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 (S-G) with symmetric abrupt and transient CO2 forcing, spanning the range 1/8x, 1/4x, 1/2x, 2x, 4x, and 8xCO(2), 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 S-G. The surface global warming at 8xCO(2) is more than one third larger than the corresponding cooling at 1/8xCO(2), and S-G is CO2 dependent, increasing non-monotonically from 1/8xCO(2) to 8xCO(2). We find similar CO2 dependence in the transient runs, forced with -1%yr(-1)CO(2) and +1%yr(-1)CO(2) up to 1/8xCO(2) and 8xCO(2), respectively. The non-logarithmic radiative forcing-not the changing feedbacks-primarily explains the dependence of S-G on CO2, particularly at low CO2 levels. The changing feedbacks, however, explain S-G's non-monotonic behavior.

Automated summary

This study investigates the impact of CO2 on climate sensitivity using two state-of-the-art fully coupled models. The results show that under CO2 forcing, there is an asymmetric response in surface temperature and climate sensitivity, with climate sensitivity increasing non-monotonically with increasing CO2 concentration.