Editing IPCC:AR6/WGI/TS (section)

== Box TS.5 | The Carbon Cycle ==

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'''The continued growthof atmospheric CO <sub>2</sub> concentrations over the industrial era is unequivocally due to emissions from human activities. Ocean and land carbon sinks slow the rise of CO <sub>2</sub> in the atmosphere. Projections show that while land and ocean sinks absorb more CO <sub>2</sub> under high emissions scenarios than low emissions scenarios, the fraction of emissions removed from the atmosphere by natural sinks decreases with higher concentrations (''high confidence''). Projected ocean and land sinks show similar responses for a given scenario, but the land sink has a much higher interannual variability and wider model spread. The slowed growth rates of the carbon sinks projected for the second half of this century are linked to strengthening carbon–climate feedbacks and stabilization of atmospheric CO <sub>2</sub> under medium-to-no-mitigation and high-mitigation scenarios, respectively (see FAQ 5.1). Links to chapters 5.2, 5.4'''

Carbon sinks for anthropogenic CO <sub>2</sub> are associated with mainly physical ocean and biospheric land processes that drive the exchange of carbon between multiple land, ocean and atmospheric reservoirs. These exchanges are driven by increasing atmospheric CO <sub>2</sub> , but are modulated by changes in climate (Box TS.5, Figure 1c,d). The Northern and Southern Hemispheres dominate the land and ocean sinks, respectively (Box TS.5, Figure 1). Ocean circulation and thermodynamic processes also play a critical role in coupling the global carbon and energy (heat) cycles. There is ''high confidence'' that this ocean carbon–heat nexus is an important basis for one of the most important carbon–climate metrics, the transient climate response to cumulative CO <sub>2</sub> emissions (TCRE; Section TS.3.2.1) used to determine the remaining carbon budget. Links to chapters 5.1, 5.2, 5.5, 9.2, Cross-Chapter Box 5.3

Based on multiple lines of evidence using interhemispheric gradients of CO <sub>2</sub> concentrations, isotopes, and inventory data, it is unequivocal that the growth in CO <sub>2</sub> in the atmosphere since 1750 (see Section TS.2.2) is due to the direct emissions from human activities. The combustion of fossil fuels and land-use change for the period 1750–2019 resulted in the release of 700 ± 75 PgC (''likely'' range, 1 PgC = 10 <sup>15</sup> g of carbon) to the atmosphere, of which about 41% ± 11% remains in the atmosphere today (''high confidence''). Of the total anthropogenic CO <sub>2</sub> emissions, the combustion of fossil fuels was responsible for about 64% ± 15%, growing to an 86% ± 14% contribution over the past 10 years. The remainder resulted from land-use change. During the last decade (2010–2019), average annual anthropogenic CO <sub>2</sub> emissions reached the highest levels in human history at 10.9 ± 0.9 PgC yr <sup>–1</sup> (''high confidence''). Of these emissions, 46% accumulated in the atmosphere (5.1 ± 0.02 PgC yr <sup>–1</sup>), 23% (2.5 ± 0.6 PgC yr <sup>–1</sup>) was taken up by the ocean and 31% (3.4 ± 0.9 PgC yr <sup>–1</sup>) was removed by terrestrial ecosystems (''high confidence''). Links to chapters 5.2.1, 5.2.2, 5.2.3

The ocean (''high confidence'') and land (''medium confidence'') sinks of CO <sub>2</sub> have increased with anthropogenic emissions over the past six decades (Box TS.5, Figure 1). This coherence between emissions and the growth in ocean and land sinks has resulted in the airborne fraction of anthropogenic CO <sub>2</sub> remaining at 44 ± 10% over the past 60 years (''high confidence''). Interannual and decadal variability of the ocean and land sinks indicate that they are sensitive to changes in the growth rate of emissions as well as climate variability and are therefore also sensitive to climate change (''high confidence''). Links to chapters 5.2.1

