High-sensitivity Earth System Models Most Consistent with Observations

Earth’s transient climate response (TCR) quantifies the global mean surface air temperature change due to a doubling of atmospheric CO_2, at the time of doubling. TCR is highly correlated with near-term climate projections, and thus of utmost relevance for climate policy, but remains poorly constrained. Within state-of-the-art Earth System Models (ESMs) participating in the Coupled Model Intercomparison Project (CMIP6), the TCR range (1.1 -2.9^oC is much too wide to offer useful guidance to policymakers on remaining carbon budgets aligneded with the Paris agreement goals. To address this issue, we here present an observation-based TCR estimate of 1.9-2.7^oC (95% confidence interval). We show that this method correctly diagnoses TCR from 22 CMIP6 ESMs if the same variables are taken from the ESMs as are available from observations. This increases confidence in the new estimate and range, which are higher and narrower, respectively, than those of the CMIP6 ensemble.