| Summary: | Effective climate sensitivity (EffCS), commonly estimated from model simulations with abrupt 4xCO(2) for 150 years, has been shown to depend on the CO2 forcing level. To understand this dependency systematically, we performed a series of simulations with a range of abrupt CO2 forcing in two climate models. Our results indicate that normalized EffCS values in these simulations are a non-monotonic function of the CO2 forcing, decreasing between 3x and 4xCO(2) in CESM1-LE (2x and 3xCO(2) in GISS-E2.1-G) and increasing at higher CO2 levels. The minimum EffCS value, caused by anomalously negative radiative feedbacks, arises mainly from sea-surface temperature (SST) relative cooling in the tropical and subtropical North Atlantic. This cooling is associated with the formation of the North Atlantic Warming Hole and Atlantic Meridional Overturning Circulation collapse under CO2 forcing. Our findings imply that understanding changes in North Atlantic SST patterns is important for constraining near-future and equilibrium global warming.
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