资源环境科技发展态势分析平台

Equilibrium climate sensitivity ‐ the equilibrium warming per CO₂ doubling ‐ increases with CO₂ concentration for thirteen of fourteen coupled general circulation models for 0.5 to 8 times the preindustrial concentration. In particular, the abrupt4xCO₂ equilibrium warming is more than twice the 2xCO₂ warming. We identify three potential causes: nonlogarithmic forcing, feedback CO₂ dependence, and feedback temperature dependence. Feedback temperature dependence explains at least half of the sensitivity increase, while feedback CO₂ dependence explains a smaller share, and nonlogarithmic forcing decreases sensitivity in as many models as it increases it. Feedback temperature dependence is positive for ten out of fourteen 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 8xCO₂.