023, notable decreases in emissions were observed in the electricity (58%), heavy industry (11%), and “waste and others” (7.2%) sectors. During the same period, increases in emissions were observed in the oil and gas (6.9%) and agriculture sectors (5.6%) (ECCC, 2025).

In accordance with UNFCCC procedures, emissions are often reported by sector (defined by the IPCC), with the breakdown for Canada in 2023 shown in Figure 9.5. Canada’s largest contributing sector is stationary combustion sources^{Footnote 7} (298 Mt CO₂-eq/yr, or 43%) followed by transport (195 Mt CO₂-eq/yr, or 28%). These are the sectors where the largest reductions in emissions will be required if we are to reach our targets of reducing emissions by 45 to 50% below the 2005 level by 2035 and achieve net-zero emissions by 2050.

Per capita emissions are not uniform across Canada (Table 9.1) (ECCC, 2025). Table 9.1 shows per capita emissions for the year 2023. Provincially, the highest per capita emissions come from Saskatchewan (61.2 t CO₂-eq/person) and Alberta (56.1 t CO₂-eq/person). The lowest emissions come from Quebec (8.9 t CO₂-eq/person), Prince Edward Island (9.2 t CO₂-eq/person), Ontario (10.2 t CO₂-eq/person), and British Columbia (10.8 t CO₂-eq/person), with other provinces and territories closer to the national average of 17.3 t CO₂-eq/person. The higher per capita emissions in Alberta and Saskatchewan are linked to the presence of the oil and gas industry and to the heavy dependence on fossil fuel combustion for electricity generation. Lower per capita emissions compared with the national average are associated with the use of nuclear energy to generate electricity in Ontario and the use of hydropower to generate electricity in Quebec and British Columbia, among other factors.

Figure take-away: Stationary combustion and transport are the two largest greenhouse-gas-emitting sectors in Canada.

Figure title: Contributions to greenhouse gas emissions from various sectors in Canada in 2023

Figure 9.5: Pie chart showing the breakdown of Canada’s emissions by Intergovernmental Panel on Climate Change (IPCC) sectors (in megatonnes of carbon dioxide equivalent units, or Mt CO₂-eq), for the year 2023. Adapted from: ECCC (2025).

Over the 1990 to 2023 period, Canada’s GHG emissions per unit of gross domestic product (GDP) decreased by about 45%. GHGs emitted to produce $1 billion in goods and services decreased from 0.52 to 0.29 Mt CO₂-eq (ECCC, 2025). The amount of GHGs emitted per person also decreased by about 26% from 2005 to 2023, from 23.5 t CO₂-eq/capita to 17.3 t CO₂-eq/capita, respectively, although Canada is still one of the highest per capita emitters in the world (ECCC, 2025). Although per-unit GDP and per capita emissions have declined substantially, Canada’s total GHG emissions have declined more slowly because both GDP and the population have increased over time.

9.3.2.1: Atmospheric greenhouse gas measurements for estimating anthropogenic emissions

The background CO₂ in Earth’s atmosphere has been measured at key locations, like Mauna Loa (Hawaii) and the South Pole, since 1957, providing accurate measurements (Keeling, 1960). Canadian measurements began in 1976 at Alert, Nunavut (Worthy et al., 1994), and expanded to over 20 federal sites across Canada at its peak. The Canadian GHG monitoring network currently consists of 14 sites across the country. Other GHG measurements are made closer to source and sink locations, using instruments that are stationary or on mobile platforms, such as aircraft, automobiles, or even bicycles (Vogel et al., 2024). Satellites began measuring atmospheric CO₂ and methane near the Earth’s surface in 2002 (Buchwitz et al., 2005), and the accuracy, precision, coverage, and spatial resolution of these measurements continue to improve (Jacobs et al., 2024). These various types of atmospheric GHG observations can be used with atmospheric transport models or other wind data to quantify GHG emissions (for example, Chan et al., 2020; Nassar et al., 2017). This approach to quantifying emissions is referred to as the top-down approach.

At present, emissions reporting by Canada and other UNFCCC signatories is based primarily on bottom-up, or activity-based, methodologies, which use the quantity of fossil fuels burned, combined with an assumed emission factor, to estimate GHG emissions. These reported emissions are typically provided on an annual basis, with a lag time of about 16 months. The accuracy of CO₂ emissions reporting is considered quite reliable at the provincial and annual scales. However, it is less reliable at finer timescales and spatial scales, such as the daily emissions in a specific urban area or facility, like a fossil fuel power plant or industrial site (Wren et al., 2023). For methane, significant uncertainties in the reported emissions at a range of spatial scales have been identified in Canada and other countries, primarily due to errors in assumptions about fugitive methane emissions. For both CO₂ and methane, there are benefits to top-down atmospheric measurement-based approaches: they help to verify and reduce uncertainty in reported anthropogenic emissions and to detect and quantify unknown emission sources.

National reporting methods must be consistent with the IPCC Task Force on Emission Inventories (IPCC-TFI) guidelines (IPCC, 2006), which were updated in 2019 (IPCC, 2019). The 2019 version of the guidelines supports the increased role of atmospheric observations, including stationary measurements, aircraft, and satellites. Countries such as the United States (The Greenhouse Gas Monitoring and Measurement Interagency Working Group, 2023), transnational governing bodies such as the European Commission (Ciais, 2015), and international bodies such as the World Meteorological Organization (WMO) have emphasized the importance of observation-based GHG emissions monitoring as a key component of emissions reporting and verification. The WMO Global Greenhouse Gas Watch (G3W) initiative (WMO, 2024) is a major effort to increase and coordinate greenhouse gas measurement and modelling capabilities across nations to support this objective.

Canada has recently begun incorporating some atmospheric observation-based approaches in its reporting practices, especially the use of aircraft (for example, Johnson et al., 2023) and other platforms for measuring methane emissions, with the realization that a combined (or hybrid) top-down and bottom-up approach may provide the most reliable emissions information (Chan et al., 2024). Satellites have also been used for quantifying anthropogenic CO₂ (Moeini et al., 2025) and methane (McLinden et al., 2024) emissions in Canada, but, so far, data limitations such as infrequent coverage have prevented their routine use. New, openly available satellite data on GHGs from the non-governmental organization MethaneSat (Miller et al., 2024), the Carbon Mapper public-private consortium (Keremedjiev et al., 2022), and the European Commission’s upcoming Copernicus Anthropogenic CO₂ Monitoring Mission (CO2M) (Kuhlmann et al., 2019) will greatly increase space-based emissions monitoring capabilities worldwide. Some advocates for greater investments in satellite observations believe that they could eventually become an integral component of emissions measurement, reporting, and verification systems (Janssens-Maenhout et al., 2020).

9.3.3: Response of the carbon cycle to anthropogenic emissions

Since the start of the industrial era in 1850, about 40% of CO₂ emitted from human activities has remained in the atmosphere (Friedlingstein et al., 2023; Le Quéré et al., 2018; Sabine et al., 2004). The remaining 60% has been taken up by the land and the ocean (Figure 9.6). Currently (2013 to 2022), land and ocean together take up about 54% of the total anthropogenic CO₂ emissions from fossil fuel combustion and land-use change (Friedlingstein et al., 2025) (Chapter 7, section 7.7; Figure 9.2). The rest of the emissions stay in the atmosphere. Uptake by the land and ocean has slowed the rate of accumulation of CO₂ in the atmosphere and the rate of climate change.

Since 2005, the Global Carbon Project (GCP) has coordinated a cooperative effort by the international science community to publish annual global carbon budgets. These budgets illustrate the proportions of anthropogenic CO₂ emissions from fossil fuels and land-use change that are taken up by the land and ocean, and the proportion that remains in the atmosphere and contributes to CO₂ growth. This is shown in Figure 9.6 for the 1850 to 2023 historical period (Friedlingstein et al., 2025). Each year, the estimates of these components of the global carbon budget are extended and updated with the latest available information. The latest available estimates from the Global Carbon Project extend the global carbon budget to the year 2023 and use projected values for the year 2024 (Friedlingstein et al., 2025). Over the 1850 to 2024 period, cumulative fossil fuel emissions are estimated to be 500 Gt C (± 25 Gt C); land-use change emissions, 225 Gt C (± 65 Gt C); ocean and land carbon uptakes, 185 Gt C (± 35 Gt C) and 225 Gt C (± 60 Gt C), respectively; and the change in atmospheric carbon burden, 290 Gt C (± 5 Gt C) (Friedlingstein et al., 2025).

Figure take-away: Not all emissions of carbon dioxide into the atmosphere from fossil fuel burning and land-use change remain in the atmosphere.

Figure title: Global carbon budget for the 1850 to 2023 period

Figure 9.6: Graph showing the partitioning of anthropogenic carbon dioxide (CO₂) emissions into their components over the historical period of 1850 to 2023. CO₂ fluxes are expressed in gigatonnes of carbon per year (Gt C/yr) and plotted against time. Fossil fuel and land-use change emissions are shown as positive values (greater than 0), and fluxes into the atmosphere, ocean, and land carbon pools as negative values. Adapted from: Friedlingstein et al. (2025).

Rising atmospheric CO₂ concentrations and the effects of climate change (primarily warming) affect both the land and ocean (Arora et al., 2020), though through different processes. On land, a higher atmospheric CO₂ concentration enhances photosynthesis (Haverd et al., 2020). This process, called the CO₂ fertilization effect, results in greater plant growth and more carbon stored on land (Walker et al., 2021). In contrast, carbon uptake by the ocean in response to an increasing atmospheric CO₂ concentration is driven primarily by a physical process: the dissolution of CO₂ in water. A higher atmospheric CO₂ concentration leads to a greater difference between the CO₂ levels in the atmosphere and the upper ocean, causing more CO₂ to dissolve into the ocean. Over tens to hundreds of years, ocean circulation carries this carbon to the deep ocean. Chapter 7, section 7.7, explains the carbon cycle processes related to ocean chemistry in more detail. The uptake of carbon by phytoplankton (section 9.2.1.2) serves as a shortcut in this respect, moving some of the carbon more quickly into the deep ocean. This occurs when phytoplankton convert dissolved inorganic carbon to organic carbon through photosynthesis and then die, sinking down in the water column and carrying the carbon directly to the deep ocean. However, unlike on land, the rate of biological uptake (by phytoplankton) does not increase in response to increasing CO₂ concentrations because there is already so much carbon in seawater. Overall, increasing atmospheric CO₂ causes both the land and the ocean to take up and store more carbon, but, in the ocean, this occurs primarily through the mechanism of dissolution.

While a higher atmospheric CO₂ concentration increases carbon uptake by both land and ocean, this is partially offset by increasing temperatures. On land, higher temperatures increase the rate at which soil organic matter decomposes, which releases more CO₂ into the atmosphere, with these effects varying regionally. Higher temperatures currently increase plant growth at high latitudes (where growth is often limited by cold temperatures) (J. Wang et al., 2023). However, in warm, tropical regions, high temperatures typically decrease plant growth, due to the heat and associated soil moisture stress (Malhi et al., 2008). Overall, climatic changes associated with increasing CO₂ (and other GHGs) reduce land carbon uptake and storage at the global scale (Canadell et al., 2021). While changes in precipitation also influence carbon uptake over land, temperature remains the dominant factor at a global scale.

Climate warming also reduces the flux of CO₂ to the ocean, since warmer water holds less dissolved CO₂ than colder water. In addition, climate change increases the stratification of the ocean by warming the surface water and melting the ice, which lowers the salinity of the surface water. The warmer and fresher surface waters are less dense than the underlying seawater, so they float on top, reducing the downward mixing of CO₂ from the surface into the deep ocean. Therefore, climate warming reduces the rate at which carbon is taken up by the ocean (Canadell et al., 2021), partially offsetting the increase in oceanic CO₂ uptake driven by higher atmospheric CO₂ concentrations.

In summary, the land and ocean respond to rising atmospheric CO₂ concentrations and resulting climate change in two opposing ways. On a global scale, the increasing atmospheric CO₂ concentration leads to greater carbon uptake by both the land and ocean, but, at the same time, the increasing temperature makes the land and ocean release some carbon back into the atmosphere. The increased amount of carbon taken up by the land and ocean as a result of increasing atmospheric CO₂ is much greater than the increased amount of carbon released into the atmosphere from the land and ocean as a result of warming (Arora et al., 2013). As a result, in the short term (about 10 to 20 years), the land and ocean are expected to remain net carbon sinks in response to continued human-caused CO₂ emissions, absorbing a substantial portion of these emissions. In the longer term (up to and beyond 100 years), the ocean is expected to continue acting as a carbon sink, although the estimated magnitude of this sink depends on the emissions scenario considered (Canadell et al., 2021). There is greater uncertainty among the models regarding the long-term status of land as a carbon sink. Model simulations suggest that the land carbon sink will weaken or even shift to a carbon source (Canadell et al., 2021; Koven et al., 2022; Li et al., 2021; Sharma et al., 2023). Changes to land and ocean carbon uptake in Canada are assessed in section 9.5.

9.3.4: Confidence terms in key messages: summary of evidence

Key Message 9.3: The increase in atmospheric carbon dioxide concentrations since the pre-industrial era has unequivocally been caused by emissions from the burning of fossil fuels and human-caused land-use change. In 2023, annual human-caused global carbon dioxide emissions reached 41 CO₂/yr (± 3 Gt CO₂/yr) (gigatonnes of carbon dioxide per year); including contributions from other greenhouse gases, total greenhouse gas emissions were 57 Gt CO₂-eq/yr (carbon dioxide equivalent units per year).

Key Message 9.4: Based on Canada’s national inventory report, total anthropogenic greenhouse gas emissions for Canada peaked in 2007 at 774 Mt CO₂-eq (megatonnes of carbon dioxide equivalent units). Notable reductions occurred in 2008 to 2009 during the global recession (714 Mt CO2-eq/yr) and in 2020 during the COVID-19 pandemic (682 Mt CO2-eq/yr). Emissions for 2023 (694 Mt CO2-eq/yr; most recent year reported) are approximately 10% lower than their 2007 peak, despite continued growth in Canada’s population and economy.

Key Message 9.5: Land and ocean together have taken up more than half of global anthropogenic carbon dioxide emissions since the pre-industrial era (very high confidence). The ocean is expected to continue taking up carbon dioxide in the long term (beyond a century), but the land carbon sink may weaken or become a carbon source (high confidence).

The evidence for the statement in Key Message 9.3 that the increase in atmospheric CO₂ is caused unequivocally by anthropogenic emissions (IPCC, 2021) is based on the unique isotopic signature of the carbon released from the burning of fossil fuels and deforestation. A confidence statement is not associated with this key message, because it is an unequivocal statement of fact. Seminal studies by Friedli et al. (1986) and Francey et al. (1995) produced decisive evidence supporting the anthropogenic cause of the rise in the atmospheric concentration of CO₂. Since then, several other studies have built on this work by using carbon isotopes to link the rise in atmospheric CO₂ to the combustion of fossil carbon, thus improving our understanding of the effects of anthropogenic emissions on atmospheric CO₂ levels (for example, Graven et al., 2020; Rubino et al., 2013). We have not assigned a confidence level to the magnitude of human-caused emissions of CO₂ and all GHGs for 2023 because they are based on single studies (Friedlingstein et al., 2025; IEA et al., 2024), although the data reported in these studies are based on multiple sources. Nevertheless, an uncertainty of approximately 8% associated with global CO₂ emissions has been noted (Friedlingstein et al., 2025), providing an indication of their reliability. This uncertainty mainly reflects the precision of fossil fuel emissions measurements and variations in the accounting methods used by different countries. There is no uncertainty estimate available for the global GHG emissions in CO₂-equivalent units from IEA et al. (2024).

