Climate Sensitivity Increases Under Higher CO2 Levels Due to Feedback Temperature Dependence

Abstract Equilibrium climate sensitivity‐the equilibrium warming per CO 2 doubling‐increases with CO 2 concentration for 13 of 14 coupled general circulation models for 0.5–8 times the preindustrial concentration. In particular, the abrupt 4 × CO 2 equilibrium warming is more than twice the 2 × CO 2 warming. We identify three potential causes: nonlogarithmic forcing, feedback CO 2 dependence, and feedback temperature dependence. Feedback temperature dependence explains at least half of the sensitivity increase, while feedback CO 2 dependence explains a smaller share, and nonlogarithmic forcing decreases sensitivity in as many models as it increases it. Feedback temperature dependence is positive for 10 out of 14 models, primarily due to the longwave clear‐sky feedback, while cloud feedbacks drive particularly large sensitivity increases. Feedback temperature dependence increases the risk of extreme or runaway warming, and is estimated to cause six models to warm at least an additional 3K under 8 × CO 2 .