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

The 2021 published study Non-Monotonic Response of the Climate System to Abrupt CO2 Forcing (10.1029/2020GL090861) involves research from the Department of Applied Physics and Applied Mathematics, NASA Goddard Institute for Space Studies, Center for Climate System Research, and the Lamont-Doherty Earth Observatory.

The study key points are:

• We examine the response of the climate system to abrupt 1.5×, 2×,3×, 4×, 5×, 6×, 7×, and 8×CO2 forcing with two different coupled models.
• Climate sensitivity, sea-ice extent, global precipitation and the atmospheric circulation respond non-monotonically across this range of CO2.
• The non-monotonicity of the response is associated with changes in ocean dynamics, notably over the North Atlantic.

The authors also noted, “The goal of this paper is simply to illustrate that the non-monotonic response to increased CO2 appears in a wide array of different metrics of the climate system”.

Abstract 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 (CESMLE). 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-monotonicresponse is found in Northern Hemisphere surface temperature, sea-ice, precipitation, the latitude ofzero precipitation-minus-evaporation, and the strength of the Hadley cell. Interestingly, the Atlanticmeridional overturning circulation collapses when non-monotonicity appears and does not recover forlarger CO2 forcings. Analyzing the climate response over the same CO2 range with slab-ocean versions ofthe same models, we demonstrate that the climate system’s non-monotonic response is linked to oceandynamics.

Plain Language Summary We perform runs with two different models using CO2concentrations in the atmosphere higher (from 1× to 8×CO2) relative to pre-industrial conditions, inorder to explore how the effective climate sensitivity (ECSeff) and the entire climate system change withincreasing CO2. We show that ECSeff is a non-monotonic function of CO2, minimizing at 3×CO2 in onemodel and 4×CO2 in the other. A similar non-monotonic response appears in precipitation, sea-ice,the edge of the dry zone, and Hadley cell strength. Interestingly, the Atlantic Meridional OverturningCirculation, which brings warm water into the North Atlantic, also shuts down at the same forcings whenECSeff is minimum and does not recover for higher forcings. We further show that the non-monotonicresponse of the climate system stems from changes in ocean dynamics.