There is no confidence level associated with the Canadian GHG emissions reported in Key Message 9.4, as they are based on a single source—Canada’s national inventory report (ECCC, 2025). The reported uncertainty surrounding Canada’s national emissions is roughly 3% when emissions from the land-use change sector are excluded, and approximately 10% when they are included.

The very high confidence in Key Message 9.5, that the land and ocean have taken up more than half of anthropogenic CO₂ emissions, is based on broad agreement among several observation- and modelling-based studies. These include (i) measurements of atmospheric CO₂ concentrations; (ii) estimates of global anthropogenic emissions derived from United Nations energy data, based on the submissions by Annex 1 countries^{Footnote 8} under the UNFCCC, and other sources (Friedlingstein et al., 2023, 2025); (iii) measurements of changes in the oceanic storage of dissolved inorganic carbon (DIC) (Gruber et al., 2019; Keppler et al., 2023; Müller et al., 2023; Sabine et al., 2004); and (iv) modelling studies based on land and ocean carbon cycle models (Friedlingstein et al., 2023, 2025). There is high confidence in continued carbon uptake by the ocean under all emissions scenarios (Canadell et al., 2021), based on multiple model results. The global ocean is expected to continue absorbing carbon from the atmosphere for some time, even after human-caused net CO₂ emissions reach zero. This is because the ocean carbon reservoir is not yet in equilibrium with the atmosphere, and the slow circulation processes that transport carbon from the surface to the deep ocean operate over timescales of centuries to millennia. In contrast, carbon uptake by the land is governed by biological processes, such as plant growth and soil carbon accumulation, which operate over much shorter (annual to decadal) timescales. As a result, there is high confidence in the greater future resilience of the ocean carbon sink than that of the land carbon sink. According to modelling studies, once human-caused net CO₂ emissions reach zero, land-based carbon uptake will slow down, and the land could potentially become a carbon source (Gillett et al., 2011; Jayakrishnan et al., 2024). This reversal to a carbon source is also expected under scenarios with higher levels of warming, causing increased ecosystem respiration and vegetation dieback to release stored carbon back into the atmosphere (Canadell et al., 2021; Koven et al., 2022).

9.4: Global climate response to carbon dioxide emissions

Key Message 9.6: There is an approximately linear relationship between global warming and cumulative carbon dioxide (CO₂) emissions (high confidence). This relationship implies a limit to the total amount of CO₂ that can be emitted into the atmosphere while keeping warming below a given level (for example, 2°C).

Key Message 9.7: There will be little to no further global warming from carbon dioxide (CO₂) after CO₂ emissions cease (medium confidence).

Key Message 9.8: In addition to carbon dioxide (CO₂), other greenhouse gases (non-CO₂ GHGs) in the atmosphere cause global warming. Limiting warming to a specific level requires emissions of long-lived GHGs such as CO₂ to reach net zero and emissions of short-lived GHGs to decline strongly (high confidence).

This section presents the near-linear relationship between the change in global temperature relative to the pre-industrial period and cumulative carbon dioxide (CO₂) emissions. It also shows that the global mean temperature will change little once CO₂ emissions cease. These findings imply that halting global warming requires CO₂ emissions to reach net zero, with the peak warming level determined by the total CO₂ emissions up to the time of net zero. However, since the climate system also responds to greenhouse gases other than CO₂ (non-CO₂ GHGs) and aerosols, the peak warming level will also depend on the rate of their emissions.

9.4.1: The linear relationship between global warming and cumulative carbon dioxide emissions

Earth system models^{Footnote 9} (ESMs) are used to assess how continued changes in anthropogenic emissions of CO₂ and other GHGs will affect the future climate. These models show that the temperature change relative to the pre-industrial era attributable to CO₂ is approximately linearly related to cumulative anthropogenic CO₂ emissions (Allen et al., 2009; Gillett et al., 2013; Matthews et al., 2009). The slope of this relationship is referred to as the transient climate response to cumulative CO₂ emissions (TCRE). This linear relationship makes it possible to calculate how much the globally averaged annual temperature will increase for every additional unit of CO₂ emitted into the atmosphere (Figure 9.7). The IPCC AR6 WGI concluded that the climate will warm by 1.65°C (best estimate) for every 1000 Gt (gigatonnes) of carbon added (or equivalently 0.45°C/1000 Gt CO₂), with a likely range from 1.0 to 2.3°C/1000 Gt C (or 0.27 to 0.63°C/1000 Gt CO₂) (Canadell et al., 2021). This estimate is based on both observations and model results. The uncertainty in this estimate is due to uncertainties in observation-based estimates of warming attributable to cumulative historical CO₂ emissions, and differences in the representation of climate and carbon cycle processes in different climate models.

The linear relationship between global warming and cumulative CO₂ emissions remains valid for a wide range of cumulative emissions (Herrington and Zickfeld, 2014). In general, it is understood that a combination of three processes, all nonlinear, creates this linear relationship (Bronselaer and Zanna, 2020; Canadell et al., 2021; Gillett, 2023; MacDougall, 2017): (i) the fact that each additional unit of CO₂ in the atmosphere causes slightly less warming than the previous unit; (ii) the less efficient heat uptake by the ocean over time; and (iii) the increased airborne fraction of CO₂ emissions at higher atmospheric CO₂ levels, as the capacity of land and ocean carbon sinks to absorb carbon gradually diminishes. These processes interact with each other over time to maintain the linear relationship between global warming and cumulative CO₂ emissions.

Figure take-away: Every additional unit of anthropogenic carbon dioxide (CO₂) emissions increases global warming.

Figure title: Near-linear relationship between cumulative carbon dioxide (CO₂) emissions and the increase in global surface temperature.

Figure 9.7: Graph showing the linear relationship between cumulative global anthropogenic carbon dioxide (CO₂) emissions (in gigatonnes, or Gt) since 1850 and the average increase in global surface air temperature (in degrees Celsius). Historical observations are shown with a solid black line. Projections from Earth system models are shown with grey shading for the historical period (historical human-caused warming) and coloured shading for the future emissions scenarios to 2050. The model results, represented by the coloured lines and shading, are provided in the form of temperature projections, based on five different shared socio-economic pathway (SSP) emissions scenarios, including the very low (SSP1-1.9), low (SSP1-2.6), intermediate (SSP2-4.5), high (SSP3-7.0), and very high (SSP5-8.5) scenarios. Coloured areas show the assessed very likely range of global surface temperature projections, and thick coloured central lines show the median estimates. Source: IPCC AR6 WGI SPM Figure SPM.10 (IPCC, 2021).

Another way to visualize the effects of CO₂ emissions on global temperature is as a function of time (Figure 9.8). Different emissions pathways (Figure 9.8a) have different effects on the globally averaged temperature (Figure 9.8b). The global temperature response shown in Figure 9.8b follows directly from the linear relationship between global temperature change and cumulative anthropogenic CO₂ emissions. When annual emissions remain constant, the global temperature will continue to rise because the cumulative CO₂ emissions into the atmosphere will continue to increase. Net zero emissions are achieved when anthropogenic CO₂ emissions are balanced by CO₂ removal from the atmosphere. When CO₂ emissions reach net zero, the global temperature is projected to stabilize. The climate response to net-zero CO₂ emissions is discussed in more detail in section 9.4.2. The global temperature will begin to decrease when emissions become negative, that is, when CO₂ drawn down or removed from the atmosphere exceeds emissions.

Figure take-away: The global temperature will stabilize when anthropogenic carbon dioxide emissions reach net zero.

Figure title: Global temperature response to different anthropogenic carbon dioxide (CO₂) emissions trajectories

Figure 9.8: Graphs illustrating a) different trajectories for carbon dioxide (CO₂) emissions over the rest of the 21st century (in gigatonnes of CO₂ per year, or Gt CO₂/year) and b) the global temperature response (in degrees Celsius) to these trajectories, as implied by the linear relationship between global temperature change and cumulative anthropogenic CO₂ emissions. The examples of possible emissions pathways, beginning with approximate emissions of 37 Gt CO₂ in 2020, as shown in a), include increasing emissions, constant emissions, declining emissions, and emissions that decline to zero and remain constant or become negative (if CO₂ drawn down or removed from the atmosphere exceeds emissions). The response of global temperature to emissions pathways, as shown in b, with a global temperature change of approximately 1.0°C above pre-industrial levels by 2020, is depicted for illustrative purposes only. The global temperature will continue to increase until emissions reach net zero. When CO₂ emissions reach net zero, the global temperature is projected to approximately stabilize. The global temperature will only decline when CO₂ emissions become negative.

The linear relationship between global warming and cumulative CO₂ emissions makes it possible to calculate the cumulative amount of anthropogenic CO₂ that can be emitted before global warming reaches a specific level, for example, 2°C higher than pre-industrial levels (Canadell et al., 2021; Rogelj et al., 2019; Zickfeld et al., 2009). The amount of CO₂ that can still be added to the atmosphere before a specific warming level is reached is referred to as the remaining carbon budget (Figure 9.9).

Figure take-away: There is a limit to how much additional carbon can be emitted to stay below a given warming level.

Figure title: Illustration of the calculation of the remaining carbon budget for a given level of global temperature increase

Figure 9.9: Diagram showing the calculation of the remaining carbon budget, or the amount of CO₂ (carbon dioxide, in gigatonnes, or Gt) that can still be added to the atmosphere before a specific warming level (expressed as the increase in temperature since the pre-industrial era, in degrees Celsius) is reached, based on the linear relationship between cumulative CO₂ emissions and the global average surface air temperature increase. The remaining carbon budget is calculated by starting from a chosen warming limit and subtracting past human-caused warming, the effect of greenhouse gases other than CO₂ (non-CO₂ gases), and the temperature change expected after CO₂ emissions cease. The warming that remains is then located on the vertical axis of the near-linear relationship between CO₂ emissions and temperature change (transient climate response to cumulative CO₂ emissions, or TCRE) traced across the line, and read down to the horizontal axis to yield allowable CO₂ emissions. This value is then adjusted downward to account for Earth system processes not fully captured in the models. The shaded area illustrates the uncertainty in the calculated value of TCRE, which leads to uncertainty in the calculation of the remaining carbon budget, as discussed in the text. This uncertainty increases as cumulative CO₂ emissions increase, as also seen in Figure 9.7. Source: IPCC AR6 WGI TS Figure TS.18 (Arias et al., 2021).

Several factors determine the size of the remaining carbon budget, as illustrated in Figure 9.9 (Canadell et al., 2021; Rogelj et al., 2019), including (i) estimated historical warming (which determines how much warming is left before reaching the chosen limit); (ii) projected warming from aerosols and non-CO₂ GHGs, such as methane and nitrous oxide (N₂O) (which determines the contribution from non-CO₂ climate forcers toward a given global temperature level); (iii) processes and feedback mechanisms in the Earth system that are not represented in the current generation of Earth system models, such as the release of CO₂ and methane from thawing permafrost soils; and (iv) the change in the global surface temperature after CO₂ emissions cease, referred to as the zero emissions commitment.

Given the uncertainty surrounding these factors, the remaining carbon budget is typically quantified by providing either an uncertainty range or the probability of limiting global warming to within a given temperature limit. The estimate of the remaining carbon budget as of January 2025, with a 50% chance of limiting global warming to 1.5°C, is 130 Gt of CO₂, or 35 Gt C, which represents about three years of global anthropogenic CO₂ emissions at the current rate. To have a 50% chance of limiting global warming to 2°C, the remaining carbon budget is estimated to be 1050 Gt CO₂ (286 Gt C), which represents about 26 years of global anthropogenic CO₂ emissions at the current rate (Forster et al., 2025).

9.4.2: Climate response to net-zero carbon dioxide emissions

As described in the previous sections (sections 9.2 and 9.3), carbon cycle processes on land and in the ocean have taken up more than half of the CO₂ released into the atmosphere by human activities. These processes operate on a range of timescales, from minutes to hundreds of thousands of years. Because of the long timescales of many of these processes, some of the CO₂ remains in the atmosphere for centuries and even millennia after it is released (Eby et al., 2009; Solomon et al., 2009). In addition, the slow response timescales of the climate system, in particular the deep ocean, delay the planet’s temperature response to a change in the atmospheric CO₂ concentration. If CO₂ emissions cease, the global average temperature will still change a little as a result of the slow decline in the atmospheric CO₂ concentration and the lag in the temperature response (Figure 9.8b, solid magenta line) (MacDougall et al., 2020). It follows that stabilizing global warming will require CO₂ emissions to reach zero (Arias et al., 2021; IPCC, 2022b; Matthews and Caldeira, 2008). The IPCC AR6 WGI report concluded that the global temperature will likely decrease by about 0.08°C during the 50-year period after CO₂ emissions cease (with a 90% chance that the change would be between 0.34°C of cooling and 0.28°C of warming) (Lee et al., 2021). However, there is low confidence about whether temperatures will rise or fall after emissions cease. A more recent study revised this estimate to slightly greater cooling of 0.19°C (compared to 0.08°C), with a range from 0.44°C of cooling to 0.04°C of warming (Borowiak et al., 2025).

Reaching zero CO₂ emissions will be challenging because emissions from some sectors (for example, aviation) are difficult to eliminate completely. Consequently, the concept of zero CO₂ emissions has been revised to net-zero CO₂ emissions—that is, the balance of total anthropogenic CO₂ emissions and anthropogenic removals. In terms of the temperature outcome, zero and net-zero CO₂ emissions are not necessarily equivalent (Zickfeld et al., 2023). For instance, afforestation may contribute to net-zero CO₂ emissions by offsetting some of the excess anthropogenic emissions. However, since forest cover is darker (that is, less reflective) than grassland or bare ground, the newly forested lands will absorb more incoming solar radiation than the previous land surface. As a result, a net-zero CO₂ scenario in which afforestation is used to offset residual fossil fuel emissions could lead to a small amount of warming, compared to a scenario in which fossil fuel emissions cease entirely (Zickfeld et al., 2023).

While the global average temperature is projected to change only a little once CO₂ emissions cease, regional changes are expected to continue. However, the pattern and magnitude of these changes, and whether they will result in regional temperature increases or decreases, are highly uncertain (Gillett et al., 2011; MacDougall et al., 2022). The ocean is expected to continue to warm and the sea level is expected to rise for a few thousand years after CO₂ emissions cease, although this will occur more slowly once emissions have ceased (Clark et al., 2016; Gillett et al., 2011; Van Breedam et al., 2020) (Chapter 7, section 7.4).