The land CO <sub>2</sub> sink is driven by carbon uptake by vegetation, with large interannual variability, for example, linked to the El Niño–Southern Oscillation (ENSO). Since the 1980s, carbon fertilization from rising atmospheric CO <sub>2</sub> has increased the strength of the net land CO <sub>2</sub> sink (''medium confidence''). During the historical period, the growth of the ocean sink has been primarily determined by the growth rate of atmospheric CO <sub>2</sub> . However, there is ''medium confidence'' that changes to physical and chemical processes in the ocean and in the land biosphere, which govern carbon feedbacks, are already modifying the characteristics of variability, particularly the seasonal cycle of CO <sub>2</sub> , in both the ocean and land. However, changes to the multi-decadal trends in the sinks have not yet been observed. Links to chapters 2.3.4, 3.6.1, 5.2.1

In AR6, ESM projections are assessedwith CO <sub>2</sub> concentrations by 2100 from about 400 ppm (SSP1-1.9) to above 1100 ppm (SSP5-8.5). Most simulations are performed with prescribed atmospheric CO <sub>2</sub> concentrations, which already account for a central estimate of climate–carbon feedback effects. Carbon dioxide emissions-driven simulations account for uncertainty in these feedbacks, but do not significantly change the projected global surface temperature changes (''high confidence''). Although land and ocean sinks absorb more CO <sub>2</sub> under high emissions than low emissions scenarios, the fraction of emissions removed from the atmosphere decreases (''high confidence''). This means that the more CO <sub>2</sub> that is emitted, the less efficient the ocean and land sinks become (''high confidence''), an effect which compensates for the logarithmic relationship between CO <sub>2</sub> and its radiative forcing, which means that for each unit increase in additional atmospheric CO <sub>2</sub> the effect on global temperature decreases. (Box TS.5, Figure 1f,g). Links to chapters 4.3.1, 5.4.5, 5.5.1.2

Ocean and land sinks show similar responses for a given scenario, but the land sink has a much higher interannual variability and wider model spread. Under SSP3-7.0 and SSP5-8.5, the initial growth of both sinks in response to increasing atmospheric concentrations of CO <sub>2</sub> is subsequently limited by emerging carbon–climate feedbacks (''high confidence'') (Box TS.5, Figure 1f). Projections show that the ocean and land sinks will stop growing from the second part of the 21st century under all emissions scenarios, but with different drivers for different emissions scenarios. Under SSP3-7.0 and SSP5-8.5, the weakening growth rate of the ocean CO <sub>2</sub> sink in the second half of the century is primarily linked to the strengthening positive feedback from reduced carbonate buffering capacity, ocean warming and altered ocean circulation (e.g., AMOC changes). In contrast, for SSP1-1.9, SSP1-2.6 and SSP2-4.5, the weakening growth rate of the ocean carbon sink is a response to the stabilizing or declining atmospheric CO <sub>2</sub> concentrations. Under SSP1-1.9, models project that combined land and ocean sinks will turn into a weak source by 2100 (''medium confidence''). Under high CO <sub>2</sub> emissions scenarios, it is ''very likely'' that the land carbon sink will grow more slowly due to warming and drying from the mid-21st century, but it is ''very unlikely'' that it will switch from being a sink to a source before 2100.

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'''Box TS.5, Figure 1 |'''

