Evaluating the Coupled GEOS‐AOGCM for Climate Research: Response to an Abrupt Quadrupling of CO ₂ Forcing

We evaluate the climate response of the NASA Goddard Earth Observing System (GEOS) coupled Atmosphere‐Ocean General Circulation Model (AOGCM) to idealized quadrupling of CO 2 concentrations. A pre‐industrial simulation and an abrupt 4 × CO 2 simulation are conducted with the GEOS AOGCM following the Coupled Model Intercomparison Project (CMIP) approach. The 4 × CO 2 run shows expected tropospheric and stratospheric climate response features, including Arctic amplification, intensification of the hydrologic cycle, Hadley Cell expansion, a poleward shift of the Southern Hemisphere jet stream, acceleration of the Brewer‐Dobson circulation, and a weaker Quasi‐Biennial Oscillation. At the ocean surface, we see a complete disappearance of Arctic sea ice in July–December, and in the Southern Hemisphere, there is a poleward shift of the surface wind stress that drives the Southern Ocean meridional overturning circulation poleward. In the ocean, there is enhanced heat content but reduced heat export to the high northern latitudes coincident with a slowing of the Atlantic Meridional Overturning Circulation. The Equilibrium Climate Sensitivity (ECS) of the GEOS AOGCM is estimated to be 2.55 K, in the lowest quartile of the CMIP5 and CMIP6 ranges. The low ECS is due to a strong negative feedback and a weak effective radiative forcing.