The asymmetry of the climate system’s response to solar forcing changes and its implications for geoengineering scenarios

Motivated by proposals to compensate CO2-induced warming with a decrease in solar radiation, this study investigates how single-forcing simulations should be combined to best represent the spatial patterns of surface temperature and precipitation of idealized geoengineering scenarios. Using instantaneous and transient simulations with changing CO2 and solar forcings, we show that a geoengineering scenario, i.e., a scenario where the solar constant is reduced as CO2 concentrations are increased, is better represented by subtracting the response pattern of a solar forcing increase simulation from the response pattern of a CO2 forcing increase simulation, than by adding the response pattern of a solar forcing decrease simulation to a CO2 forcing increase simulation. The reason is a asymmetric response of the climate system to a forcing increase or decrease between both hemispheres. In particular, the Atlantic meridional overturning circulation responds faster to a solar forcing decrease compared to a solar forcing increase. Further, the climate feedbacks are state and region dependent, which is particularly apparent in the polar regions due to the sea ice-albedo feedback. The importance of understanding the local response of the climate system to geoengineering and single-forcing scenarios is highlighted, since these aspects are hardly discernible when only global mean values are considered.