'''Carbon cycle processes and projections.''' ''The intent of this figure is to show the response of the carbon cycle to carbon dioxide (CO'' 2 '') emissions and climate and its role in determining future CO'' 2 ''levels through projected changes to sinks and sink fractions.'' The figure shows changes in carbon storage in response to elevated CO <sub>2</sub> '''(a, b)''' and the response to climate warming '''(c, d)''' . Maps show spatial patterns of changes in carbon uptake during simulations with 1% per year increase in CO <sub>2</sub> ([[IPCC:Wg1:Chapter:Chapter-5#5.4.5.5|Section 5.4.5.5]]), and zonal mean plots show distribution of carbon changes is dominated by the land (green lines) in the tropics and Northern Hemisphere and ocean (blue lines) in the Southern Hemisphere. Hatching indicates regions where fewer than 80% of models agree on the sign of response. '''(e)''' Future CO <sub>2</sub> projections: projected CO <sub>2</sub> concentrations in the Shared Socio-economic Pathway (SSP) scenarios in response to anthropogenic emissions, results from coupled Earth system models for SSP5-8.5 and from the MAGICC7 emulator for other scenarios ([[IPCC:Wg1:Chapter:Chapter-4#4.3.1|Section 4.3.1]]). '''(f)''' Future carbon fluxes: projected combined land and ocean fluxes (positive downward) up to 2100 for the SSP scenarios, and extended to 2300 for available scenarios, 5–95% uncertainty plumes shown for SSP1-2.6 and SSP3-7.0 (Sections 4.3.2.4, 5.4.5.4 and 5.4.10). The numbers near the top show the number of model simulations used. '''(g)''' Sink fraction: the fraction of cumulative emissions of CO <sub>2</sub> removed by land and ocean sinks. The sink fraction is smaller under conditions of higher emissions. Links to chapters Figure 4.3; 5.4.5; Figures 5.25, 5.27 and 5.30
Climate change alone is expected to increase land carbon accumulation in the high latitudes (not including permafrost, which is assessed in Sections TS.2.5 and TS.3.2.2), but also to lead to a counteracting loss of land carbon in the tropics (''medium confidence''). Earth system model projections show that the overall uncertainty of atmospheric CO <sub>2</sub> by 2100 is still dominated by the emissions pathway, but carbon–climate feedbacks (see Section TS.3.3.2) are important, with increasing uncertainties in high emissions pathways (Box TS.5, Figure 1e). Links to chapters 4.3.2, 5.4.1, 5.4.2, 5.4.4, 5.4.5, 11.6, 11.9, Cross-Chapter Box 5.1, Cross-Chapter Box 5.3

Under three SSP scenarios with long-term extensions until 2300 (SSP5-8.5, SSP5-3.4-OS, SSP1-2.6), ESMs project a change of the land from a sink to a source (''medium confidence''). The scenarios make simplified assumptions about emissions reductions, with SSP1-2.6 and SSP5-3.4-OS reaching about 400 ppm by 2300, while SSP5-8.5 exceeds 2000 ppm. Under high emissions, the transition is warming-driven, whereas it is linked to the decline in atmospheric CO <sub>2</sub> under net negative CO <sub>2</sub> emissions. The ocean remains a sink throughout the period to 2300 except under very large net negative emissions. The response of the natural aspects of the carbon cycle to carbon dioxide removal is further developed in Section TS.3.3.2. Links to chapters 5.4.9
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=== TS.2.6 Land Climate, Including Biosphere and Extremes ===

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'''Land surface air temperatures have risen faster than the global surface temperature since the 1850s, and it is ''virtually certain'' that this differential warming will persist into the future. It is ''virtually certain'' that the frequency and intensity of hot extremes and the intensity and duration of heatwaves have increased since 1950 and will further increase in the future even if global warming is stabilized at 1.5°C. The frequency and intensity of heavy precipitation events have increased over a majority of those land regions with good observational coverage (''high confidence'') and will ''extremely likely'' increase over most land regions with additional global warming.''' '''Over the past half century, key aspects of the biosphere have changed in ways that are consistent with large-scale warming: climate zones have shifted poleward, and the growing season length in the Northern Hemisphere extratropics has increased (''high confidence''). The amplitude of the seasonal cycle of atmospheric CO <sub>2</sub> poleward of 45°N has increased since the 1960s (''very high confidence''), with increasing productivity of the land biosphere due to the increasing atmospheric CO <sub>2</sub> concentration as the main driver (''medium confidence''). Global-scale vegetation greenness has increased since the 1980s (''high confidence''). Links to chapters 2.3, 3.6, 4.3, 4.5, 5.2, 11.3, 11.4, 11.9, 12.4'''

