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

Abstract:

The equilibrium warming per CO₂ doubling increases with CO₂ concentration between 0.5x to 8x preindustrial for 13 out of 14 coupled atmosphere-ocean general circulation models. To understand this increase, we consider three nonlinearities: nonlogarithmic forcing, feedback CO₂ dependence, and feedback temperature dependence. Nonlogarithmic forcing decreases warming as often as it increases it, but feedback CO₂ dependence is likely positive. Feedback temperature dependence is positive for 11 out of 14 models; it increases the warming per doubling by more than the other two nonlinearities combined, and increases the risk of extreme warming, causing six models to warm at least an additional 3K under 8xCO₂. It is positive due to the longwave clear-sky component, while its model spread is due to the shortwave cloud component. Nonlinearities play a negligible role in projections of warming by 2100, but feedback temperature dependence increases the risk of high or even runaway warming by 2300.