Nonlinearity in the Climate Response to Greenhouse Gas and Aerosol Forcing

We examine the nonlinearity in the climate impacts of greenhouse gases and scattering aerosols by analyzing the response to a doubling of CO₂ and to a zonally symmetric solar forcing perturbation simulated with a slab ocean model. The solar forcing has a similar latitudinal variation to the realistic aerosol forcing, peaking in the Northern Hemisphere midlatitudes. The zonal mean circulation response to solar forcing closely resembles the aerosol-induced change. When forced by CO₂ and solar forcing simultaneously, the model simulated warming is less than the linear sum of the response to individual forcing in the Northern Hemisphere. As a result, a significant nonlinearity arises in the response of the subtropical jet and the Hadley Cell. The nonlinearity largely comes from cloud feedbacks. The results suggest that the nonlinear climate response to greenhouse gas and aerosol forcings is a robust feature that cannot be neglected in climate change attribution studies.