Revisiting a Constraint on Equilibrium Climate Sensitivity From a Last Millennium Perspective

Despite decades of effort to constrain equilibrium climate sensitivity (ECS), current best estimates still exhibit a large spread. Past studies have sought to reduce ECS uncertainty through a variety of methods including emergent constraints. One example uses global temperature variability over the past century to constrain ECS. While this method shows promise, it has been criticized for its susceptibility to the influence of anthropogenic forcing and the limited length of the instrumental record used to compute temperature variability. Here, we investigate the emergent relationship between ECS and two metrics of global temperature variability using model simulations and paleoclimate reconstructions over the last millennium (850-1999). We find empirical evidence in support of these emergent relationships. Observational constraints suggest a central ECS estimate of 2.6-2.8 K, consistent with the Intergovernmental Panel on Climate Change's consensus estimate of 3K. Moreover, they suggest ECS "likely" ranges of 1.8-3.3 K and 2.0-3.6 K. Future changes in global-mean temperatures have substantial implications for climate-related risks, and global-mean temperatures will continue to increase in response to carbon emissions (a property known as climate sensitivity). In addition to long-term, human-driven warming, the global surface temperature also exhibits short-term swings due to natural climate variability. Recently, climate model experiments have shown some evidence of a linear relationship between climate sensitivity and climate variability, with potential applications for climate model development and future predictions. We show that this relationship holds when applied to paleoclimate data from the Common Era (850-1999) and use the reconstructed temperature record over this period to estimate climate sensitivity. Our analysis considers a period (similar to 1,000 years) that is much longer than what has been used in prior studies (similar to 100 years) and thus paints a fuller picture of past temperature variability. We show that interannual and decadal fluctuations in climate could offer a novel technique for assessing the Earth's sensitivity to external forcing. Our results are consistent with other lines of evidence, increasing confidence in our approach. We hope our findings add value to the existing collection of climate sensitivity estimates and encourage future use of paleoclimate data to verify proposed constraints on climate quantities. We use model output and temperature reconstructions from 850 to 1999 to test a proposed emergent constraint on climate sensitivityThe proposed emergent constraint largely holds for this period, which is longer and more stationary than that of prior workOur central estimates of equilibrium climate sensitivity are 2.6-2.8 K, consistent with prior results and the IPCC's consensus estimate