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

=== 5.4.2 Direct CO <sub>2</sub> Effects on Projected Ocean Carbon Uptake ===

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In AR5 (WGI, Section 6.4.2) there was ''high agreement'' that CMIP5 ESMs project continued ocean CO <sub>2</sub> uptake through to 2100, with higher uptake corresponding to higher concentration or emissions pathways. There has been no significant change in the magnitude of the sensitivity of ocean carbon uptake to increasing atmospheric CO <sub>2</sub> , or in the inter-model spread, between the CMIP5 and CMIP6 era ( [[#Arora--2020|Arora et al., 2020]] ). The analysis from emissions and concentration-driven CMIP5 model projections show that the ocean sink stops growing beyond 2050 across all emissions scenarios ( [[#5.4.5.3|Section 5.4.5.3]] ). CMIP6 models also show a similar time evolution of global ocean CO <sub>2</sub> uptake to CMIP5 models over the 21st century (Figure 5.25) with decreasing net ocean CO <sub>2</sub> uptake ratio to anthropogenic CO <sub>2</sub> emissions under SSP5-8.5.

The projected weakening of ocean carbon uptake is driven by a combination of decreasing carbonate buffering capacity and warming, which are positive feedbacks under weak to no mitigation scenarios (SSP4 and 5). In high mitigation scenarios (SSP1-2.6), weakening ocean carbon uptake is driven by decreasing emissions (Cross-Chapter Box 5.3). The detailed understanding of carbonate chemistry in seawater that has accumulated over more than half a century (e.g., [[#Revelle--1957|Revelle and Suess, 1957]] ; [[#Egleston--2010|Egleston et al., 2010]] ), provides ''high confidence'' that the excess CO <sub>2</sub> dissolved in seawater leads to a non-linear reduction of the CO <sub>2</sub> buffering capacity, that is smaller dissolved inorganic carbon (DIC) increase with respect to ''p'' CO <sub>2</sub> increase along with the increase in cumulative ocean CO <sub>2</sub> uptake. Recent studies ( [[#Katavouta--2018|Katavouta et al., 2018]] ; [[#Jiang--2019|Jiang et al., 2019]] ; [[#Arora--2020|Arora et al., 2020]] ; [[#Rodgers--2020|Rodgers et al., 2020]] ) suggest with ''medium confidence'' that the decrease in the ocean CO <sub>2</sub> uptake ratio to anthropogenic CO <sub>2</sub> emissions, under low to no mitigation scenarios over the 21st century, is predominantly attributable to the ocean carbon-concentration feedback through the reduction of the seawater CO <sub>2</sub> buffering capacity, but with contributions from physical drivers such as warming and wind stress ( ''medium confidence'' ) and biological drivers ( ''low confidence'' ) (Sections 5.2.1.3.3 and 5.4.4).

Projected increases in ocean DIC due to anthropogenic CO <sub>2</sub> uptake amplify the sensitivity of carbonate system variables to perturbations of DIC in the surface ocean, for example via the amplitude of the seasonal cycle of ''p'' CO <sub>2</sub> , which impacts the mean annual air–sea fluxes ( [[#Hauck--2015|Hauck et al., 2015]] ; [[#Fassbender--2018|Fassbender et al., 2018]] ; [[#Landschützer--2018|Landschützer et al., 2018]] ; SROCC, [[#5.2.2.3|Section 5.2.2.3]] ). A larger amplification of the surface ocean ''p'' CO <sub>2</sub> seasonality occurs in the subtropics where ''p'' CO <sub>2</sub> seasonality is dominated by temperature seasonality, with the summer increase in the difference in ''p'' CO <sub>2</sub> <sup></sup> between surface water and the overlying atmosphere reaching 3μatm per decade between 1990 and 2030 under RCP8.5 ( [[#Schlunegger--2019|Schlunegger et al., 2019]] ; [[#Rodgers--2020|Rodgers et al., 2020]] ). In contrast, the impact of biological production on the seasonal cycle of ''p'' CO <sub>2</sub> in summer in the Southern Ocean strengthens the drawdown of CO <sub>2</sub> ( [[#Hauck--2015|Hauck et al., 2015]] ).

Overall, there is ''medium confidence'' on three outcomes in the ocean from projected CO <sub>2</sub> uptake under medium to high CO <sub>2</sub> concentration scenarios: (i) a weakening of the buffering capacity, which impacts the airborne fraction via the reduction of the ocean CO <sub>2</sub> buffering capacity due to cumulative ocean CO <sub>2</sub> uptake, which reduces the net ocean CO <sub>2</sub> uptake ratio to anthropogenic CO <sub>2</sub> emissions ( [[#Katavouta--2018|Katavouta et al., 2018]] ; [[#Arora--2020|Arora et al., 2020]] ; [[#Rodgers--2020|Rodgers et al., 2020]] ); (ii) an amplification of the seasonal cycle of CO <sub>2</sub> variables, which impacts both the ocean sink and ocean acidification ( [[#Hauck--2015|Hauck et al., 2015]] ); (iii) a decrease in the aragonite and calcite saturation levels in the ocean, which negatively impacts the calcification rates of marine organisms ( ''high confidence'' ) and forms a negative feedback on the uptake of CO <sub>2</sub> ( [[#McNeil--2016|McNeil and Sasse, 2016]] ) (Cross-Chapter Box 5.3).

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