Suppressed Late‐20th Century Warming in CMIP6 Models Explained by Forcing and Feedbacks.

For the 1960–2000 period, the latest generation of climate models (Coupled Model Intercomparison Project Phase 6 [CMIP6]) shows less global mean surface temperature change relative to pre‐industrial than that seen in observations. In contrast, the previous generation of models (CMIP5) performed well over this period. It has been hypothesized that this suppressed late‐20th Century warming seen in CMIP6 is caused by a stronger aerosol forcing. However, we find this to be only part of the story. Not only is the aerosol forcing marginally more negative in CMIP6 compared to CMIP5, the greenhouse gas forcing in CMIP6 is also weaker than in CMIP5. These forcing differences are amplified by differences in climate sensitivity between the CMIP5 and CMIP6 ensemble, which leads to both a stronger aerosol cooling over 1960–1990 and a stronger greenhouse gas induced warming from 1990, returning the warming post‐2000 toward the observed level. Plain Language Summary: Climate models are our best tools for predicting how the climate will change in the future. Confidence in future projections relies on the ability to accurately simulate the past. Many of the latest climate models show less warming than observations around the 1960–2000 period, so understanding why is key to making more confident projections. Models respond to human and natural forcings such as greenhouse gases, air pollutants (aerosols), volcanic eruptions, and solar activity. We show that the latest models simulate a lower forcing from greenhouse gases and aerosols than older models, which is part of the reason why the climate does not warm in line with observations in the late 20th Century. The other part of the reason for the lower warming is that the newer models are more sensitive: they project a greater temperature change for a given amount of forcing. The higher sensitivity is found to result in more cooling from aerosols in the new models, but it opposes the lower forcing from greenhouse gases. The aerosol effects win out, causing suppressed warming in the late 20th Century. Key Points: Coupled Model Intercomparison Project Phase 6 (CMIP6) models warm less in the multimodel mean than CMIP5 models in the 1960–2000 periodImplied forcing from greenhouse gases and aerosols is lower (more negative) in CMIP6 than in CMIP5Stronger climate feedbacks in CMIP6 amplifies the cooling from aerosols [ABSTRACT FROM AUTHOR]