Climate forcing-response relationships for greenhouse and shortwave radiative perturbations

The earth's climate system is subject to radiative forcings caused by perturbations in the infrared 'greenhouse' effect and absorbed solar energy. The forcings can be classified as being global in extent (e.g., increase of CO₂) or spatially confined to the northern hemisphere midlatitudes (e.g., anthropogenic sulfate aerosols). Of central importance to climate change assessments are the characteristics of the global and latitudinal changes, and the forcing-response relationships for different kinds of perturbations. Using a general circulation climate model with fixed cloud distributions and microphysical properties, we analyze the equilibrium climate responses to different perturbations representing global and spatially localized radiative forcings. The total climate feedback in the various experiments does not differ significantly, and the global-mean climate sensitivity (ratio of the equilibrium global-mean surface temperature change to the global-mean imposed radiative forcing) behaves in a near-invariant manner for both global and spatially confined forcings. However, relative to the global perturbation cases, forcings confined to the northern hemisphere midlatitudes exhibit a steepening of the meridional gradient of the temperature response in that hemisphere.