Editing IPCC:AR6/WGI/Chapter-5 (section)

=== Future Projections of Carbon Feedbacks on Climate Change ===

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'''Oceanic and terrestrial carbon sinks are projected to continue to grow with increasing atmospheric concentrations of CO''' <sub>2</sub> ''', but the fraction of emissions taken up by land and ocean is expected to decline as the CO''' <sub>2</sub> '''concentration increases''' ( ''high confidence'' ''').''' ESMs suggest approximately equal global land and ocean carbon uptake for each of the SSP scenarios. However, the range of model projections is much larger for the land carbon sink. Despite the wide range of model responses, uncertainty in atmospheric CO <sub>2</sub> by 2100 is dominated by future anthropogenic emissions rather than uncertainties related to carbon–climate feedbacks ( ''high confidence'' ). {5.4.5; Figure 5.25, 5.26}

'''Increases in atmospheric CO''' <sub>2</sub> '''lead to increases in land carbon storage through CO''' <sub>2</sub> '''fertilization of photosynthesis and increased water use efficiency''' ( ''high confidence'' ''').''' However, the overall change in land carbon also depends on land-use change and on the response of vegetation and soil to continued warming and changes in the water cycle, including increased droughts in some regions that will diminish the sink capacity. Climate change alone is expected to increase land carbon accumulation in the high latitudes (not including permafrost) and also to lead to a counteracting loss of land carbon in the tropics ( ''medium confidence'' , Figure 5.25). More than half of the latest CMIP6 ESMs include nutrient limitations on the carbon cycle, but these models still project increasing tropical land carbon ( ''medium confidence'' ) and increasing global land carbon ( ''high confidence'' ) through the 21st century. {5.4.1, 5.4.3, 5.4.5; Figure 5.27; Cross-Chapter Box 5.1}

'''Future trajectories of the ocean CO''' <sub>2</sub> '''sink are strongly emissions-scenario dependent''' ( ''high confidence'' ''').''' Emissions scenarios SSP4-6.0 and SSP5-8.5 lead to warming of the surface ocean and large reductions of the buffering capacity, which will slow the growth of the ocean sink after 2050. Scenario SSP1-2.6 limits further reductions in buffering capacity and warming, and the ocean sink weakens in response to the declining rate of increasing atmospheric CO <sub>2</sub> . There is ''low confidence'' in how changes in the biological pump will influence the magnitude and direction of the ocean carbon feedback. {5.4.2, 5.4.4, Cross-Chapter Box 5.3}

'''Beyond 2100, land and ocean may transition from being a carbon sink to a source under either very high emissions or net negative emissions scenarios, but for different reasons.''' Under very high emissions scenarios such as SSP5-8.5, ecosystem carbon losses due to warming lead the land to transition from a carbon sink to a source ( ''medium confidence'' ), while the ocean is expected to remain a sink ( ''high confidence'' ). For scenarios in which CO <sub>2</sub> concentration stabilizes, land and ocean carbon sinks gradually take up less carbon as the increase in atmospheric CO <sub>2</sub> slows down. In scenarios with moderate net negative CO <sub>2</sub> emissions, and CO <sub>2</sub> concentrations declining during the 21st century (e.g., SSP1-2.6), the land sink transitions to a net source in decades to a few centuries after CO <sub>2</sub> emissions become net negative, while the ocean remains a sink ( ''low confidence'' ). Under scenarios with large net negative CO <sub>2</sub> emissions and rapidly declining CO <sub>2</sub> concentrations (e.g., SSP5-3.4-OS (overshoot)), both land and ocean switch from a sink to a transient source during the overshoot period ( ''medium confidence'' ). {5.4.10, 5.6.2.1.2; Figures 5.30, 5.33}

'''Thawing terrestrial permafrost will lead to carbon release''' ( ''high confidence'' '''), but there is''' ''low confidence'' '''in the timing, magnitude and the relative roles of CO''' <sub>2</sub> '''versus CH''' <sub>4</sub> '''as feedback processes''' . CO <sub>2</sub> release from permafrost is projected to be 3–41 PgC per 1°C of global warming by 2100, based on an ensemble of models. However, the incomplete representation of important processes such as abrupt thaw, combined with weak observational constraints, only allow ''low confidence'' in both the magnitude of these estimates and in how linearly proportional this feedback is to the amount of global warming. It is ''very unlikely'' that gas clathrates in terrestrial and subsea permafrost will lead to a detectable departure from the emissions trajectory during this century. {5.4.9; Box 5.1}

'''The net response of natural CH''' <sub>4</sub> '''and N''' <sub>2</sub> '''O sources to future warming will be increased emissions''' ( ''medium confidence'' ''').''' Key processes include increased CH <sub>4</sub> emissions from wetlands and permafrost thaw, as well as increased soil N <sub>2</sub> O emissions in a warmer climate, while ocean N <sub>2</sub> O emissions are projected to decline at centennial time scale. The magnitude of the responses of each individual process and how linearly proportional these feedbacks are to the amount of global warming is known with ''low confidence'' due to incomplete representation of important processes in models combined with weak observational constraints. Models project that, over the 21st century, the combined feedback of 0.02–0.09 W m <sup>–2</sup> °C <sup>–1</sup> is comparable to the effect of a CO <sub>2</sub> release of 5–18 petagrams of carbon equivalent per °C (PgCeq °C <sup>–1</sup> ) ( ''low confidence'' ). {5.4.7, 5.4.8; Figure 5.29}

'''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'' ''').''' The probability of crossing uncertain regional thresholds (e.g., high severity fires, forest dieback) increases with climate change ( ''high confidence'' ). Possible abrupt changes and tipping points in biogeochemical cycles lead to additional uncertainty in 21st century GHG concentrations, but these are ''very likely'' to be smaller than the uncertainty associated with future anthropogenic emissions ( ''high confi'' ''dence'' ). {5.4.9}

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