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Estimating the equilibrium climate sensitivity (ECS; the equilibrium warming in response to a doubling of CO2) from observations is one of the big problems in climate science. Using observations of interannual climate variations covering the period 2000 to 2017 and a model-derived relationship between interannual variations and forced climate change, we estimate that ECS is likely 2.4-4.6 K (17-83% confidence interval), with a mode and median value of 2.9 and 3.3 K, respectively. This analysis provides no support for low values of ECS (below 2 K) suggested by other analyses. The main uncertainty in our estimate is not observational uncertainty but rather uncertainty in converting observations of short term, mainly unforced climate variability to an estimate of the response of the climate system to long-term forced warming.

Plain Language Summary Equilibrium climate sensitivity (ECS) is the amount of warming resulting from doubling carbon dioxide. It is one of the important metrics in climate science because it is a primary determinant of how much warming we will experience in the future. Despite decades of work, this quantity remains uncertain: the last Intergovernmental Panel on Climate Change report stated a range for ECS of 1.5-4.5 degrees C. Using observations of interannual climate variations covering the period 2000 to 2017, we estimate that ECS is likely 2.4-4.6 K. Thus, our analysis provides no support for the bottom of the Intergovernmental Panel on Climate Change's range.