9.4.3: Effect of non-carbon dioxide greenhouse gases on peak warming

The linear relationship between global warming and cumulative emissions only applies to greenhouse gases with long atmospheric lifetimes, such as CO₂, and not to short-lived, non-CO₂ GHGs and aerosols. While much of the released CO₂ will remain in the atmosphere for centuries and even millennia, other GHGs, such as methane and some hydrofluorocarbons, as well as aerosols released into the atmosphere by human activities, have much shorter atmospheric lifetimes. Because these non-CO₂ gases and aerosols are shorter-lived, their atmospheric concentrations and the associated temperature change will decline rapidly once their emissions cease. Therefore, the effect of short-lived, non-CO₂ GHGs on warming at any given time—contrary to that of CO₂—is better characterized by their rate of emissions over a short period leading up to that time, rather than by cumulative emissions up to that time (Forster et al., 2021; S. M. Smith et al., 2012). As a result, if emissions of both short- and long-lived GHGs are reduced concurrently, temperatures will peak and then decline. If the mitigation of short-lived GHGs occurs much earlier than that of long-lived GHGs, temperatures will stabilize very near peak values, without a short-term decline after the peak (Forster et al., 2021). While emissions of long-lived greenhouse gases such as CO₂ need to reach net zero for the global average temperature to stabilize, emissions of short-lived non-CO₂ GHGs need to be strongly reduced, but not fully eliminated (Arias et al., 2021; Forster et al., 2021).

9.4.4: Confidence terms in key messages: summary of evidence

Key Message 9.6: There is an approximately linear relationship between global warming and cumulative carbon dioxide (CO₂) emissions (high confidence). This relationship implies a limit to the total amount of CO₂ that can be emitted into the atmosphere while keeping warming below a given level (for example, 2°C).

Key Message 9.7: There will be little to no further global warming from carbon dioxide (CO₂) after CO₂ emissions cease (medium confidence).

Key Message 9.8: In addition to carbon dioxide (CO₂), other greenhouse gases (non-CO₂ GHGs) in the atmosphere also cause global warming. Limiting warming to a specific level requires emissions of long-lived GHGs such as CO₂ to reach net zero and emissions of short-lived GHGs to decline strongly (high confidence).

There is high confidence in Key Message 9.6 for the near-linear relationship between global warming caused by CO₂ and its cumulative emissions, because of agreement among the several different lines of evidence used to investigate this relationship. These include observation-based studies, simulations by climate models of intermediate and full complexity, and theoretical frameworks (Canadell et al., 2021). The second sentence of Key Message 9.6, that this near-linear relationship implies a limit to the total amount of CO₂ that can be emitted for a given global warming level, does not have an associated confidence statement. Such a calculation derives directly from this linear relationship and is therefore a factual statement. However, the exact amount of CO₂ that can be emitted (that is, the calculated remaining carbon budget) is uncertain. This uncertainty results from the uncertainty in the estimation of the transient climate response to cumulative CO₂ emissions (TCRE) (1.0 to 2.3°C/1000 Gt C; section 9.4.1), as well as uncertainty in the other factors, such as the projected warming from non-CO₂ climate drivers and feedbacks not represented in the current generation of Earth system models (section 9.4.1).

In the context of Key Message 9.7, Earth system models have been used to explore the effect of the cessation of anthropogenic CO₂ emissions on global surface air temperatures. MacDougall et al. (2020) modelled a small average decrease in temperature of 0.08°C (with a range from -0.34°C to 0.28°C) 50 years after CO₂ emissions cease. On the basis of these results, the IPCC assessed the absolute magnitude of the temperature change 50 years after the cessation of CO₂ emissions as likely less than 0.3°C, but with low confidence about whether the globally averaged surface temperature will increase or decrease following the cessation of emissions (Lee et al., 2021). In a recent study, the estimate from MacDougall et al. (2020) was revised slightly downward to a decrease of 0.19°C (with a range from ‑0.44°C to 0.04°C) (Borowiak et al., 2025). On the basis of this evidence, we have medium confidence that little to no further global warming from CO₂ would occur after CO₂ emissions cease.

A confidence statement is not associated with the first sentence of Key Message 9.8 that global warming is also caused by non-CO₂ GHGs, because that is a statement of fact. The IPCC AR6 WGI report (Arias et al., 2021) states that, along with the increase in atmospheric CO₂, the increases in atmospheric concentrations of methane and nitrous oxide are also unequivocally due to human activities, and that human influence is the main driver of changes in the Earth system, including global warming. The first part of the second sentence of Key Message 9.8 regarding long-lived GHGs is based on the linear relationship between global warming and cumulative CO₂ emissions, as also noted in the Summary for Policy Makers of the IPCC AR6 WGI report (IPCC AR6 WGI SPM D.1.1) (IPCC, 2021), as well as the extension of the TCRE approach to all long-lived climate drivers (Allen et al., 2022). The second part of the second sentence of Key Message 9.8 that mentions the role of reducing short-lived non-CO₂ GHG emissions (as opposed to their cumulative emissions) in limiting warming to a given level, is based on evidence from climate simulations and theoretical considerations that warming from GHGs with shorter atmospheric lifetimes is related to their rate of emissions (Forster et al., 2021; S. M. Smith et al., 2012). Taken together with Key Message 9.7, this results in high confidence in the finding, conveyed in Key Message 9.8, that limiting warming to a specific level will require emissions of long-lived GHGs, including CO₂, to reach net zero and radiative forcing from short-lived non-CO₂ GHGs to decline (Arias et al., 2021; IPCC, 2022b; Matthews and Caldeira, 2008).

9.5: The carbon cycle in Canada

Key Message 9.9: Carbon in Canada’s oceans is mainly stored in seawater and marine sediments. On land, almost all (about 95%) of the carbon in Canada is stored in soils, including permafrost and peatlands, with the rest (about 5%) stored in vegetation (very high confidence).

Key Message 9.10: Since the last Ice Age, there has been a net uptake of carbon by Canada’s land mass. At present (2000 to 2019), the land continues to be a carbon sink (medium confidence). However, extreme events like the 2023 fire season can cause the land to become a temporary source of carbon (low confidence). In the future, both carbon uptake from increased vegetation growth (medium confidence) and carbon emissions from permafrost thaw, wildfires, and other disturbances (high confidence) are expected to increase. However, their net effect on the carbon balance of Canada’s land ecosystems remains uncertain.

Key Message 9.11: The annual forest area burned by fires in Canada has increased since the early 1980s (medium confidence). The area burned is projected to increase further as the climate continues to warm (high confidence).

Key Message 9.12: On average, Canada’s oceans (inside the Exclusive Economic Zone) take up carbon dioxide (CO₂) (medium confidence), although some nearshore areas emit CO₂ (low confidence). Overall, Canada’s oceans will continue to take up CO₂, as long as atmospheric CO₂ concentrations continue to increase (medium confidence).

This section assesses the current state of the natural carbon cycle in Canada, including its land mass and oceans, and the projected response of the land and ocean to future changes in climate. Limited information is available to fully quantify the exchanges of carbon dioxide (CO₂) between the atmosphere and Canada’s land mass and oceans, although more data are available for the land than for the oceans. Assessing natural exchanges of CO₂ across Canada’s land mass and oceans has two main purposes. First, both the land and ocean currently act as CO₂ sinks; quantifying the size of these sinks—or at least the size of the land-based sink—helps to elucidate Canada’s contribution to the global land-based CO₂ sink. Second, estimating natural CO₂ exchanges concurrently with Canada’s greenhouse gas (GHG) emissions provides an important context for evaluating avoided emissions (potential emission increases prevented by mitigation measures) and assessing the atmospheric CO₂ removal potential of nature-based climate solutions (NBCS).

9.5.1: The carbon cycle of Canada’s land ecosystems

Canada’s land mass stores about 280 to 330 Gt C (gigatonnes of carbon), primarily (about 95%) in the top layer (1 m) of its soils, with the remainder mainly stored in plant material (about 5%) and inland waters (small amounts) (Domke et al., 2018; Lajtha et al., 2018; Sothe et al., 2022). The amount of carbon in Canada’s soils is substantial. Canadian soils hold roughly 20% of the world’s total soil organic carbon in their top 1 m layer.

Canada’s primary ecosystems include forests, tundra, croplands, grasslands, and inland waters (including wetlands, lakes, and rivers). Forests cover 367 million ha, or nearly 40% of Canada’s land area (Canadian Forest Service, 2024). Approximately 72% of Canada’s land is unmanaged (Ogle et al., 2018). The remaining 28% is managed for products such as food, fibre, and fuel, or for social and ecological functions. In the following subsections, forests, permafrost, wetlands, croplands, and inland waters are considered individually. A further section is devoted to wildfire, given the large carbon emissions from Canada’s landscape during extreme wildfire seasons,^{Footnote 10} such as the one that occurred in 2023 (Byrne et al., 2024).

This section assesses historical and future carbon exchanges between the land and atmosphere in managed and unmanaged terrestrial ecosystems and freshwater ecosystems in Canada, using top-down and bottom-up approaches. Top-down approaches estimate carbon fluxes from atmospheric concentrations of CO₂ and methane (CH₄). Bottom-up approaches estimate carbon fluxes using on-site observations and models that consider underlying ecosystem processes. For managed lands in Canada, carbon flux estimates are largely based on bottom-up approaches, using national and provincial statistics and other methods (section 9.5.1.1). They are presented in the country’s National Inventory Report, which is published annually (for example, ECCC, 2025), in accordance with Canada’s obligations under the United Nations Framework Convention on Climate Change (UNFCCC) and its reporting guidelines.

9.5.1.1: Forests

Canada’s forests make up 9% of the world’s forests, including 25% of the world’s boreal forest (Brandt et al., 2013; Canadian Forest Service, 2024; FAO, 2020). About two thirds of Canada’s forest is managed, and the remaining one third is unmanaged. The former includes lands managed for wood fibre (for example, lumber and pulp) and wood-based bioenergy, as well as lands protected to limit insect and fire disturbances and to conserve forest ecological values (for example, national and provincial parks) (Stinson et al., 2011).

In Canada, approximately 21 Gt C is stored in living and dead terrestrial plant biomass (Sothe et al., 2022). Forests account for over 99% of this carbon storage, while plants in other ecosystems, such as grasslands, tundra, peatlands, and agricultural land, account for only 0.2 Gt C (Sothe et al., 2022) (Figure 9.10a). Canada’s forest ecosystems also store large amounts of carbon in the soil (Figure 9.10b). For example, another study estimates that Canada’s managed forests stored about 46 Gt C in 2018, 40% of this in soil organic matter (Andrews et al., 2022).

Forests are responsible for most of the CO₂ absorbed through photosynthesis in Canada (for example, Gonsamo et al., 2013). At the same time, large amounts of carbon are lost from Canada’s forests each year through natural processes, including wildfire, the decomposition of dead plant material, and insect outbreaks. Carbon is also lost through direct anthropogenic activities, such as prescribed burns; harvesting and the resulting decomposition of harvest residues and harvested wood products (HWP); and the conversion of forest land to other uses (ECCC, 2025).

Figure take-away: More carbon is stored in plant material in forests than in other ecosystems.

Figure title: Geographical distribution of carbon in plant material and in soil organic matter

Figure 9.10: Maps showing the distribution of carbon stocks in plant material (in kg of carbon per m², or kg C m^-2) in a) forest and non-forest lands in Canada and b) soil organic matter to a depth of 1 m. Source: Sothe et al. (2022).

This balance between carbon uptake and losses in Canada’s forests has shifted over time. Since the last Ice Age, land ecosystems have taken up carbon in Canada. This accumulation of carbon occurred as the glaciers retreated. Models suggest that, in the late 1800s, during the pre-industrial era, Canada’s managed and unmanaged forests were net sources of carbon, due to forest fires, forest insect-induced mortality, and harvesting (J. Chen et al., 2000). As forests regrew in the 20th century, climate warming, rising CO₂ concentrations, and nitrogen deposition from pollution may have enhanced tree growth and resulted in more carbon being gained than lost from both managed and unmanaged forests (J. Chen et al., 2000).

During the 2000 to 2019 period, Canada’s total land area appears to have been a net carbon sink (section 9.5.1.6). Although the analysis by Poulter et al. (2025) (in section 9.5.1.6) does not report the contribution from forests separately from that from other ecosystems, Canada’s managed and unmanaged forests are expected to make up a substantial portion of this carbon sink, since they dominate CO₂ exchanges between the land and atmosphere (Gonsamo et al., 2013; Poulter et al., 2025).

In Canada’s managed forests (Figure 9.11a), CO₂ drawdown from the atmosphere and emissions of CO₂, methane, carbon monoxide (CO), and nitrous oxide (N₂O) are closely tracked in CO₂-equivalent (CO₂-eq) units using inventory-based methods (Figure 9.11b) (ECCC, 2025), which take into account the direct effects of forest management actions. The methodology used to estimate GHG emissions and removals from managed forests reports emissions and removals separately for forest stands impacted by anthropogenic drivers versus those impacted by natural drivers—referred to as the anthropogenic component and natural disturbance component, respectively. These estimates are produced by Natural Resources Canada (NRCan) and published by Environment and Climate Change Canada (ECCC). Only the emissions and removals associated with the anthropogenic component are reported to UNFCCC but, to ensure transparency, all emissions and removals, including those related to the disturbance component, are also tracked separately. Since the NRCan methodology relies on historical growth and yield data, it does not explicitly account for changes in forest productivity driven by elevated atmospheric CO₂ or climate change, which are considered natural effects outside the scope of anthropogenic GHG reporting under the UNFCCC. The natural disturbance component includes GHG emissions associated with severe natural disturbances, such as wildfires or insect outbreaks, and the GHG removals from the atmosphere that occur as the stands regrow naturally. The anthropogenic component includes carbon change in the HWP pool, as well as emissions and removals associated with forest stands directly affected by past forest management activities (for example, harvesting), mature stands affected by low-mortality natural disturbances, and mature stands affected by stand-replacing natural disturbances in the past that have now reached commercial maturity. The separate and combined estimates of GHG emissions and removals from these two components are shown in Figure 9.11b.

As a result of the increase in natural disturbances (dashed red line in Figure 9.11b), Canada’s managed forests have shifted from being a carbon sink to a carbon source in recent years (values below zero indicate a sink, while positive values indicate a source). The natural disturbance component of the managed forest is estimated to have shifted from being a sink of approximately 60 Mt (megatonnes) CO₂-eq/yr during the 1990−1999 period, when post-fire regrowth exceeded the emissions from combustion and decay associated with fire and insect damage, to a source of around 10 Mt CO₂-eq/yr in 2000−2009 and 130 Mt CO₂-eq/yr in 2010−2019. Year-to-year variations largely depend on the extent of these natural disturbances, which have increased over this period. For example, a major pine beetle infestation from 1999 to 2015, which peaked in 2005, affected 18 million ha of forest (Perrakis et al., 2014). The sizable net emissions of 1100 Mt CO₂-eq from managed forests (Figure 9.11b, dashed red line) in 2023 are attributable to the largest fire season on record (Chapter 8, Box 8.4; section 9.5.1.7).

