Climate Sensitivity Increases Under Higher CO ₂ Levels Due to Feedback Temperature Dependence

Equilibrium climate sensitivity-the equilibrium warming per CO2 doubling-increases with concentration for 13 of 14 coupled general circulation models 0.5–8 times the preindustrial concentration. In particular, abrupt 4 × is more than twice 2 warming. We identify three potential causes: nonlogarithmic forcing, feedback dependence, and temperature dependence. Feedback dependence explains at least half sensitivity increase, while a smaller share, forcing decreases in as many it increases it. positive 10 out models, primarily due to longwave clear-sky feedback, cloud feedbacks drive particularly large increases. risk extreme or runaway warming, estimated cause six warm an additional 3K under 8 CO2.