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

=== 5.1.1 The Physical and Biogeochemical Processes in Carbon–Climate Feedbacks ===

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The influence of anthropogenic CO <sub>2</sub> emissions and emissions scenarios on the carbon–climate system is the primary driver of ocean and terrestrial sinks as the major negative feedbacks that determine the atmospheric CO <sub>2</sub> levels, which then drive climate feedbacks through radiative forcing (Figure 5.2) ( [[#Friedlingstein--2006|Friedlingstein et al., 2006]] ; [[#Jones--2013|]] [[#Jones--2013|Jones et al., 2013]] ; [[#Jones--2020|Jones and Friedlingstein, 2020]] ). Biogeochemical feedbacks follow as an outcome of both carbon and climate forcing on the physics and the biogeochemical processes of the ocean and terrestrial carbon cycles (Figure 5.2) ( [[#Katavouta--2018|Katavouta et al., 2018]] ; [[#Williams--2019|Williams et al., 2019]] ; [[#Jones--2020|Jones and Friedlingstein, 2020]] ). Together, these carbon–climate feedbacks can amplify or suppress climate change by altering the rate at which CO <sub>2</sub> builds up in the atmosphere through changes in the land and ocean sources and sinks (Figure 5.2; C.D. [[#Jones--2013|]] [[#Jones--2013|Jones et al., 2013]] ; [[#Raupach--2014|Raupach et al., 2014]] ; [[#Williams--2019|Williams et al., 2019]] ). These changes depend on the, often non-linear, interaction of the drivers (CO <sub>2</sub> and climate) and processes in the ocean and land as well as the emissions scenarios (Figure 5.2; Sections 5.4 and 5.6) ( [[#Raupach--2014|Raupach et al., 2014]] ; [[#Schwinger--2014|Schwinger et al., 2014]] ; [[#Williams--2019|Williams et al., 2019]] ). There is ''high'' ''confidence'' that carbon–climate feedbacks and their century scale evolution play a critical role in two linked climate metrics that have significant climate and policy implications: (i) the fraction of anthropogenic CO <sub>2</sub> emissions that remains in the atmosphere, the so-called airborne fraction of CO <sub>2</sub> (AF; [[#5.2.1.2|Section 5.2.1.2]] , Figures 5.2 and 5.7, and FAQ 5.1); and (ii) the quasi-linear trend characteristic of the transient temperature response to cumulative CO <sub>2</sub> emissions (TCRE; [[#5.5|Section 5.5]] ; [[#MacDougall--2016|MacDougall, 2016]] ; [[#Williams--2016|Williams et al., 2016]] ; [[#Jones--2020|Jones and Friedlingstein, 2020]] ) and other GHGs (CH <sub>4</sub> and N <sub>2</sub> O). This chapter assesses the implications of these issues from the perspective of carbon cycle processes (Figure 5.2) in [[#5.2|Section 5.2]] (historical and contemporary), [[#5.3|Section 5.3]] (changing carbonate chemistry), [[#5.4|Section 5.4]] (future projections), [[#5.5|Section 5.5]] (remaining carbon budget) and [[#5.6|Section 5.6]] (response to carbon dioxide removal and solar radiation modification).

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[[File:439c36b87d834f98da0d29ab09480871 IPCC_AR6_WGI_Figure_5_2.png]]

'''Figure 5.2 |''' '''Key compartments, processes and pathways that govern historical and future CO''' <sub>2</sub> '''concentrations and carbon–climate feedbacks through the coupled Earth system.''' The anthropogenic CO <sub>2</sub> emissions, including land-use change, are partitioned via negative feedbacks (turquoise dotted arrows) between the ocean (23%), the land (31%) and the airborne fraction (46%) of anthropogenic CO <sub>2</sub> that sets the changing CO <sub>2</sub> concentration in the atmosphere (2010–2019; Table 5.1). This regulates most of the radiative forcing that creates the heat imbalance that drives the climate feedbacks to the ocean (blue) and land (green). Positive feedbacks (red arrows) result from processes in the ocean and on land (red text). Positive feedbacks are influenced by both carbon-concentration and carbon–climate feedbacks simultaneously. Additional biosphere processes have been included, but these have an as-yet-uncertain feedback impact (blue-dotted arrows). CO <sub>2</sub> removal from the atmosphere into the ocean, land and geological reservoirs, necessary for negative emissions, has been included (grey arrows). Although this schematic is built around CO <sub>2</sub> (the dominant greenhouse gas), some of the same processes also influence the fluxes of CH <sub>4</sub> and N <sub>2</sub> O and the strength of the positive feedbacks from the terrestrial and ocean systems.

