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

==== 5.6.3.2 Consequences of SRM and its Termination on Atmospheric CO <sub>2</sub> burden ====

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Modelling studies consistently show that, relative to a high-CO <sub>2</sub> world without SRM, SRM-induced cooling ( [[IPCC:Wg1:Chapter:Chapter-4#4.6.3.3|Section 4.6.3.3]] ) would reduce plant and soil respiration, and also reduce the negative effects of warming on ocean carbon uptake ( [[#Tjiputra--2016|Tjiputra et al., 2016]] ; [[#Xia--2016|Xia et al., 2016]] ; [[#Cao--2017|Cao and Jiang, 2017]] ; [[#Jiang--2018|Jiang et al., 2018]] ; [[#Muri--2018|Muri et al., 2018]] ; [[#Sonntag--2018|Sonntag et al., 2018]] ; [[#Plazzotta--2019|Plazzotta et al., 2019]] ; C.-E. [[#Yang--2020|]] [[#Yang--2020|Yang et al., 2020]] ). A multi-model study ( [[#Plazzotta--2019|Plazzotta et al., 2019]] ) indicates that, relative to a high-CO <sub>2</sub> concentration world without SRM, stratospheric sulphur dioxide (SO <sub>2</sub> ) injection increases the allowable CO <sub>2</sub> emissions by enhancing CO <sub>2</sub> uptake by both land and ocean (Figure 5.37). As a result of enhanced global carbon uptake, SRM would reduce the burden of atmospheric CO <sub>2</sub> ( ''high confidence'' ). However, the amount of SRM-induced reduction in atmospheric CO <sub>2</sub> depends on the future emissions scenario and modelled oceanic and terrestrial carbon sinks, which differ widely between models ( [[#Tjiputra--2016|Tjiputra et al., 2016]] ; [[#Xia--2016|Xia et al., 2016]] ; [[#Cao--2017|Cao and Jiang, 2017]] ; [[#Muri--2018|Muri et al., 2018]] ). Models that include the terrestrial nitrogen cycle usually report a much smaller reduction of atmospheric CO <sub>2</sub> in response to SRM than models without the nitrogen cycle, mainly because nitrogen limitation leads to a weaker terrestrial carbon sink ( [[#Tjiputra--2016|Tjiputra et al., 2016]] ; [[#Muri--2018|Muri et al., 2018]] ; C.-E. [[#Yang--2020|]] [[#Yang--2020|Yang et al., 2020]] ). Large-scale application of SAI is found to reduce the rate of release of CO <sub>2</sub> and CH <sub>4</sub> from permafrost thaw ( [[#Lee--2019|Lee et al., 2019]] ; [[#Chen--2020|Chen et al., 2020]] ).

A hypothetical sudden and sustained termination of SRM would cause a rapid increase in global warming that would pose great risks to biodiversity ( [[#Jones--2013|]] [[#Jones--2013|A. Jones et al., 2013]] ; [[#McCusker--2014|McCusker et al., 2014]] ; [[#Trisos--2018|Trisos et al., 2018]] ) ( [[IPCC:Wg1:Chapter:Chapter-4#4.6.3.3|Section 4.6.3.3]] ). It would also weaken carbon sinks, accelerating atmospheric CO <sub>2</sub> accumulation and inducing further warming (Figure 5.37; [[#Matthews--2007|Matthews and Caldeira, 2007]] ; [[#Tjiputra--2016|Tjiputra et al., 2016]] ; [[#Muri--2018|Muri et al., 2018]] ; [[#Plazzotta--2019|Plazzotta et al., 2019]] ). However, a scenario with gradual phase-out of SRM under emissions reduction could reduce the large negative effect of sudden SRM termination ( [[#MacMartin--2014|MacMartin et al., 2014]] ; [[#Keith--2015|Keith and MacMartin, 2015]] ; [[#Tilmes--2016|Tilmes et al., 2016]] ),though this would be limited by how rapidly emissions reductions can be scaled-up ( [[#Ekholm--2016|Ekholm and Korhonen, 2016]] ).

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