A key uncertainty in projecting future climate change is the magnitude of equilibrium climate sensitivity (ECS), that is, the eventual increase in global annual average surface temperature in response to a doubling of atmospheric CO2 concentration. The lower bound of the likely range for ECS given in the IPCC Fifth Assessment Report (AR5; refs 1, 2) was revised downwards to 1.5 °C, from 2 °C in its previous report3, mainly as an effect of considering observations over the warming hiatus—the period of slowdown of global average temperature increase since the early 2000s. Here we analyse how estimates of ECS change as observations accumulate over time and estimate the contribution of potential causes to the hiatus. We find that including observations over the hiatus reduces the most likely value for ECS from 2.8 °C to 2.5 °C, but that the lower bound of the 90% range remains stable around 2 °C. We also find that the hiatus is primarily attributable to El Niño/Southern Oscillation-related variability and reduced solar forcing.
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D.J.A.J. wants to thank the Swedish Energy Agency and Carl Bennet AB for financial support. C.T. was supported by the Regional and Global Climate Modeling Program (RGCM) of the US Department of Energy’s Office of Science (BER), Cooperative Agreement DE-FC02-97ER62402. O.H. was supported by the Knut and Alice Wallenberg Foundation and the Swedish Research Council. C. Azar is acknowledged for useful comments.
The authors declare no competing financial interests.
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Johansson, D., O’Neill, B., Tebaldi, C. et al. Equilibrium climate sensitivity in light of observations over the warming hiatus. Nature Clim Change 5, 449–453 (2015). https://doi.org/10.1038/nclimate2573
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