A comparison of the efficacy of greenhouse gas forcing and solar forcing

Abstract

The efficacy (E) of a forcing is a measure of its capacity to generate a temperature response in the earth’s system. Most Coupled Model Intercomparison Project Phase 5 (CMIP5) climate models assume that the efficacy of a solar forcing is close to the efficacy of a similar sized Green House Gas (GHG) forcing. This paper examines the possibility that a change in short wave solar forcing may more readily contribute to ocean heat content (OHC) than a similar change in long wave GHG forcing. If this hypothesis is shown to be correct, then it follows that equilibrium restoration times at the top of the atmosphere (TOA) are likely to be considerably faster, on average, for a change in GHG forcing than for a similar change in solar forcing. A crude forcings model has been developed that matches almost perfectly (R2=0.89) the National Oceanic and Atmospheric Administration (NOAA) temperature series from 1880 to 2010. This model is compared to and performs much better over this period than the United Kingdom Met Office’s (HadGEM2) contribution to the CMIP5 (R 2 =0.16). It is concluded, by implication that the efficacy of a GHG forcing is likely to be