The anthropogenic component of the managed forest includes GHG emissions and drawdown associated with forest stands and the carbon change in the HWP pool associated with annual forest product production. Over the 1990 to 2023 period, the anthropogenic component (solid blue line in Figure 9.11b) was a source of 46 Mt CO₂-eq/yr (Figure 6-2a in ECCC, 2025). Wood products harvested from forests are tracked as part of the harvested wood products (HWP) carbon pool. Emissions of CO₂ and non-CO₂ GHGs (greenhouse gases other than CO₂) from these wood products may occur decades after the wood has been harvested and may not occur within Canada. Approximately one third of wood product emissions come from long-lived wood products that have reached the end of their useful life and are transferred to the waste stream, where they may decompose or be incinerated. The remaining two thirds of these emissions come from short-lived products, such as pulp and paper and bioenergy products (ECCC, 2025).

Figure take-away: Carbon uptake by managed forests has declined over time, and in recent years managed forests have become a source of carbon.

Figure title: Net greenhouse gas balance of Canada’s managed forests

Figure 9.11: a) Map of managed and unmanaged forests in Canada (Canadian Forest Service, 2024) and b) net greenhouse gas (GHG) flux for Canada’s managed forests (in megatonnes of carbon dioxide equivalent units per year, or Mt CO₂-eq/yr) during the 1990 to 2023 period. The solid blue line shows the anthropogenic component, or the combined value of GHG emissions and removals in the forest land category and the carbon change in the harvested wood products (HWP) pool, which are reported to the United Nations Framework Convention on Climate Change (same as in Figure 6-2 in ECCC, 2025). The dashed red lines show emissions and removals associated with severe natural disturbances, including insects, fire, and regrowth after disturbances (same as in Figure 6-3 in ECCC, 2025). The solid black line shows the total net GHG flux due to these natural and anthropogenic factors. Positive values indicate net emissions to the atmosphere (that is, source to the atmosphere); negative values indicate net removals from the atmosphere (that is, flux to the land) Map source: Canadian Forest Service (2024); data source: ECCC (2025).

The future strength of the carbon sink in Canada’s managed and unmanaged forests will depend on natural disturbance patterns, forest stand age, the atmospheric CO₂ concentration, and climate, among other factors. In Canada’s managed and unmanaged forests, trees younger than 150 years account for about 94% of the treed area and their average age is approximately 70 years (Maltman et al., 2023). Net carbon uptake by forests is projected to decrease in the future, due to the fact that, as forests age, they become less productive and, when they are disturbed and regenerate as young stands, they take time to reach peak productivity (Böttcher et al., 2008; Metsaranta et al., 2011).

In the future, forest growth and carbon uptake in high-latitude regions, including Canada, are expected to continue to be enhanced by CO₂ fertilization (increased plant growth from elevated atmospheric CO₂) and by warmer and longer growing seasons (Haverd et al., 2020; J. Wang et al., 2023), provided that such growth is not limited by a lack of water (Ma et al., 2012; Reich et al., 2018; Wotherspoon et al., 2024). Warming can increase evaporation, reduce soil moisture, and cause water stress in trees, increasing natural mortality rates (J. Wang et al., 2023). There is evidence that increasing water stress is already reducing forest carbon uptake, due to higher tree mortality rates. The trend in drought-induced tree mortality has accelerated since 2002, predominantly in western regions of the country, and is expected to limit the carbon absorption capacity of Canada’s boreal forest in the future (Q. Liu et al., 2023). Natural disturbances, including forest fires (section 9.5.1.7) and insect outbreaks, are also expected to increase in the future and to have continuing adverse impacts on forest carbon stocks (Curasi et al., 2024; Kurz, Dymond, et al., 2008; Kurz, Stinson, et al., 2008).

The net effect of the positive and negative effects of future changes on the carbon balance of Canadian forests is difficult to quantify. However, a recent study that investigated the temperature response of the six most abundant boreal tree species in Canada, based on more than 20,000 sample plots, found that the projected gains in tree productivity (of over 20%) until the 2050s, associated with warmer temperatures, may significantly offset some of the negative effects of projected increases in drought and wildfire (J. Wang et al., 2023).

In summary, Canada’s managed forests and the carbon change in the HWP pool are estimated to have been a net source of carbon to the atmosphere since 2001, based on the methodology used in national inventory reporting. A recent multi-model study (Poulter et al., 2025), which combines the bottom-up and top-down approaches, indicates that Canada’s land ecosystems as a whole were a net carbon sink in 2010 to 2019 (section 9.5.1.6), although it does not report the contributions of forests separately from those of other ecosystems. Because forests dominate land-atmosphere CO₂ exchange in Canada, a substantial portion of this carbon sink is expected to lie within forests (both managed and unmanaged). Reconciling these different estimates and methods remains an area of active research. The future carbon balance of Canada’s forests remains uncertain, due to complex interactions between stand age, climate change, CO₂ fertilization, and disturbances such as fire, insect outbreaks, and land-use changes.

9.5.1.2: Permafrost

Permafrost is ground that remains frozen for at least two years (Chapter 6, section 6.7.1). Permafrost regions have been long-term carbon sinks, which has allowed them to build up large amounts of carbon in their soils over thousands of years. Estimates based on observations and modelling studies indicate that some of Canada’s permafrost regions continue to act as carbon sinks (for the period 2002 to 2014) (Figure 9.12). Estimates of carbon uptake in the eastern and western boreal regions (Figure 9.12b) range from roughly 25 to 50 grams of carbon per m² per year (g C per m² per yr). However, considerable uncertainty surrounds these estimates, due both to limited observations in remote regions (Falvo et al., 2025) and to missing processes in models (Treat et al., 2024).

The tundra region generally has more modest carbon fluxes than forested regions. Estimates using the different methods shown in Figure 9.12a appear to be relatively evenly divided between carbon losses and gains, indicating that the tundra region is not a substantial sink or source of carbon (Treat et al., 2024; Virkkala et al., 2025). There is a general pattern of stronger regional CO₂ sinks in the southern parts of the permafrost zone, with regional sinks decreasing or becoming small sources toward the north. Quantifying the carbon balance for these remote regions remains challenging, due to sparse observations over a large and diverse area (compare observation sites in Figure 9.12b) and the limited ability to represent and evaluate permafrost processes in models.

Figure take-away: Much of Canada’s permafrost-affected region continues to function as a CO₂ sink.

Figure title: Status of current ecosystem carbon exchange in Canada’s permafrost regions, based on five estimation methods

Figure 9.12: a) Average annual ecosystem carbon exchange (in g C per m² per yr) estimated by different methods for three Canadian permafrost-affected regions during the 2002−2014 period (Treat et al., 2024); and b) map showing the three regions in question along with the observation sites used to estimate the in situ and upscaling estimates. Negative values indicate that the region is a net carbon sink, and positive values, that the region is a net carbon source. The points show the mean value. The uncertainty bounds, indicated by the vertical bars on either side of the points, represent the average standard deviation from the mean and were derived for each method. The methods used included (i) on-site measurements from observation towers or chambers (in situ); (ii) upscaling methods where the on-site measurements were used in statistical frameworks to estimate the ecosystem carbon exchange in the regions as a whole; (iii) inversion (top-down) methods where atmospheric CO₂ concentrations were used in modelling frameworks to trace the CO₂ back to its origin at the land surface; (iv) land surface models (LSMs), a form of bottom-up (process-based) modelling that directly simulates CO₂ fluxes; and (v) Earth system models (ESMs), which run simulations from the pre-industrial era to the present day and assess the results against observation-based changes. The different methods consistently estimate a sink for the Eastern and Western Boreal regions but show less agreement for the Tundra region. In the map in b), the numbers beside the region labels indicate the number of field sites used to derive in situ estimates. Data source: Treat et al. (2024).

In addition to emitting CO₂, permafrost regions also emit methane. Circumpolar permafrost regions are a net source of atmospheric methane (Natali et al., 2015; Olefeldt et al., 2013; Saunois et al., 2024). Methane emissions from permafrost regions result mainly from the decomposition of organic matter under anaerobic (oxygen poor) conditions in thawed soils. These emissions can be enhanced by the warming and thawing of permafrost-affected soils; changes in the amount of moisture in the soil; changes in the landscape due to thaw, including ground subsidence; the formation of lakes and wetlands; and ground movement from landslides triggered by permafrost thaw (thaw slumps) (Chapter 6, section 6.2.2). Additionally, emissions of old geological methane can occur from oil and gas reservoirs deep within the ground. Continuous permafrost acts as a seal that prevents the migration of geological methane upward from the subsurface, but unfrozen ground (such as taliks^{Footnote 11} beneath water bodies) can act as a pathway for the geological methane to be emitted to the atmosphere (Dallimore et al., 2024).

Canada’s Mackenzie Delta has large amounts of oil and gas deposits, and measurements made at hotspots (discrete locations of higher methane emissions to the atmosphere) indicate that at least some of the methane emitted in the region is of geologic origin (Wesley et al., 2023). Airborne measurements over a 10,000 km² study region in the Mackenzie Delta suggest that geological methane emissions could represent about 17% of the region’s total annual methane emissions (Kohnert et al., 2017). The total amount of onshore geological methane emitted in Canada is unknown. However, globally, estimates put geological onshore sources at roughly 38 megatonnes of methane per year (Mt CH₄/yr) (or 1064 Mt CO₂-eq/yr) (range of 13 to 53 Mt CH₄/yr, or 364 to 1484 Mt CO₂-eq/yr), which makes onshore geological emissions a relatively small source, representing around 7% of total global methane emissions (Saunois et al., 2024).

Not all parts of the permafrost landscape produce methane. Drier, upland tundra soils are richer in oxygen, allowing them to act as a methane sink. The methane taken up by the soil—either from the atmosphere or produced deeper in the soil column—is broken down by bacteria into CO₂ and water. Globally, methane uptake by upland soils is much smaller (~33 Mt CH₄/yr or ~920 Mt CO₂-eq/yr) than methane emissions from wetlands (~162 Mt CH₄/yr or ~4.5 Gt CO₂-eq/yr) (Saunois et al., 2024). Estimates of permafrost methane emissions and methane consumption are highly uncertain, due to limited observational data. This uncertainty is further increased by methane emissions from wetlands and freshwater bodies that occur near, or directly over, locations where methane is released from soil that recently thawed at depth and is now available for decomposition.

Presently, estimates of permafrost methane emissions in Canada are unavailable. However, if we assume that all estimated wetland and lake methane emissions in the entire global permafrost domain can be attributed to recent permafrost thaw, this allows us to place an upper bound on the amount of methane emitted that could be attributed to historical permafrost thaw. These global-scale estimates based on bottom-up, top-down, and data-driven approaches fall within the range of 5.3 to 37.5 Mt CH₄/yr (~148 to 1080 Mt CO₂-eq/yr), with most estimates around 20 Mt CH₄/yr(560 Mt CO₂-eq/yr) (Bruhwiler et al., 2021; McNicol et al., 2023; Peltola et al., 2019; Ramage et al., 2024; Saunois et al., 2020, 2024). This upper limit represents around 3% of total global methane emissions (~575 Mt CH₄/yr, or 16.1 Gt CO₂-eq/yr, in 2010 to 2019) (Saunois et al., 2024), around two thirds of which is anthropogenic and one third is natural (Saunois et al., 2024). This indicates that permafrost thaw currently represents, at most, a small source of atmospheric methane emissions in the global context.

Additional permafrost thaw could potentially release large amounts of carbon from decomposing organic matter in the soil to the atmosphere in the form of either CO₂ or methane, so there is interest in quantifying future emissions from permafrost regions. However, the rate of CO₂ and methane emissions in the future from permafrost thaw remains highly uncertain, due to a multitude of interacting processes, some of which depend on how the thaw occurs.

Permafrost can thaw in two ways: abruptly or gradually. Abrupt thaw, commonly associated with the development of thermokarst terrain, occurs in soils that contain ice in excess of what they can hold within their pore spaces, meaning that the loss of the ice reduces the structural integrity of the soil (Webb et al., 2025). In turn, this leads to a landscape with irregular ground, due to slumping (downslope movement) of the soil and subsidence (settling or sinking) of the surface. Gradual thaw, in contrast, occurs more slowly, at the rate at which heat can penetrate the ground and cause it to thaw at depth. Long-term monitoring of ground temperatures and the thickness of the active layer (the ground above permafrost that seasonally freezes and thaws) shows increases in these metrics in most of the permafrost region (Chapter 6, section 6.2.1).

Process-based numerical models have been used to project the amount of carbon that will be released by gradual thaw. However, substantial losses of ecosystem carbon to the atmosphere are not expected to occur prior to the year 2100, irrespective of the emissions scenario. This is primarily due to the slow nature of gradual thaw processes, but also because enhanced productivity from vegetation will offset some of the carbon losses from soils (McGuire et al., 2018).

Disturbances related to abrupt thaw (such as the downhill movement of previously frozen soil, soil subsidence, and the formation of lakes) have been observed extensively (Chapter 6, section 6.7.2). The quantification of the trend over time in all of Canada’s permafrost regions as a whole is not yet possible (Treat et al., 2024), although regional estimates are available (Chapter 6, section 6.7.2). Abrupt thaw typically releases a larger amount of carbon into the atmosphere than gradual thaw, due to the land disturbance it causes (Ramage et al., 2024; Turetsky et al., 2020). The quantification of future emissions associated with abrupt thaw remains highly uncertain, due to limitations in the modelling of abrupt thaw processes and their resulting emissions. The uncertainty surrounding CO₂ exchange in permafrost regions also stems from the response of the vegetation to climate warming. Extended growing seasons and changes in the distribution of tundra vegetation have complex effects on the carbon cycle and may help to offset some of the carbon emissions associated with a warmer climate. Satellite-based estimates of vegetation activity have revealed areas of increased vegetation productivity (including both greening and expansion), as well as areas of decreased productivity (including browning and die-off). These regional trends of greening and browning are difficult to simulate accurately using current models (Alfaro-Sánchez et al., 2024; Berner et al., 2020; Myers-Smith et al., 2020; Ogden et al., 2023; Reid et al., 2022). In contrast to increased vegetation growth (greening), which leads to greater carbon uptake, an increase in the depth of seasonal thaws (Chapter 6, section 6.7.2) can produce more carbon emissions. This occurs during spring and fall, when temperatures in the deeper soil layers remain stable, at around 0°C (Arndt et al., 2023). Until the soil freezes completely, microbial decomposition continues to occur at low rates. This can lead to enhanced emissions (up to 50% of the annual carbon emissions in some cases) (Natali et al., 2019; Zona et al., 2016).

In summary, permafrost regions are expected to undergo extensive changes as the climate warms, releasing more CO₂ and methane than at present. However, the complex and poorly understood interactions between permafrost thaw, disturbances, and increased vegetation growth in these regions limit our ability to accurately project these changes and their effects on carbon exchange between the land and the atmosphere.

