Abstract
Equilibrium Climate Sensitivity (ECS) characterizes the response of Earth’s temperature to a doubling of atmospheric CO2 and is one of the most important and most studied metrics in climate science. For decades, estimates of ECS have been stable around 1.5°C to 4.5°C. In the most recent coupled model intercomparison project (CMIP6), however, many state-of-the-art climate models calculated ECS to be “hotter” than the upper bound of the consensus range; if correct, this would mean even more dire consequences for our planet than previously anticipated. The surprising CMIP6 results quickly became one of the highest-profile issues in climate science and a focus of intensive research, as scientists tried to determine why the models produced these unexpected results and whether they were erroneous. Our symposium explores several key epistemological and methodological issues arising from this high-profile case: the handling of discordant results; the validation of paleoclimate data used in both climate model evaluation and estimating ECS; the interpretation of climate model projections; holism and underdetermination in complex simulation models; and the end of climate science’s long-standing practice of “model democracy”, in which each state-of-the-art model gets equal weight in assessments of future warming.
