Using historical temperature to constrain the climate sensitivity and aerosol-induced cooling

The most recent generation of climate models that has informed the 6 ^th 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 4xCO ₂ 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.