9.5.1.3: Wetlands

Wetlands are terrestrial ecosystems with poorly drained soils and plants adapted to the waterlogged conditions. Canada is a wetland-rich country, with about 1.29 million km² of wetlands covering 13% of its land area (ECCC, 2016). Figure 9.13 shows the geographical distribution of wetlands in Canada. Mineral wetlands include marshes, swamps, and shallow water bodies. Organic wetlands (peatlands) include bogs, fens, and some swamps that can accumulate partially decomposed plant material (peat). Peatlands in Canada store approximately 150 Gt C, or half of the soil organic carbon in Canada (Tarnocai et al., 2009). For example, the Hudson Bay Lowlands region stores 30 Gt C and is the second-largest continuous peatland region in the world (Packalen et al., 2014).

Data from peat cores from northern peatlands around the world, including boreal peatlands in Canada, show that, over thousands of years, these ecosystems act as long-term net carbon sinks (Loisel et al., 2014), due to a small but persistent difference between slow plant growth (carbon uptake) and even slower decomposition (carbon loss) under the waterlogged conditions. Canada’s peatlands south of the sporadic discontinuous permafrost zone (Chapter 6, section 6.7.1), which represent about half of the peatland area in Canada, have been estimated to be a net carbon sink of 19 Mt C/yr (70 Mt CO₂/yr) during the 1990 to 2019 period. This estimate takes into account the uptake of CO₂ due to peatland plant growth, minus emissions of CO₂, methane, and carbon monoxide (CO) due to decomposition, forest harvesting, and wildfire (Bona et al., 2024).

Of Canada’s ecosystems, wetlands are the largest natural source of atmospheric methane (Saunois et al., 2024), as a result of the decomposition of organic matter under waterlogged conditions. Recent estimates of these emissions, constrained by measurements of atmospheric methane concentrations, suggest that natural sources in Canada, which include wetlands and inland waters, emit around 11 to 14 Mt CH₄/yr (310 to 390 Mt CO₂-eq/yr) (Ishizawa et al., 2024; Poulter et al., 2025) (Table 9.2). Generally, the amount of carbon lost from northern peatlands in the form of methane is less than the CO₂ taken up, and methane is relatively short-lived in the atmosphere compared to CO₂. As a result, despite methane being a stronger GHG than CO₂, peatlands generally have a net cooling effect^{Footnote 12} on the climate over hundreds to thousands of years (Frolking et al., 2006).

Northern peatlands are resilient ecosystems that are thousands of years old. However, the flux of carbon between the atmosphere and peatland ecosystems may change as the climate warms. If peatlands dry up, their methane emissions will decline, but the peat could become more susceptible to aerobic decomposition and fire (section 9.5.1.7), which could increase CO₂ emissions (Bona et al., 2024; McLaughlin and Webster, 2014). Warmer temperatures increase both aerobic and anaerobic decomposition rates; if carbon emissions from decomposition are not offset by greater carbon uptake from increasing plant productivity, then peatlands could become a carbon source.

Peatlands with permafrost have a complex response to climate change (Chapter 6, section 6.7). As peat slowly thaws, it can decompose and reduce the carbon sink capacity of the peatland (section 9.5.1.2). However, the thawing of ice-rich permafrost, combined with thermokarst processes, can cause raised peatland surfaces that were previously dry to collapse and form irregular topography. The resulting thermokarst peatlands are much wetter, and their formation can lead to increased methane emissions. However, over time, shifts in vegetation may reestablish the CO₂ sink capacity (Hugelius et al., 2020).

Broad inferences can be drawn about the response of peatlands to future changes in the climate, based on modelling studies. As an example, Qiu et al. (2022) projected that CO₂ uptake and methane emissions from northern peatlands will remain relatively stable under the low emissions scenario (RCP 2.6). However, under the very high emissions scenario (RCP 8.5), peatland regions in Canada are projected (with low certainty) to be carbon neutral by the year 2100 and to become sources of roughly 80 g C per m² per yr by 2200 (Qiu et al., 2022). If broadly extrapolated to all wetlands in Canada, this yields emissions of around 100 Mt C/yr (367 Mt CO₂/yr) by 2200 under the very high emissions scenario (RCP 8.5).

In summary, peatlands are an important carbon store, containing more than half of the total soil organic matter in Canada. Given their vast area and vulnerability to warming and drying, they play a critical role in determining the future land carbon balance. Peatlands are projected to become carbon sources by the end of the century or beyond under the very high emissions scenario (SSP5-8.5). Under more modest scenarios, however, peatlands are projected to remain carbon sinks (Chaudhary et al., 2020; Gallego-Sala et al., 2018; Hugelius et al., 2020; McLaughlin and Webster, 2014; Qiu et al., 2019, 2020, 2022; Zhao and Zhuang, 2023).

Figure take-away: Wetlands cover 13% of Canada’s land area and are especially common along the southwestern shores of Hudson Bay and James Bay.

Figure title: The distribution of wetlands in Canada

Figure 9.13: Map showing the distribution of wetlands in Canada. Darker colours indicate that a higher fraction of the area is covered by wetlands. Source: ECCC (2016).

9.5.1.4: Inland waters

Freshwater ecosystems are an important part of the natural carbon cycle in terrestrial ecosystems. Some of the carbon sequestered on land enters freshwater streams, rivers, and lakes as organic matter (Poulter et al., 2025). This organic matter supports aquatic food webs (Butman et al., 2018). Particulate organic matter can also shape habitat conditions, such as water clarity and oxygen concentration, which govern where species live and how they interact. In turn, aquatic plants, animals, and bacteria that live in freshwater play important roles in determining how much organic carbon is recycled back to the atmosphere, released into the ocean, or buried in sediments in freshwater systems (Cole et al., 2007).

Most inland waters are net sources of CO₂ to the atmosphere, with global estimates of CO₂ emissions from inland waters ranging from 1.0 to 2.5 Gt C/yr (Lauerwald et al., 2023; Tian et al., 2023). A small fraction of the carbon that enters lakes and reservoirs becomes buried in the sediments, creating a small long-term sink of 0.15 Gt C/yr (0.06 to 0.25 Gt C/yr) globally (Mendonça et al., 2017). For comparison, a similar amount of carbon is buried in global ocean sediments (0.2 to 0.4 Gt C/yr) (Middelburg, 2019). Of the 2 to 3 Gt C/yr that enters global river networks, about 1 Gt C/yr is transported to the ocean (M. Liu et al., 2024). This natural flux of carbon from the land to the ocean through rivers is part of the global carbon cycle.

Canada has the largest number of lakes in the world (Messager et al., 2016), and inland waters play an important role in Canada’s carbon budget. Rivers and lakes cover nearly 9% of the land surface in Canada (Chapter 5), with this proportion as high as 25% in some boreal regions (Messager et al., 2016), and over 30% in some Arctic regions (Plug et al., 2008). Canada’s inland waters are estimated to have emitted 193 Mt C/yr (710 Mt CO₂/yr) into the atmosphere and exported 25 Mt C/yr to the ocean during the 2010 to 2019 period (Poulter et al., 2025), which is about 2.5% of the 1 Gt C exported annually to the global oceans (M. Liu et al., 2024). Over the past few decades, freshwater browning, or darkening water colour—often related to increasing dissolved organic matter and carbon content in the water (Blanchet et al., 2022; Roulet and Moore, 2006)—has been observed in Canadian lakes (Rodríguez-Cardona et al., 2023). In some cases, freshwater browning has been linked to increasing terrestrial organic carbon exports to these waters, in response to declining atmospheric acid deposition, increasing temperature, and increasing precipitation (for example, Rodríguez-Cardona et al., 2023).

Carbon exports from terrestrial to freshwater ecosystems may also increase when runoff and streamflow increase (Chapter 5, section 5.3). Increasing plant productivity on land, associated with increasing atmospheric CO₂ and nitrogen inputs, as well as changes to how water moves through soil, can also affect carbon exports. For example, dissolved organic carbon exported from the Mackenzie River drainage basin to the Arctic Ocean increased by 39% over the 1978 to 2012 period (Tank et al., 2016). This increase was associated with greater productivity in lakes, permafrost degradation that resulted in wetland expansion (and more connectivity between water bodies across the landscape), deeper flow through thawed soil over longer periods of the year, and mechanical erosion of soils. These processes can lead to the release of carbon that was recently taken up by land plants and stored as organic matter in soils, as well as the release of organic carbon that was sequestered hundreds to thousands of years ago (Tank et al., 2016). Land-use changes also affect carbon exports to inland waters. Forest disturbances, such as wood harvesting (Webster et al., 2015) and wildfires (Bélair et al., 2025), can increase the concentration of dissolved organic carbon in lakes and rivers.

The riverine transport (that is, by streams and rivers) of carbon from land to ocean is expected to increase in the future as a result of climate change and changes in land use and disturbances, such as greater rates of soil organic matter decomposition in a warmer climate, enhanced vegetation productivity due to CO₂ fertilization (Haverd et al., 2020), more wildfires, and enhanced runoff due to increased precipitation (Chapter 5, section 5.3). While modelling results support this assessment, they only show a marginal increase in riverine carbon exports to the ocean (M. Liu et al., 2024). Long-term observational datasets are scarce, which makes it difficult to detect long-term trends (M. Liu et al., 2024). In summary, the riverine transport of carbon to the ocean is expected to increase in the future, but the magnitude of the change remains uncertain.

9.5.1.5: Croplands and grasslands

Land managed for agricultural purposes in Canada includes croplands (46 million hectares [Mha]) and grasslands (7.5 Mha). Croplands and grasslands cover less than 5% of Canada’s land area and are concentrated largely in Western Canada (ECCC, 2025). Cropland includes land used to produce annual and perennial food crops and forage, while grasslands include unimproved pasture and rangeland used for grazing livestock (ECCC, 2025). Carbon drawdown and emissions from cropland and grasslands are tracked for UNFCCC reporting purposes as changes in the carbon stored in soils and woody biomass (ECCC, 2025). Crops take up CO₂ from the atmosphere as they grow, and the carbon in crop residues becomes incorporated into soils. As crop residues and soil organic matter decompose, CO₂ is emitted back into the atmosphere. Cropland management decisions may either increase or decrease carbon sequestration (the process of capturing and storing CO₂ from the atmosphere) in the soil. Practices like manure application, conversion to perennial crops, the use of no-till or reduced tillage practices, and reduction in summer fallow increase carbon sequestration, while conversion to annual crops, intensive tillage practices, and prolonged fallow periods decrease sequestration (ECCC, 2025). While grasslands are not a substantial carbon source or sink, croplands have been a small net carbon sink on average for the 1990 to 2022 period (19 Mt CO₂-eq/yr) (ECCC, 2025).

It is important to note that this assessment of croplands and grasslands as carbon sources and sinks does not include emissions and drawdown associated with livestock production, fertilization, liming, and other management activities. These activities result in significant GHG emissions, making the overall agriculture sector a net GHG emitter, according to Canada’s annual national inventory report (ECCC, 2025) (section 9.3.2, Figure 9.5). For example, the application of nitrogen fertilizer on agricultural soils makes them the primary source of anthropogenic nitrous oxide emissions in Canada. Methane emissions associated with animal production, specifically manure management and enteric fermentation, are the second-largest source of anthropogenic methane emissions in Canada (ECCC, 2025).

Year-to-year differences in weather affect crop productivity and drive the variability in the net sink or source status of croplands in Canada. For example, a significant drought in 2021 reduced crop productivity and carbon inputs to soils from crop residues, resulting in a temporary switch from carbon sink to carbon source of 20 Mt CO₂-eq/yr in 2022. In 2023, croplands returned to being a carbon sink of 27 Mt CO₂-eq/yr (ECCC, 2025).

Future climate change is expected to affect soil carbon storage in agricultural areas in Canada. Climate warming is projected to increase soil carbon decomposition rates by 2.5 to 3.9% in agricultural areas globally, contributing to carbon flux to the atmosphere (Poeplau and Dechow, 2023). Conversely, warmer weather and earlier and longer growing seasons, if not limited by moisture or heat stress, are projected to increase crop yields in Canada (Jégo et al., 2024, 2025; W. N. Smith et al., 2013) and thus boost crop residue inputs to soil. Climate warming is also expected to create opportunities for agriculture in new regions where the soils, precipitation, terrain, and infrastructure are suitable (Jiang et al., 2023; Krishna Bahadur et al., 2021). However, the conversion of unmanaged lands—such as native grasslands, wetlands, and forests—to cropland has resulted in considerable losses of carbon from these natural ecosystems, which continues long after their conversion (Byun et al., 2018; Guo and Gifford, 2002). For example, Byun et al. (2018) estimated that the conversion of 56% of the wetland area in Southern Ontario (roughly 84,000 km²) to other land uses, including cropland, was associated with a loss of approximately 1.9 Gt C of total ecosystem carbon (~60% of the original amount) since 1850. More recently, land conversion to agriculture has declined in Canada. In the 20 years prior to 1990, 1.3 Mha of forested land was converted to cropland, while 0.38 Mha was converted in the 20 years prior to 2023 (ECCC, 2025).

Agricultural expansion also affects the land surface energy balance—the exchange of energy at the Earth’s surface between solar radiation, heat, and evaporation. For example, land-use change involving the conversion of forests to croplands makes the land surface brighter, which reflects more solar radiation and causes regional cooling (Betts, 2000; J. Liu et al., 2022). However, less moisture is evaporated from croplands than from forests (O’Connor et al., 2019; Verstraeten et al., 2005), and this can contribute to warming. The net effect of these opposing processes depends on the geographical location of the land-use change (Arora and Montenegro, 2011). In high-latitude regions with significant snow cover, such as Canada, the cooling from the increased reflectivity of cropland typically outweighs the warming from reduced evaporation. As a result, converting forests to croplands in these regions is expected to cause regional cooling (Betts, 2000). The reverse is true for afforestation, which can lead to regional warming (Arora and Montenegro, 2011). Given these strong local controls, the effects of changes in cropland area on the land surface energy balance vary spatially and are generally not well quantified (Desjardins et al., 2007). Consequently, the global effect of land-cover change on energy balance also remains uncertain (Jia et al., 2019).

Overall, croplands are a much smaller carbon sink than forests, and the agriculture sector as a whole is a net GHG emitter. The conversion of 0.38 Mha of forest land to cropland over the 20 years prior to 2023—a small amount when compared with Canada’s 367 Mha of forest and 46 Mha of cropland—suggests that cropland expansion has largely stabilized. Climate change is projected to increase crop yields in areas where soil moisture is not limiting, opening new regions to agriculture, provided that other conditions are suitable.

9.5.1.6: Overall assessment of Canada’s land ecosystems

Despite the uncertainties and the need to rely upon several disparate sources of information,^{Footnote 13} it is possible to make an overall assessment of the movement of carbon into and out of Canada’s land ecosystems. Such an assessment provides an important starting point for understanding the contribution of Canada’s land ecosystems to the global land carbon sink. The annual National Inventory Report (ECCC, 2025) only assesses the direct effects of management actions on managed lands in Canada. Roughly 72% of Canada’s land is unmanaged and thus not considered in formal reporting. As a result, other approaches are needed for a full accounting.

