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

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