Observed temperatures over land have increased by 1.59 [1.34–1.83] °C between the period 1850–1900 and 2011–2020. Warming of the land is about 45% larger than for global surface temperature and about 80% larger than warming of the ocean surface. Warming of the land surface during the period 1971–2018 contributed about 5% of the increase in the global energy inventory (Section TS.3.1), nearly twice the estimate in AR5 (''high confidence''). It is ''virtually certain'' that the average surface warming over land will continue to be higher than over the ocean throughout the 21st century. The warming pattern will ''likely'' vary seasonally, with northern high latitudes warming more during winter than summer (''medium confidence''). Links to chapters 2.3.1, 4.3.1, 4.5.1, 7.2.2, Box 7.2, Cross-Chapter Box 9.1, 11.3, Atlas 11.2

The frequency and intensity of hot extremes (warm days and nights) and the intensity and duration of heatwaves have increased globally and in most regions since 1950, while the frequency and intensity of cold extremes have decreased (''virtually certain''). There is ''high confidence'' that the increases in frequency and severity of hot extremes are due to human-induced climate change. Some recent extreme events would have been ''extremely unlikely'' to occur without human influence on the climate system. It is ''virtually certain'' that further changes in hot and cold extremes will occur throughout the 21st century in nearly all inhabited regions, even if global warming is stabilized at 1.5°C (Table TS.2, Figure TS.12a). Links to chapters 1.3, Cross-Chapter Box 3.2, 11.1.4, 11.3.2, 11.3.4, 11.3.5, 11.9, 12.4

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'''Figure TS.12 |''' '''Land-related changes relative to the 1850-1900 as a function of global warming levels.''' ''The intent of this figure is to show that extremes and mean land variables change consistently with warming levels and to show the changes with global warming levels of water cycle indicators (i.e., precipitation and runoff) over tropical and extratropical land in terms of mean and interannual variability (interannual variability increases at a faster rate than the mean)'' ''.'' (a) Changes in the frequency (left scale) and intensity (in °C, right scale) of daily hot extremes occurring every 10 and 50 years. (b) as (a), but for daily heavy precipitation extremes, with intensity change in %. (c) Changes in 10-year droughts aggregated over drought-prone regions (WNA, CNA, NCA, SCA, NSA, NES, SAM, SWS, SSA, WCE, MED, WSAF, ESAF, MDG, SAU, and EAU; for definitions of these regions, see Figure Atlas.2), with drought intensity (right scale) represented by the change of annual mean soil moisture, normalized with respect to interannual variability. Limits of the 5% − 95% confidence interval are shown in panels (a–c). (d) Changes in Northern Hemisphere spring (March–April–May) snow cover extent relative to 1850–1900; (e,f) Relative change (%) in annual mean of total precipitable water (grey line), precipitation (red solid lines), runoff (blue solid lines) and in standard deviation (i.e., variability) of precipitation (red dashed lines) and runoff (blue dashed lines) averaged over (e) tropical and (f) extratropical land as function of global warming levels. Coupled Model Intercomparison Project Phase 6 (CMIP6) models that reached a 5°C warming level above the 1850–1900 average in the 21st century in SSP5-8.5 have been used. Precipitation and runoff variability are estimated by respective standard deviation after removing linear trends. Error bars show the 17–83% confidence interval for the warmest +5°C global warming level. Links to chapters Figures 8.16, 9.24, 11.6, 11.7, 11.12, 11.15, 11.18 and Atlas.2

Greater warming over land alters key water cycle characteristics (Box TS.6). The rates of change in mean precipitation and runoff, and their variability, increase with global warming (Figure TS.12e,f). Human-induced climate change has contributed to increases in agricultural and ecological droughts in some regions due to increases in evapotranspiration (''medium confidence''). More regions are affected by increases in agricultural and ecological droughts with increasing global warming (''high confidence'' ; see also Figure TS.12c).