The primary way to accomplish this accounting is through the use of two different modelling approaches: bottom-up (based on process-based models) and top-down (based on atmospheric inversions). The second Regional Carbon Cycle Assessment and Processes Study (RECCAP2) (Poulter et al., 2025) adopts a multi-model approach, using results obtained with both bottom-up and top-down approaches. Based on results from RECCAP2, Canada’s land ecosystems as a whole consistently acted as a carbon sink from 2000 to 2019, taking up roughly 600 to 1500 Mt CO₂/yr (Figure 9.14). While the estimated size of the land sink differs between the bottom-up (~600 Mt CO₂/yr) and top-down (~1500 Mt CO₂/yr) models, there is strong agreement that Canada’s land ecosystems function as a relatively large sink of carbon. The lack of agreement between the bottom-up and top-down approaches can be attributed to difficulties in quantifying the horizontal transfer of carbon from land to rivers and in accounting for crops and wood harvesting (Pacala et al., 2001). While Canada’s land carbon sink was relatively consistent during the 2000 to 2019 period (Figure 9.14), it can become a source of CO₂ in years with extreme disturbances, such as the 2023 fire season (Chapter 8, Box 8.4). According to national inventory reporting, Canada’s managed forests shifted from a GHG sink to a GHG source during the 2010s (Figure 9.11b). This transition from sink to source is attributed to increasing levels of natural disturbances (ECCC, 2025).

Figure take-away: Canada’s land ecosystems (managed and unmanaged combined) function as a net CO₂ sink.

Figure title: Estimated annual fluxes of carbon dioxide for all Canadian land ecosystems (managed and unmanaged) during the 2000 to 2019 period, based on the Regional Carbon Cycle Assessment and Processes Study (RECCAP2)

Figure 9.14: Graph of the model ensemble average (mean annual net CO₂ flux in megatonnes [1,000,000 metric tonnes] of carbon dioxide per year, or Mt CO₂/yr) reconstructed for Canada from global-scale simulations compiled by RECCAP2, based on 19 bottom-up (process-based) models (Sitch et al., 2024) and 7 top-down (atmospheric inversion) models (Friedlingstein et al., 2022). Shaded regions indicate one standard deviation above and below the average. The estimates from both the bottom-up and top-down model ensembles indicate that Canada’s land ecosystems are a carbon sink (indicated by negative values). Data source: RECCAP2 (Poulter et al., 2025).

Canada’s anthropogenic methane emissions are estimated to be 4 to 6 Mt CH₄/yr (112 to 168 Mt CO₂-eq/yr) (Table 9.2), representing about 1.3% of global anthropogenic methane emissions (approximately 360 Mt CH₄/yr; 10.1 Gt CO₂-eq/yr) (Saunois et al., 2024). The rate of natural methane emissions, including primarily those from wetlands and inland waters (lakes, ponds, and rivers), is more uncertain. The bottom-up approach estimates natural methane emissions at 33 Mt CH₄/yr (924 Mt CO₂-eq/yr), more than twice the estimate made using the top-down approach (around 11 to 14 Mt CH₄/yr; 310 to 302 Mt CO₂-eq/yr). The higher estimates provided by the bottom-up models could be due to double counting, in which some inland lakes and wetland areas were included in both inventories, resulting in inflated estimates of methane emissions from these ecosystems (Saunois et al., 2020).

The future carbon budget for Canada’s land ecosystems will be determined by shifts in rates of carbon uptake and release. These shifts will be driven by increasing atmospheric CO₂, climate warming, permafrost thaw, hydrological changes, human-caused and natural ecosystem disturbances, and land management decisions, among other factors (Braghiere et al., 2023). Although the strength of the global terrestrial carbon sink is projected to increase, due to the fertilization effect of increasing atmospheric CO₂ concentrations (Canadell et al., 2021; Walker et al., 2021), the strength of this sink in northern high latitudes is uncertain. Several factors, particularly carbon release driven by permafrost thaw and insect and fire disturbances, are poorly understood and not represented in many Earth system models (Jactel et al., 2019; Seiler et al., 2024).

9.5.1.7: Wildfire

Wildfires are a recurrent feature of the Canadian landscape, especially in the boreal forest and taiga, where less frequent, but very large, fires can burn vast areas over relatively short periods (Stocks et al., 2002). Wildfires are a common natural disturbance in forest ecosystems, where they can help rejuvenate habitats, promote new plant growth, and maintain species diversity (Bergeron and Fenton, 2012). However, fires can also be detrimental if their frequency and intensity are altered by human activities and climate change (Johnstone and Chapin, 2006). Since 1959, when records of burned area became available, the area burned by forest fires and the number of large fires (> 200 ha) in Canada have both increased, along with the size of large fires and the length of the fire season (Hanes et al., 2019) (Chapter 8, section 8.7.1). It should be noted that, prior to the satellite era (that is, before about 1980), estimates of area burned were potentially biased, due to unobserved or poorly mapped fires and inconsistent methodologies. Large fires account for approximately 99% of the total burned area. Figure 9.15 shows the area burned by large fires and their numbers in Canada from 1959 to 2023. The linear trend for area burned indicates an average increase of almost 30,000 hectares per year over this period.

Fire occurrence depends on three factors: availability and type of fuel, fuel dryness (which depends on the weather), and the presence of an ignition source. The severity and spread of fires depend on fuel, weather, and topography (Coogan et al., 2021). Weather is the dominant factor controlling the wildfire- burned area and how it changes from year to year. For example, the increase in wildfire-burned area in Canada has been found to be primarily driven by an increase in fire season (May–October) temperatures and a decrease in atmospheric humidity (Gillett et al., 2004; Parisien et al., 2011). Since global temperature is rising with increasing anthropogenic CO₂ emissions, this trend is expected to continue (Boulanger et al., 2018; Girardin and Mudelsee, 2008; X. Wang et al., 2022).

Fuel loads are one of the three key drivers of wildfire, and fuel management provides a potentially critical lever for mitigating wildfire impacts. Fuel management to reduce wildfire risk is not assessed in this chapter; however, it is discussed here in the context of Indigenous fire stewardship. Cultural burning, practiced by Indigenous peoples for millennia, has been used to support biodiversity and forest resilience, and to maintain areas for medicinal plants and hunting habitats (Hoffman et al., 2022; Parks Canada, 2025). However, over the past century, fire suppression—including the prohibition of cultural burning—has been enforced almost everywhere in North America (Parks et al., 2025), including in Canada’s national parks (Parks Canada, 2025). Prolonged fire suppression can lead to fuel build-up and greater vulnerability to disease, pests, and high-severity fires (Kreider et al., 2024). Today, fuel management practices, such as prescribed burns, are used by Parks Canada and provincial fire agencies in an attempt to reduce wildfire risk and intensity (Ontario Ministry of Natural Resources and Forestry, 2017; Parks Canada, 2025). Modern-day fuel management practices primarily attempt to reduce fuel loads, but they differ from Indigenous practices in their timing, frequency, and cultural context. Despite the unique vulnerabilities of Indigenous communities to large, high-intensity wildfires that occur predominately in remote forested regions, Indigenous fire stewardship in Canada faces significant barriers (Hoffman et al., 2022). Case Story 9.1 profiles one example of efforts to revitalize Indigenous fire stewardship in Canada.

The interannual variability in burned area is influenced by the variability in both climate and ignition sources, including human and lightning activity (Beverly et al., 2011; Macias Fauria and Johnson, 2006). In Canada, the number of human-caused fires has been declining, while the proportion of lightning-caused fires has increased (Hanes et al., 2019). Notably, lightning-caused fires are responsible for over 90% of the historical area burned (Coogan et al., 2021; Hanes et al., 2019). Recent observations show that western and northern Canada have seen increasing lightning activity over the last couple of decades, while Eastern Canada has experienced a decrease in lightning activity (Kochtubajda and Burrows, 2020).

Figure take-away: Fire is an important, and increasing, ecosystem disturbance across most of Canada.

Figure title: Area burned and number of large wildfires in Canada from 1959 to 2023

Figure 9.15: Graph showing the area burned by large fires (in millions of ha) and the number of large fires, based on data from the Canadian National Fire Database (CFNDB) for the 1959 to 2023 period. Large fires are defined as those burning more than 200 ha. The linear trend for area burned was estimated using the Kendall-Mann approach (F. Wang et al., 2020). Data source: Natural Resources Canada (2025).

A substantial fraction (~90%) of the direct emissions from the burning of vegetation and organic matter in soils occurs in the form of CO₂. Fires also emit carbon in the form of carbon monoxide, CO (~5 to 10%), and methane (< 1%) gases, along with black carbon and organic carbon particulates. Wildfires also emit CO₂ indirectly, by killing vegetation that subsequently decomposes and by altering surface conditions, which results in increased permafrost thaw (Cao and Furuya, 2024; S. L. Smith et al., 2015). After a fire, the affected area emits CO₂ into the atmosphere as the dead vegetation decomposes. Over time, however, the fire-affected area typically transitions to a carbon sink as the vegetation regrows and begins to take up carbon from the atmosphere. The amount of carbon emitted and the subsequent uptake from the atmosphere are highly variable and depend on fire severity and frequency, climate, and vegetation characteristics, among other factors (Goetz et al., 2012). Direct CO₂ emissions from fires in Canada between 1985 and 2022 were estimated to be 88 Mt CO₂/yr (± 62 Mt CO₂/yr) on average (Curasi et al., 2024). The average burned area during the 1986 to 2022 period was 2.1 Mha per year (Skakun et al., 2022). Notably, the 2023 Canadian fire season resulted in a record-high burned area that was more than eight times the average (Chapter 8, Box 8.4). The estimated emissions from the 2023 Canadian wildfires vary from as low as 700 Mt CO₂ (Kirchmeier-Young et al., 2024) to as high as 2370 Mt CO₂ (Byrne et al., 2024). These estimates are larger than the emissions from all other anthropogenic sectors in Canada combined for the year 2023 (694 Mt CO₂-eq).

Projections of wildfire-burned area in Canada are based on a wide range of approaches. These approaches range from simple, regression-based methods, which attempt to project change based on statistical relationships between burned area and fire weather indices, to more complex, process-based model simulations. Regardless of the approach, most projections estimate substantial increases in burned area under the high and very high emissions scenarios (SSP3-7.0 and SSP5-8.5, respectively), because burned area is tightly linked to climate (Boulanger et al., 2014, 2017, 2018; Curasi et al., 2024; M. D. Flannigan et al., 2005; Wotton et al., 2010).

As an example, using a process-based model, Curasi et al. (2024) projected the burned area for Canada under three emissions scenarios. They calculated that, compared to the annual burned area during the historical period (1998 to 2014: 2.67 ± 0.70 Mha [average and 95% confidence interval]), the annual burned area during the 2081 to 2100 period would increase to 10.32 Mha (± 3.92 Mha) and 11.68 Mha (± 4.60 Mha) under the high and very high emissions scenarios (SSP3-7.0 and SSP5-8.5), respectively. However, under the low emissions scenario (SSP1-2.6), although the area burned is projected to increase to 3.91 Mha (± 1.12 Mha) during the same period (2081 to 2100), it is not statistically different from the burned area simulated for their chosen historical period (1998 to 2014). This indicates that, if warming remains below 2°C globally (that is, the SSP1-2.6 emissions scenario), the area burned in Canada is projected to be similar to that experienced in the early 21st century (Curasi et al., 2024). Another study, using a regression-based approach that takes account of fire size and the number of days on which fires can spread (X. Wang et al., 2020), estimated that the area burned in Canada during an average fire year will increase to around 11 Mha by the end of the 21st century under the very high emissions scenario (SSP5-8.5), which is consistent with the estimate by Curasi et al. (2024).

Changes in future lightning activity are important, due to the role of lightning in starting fires, the often remote locations of lightning-caused fires, and the number of fires that can be ignited by lightning in a short time (Janssen et al., 2023; Veraverbeke et al., 2017). Some projections show an increase in lightning activity over Canada under the high and very high emissions scenarios (SSP3-7.0 and SSP5-8.5, respectively) (Y. Chen et al., 2021; Finney et al., 2018; Whaley et al., 2024). However, the amount and geographical distribution of the projected change are highly variable and uncertain, and lightning activity might also decrease in some parts of Canada.

In addition, the fire season is projected to lengthen (Chapter 8, section 8.7.1) (M. Flannigan et al., 2009; Jain et al., 2017; Van Vliet et al., 2024; X. Wang et al., 2022), the number of days over which fires actively grow is projected to increase (Jain et al., 2020; X. Wang et al., 2015, 2017), and the number of fires is also projected to increase (X. Wang et al., 2022; Wotton et al., 2010), along with the average fire size (X. Wang et al., 2020, 2022). As the likelihood of climate extremes relevant to fire increases with warming (Chapter 8, section 8.7.1), concomitant increases in fire danger and extreme fire behaviour are also expected (Jain et al., 2024; Kirchmeier-Young et al., 2024).

Increases in the size and frequency of wildfires have far-reaching consequences for air quality and human health in Canada, as well as for the structure and function of boreal forest ecosystems. Larger and more frequent wildfires are expected to alter the mix of species, the age distribution of trees in the forest, wood production, nutrient turnover, and vulnerability to insect and pathogen attack (Weber and Flannigan, 1997). Peatlands will also be affected by rapidly changing fire regimes (section 9.5.1.3), as Canada’s boreal forest is a peatland-rich region (Tarnocai et al., 2011). Pristine peatlands can often recover the carbon lost to fire (averaging 1 to 5 kg C/m²) (Turetsky et al., 2011; Wilkinson et al., 2018) within a relatively short period (~10 to 30 years) (Wieder et al., 2009), if there is a long interval between fires. However, if a peatland has been drained (degraded), if fires become too frequent due to climate change, or if fires accelerate permafrost thaw (in permafrost peatlands), peatland fires can result in large carbon emissions (Gibson et al., 2018; Mickler et al., 2017) and can greatly reduce the carbon sink potential of peatlands in the future (Wilkinson et al., 2023).

Overall, the area burned by wildfires in Canada has increased by nearly 30,000 ha per year since 1959 (Figure 9.15), based on a linear trend fitted to annual burned area data, although estimates from the pre-satellite era may be less reliable. This upward trend, and the resulting increase in fire-related emissions of CO₂, other co-emitted gases, and particulate air pollutants, is expected to continue, as the length of fire season in Canada is projected to increase.

9.5.2: Oceans

CO₂ is exchanged between the atmosphere and the surface ocean. Globally, the ocean stores more CO₂ than any other Earth system component that exchanges CO₂ directly with the atmosphere (Figure 9.2; Chapter 7, section 7.7). The ocean has taken up about 25% of anthropogenic CO₂ (Friedlingstein et al., 2023), which has reduced the rate of increase in atmospheric CO₂, but has resulted in ocean acidification (Chapter 7, section 7.7.1.1). The magnitude and direction of the flux (into or out of the ocean) are controlled by the difference between the concentration of CO₂ in the atmosphere and in the surface ocean. CO₂ uptake by the ocean is increasing, as the concentration of CO₂ increases in the atmosphere.

Water temperature and the growth of marine plants and phytoplankton affect how much CO₂ the ocean can absorb. A larger amount of CO₂ dissolves in colder water than in warmer water. Marine phytoplankton take up CO₂ as they grow, as do Canada’s vegetated coastal ecosystems, which include seagrass meadows, salt marshes, and kelp beds (Figure 9.16a−c).

