Asymmetric Warming/Cooling Response to CO2 Increase/Decrease Mainly Due To Non‐Logarithmic Forcing, Not Feedbacks

We explore the CO2 dependence of effective climate sensitivity (SG) with symmetric abrupt and transient CO2 forcing, spanning the range 1/8×, 1/4×, 1/2×, 2×, 4×, and 8×CO2, using two state‐of‐the‐art fully coupled atmosphere‐ocean‐sea‐ice‐land models. In both models, under abrupt CO2 forcing, we find an asymmetric response in surface temperature and SG. The surface global warming at 8×CO2 is more than one third larger than the corresponding cooling at 1/8×CO2, and SG is CO2 dependent, increasing non‐monotonically from 1/8×CO2 to 8×CO2. We find similar CO2 dependence in the transient runs, forced with −1%yr−1CO2 and +1%yr−1CO2 up to 1/8×CO2 and 8×CO2, respectively. The non‐logarithmic radiative forcing—not the changing feedbacks—primarily explains the dependence of SG on CO2, particularly at low CO2 levels. The changing feedbacks, however, explain SG's non‐monotonic behavior.