Cloud Responses to Abrupt Solar and CO2 Forcing Part I: Temperature Mediated Cloud Feedbacks

The third phase of the Cloud Feedback Model Intercomparison Project requested that modeling centers perform a pair of simulations where the climate system is subjected to an abrupt change of the solar constant by +/- 4%. The forcing is designed to loosely match the amount of radiative forcing incurred by quadrupling atmospheric CO2 concentrations. Using these simulations, we examine how clouds respond to changes in solar forcing and act as a feedback on global surface temperature. Specifically, in this paper, we study the temperature mediated cloud changes that occur following an abrupt increase and decrease of the solar constant and compare with temperature mediated cloud changes that occur following quadrupling and halving of CO2. We seek to answer two primary questions: 1) How do cloud feedbacks differ in response to abrupt changes in CO2 and solar forcing? And 2) Are there symmetrical (equal and opposite) cloud feedbacks to an increase and a decrease in solar forcing? We find that temperature mediated cloud changes are similar from increasing solar and CO2 forcing, with the only robust difference being that there is a larger reduction of low cloud amount following solar forcing; and we find that cloud responses to warming and cooling are not symmetric, due primarily to non-linearity introduced by phase changes in mid-to-high latitude low clouds, and sea ice loss/formation.