Figure take-away: Vegetated coastal ecosystems are widely distributed along Canada’s coastline, and organic carbon is stored in both vegetated and unvegetated sediments.

Figure title: Distribution of vegetated coastal ecosystems and sedimentary organic carbon in Canadian coastal waters

Figure 9.16: Maps showing distribution of vegetated coastal ecosystems in Canada and organic carbon density (in kg of carbon per m³, or kg C m^-3) in unvegetated sediment: a) seagrass (adapted from Murphy et al., 2021); b) salt marsh (adapted from Rabinowitz and Andrews, 2022); c) kelp (adapted from United Nations Environment Programme and Norwegian Blue Forests Network, 2023); and d) organic carbon density in unvegetated sediment (adapted from Epstein et al., 2024).

Some of the carbon taken up by phytoplankton and marine plants is stored in the sediment at the bottom of the ocean, along with the organic carbon that washes into the ocean from the land (Figure 9.16d). Storage of organic carbon in sediment reduces the amount of CO₂ that remains in the atmosphere (Volk and Hoffert, 1985). The concentration of organic carbon in sediment varies regionally (Figure 9.16d), with higher concentrations generally associated with finer sediment particles (Duke, Richaud, et al., 2023). The highest organic carbon concentrations in Canadian sediments are found in coastal regions, such as the Strait of Georgia and the Gulf of St. Lawrence (Figure 9.16d). Air-sea fluxes of CO₂ in Canada’s territorial waters are sizable, but highly variable, both spatially and temporally (seasonally and during individual weather events) (Ianson and Allen, 2002). Concentrations of CO₂ in surface seawater are strongly influenced by local wind patterns and intermittent biological uptake, especially near shore (for example, Duke, Hamme, et al., 2023; Fennel et al., 2019; Jarníková et al., 2022).

Both phytoplankton growth and water temperatures in Canada’s oceans show strong seasonal cycles, which cause seasonal changes in the carbon flux. During spring and early summer, light and nutrients are abundant. As a result, phytoplankton productivity is high, and the ocean takes up atmospheric CO₂, even though temperatures are higher at that time of year (Fennel et al., 2019).

In contrast, in winter, biological productivity in the ocean is low, but water temperatures are also low. Some coastal regions act as a source of CO₂ during this time, while others take up CO₂ from the atmosphere, acting as a sink (Fennel et al., 2019). Seasonal cycles are further amplified by the presence of sea ice, especially in the Canadian Arctic (section 9.5.2.3).

In the Pacific, the narrow band of ocean inside Canada’s Exclusive Economic Zone (EEZ) is a source of CO₂ emissions to the atmosphere in the winter and a sink in the summer. The open ocean portion of these waters is a stronger sink than the nearshore area along the coast, outweighing the coastal source (Duke et al., 2024).

In summary, Canada’s oceans are a net CO₂ sink (Duke, Richaud, et al., 2023).

9.5.2.1 Sources and sinks in the open ocean

The open ocean inside Canada’s EEZ (within the 200-nautical-mile limit; ~370 km) is generally a net carbon sink (Duke, Hamme, et al., 2023; Fennel et al., 2019) (Chapter 7, section 7.7.1.1.). The surface water cools seasonally, allowing more CO₂ to enter the surface ocean from the atmosphere (Gruber et al., 2019). In the North Atlantic, the cooled surface water becomes dense enough to sink into the deep ocean (Hamme et al., 2019). Organic particles (mainly from phytoplankton) sink into this deep water as it makes its way from the North Atlantic to the Southern Ocean and then slowly back to the North Pacific. The particles break down along the way, releasing CO₂. As a result, the deep waters of the North Pacific have a much higher concentration of CO₂ than the deep waters of the North Atlantic (Emerson and Hamme, 2022; Gruber et al., 2019) (Chapter 7).

However, the seawater in the upper layer of the northeast Pacific Ocean is much fresher than that in the other large ocean basins (for example, North Atlantic). The fresher surface water limits vertical mixing (Gargett, 1991) and helps to isolate the carbon-rich subsurface waters. Consequently, although the deep water of the North Pacific is carbon-rich, the surface water is not, and this region, like the North Atlantic, appears to be a net sink of atmospheric CO₂ (Duke et al., 2024; Duke, Hamme, et al., 2023; Franco et al., 2021; Wong et al., 2010).

9.5.2.2 Sources and sinks in coastal waters

Unlike the open ocean, some nearshore coastal waters act as net carbon sources over the course of a year (Bauer et al., 2013). Canada has one of the longest coastlines in the world, and much of our coastal ocean has not been studied. However, the coastal areas that have been studied (for example, the Salish Sea, Bay of Fundy, Gulf of St. Lawrence, Hudson Bay, and the Beaufort Shelf) are net sources of CO₂ to the atmosphere (Capelle et al., 2020; Duke et al., 2024; Duke, Richaud, et al., 2023).

Nearshore coastal regions are dynamic, and their CO₂ concentrations are highly variable in both space and time. Tidal mixing exposes subsurface waters to the atmosphere in some regions and can cause CO₂ to move from the ocean to the atmosphere year-round (Clarke Murray et al., 2015; Evans et al., 2012; Nemcek et al., 2008), while other coastal areas may be carbon sinks (Fennel et al., 2019).

Organic carbon from both land and ocean sources (for example, phytoplankton and fragments of land plants) is buried in coastal sediments. This burial results in the drawdown of carbon from the atmosphere on a timescale of a few years to thousands of years (for example, Johannessen, 2022).

However, some of the organic carbon in nearshore waters is broken down by marine organisms, releasing CO₂ (for example, Capelle et al., 2020), so the input of carbon from the land via rivers can also cause coastal regions to be a net source of CO₂ (Fennel et al., 2019; Najjar et al., 2018). In addition, some river water contains high concentrations of dissolved CO₂ (for example, Moore-Maley et al., 2018), so outgassing can occur year-round in some coastal waters (Jarníková et al., 2022).

9.5.2.3: Sources and sinks in the Arctic Ocean

The Arctic Ocean is physically and chemically complex, due to the presence of sea ice and the dramatic seasonal change in daylight at high latitudes (Carmack et al., 2006, 2015). Both strong seasonality and substantial year-to-year variability occur, particularly in spring and fall. When light begins to return to the Arctic in the spring, macroalgae grow on the bottom surface of the ice and phytoplankton grow in the water underneath, which take up CO₂ from the seawater, causing low-carbon water to accumulate beneath the ice. Once the ice melts in spring, this can cause a strong uptake of carbon, although, in some areas, cracks in the ice permit CO₂ exchange all winter, reducing the uptake in spring (Steiner et al., 2013).

During the summer, when much of the sea ice has melted, the Arctic Ocean receives a strong input of freshwater from land runoff, as well as meltwater from the ice (for example, Déry et al., 2005; Macdonald et al., 1999). This fresher inflow is less dense, so it floats on top of the denser, saltier seawater. The difference in density limits mixing of the surface waters, preventing the carbon-rich subsurface water from exchanging with the atmosphere. In addition, phytoplankton growth is high at this time, due partly to nutrients delivered through river runoff and coastal erosion (Terhaar et al., 2021; Zhang et al., 2024). The high productivity, cool water, and export of organic carbon to deep water together result in low surface ocean CO₂ concentrations in summer (Cai et al., 2010; Steiner et al., 2014).

In the fall, some areas of the Arctic Ocean release CO₂ into the atmosphere. In the winter, sea ice cover (for example, Butterworth and Miller, 2016) reduces air-sea gas exchanges and the potential outgassing of the carbon taken up in the summer (for example, Bates and Mathis, 2009).

Hudson Bay is one exception to the regional picture. It experiences a high runoff of dissolved organic carbon from the land, much of which is broken down by microorganisms, releasing CO₂. Together with the environmental conditions in that area (temperature, salinity, etc.), the breakdown of organic carbon from land makes Hudson Bay a source, albeit a weak one, of CO₂ to the atmosphere at certain times of the year (Ahmed et al., 2021; Capelle et al., 2020). Capelle et al. (2020) point out that nearshore areas in the rest of the Arctic Ocean also receive a large amount of dissolved organic matter from land. However, there is no evidence yet that other nearshore Arctic areas are net sources of CO₂ (Fennel et al., 2019).

Long-term records of trends in air-sea CO₂ flux are not available for northern Canada. However, in the Labrador Sea, dissolved CO₂ in the surface water increased by 1.1% (23 µmol [one millionth of a mole] C/kg) between 1996 and 2023, resulting in a 17% increase in acidity (0.07 pH unit decrease) (Chapter 7, section 7.7.1.1) (Raimondi et al., 2021). Overall, the Canadian Arctic Ocean appears to currently be a net carbon sink (Bates and Mathis, 2009; Else et al., 2013; Fennel et al., 2019), even though some outgassing occurs in late summer in some areas (Ahmed et al., 2021; Else et al., 2013). However, the capacity of this water body to continue to take up atmospheric CO₂ is limited (Cai et al., 2010; Steiner et al., 2013).

9.5.2.4 Carbon cycle trends in Canada’s oceans

We lack the data needed to assess the trend in carbon uptake in Canada’s coastal waters overall, but there is some information for the west coast and the northeast Pacific Ocean.

In the Salish Sea, a semi-enclosed sea on the west coast, CO₂ concentrations are controlled mainly by the exchange of seawater with the open ocean (Jarníková et al., 2022). The Salish Sea acts as a small source of CO₂ to the atmosphere (Duke, Richaud, et al., 2023; Jarníková et al., 2022), but the strength of this source appears to have weakened slightly (by ~2%) since pre-industrial times (Jarníková et al., 2022).

In the northeast Pacific Ocean, the amount of carbon stored in subsurface waters is projected to increase as a result of increasing atmospheric CO₂ (Holdsworth et al., 2021). Farther offshore, in the open ocean, the increase in CO₂ concentrations in surface water has been slower than the increase in CO₂ concentrations in the atmosphere in recent years (1998−2019) (Duke, Hamme, et al., 2023), resulting in higher carbon uptake.

Although the Arctic Ocean is currently a strong carbon sink, models indicate that its capacity to take up CO₂ is limited. Sea ice extent has declined since the 1970s (Meier, 2017), particularly in the late summer and early fall. Extended periods of open water in fall allow the wind and waves to mix carbon-rich water from the lower depths up to the surface, potentially turning the Canadian Arctic into a seasonal CO₂ source in the future (Steiner et al., 2013).

The limited information currently available suggests that Canada’s oceans will, on average, remain a carbon sink in the future, as long as atmospheric CO₂ continues to increase. However, a seasonal shift to a source is expected in some regions, and the overall rate of uptake will likely slow down, due to ocean acidification (Chapter 7, section 7.7.2).

9.5.3: Confidence terms in key messages: summary of evidence

Key Message 9.9: Carbon in Canada’s oceans is mainly stored in seawater and marine sediments. On land, almost all (about 95%) of the carbon in Canada is stored in soils, including permafrost and peatlands, with the rest (about 5%) stored in vegetation (very high confidence).

Key Message 9.10: Since the last Ice Age, there has been a net uptake of carbon by Canada’s land mass. At present (2000−2019), the land continues to be a carbon sink (medium confidence). However, extreme events like the 2023 fire season can cause the land to become a temporary source of carbon (low confidence). In the future, both carbon uptake from increased vegetation growth (medium confidence) and carbon emissions from permafrost thaw, wildfires, and other disturbances (high confidence) are expected to increase. However, their net effect on the carbon balance of Canada’s land ecosystems remains uncertain.

Key Message 9.11: The annual forest area burned by fires in Canada has increased since the early 1980s (medium confidence). The area burned is projected to increase further as the climate continues to warm (high confidence).

Key Message 9.12: On average, Canada’s oceans (inside the Exclusive Economic Zone) take up carbon dioxide (CO₂) (medium confidence), although some nearshore areas emit CO₂ (low confidence). Overall, Canada’s oceans will continue to take up CO₂, as long as atmospheric CO₂ concentrations continue to increase (medium confidence).

The first sentence of Key Message 9.9 is not associated with a confidence statement, because it is a statement of fact. There is very high confidence in the second sentence of Key Message 9.9. On land, the very high confidence in the general partitioning of carbon between soils and living vegetation stems from the large number of field observations of both soils and vegetation and the good spatial coverage that they provide (Domke et al., 2018; Lajtha et al., 2018; Sothe et al., 2022). We have lower confidence in the precise determination of how much carbon is found in soils, especially at depths below 1 m, as many observations of soil carbon do not penetrate deeply enough to fully quantify the carbon stored there (Sothe et al., 2022). Any attempt to quantify the amount of carbon in the vegetation and soil pools in Canada as a whole requires some form of modelling or upscaling from point measurements. Uncertainties are associated with both modelling and upscaling, but there remains very high confidence in the partitioning of land carbon between vegetation and soils at the Canada-wide scale.

The historical net uptake of carbon by land in Canada is a well-established fact. This net accumulation of carbon occurred because much of Canada was covered by glaciers during the last Ice Age. Glaciation stripped away much of the soil and vegetation, leaving a relatively barren landscape once the glaciers retreated. In these regions, the carbon currently stored in the vegetation and soils is due to carbon uptake over the last roughly 15,000 years, during postglacial ecosystem development. The modern-day land CO₂ sink is more difficult to quantify, as its estimation relies upon a combination of observations and modelling efforts. Most models, whether they use top-down or bottom-up approaches, consistently indicate a sink, albeit with different estimated strengths, giving medium confidence that the historical sink has continued. In Key Message 9.10, there is low confidence in whether extreme events in any given year can turn the Canadian landscape into a short-term carbon source. This low confidence stems from the difficulties in quantifying the carbon released as a result of disturbances and the uncertainties associated with estimating post-disturbance carbon fluxes for land ecosystems. For example, estimates of carbon emissions from the 2023 Canadian wildfires range from as low as 700 Mt CO₂ (Kirchmeier-Young et al., 2024) to as high as 2370 Mt CO₂ (Byrne et al., 2024). There is medium confidence that vegetation productivity will increase in the future due to the CO₂ fertilization effect and warmer temperatures that promote vegetation growth in temperature-limited high-latitude ecosystems. There is high confidence that carbon losses associated with permafrost thaw and wildfires will increase in the future. As a result, the future of Canada’s CO₂ land sink remains uncertain. Many processes believed to be important (for example, abrupt permafrost thaw, wildfires, peatland dynamics, and insect damage) are poorly represented, or not represented at all, in the current state-of-the-art models, due to an incomplete understanding of the processes involved or the lack of adequate observational data to constrain the models.

The quality of burned area data has changed over time. Fire records from the pre-satellite era (before ca. 1980) potentially contain biases, due to unobserved fires, poorly mapped fire extents, and inconsistent methodologies. These potential biases in the fire records result in medium confidence in Key Message 9.11, in the context of the historical trend in burned area. Model simulations are consistent in their projections of fire metrics. Given that the relationship between climate and fire behaviour is well established, there is high confidence that, in the future, there will be increases in the average annual burned area.