The airborne fraction is an important constraint for adjustments in carbon–climate feedbacks and reflects the partitioning of CO <sub>2</sub> emissions between reservoirs by the negative feedbacks, which were 31% on land and 23% in the ocean for the decade 2010–2019 and also dominated the historical period (Figure 5.2; Table 5.1) ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ). During the period 1959–2019, the airborne fraction has largely followed the growth in anthropogenic CO <sub>2</sub> emissions with a mean of 44% and a large interannual variability ( [[#Ballantyne--2012|Ballantyne et al., 2012]] ; [[#Ciais--2019|Ciais et al., 2019]] ; [[#Friedlingstein--2020|Friedlingstein et al., 2020]] , [[#5.2.1.2|Section 5.2.1.2]] ; Table 5.1). The negative feedback to CO <sub>2</sub> concentrations is associated with its impact on the air–sea and air–land CO <sub>2</sub> exchange through strengthening of partial pressure of CO <sub>2</sub> ( ''p'' CO <sub>2</sub> ) gradients as well as the internal processes that enhance uptake. Two of these key processes are the buffering capacity of the ocean and the CO <sub>2</sub> fertilization effect on gross primary production (Sections 5.4.1–5.4.4).

Positive and negative climate and carbon feedbacks involve: (i) fast processes on land and oceans at time scales from minutes to years, such as photosynthesis, soil respiration, net primary production, shallow ocean physics and air–sea fluxes; and (ii) slower processes taking from decades to millennia, such as changing ocean buffering capacity, ocean ventilation, vegetation dynamics, permafrost changes, peat formation and decomposition (Figure 5.2; [[#Ciais--2013|Ciais et al., 2013]] ; [[#Forzieri--2017|Forzieri et al., 2017]] ; [[#Williams--2019|Williams et al., 2019]] ). Depending on the particular combination of driver process and response dynamics, they behave as positive or negative feedbacks that amplify or dampen the magnitude and rates of climate change, respectively ( [[#Cox--2000|Cox et al., 2000]] ; [[#Friedlingstein--2003|Friedlingstein et al., 2003]] , [[#Friedlingstein--2006|2006]] ; [[#Hauck--2015|Hauck and Völker, 2015]] ; [[#Williams--2019|Williams et al., 2019]] ); red and turquoise arrows in Figure 5.2 and Table 5.1).

Carbon cycle feedbacks co-exist with climate (heat and moisture) feedbacks (Cross-Chapter Boxes 5.1 and 5.3), which together drive contemporary ( [[#5.2|Section 5.2]] ) and future ( [[#5.4|Section 5.4]] ) carbon–climate feedbacks ( [[#Williams--2019|Williams et al., 2019]] ). The excess heat generated by radiative forcing from increasing concentration of atmospheric CO <sub>2</sub> and other GHGs is mostly taken up by the ocean (&gt;90%) and the residual balance partitioned between atmospheric, terrestrial and ice melting (Cross-Chapter Box 9.2; [[#Frölicher--2015|Frölicher et al., 2015]] ). The combined effect of these two large-scale negative feedbacks of CO <sub>2</sub> and heat are reflected in the TCRE ( [[#5.5|Section 5.5]] and Cross-Chapter Box 5.3), which points to a quasi-linear and quasi-emission-path independent relationship between cumulative emissions of CO <sub>2</sub> and global warming, which is used as the basis to estimate the remaining carbon budget ( [[#5.5|Section 5.5]] ; [[#MacDougall--2015|MacDougall and Friedlingstein, 2015]] ; [[#MacDougall--2017|MacDougall, 2017]] ; [[#Bronselaer--2020|Bronselaer and Zanna, 2020]] ; [[#Jones--2020|Jones and Friedlingstein, 2020]] ). There is still ''low confidence'' on the relative roles and importance of the ocean and terrestrial carbon processes on TCRE variability and uncertainty on centennial time scales ( [[#MacDougall--2016|MacDougall, 2016]] ; [[#MacDougall--2017|MacDougall et al., 2017]] ; [[#Williams--2017|Williams et al., 2017]] ; [[#Katavouta--2018|Katavouta et al., 2018]] , [[#Katavouta--2019|2019]] ; [[#Jones--2020|Jones and Friedlingstein, 2020]] ) (Sections 5.5.1.1, 5.5.1.2).

The combined effects of climate and CO <sub>2</sub> concentration feedbacks on the global carbon cycle are projected by ESMs to modify both the processes and natural reservoirs of carbon on a regional and global scale that may result in positive feedbacks (red arrows in Figure 5.2), which could weaken the major terrestrial and ocean sinks and disrupt the airborne fraction and TCRE under medium- to high-emissions scenarios ( [[#5.4.5|Section 5.4.5]] and Figure 5.25).

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