There is ''low confidence'' that the increase of plant water-use efficiency due to higher atmospheric CO <sub>2</sub> concentration alleviates extreme agricultural and ecological droughts in conditions characterized by limited soil moisture and increased atmospheric evaporative demand. Links to chapters 2.3.1, Cross-Chapter Box 5.1, 8.2.3, 8.4.1, 11.2.4, 11.4, 11.6, Box 11.1

Northern Hemisphere spring snow cover has decreased since at least 1978 (''very high confidence''), and there is ''high confidence'' that trends in snow cover loss extend back to 1950. It is ''very likely'' that human influence contributed to these reductions. Earlier onset of snowmelt has contributed to seasonally dependent changes in streamflow (''high confidence''). A further decrease of Northern Hemisphere seasonal snow cover extent is ''virtually certain'' under further global warming (Figure TS.12d). Links to chapters 2.3.2, 3.4.2, 8.3.2. 9.5.3, 12.4, 9.2, 11.2, [[IPCC:Wg1:Chapter:Atlas|Atlas]] 8.2

The frequency and intensity of heavy precipitation events have increased over a majority of land regions with good observational coverage since 1950 (''high confidence,'' Box TS.6, Table TS.2). Human influence is ''likely'' the main driver of this change (Table TS.2). It is ''extremely likely'' that on most land regions heavy precipitation will become more frequent and more intense with additional global warming (Table TS.2, Figure TS.12b). The projected increase in heavy precipitation extremes translates to an increase in the frequency and magnitude of pluvial floods (''high confidence'') (Table TS.2). Links to chapters Cross-Chapter Box 3.2, 8.4.1, 11.4.2, 11.4.4, 11.5.5, 12.4

Theprobability of compound extreme events has ''likely'' increased due to human-induced climate change. Concurrent heatwaves and droughts have become more frequent over the last century, and this trend will continue with higher global warming (''high confidence''). The probability of compound flooding (storm surge, extreme rainfall and/or river flow) has increased in some locations and will continue to increase due to both sea level rise and increases in heavy precipitation, including changes in precipitation intensity associated with tropical cyclones (''high confidence''). Links to chapters 11.8.1, 11.8.2, 11.8.3

Changes in key aspects of the terrestrial biosphere, such as an increase of the growing season length in much of the Northern Hemisphere extratropics since the mid-20th century (''high confidence''), are consistent with large-scale warming. At the same time an increase in the amplitude of the seasonal cycle of atmospheric CO <sub>2</sub> poleward of 45°N since the early 1960s (''high confidence'') and a global-scale increase in vegetation greenness of the terrestrial surface since the early 1980s (''high confidence'') have been observed. Increasing atmospheric CO <sub>2</sub> , warming at high latitudes, and land management interventions have contributed to the observed greening trend, but there is ''low confidence'' in their relative roles. There is ''medium confidence'' that increased plant growth associated with CO <sub>2</sub> fertilization is the main driver of the observed increase in amplitude of the seasonal cycle of atmospheric CO <sub>2</sub> in the Northern Hemisphere. Reactive nitrogen, ozone and aerosols affect terrestrial vegetation and carbon cycle through deposition and effects on large-scale radiation (''high confidence''), but the magnitude of these effects on the land carbon sink, ecosystem productivity and indirect CO <sub>2</sub> forcing remains uncertain. Links to chapters 2.3.4, 3.6.1, 5.2.1, 6.4.5, 12.3.7, 12.4

Over the last century, there has been a poleward and upslope shift in the distribution of many land species (''very high confidence'') as well as increases in species turnover within many ecosystems (''high confidence''). There is ''high confidence'' that the geographical distribution of climate zones has shifted in many parts of the world in the last half century. The SRCCL concluded that continued warming will exacerbate desertification processes (''medium confidence'') and that ecosystems will become increasingly exposed to climates beyond those that they are currently adapted to (''high confidence''). There is ''medium confidence'' that climate change will increase disturbance by, for example, fire and tree mortality, across several ecosystems. Increases are projected in drought, aridity and fire weather in some regions (Section TS.4.3; ''high confidence''). There is ''low confidence'' in the magnitude of these changes, but the probability of crossing uncertain regional thresholds (e.g., fires, forest dieback) increases with further warming (''high confidence''). The response of biogeochemical cycles to the anthropogenic perturbation can be abrupt at regional scales, and irreversible on decadal to century time scales (''high confidence''). Links to chapters 2.3.4, 5.4.3, 5.4.9, 11.6, 11.8, 12.5, SRCCL 2.2, SRCCL 2.5, SR1.5 3.4

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