Emergent constraint on equilibrium climate sensitivity is an artefact of historical forcing

Equilibrium climate sensitivity (ECS) is widely used in assessments of anthropogenic climate change, and the Intergovernmental Panel on Climate Change¹ has estimated the likely range of ECS to be between 1.5 and 4.5 K, with a best estimate of 3 K. Estimates of ECS from observational data are uncertain, and long time series are required due to the slow thermal response of the deep oceans. The estimates are therefore largely based on experiments in Earth system models (ESMs), and part of the uncertainty in the ECS reflects the disagreement among models. Recently, Cox et al.² presented a method for weighting the ESMs based on how well they reproduce a theoretically informed metric of the instrumental temperature record for the years 1880 to 2016. By demonstrating that the models with higher ECS deviate more from the instrumental temperature record according to this metric, they narrowed the likely range to be between 2.2 and 3.4 K, which would imply a reduced risk of dangerous global warming. This result, however, is an artefact induced by the anthropogenic trend manifest in the last forty years of the temperature record. When the analysis is repeated using data for the years 1880-1975 there is no emergent constraint on ECS.