Using historical temperature to constrain the climate sensitivity and aerosol-induced cooling
Abstract. The most recent generation of climate models that has informed the 6th Assessment Report (AR6) of IPCC is characterized by the presence of several models with anomalously large equilibrium climate sensitivities (ECSs) relative to the previous generation. Partly as a result, AR6 did not use any direct quantifications of ECSs based on 4xCO2 simulations and relied on other evidence when assessing the Earth’s actual ECS. Here I use the historical observed global-mean surface air temperature and simulations produced under the Detection and Attribution Model Intercomparison Project to constrain the ECS and historical aerosol-related cooling. Based on 15 largely independent models I obtain an average adjusted ECS of 3.4±0.8 K (at 68 % confidence), which is very consistent with the AR6 estimate. Furthermore, importantly I find that the optimal cooling due to anthropogenic aerosols consistent with the observed temperature record should on average be about 34±31 % of what these models simulate, yielding a multi-model-mean, global-, and annual-mean aerosol-related cooling for 2000–2014, relative to 1850–1899, of -0.19±0.14 K (at 68 % confidence), when these models simulate on average -0.63±0.28 K. For 12 models the reduction in aerosol-related cooling equals or exceeds 50 %. There is a correlation between the models’ ECS and their aerosol-related cooling, whereby large-ECS models tend to be associated also with large aerosol-related cooling. The results imply that a large reduction of the aerosol-related cooling, along with a more moderate adjustment of the greenhouse-gas related warming, for most models would bring the historical global mean temperature simulated by these models into better agreement with observations.
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