Using an AGCM to Diagnose Historical Effective Radiative Forcing and Mechanisms of Recent Decadal Climate Change

Abstract

n atmospheric general circulation model is forced with observed monthly sea surface temperature and sea ice boundary conditions, as well as forcing agents that vary in time, for the period 1979?2008. The simulations are then repeated with various forcing agents, individually and in combination, fixed at preindustrial levels. The simple experimental design allows the diagnosis of the model?s global and regional time-varying effective radiative forcing from 1979 to 2008 relative to preindustrial levels. Furthermore the design can be used to (i) calculate the atmospheric model?s feedback/sensitivity parameters to observed changes in sea surface temperature and (ii) separate those aspects of climate change that are directly driven by the forcing from those driven by large-scale changes in sea surface temperature. It is shown that the atmospheric response to increased radiative forcing over the last 3 decades has halved the global precipitation response to surface warming. Trends in sea surface temperature and sea ice are found to contribute only ~60% of the global land, Northern Hemisphere, and summer land warming trends. Global effective radiative forcing is ~1.5 W m?2 in this model, with anthropogenic and natural contributions of ~1.3 and ~0.2 W m?2, respectively. Forcing increases by ~0.5 W m?2 decade?1 over the period 1979?2008 or ~0.4 W m?2 decade?1 if years strongly influenced by volcanic forcings?which are nonlinear with time?are excluded from the trend analysis. Aerosol forcing shows little global decadal trend due to offsetting regional trends whereby negative aerosol forcing weakens in Europe and North America but continues to strengthen in Southeast Asia.