There is medium confidence in the first part of the first sentence of Key Message 9.12. Models and limited observations indicate that Canada’s oceans are a net carbon sink overall (Duke, Richaud, et al., 2023; Fennel et al., 2019). However, regional and seasonal variations in the net atmosphere-ocean CO₂ flux are not well documented for many regions, and the aggregate flux into Canada’s national waters is surrounded by considerable uncertainty (Duke, Richaud, et al., 2023). Nearshore areas can be sources of CO₂ to the atmosphere, largely because of the breakdown of organic carbon that is washed into coastal waters from land (Capelle et al., 2020); however, only a few Canadian nearshore areas have been studied in detail, so there is low confidence associated with that part. Globally, if atmospheric CO₂ continues to increase, the ocean will continue to take up CO₂, because the flux is driven by the difference between the concentrations in the atmosphere and in the surface ocean. Models show that the open-ocean portion of Canada’s EEZ will continue to take up CO₂ (Holdsworth et al., 2021) and that the uptake could even accelerate in some areas (Duke, Hamme, et al., 2023). Combining the open ocean result with the uncertainty about fluxes in nearshore areas results in the medium confidence in the last sentence in Key Message 9.12.

9.6: Nature-based approaches to climate change mitigation

Key Message 9.13: Nature-based climate solutions (NBCS) play a complementary rather than a central role in achieving climate change mitigation goals (very high confidence).

Key Message 9.14: In Canada, the protection of existing nature-based carbon stocks and carbon dioxide (CO₂) drawdown resulting from the expansion of nature-based carbon sinks is estimated to reach as much as 78 Mt CO₂-eq/yr (CO₂-equivalent units per year) after 10 years of implementation (low confidence).

The climate is warming due to anthropogenic greenhouse gas (GHG) emissions from the burning of fossil fuels and from land-use changes (section 9.3). When anthropogenic emissions of long-lived GHGs such as carbon dioxide (CO₂) reach net zero, and emissions of short-lived GHGs are strongly reduced, then the global temperature will stabilize (Matthews and Caldeira, 2008) (section 9.4). However, much of the excess CO₂ will remain in the atmosphere for thousands to hundreds of thousands of years (Archer et al., 2009).

When accompanied by a reduction in anthropogenic CO₂ emissions, carbon dioxide removal (CDR) potentially provides an option to reduce the atmospheric concentration of CO₂. CDR refers to deliberate human interventions to draw down CO₂ from the atmosphere. CDR is not an alternative to reducing emissions. Rather, it offers to help offset residual, hard-to-mitigate emissions, or possibly to help achieve net negative emissions in order to reduce global temperature, in case a target for global maximum temperature is exceeded. CDR methods can be grouped into nature-based and technological methods (S. M. Smith et al., 2024). Technological CDR options include large-scale interventions, such as direct air-carbon capture and storage, or large-scale alkalinization of the ocean (S. M. Smith et al., 2024). Nature-based CDR options aim to enhance organic carbon storage in land- and ocean-based ecosystems (Council of Canadian Academies, 2022).

Nature-based climate solutions (NBCS) refer to both nature-based CDR and the protection of existing carbon stocks in ecosystems. The use of NBCS to reduce or offset anthropogenic emissions has received considerable public attention. This section assesses the limited literature available that quantifies the climate change mitigation potential of NBCS in Canada, through the protection of ecosystems and the enhancement of CO₂ drawdown from the atmosphere. The objective is to situate NBCS in the context of Canada’s anthropogenic greenhouse gas emissions and emissions reduction targets. The co-benefits and trade-offs associated with various NBCS are acknowledged but not assessed in any detail. The implementation and feasibility of NBCS depend on regional ecosystems, local laws, municipal regulations, community acceptance, and other location-specific factors. Regional evaluation of NBCS in light of these considerations is beyond the scope of this assessment. Technological CDR methods have been assessed comprehensively elsewhere (IPCC, 2022a; Minx et al., 2018; S. M. Smith et al., 2024) and are not discussed further in this report.

9.6.1: Nature-based climate solutions as carbon dioxide removal methods

There is some confusion over the role of ecosystem carbon storage in climate mitigation. Methods that protect existing ecosystem carbon stocks are often presented interchangeably with those that enhance CO₂ drawdown from the atmosphere. Protecting existing stocks helps to avoid additional future emissions that might otherwise have resulted from ecosystem degradation or loss. However, protection is not considered a form of CDR, because it does not increase the drawdown of atmospheric CO₂ relative to current conditions.

To use NBCS for CO₂ removal, the uptake of CO₂ must be increased by restoring or enhancing natural ecosystems. Examples of these types of NBCS include the restoration or expansion of forests and vegetated coastal ecosystems, as well as the modification of management practices in managed forests, croplands, grasslands, wetlands, and coastal ecosystems.

Given the timescales associated with biological processes, NBCS can result in carbon storage on a timescale of decades to centuries, before that carbon is returned to the atmosphere through decomposition and other natural processes. The potential carbon storage offered by NBCS is limited, because most natural ecosystems, such as forests and grasslands, do not continue to take up CO₂ indefinitely. These ecosystems tend to become carbon neutral once they reach maturity (Bai and Cotrufo, 2022; Minx et al., 2018; P. Smith, 2014), as long as environmental conditions remain relatively stable and no substantial disturbances occur (McGrath et al., 2024; Turner et al., 2015).

Disturbances disrupt the carbon balance of ecosystems by releasing stored carbon back into the atmosphere. For example, the release of carbon by forest fires and insect infestations can negate the increased carbon uptake resulting from afforestation and reforestation efforts (section 9.5.1). Similarly, marine heatwaves can cause the release of carbon stored in vegetated coastal ecosystems (Arias-Ortiz et al., 2018). The frequency of these disturbances and the conditions favouring them are increasing with climate change in Canada (Chapter 8).

In addition, some NBCS could compete with one another for the use of the same land (for example, expanded forest versus expanded cropland for soil carbon storage), making it impossible to realize the full potential of all the methods combined (Council of Canadian Academies, 2022). NBCS also compete with other land-use requirements. For example, implementation of afforestation and forest cover restoration may be regionally constrained due to a variety of factors, including agricultural demand, infrastructure development, and extractive industries (Council of Canadian Academies, 2022).

Like other CO₂ removal methods, NBCS also carry some risks and trade-offs (Cobo et al., 2022). For example, afforestation could displace agriculture (Council of Canadian Academies, 2022). Afforestation also changes the reflectivity of the land surface, making it darker compared to grasslands or bare soil, which makes the land absorb additional solar radiation that would otherwise have been reflected back to space (Arora and Montenegro, 2011). As a result, in high-latitude countries like Canada, the net effect of afforestation is a slight warming of the local climate (section 9.5.1.5). The restoration of wetlands and other ecosystems can increase the release of greenhouse gases other than CO₂ (non-CO₂ GHGs), such as methane and nitrous oxide; a full assessment of the effect of wetland restoration on the climate requires the consideration of these emissions (Rosentreter et al., 2021; Zickfeld et al., 2023).

The global carbon sequestration potential of land-based mitigation methods is unclear. Some authors have estimated that NBCS could reduce net anthropogenic emissions by as much as 14 to 26% (Roe et al., 2021; Strassburg et al., 2020). However, others consider these estimates to be too high, due to the overly optimistic assumptions about land availability and limitations in the quality of the underlying data (Doelman and Stehfest, 2022; Reise et al., 2021). Overestimating the role of ecosystem restoration could undermine mitigation efforts and distract from the core task of reducing anthropogenic GHG emissions (Doelman and Stehfest, 2022). While nature-based CDR could help to mitigate anthropogenic GHG emissions, climate mitigation goals at the global scale will only be met by coordinated, global action to make deep and sustained reductions in fossil fuel emissions and to reach net zero CO₂ emissions (IPCC, 2021, 2022c, 2023). The role of NBCS is complementary rather than central to achieving global climate change mitigation targets (Anderson et al., 2019; Ellis et al., 2024; Seddon et al., 2020, 2021).

9.6.2: Nature-based climate solutions in Canada

The mitigation potential of NBCS in Canada is highly uncertain, due to the limited number of studies conducted. One study (Drever et al., 2021) estimated that protecting existing carbon stocks in Canadian ecosystems over a 10-year period could influence future emissions by up to 78 megatonnes of carbon dioxide equivalent units per year (Mt CO₂-eq/yr), from a combination of effects. More specifically, NBCS could remove 3.9 Mt CO₂-eq/yr from the atmosphere, prevent 30 Mt CO₂-eq/yr of future emissions by protecting existing ecosystems, and improve forestry and agricultural management to achieve a total of 44.3 Mt CO2-eq/yr through a combination of avoided emissions and CDR. Together, these three components add up to 78 Mt CO₂-eq/yr, although they have differing effects on atmospheric CO₂, as explained above. To put the mitigation potential of NBCS in context, Canada’s total anthropogenic emissions were 694 Mt CO₂-eq/yr in 2023 (section 9.5.1), and Canada has made a commitment to reduce its annual emissions by 45 to 50% below the 2005 level (759 Mt CO₂-eq (ECCC, 2025)) by 2035, or to 380 to 417 Mt CO₂‑eq/yr (ECCC, 2024).

A more optimistic estimate of NBCS potential in Canada—400 Mt CO₂-eq/yr by Roe et al. (2021)—is considered to be significantly exaggerated (more than 10 times too high) (Council of Canadian Academies, 2022). This is consistent with comments by (Doelman and Stehfest, 2022) and (Reise et al., 2021) on the global estimates provided in the same study (Roe et al., 2021). For Canada, the estimates of mitigation potential by (Drever et al., 2021) are considered more credible, providing a useful baseline for policymakers (Council of Canadian Academies, 2022). The Drever et al. (2021) estimates are based on a range of approaches for preserving carbon stores (including preventing the conversion of grasslands to croplands and the conversion of forests to agricultural land) and for enhancing carbon uptake (including the improved management and restoration of forests). These authors also considered the cost effectiveness of NBCS, by taking into account the resource investment required for each tonne of CO₂-eq removed from the atmosphere.

In the short term (by 2030), NBCS appears to have the highest climate change mitigation potential in Canada through measures targeting agricultural lands, followed by wetlands, grasslands, and forests; in the medium term (by 2050), measures targeting forests appear to have the highest potential (Drever et al., 2021). Estimates of potential CO₂ drawdown are highly uncertain, because they are based on a limited number of studies and the calculations rely heavily on assumptions about Canada’s future land-use decisions.

In Canada, ecosystem carbon stocks are located on the traditional territory of Indigenous Peoples. As a result, the implementation of NBCS in Canada critically depends on the involvement of Indigenous communities and coordination with ongoing Indigenous-led land and resource management efforts (Council of Canadian Academies, 2022). An analysis of Canadian climate policy, planning, and science documents concluded that NBCS discussions must recognize the central role of Indigenous perspectives in achieving meaningful NBCS implementation (Reed et al., 2022).

Despite the relatively low potential of NBCS in climate change mitigation, NBCS measures offer important ecological and socio-economic benefits, including contributing to climate change adaptation (Molnar et al., 2021). For example, seagrass meadows and salt marshes provide coastal flood control, food security, and eco-tourism opportunities, as well as critical habitat for invertebrates, juvenile fish, and marine birds (Hejnowicz et al., 2015). The use of NBCS in land-based ecosystems can improve air, soil, and water quality; increase crop growth; provide protection from soil erosion; reduce urban heat island effects; and conserve biodiversity (Adams et al., 2013; Cook-Patton et al., 2021). Additionally, protecting ecosystems can improve their resilience to disturbance—for example, intact peatlands are better able to withstand severe fires and thus preserve their stores of carbon (section 9.5.1.3) (Kuntzemann et al., 2023; Wilkinson et al., 2023).

For a more detailed discussion of the potential of NBCS in Canada, see the comprehensive report by the Council of Canadian Academies (Council of Canadian Academies, 2022).

9.6.3: Confidence terms in key messages: summary of evidence

Key Message 9.13: Nature-based climate solutions (NBCS) play a complementary rather than a central role in achieving climate change mitigation goals (very high confidence).

Key Message 9.14: In Canada, the protection of existing, nature-based carbon stocks and carbon dioxide (CO₂) drawdown resulting from the expansion of nature-based carbon sinks is estimated to reach up to 78 Mt CO₂-eq/yr (CO₂-equivalent units per year) after 10 years of implementation (low confidence).

There is very high confidence in Key Message 9.13, that while NBCS can be a component of climate change mitigation strategies, NBCS are not sufficient on their own. IPCC reports have consistently stated that achieving global warming levels of 1.5°C and 2°C will require deep, rapid, and sustained reductions in GHG emissions, and limiting global warming at any level will require net-zero emissions of long-lived GHGs and declining emissions of short-lived GHGs (IPCC, 2021, 2022c, 2023). These requirements for limiting global warming are discussed in detail in section 9.4. Key Message 9.13 is also supported by scientific studies that focus explicitly on the role of NBCS in achieving climate change mitigation goals (Anderson et al., 2019; Ellis et al., 2024; Seddon et al., 2020, 2021). These studies caution that the benefits of NBCS do not decrease the imperative to mitigate climate change by reducing emissions from the energy and industrial sectors (Anderson et al., 2019) and that NBCS should not curtail or distract from the urgent need to rapidly decarbonize our economy (Seddon et al., 2020).

There is low confidence in key message 9.14 due to the limited number of Canada-specific studies upon which the mitigation potential of NBCS can be based. The comprehensive report by the Council of Canadian Academies (Council of Canadian Academies, 2022) draws a similar conclusion: that national estimates of the mitigation potential of NBCS in Canada are based on limited evidence and remain highly uncertain.

Frequently Asked Question

FAQ 9.1 How does global temperature respond to increasing, decreasing, and net-zero carbon dioxide emissions?

The global temperature will rise at a faster rate with increasing emissions, at a constant rate with constant emissions, and at a slower rate with decreasing emissions. The global temperature will approximately stabilize once emissions reach net zero. This concept is illustrated in FAQ 9.1 Figure 1.

Figure take-away: The global temperature will stabilize when anthropogenic carbon dioxide emissions reach net zero.

Figure title: Global temperature response to different anthropogenic carbon dioxide (CO₂) emissions trajectories

FAQ 9.1 Figure 1: Graphs illustrating a) different trajectories for carbon dioxide (CO₂) emissions over the rest of the 21st century (in gigatonnes of CO₂ per year, or Gt CO₂/year) and b) the illustrative global temperature response (in degrees Celsius) to these trajectories. The examples of possible emissions pathways begin with approximate emissions of 37 Gt CO₂ in 2020, as shown in a). Net-zero emissions are achieved when anthropogenic CO₂ emissions are balanced by CO₂ removal from the atmosphere, and negative emissions occur when removals exceed emissions. The global temperature change of approximately 1.0°C above pre-industrial levels by 2020 in panel b) is depicted for illustrative purposes only. The global temperature will continue to increase until emissions reach net zero. If CO₂ emissions reach net zero, the global temperature is projected to approximately stabilize. The global temperature will only decline when CO₂ emissions become negative. Source: Figure 9.8.

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