Editing IPCC:AR6/WGI/Chapter-4 (section)

==== 4.3.2.3 Atlantic Meridional Overturning Circulation ====

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The AR5 assessed from CMIP5 simulations that the Atlantic Meridional Overturning Circulation (AMOC) will ''very likely'' weaken over the 21st century, and the projected weakening of the AMOC is consistent with CMIP5 projections of an increase of high-latitude temperature and high-latitude precipitation, with both effects causing the surface waters at high latitudes to become less dense and therefore more stable ( [[#Collins--2013|Collins et al., 2013]] ).

Based on CMIP6 models, we find that over the 21st century, AMOC strength, relative to 1995–2014, shows a multi-model mean decrease in each of the SSP scenarios but with a large spread across the individual simulations (Figure 4.6). We also note that the magnitude of the ensemble-mean strength decrease is approximately scenario independent up to about 2060 ( [[#Weijer--2020|Weijer et al., 2020]] ). A more detailed assessment of these projected AMOC changes, and the mechanisms involved, can be found in [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] (Section 9.2.3).

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[[File:bac164475cd281d483ffa55cbe94c78f IPCC_AR6_WGI_Figure_4_6.png]]

'''Figure 4.6 |''' '''CMIP6 annual mean Atlantic Meridional Overturning Circulation (AMOC) strength change in historical and scenario simulations.''' Changes are relative to averages from 1995–2014. The curves show ensemble averages and the shadings the 5–95% ranges across the SSP1-2.6 and SSP3-7.0 ensembles. The circles to the right of the panel show the anomalies averaged from 2081–2100 for each of the available model simulations. The numbers inside the panel are the number of model simulations. Here, the strength of the AMOC is computed as the maximum value of annual mean ocean meridional overturning mass stream function in the Atlantic at 26°N. Results are from concentration-driven simulations. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

In summary, we assess from the CMIP6 models that AMOC weakening over the 21st century is ''very likely'' ; the rate of weakening is approximately independent of the emissions scenario ( ''high confidence'' ).

Based on a large initial condition ensemble of simulations with a CMIP5 model (CanESM2) with emissions scenarios leading to stabilization of global warming of 1.5°C, 2.0°C, or 3.0°C relative to 1850–1900, AMOC continues to decline for 5–10 years after GSAT is effectively stabilized at the given GWL ( [[#Sigmond--2020|Sigmond et al., 2020]] ). This is followed by a recovery of AMOC strength for about the next 150 years to a level that is approximately independent of the considered stabilization scenario. These results are replicated in simulations in a CMIP6 model (CanESM5) with emissions cessation after diagnosed CO <sub>2</sub> emissions reach 750 Gt, 1000 Gt, or 1500 Gt. These emissions levels lead to global warming stabilization at 1.5°C, 2.0°C, or 3.0°C relative to 1850–1900. In summary, in these model simulations the AMOC recovers over several centuries after the cessation of CO <sub>2</sub> emissions ( ''medium confidence'' ).

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