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