Non-Monotonic Response of the Climate System to Abrupt CO₂ Forcing

We explore the climate system response to abrupt CO₂ forcing, spanning the range 1× to 8×CO₂, 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 CO₂ in both models, reaching a minimum at 3×CO₂ for GISS-E2.1-G, and 4×CO₂ 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 CO₂ forcings. Analyzing the climate response over the same CO₂ range with slab-ocean versions of the same models, we demonstrate that the climate system’s non-monotonic response is linked to ocean dynamics.