Speaker: Ivan Mitevski (Columbia/APAM)
Title: "Non-monotonic Response of the Climate System to Abrupt CO2 Forcing"

Abstract: We explore the climate system response to abrupt CO2 forcing, spanning the range 1x to 8xCO2, 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 xCO2 for GISS-E2.1-G, and 4xCO2 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.