Non‐Monotonic Response of the Climate System to Abrupt CO ₂ Forcing

We explore the climate system response to abrupt CO 2 forcing, spanning the range 1× to 8×CO 2 , with two state‐of‐the‐art coupled atmosphere‐ocean‐sea‐ice‐land models: the NASA Goddard Institute for Space Studies Model E2.1‐G (GISS‐E2.1‐G) and the Community Earth System Model (CESM‐LE). We find that the effective climate sensitivity is a non‐monotonic function of CO 2 in both models, reaching a minimum at 3×CO 2 for GISS‐E2.1‐G, and 4×CO 2 for CESM‐LE. A similar non‐monotonic response is found in Northern Hemisphere surface temperature, sea‐ice, precipitation, the latitude of zero precipitation‐minus‐evaporation, and the strength of the Hadley cell. Interestingly, the Atlantic meridional overturning circulation collapses when non‐monotonicity appears and does not recover for larger CO 2 forcings. Analyzing the climate response over the same CO 2 range with slab‐ocean versions of the same models, we demonstrate that the climate system’s non‐monotonic response is linked to ocean dynamics.