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

Abstract:

We explore the state-dependence of the response of the climate system to abrupt CO₂ forcing, spanning the range 1.5-to-8xCO₂ with the coupled atmosphere-ocean NASA Goddard Institute for Space Studies (GISS) Model E2.1. We find that the equilibrium climate sensitivity increases non-monotonically with CO₂, reaching a minimum at 3xCO₂. 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 at 3xCO₂ and does not recover for larger CO₂ forcings. Analyzing the climate response over the same CO₂ range with a slab-ocean version of the same model, we demonstrate that the non-monotonic response of the climate system stems from changes in ocean heat transport. Furthermore, we decompose the feedbacks and explore which one contributes to the non-monotonicity. Lastly, we contrast the climate system response to the abrupt CO₂ forcing with the transient response from an integration subject to a 1% yr^-1 CO₂ increase.