Internal Variability and Disequilibrium Confound Estimates of Climate Sensitivity From Observations

An emerging literature suggests that estimates of equilibrium climate sensitivity (ECS) derived from recent observations and energy balance models are biased low because models project more positive climate feedback in the far future. Here we use simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) to show that across models, ECS inferred from the recent historical period (1979-2005) is indeed almost uniformly lower than that inferred from simulations subject to abrupt increases in CO2 radiative forcing. However, ECS inferred from simulations in which sea surface temperatures are prescribed according to observations is lower still. ECS inferred from simulations with prescribed sea surface temperatures is strongly linked to changes to tropical marine low clouds. However, feedbacks from these clouds are a weak constraint on long-term model ECS. One interpretation is that observations of recent climate changes constitute a poor direct proxy for long-term sensitivity. Plain Language Summary Even if we remove the uncertainty associated with human behavior, we still don't know exactly how hot it is going to get. This is because warming associated with increased atmospheric carbon dioxide triggers climate changes that themselves can accelerate or decelerate the warming. The equilibrium climate sensitivity (ECS) is defined as the eventual warming in response to a doubling of atmospheric CO2, and it is tempting to estimate this quantity from recent observations. However, in climate models, the ECS inferred from recent decades is lower than the eventual warming for two reasons. First, it takes the climate many centuries to fully come to equilibrium, and models indicate that we should expect even more warming in the future. Second, the conditions experienced in the real world seem to have given rise to especially low estimates of ECS, perhaps purely by chance. Climate models indicate that not only are ECS estimates based on recent decades lower than the eventual warming, but they may not even be predictive of that warming. A climate model that shows strong warming in response to recent real-world conditions does not necessarily have high long-term sensitivity, and vice versa.