How Accurately do we Know the Atmospheric Absorption of Solar Radiation by Greenhouse Gases?

Abstract:

The exact magnitude of absorbed shortwave radiation by the atmosphere (SWA) has long been a subject of great uncertainty. Even today, IPCC AR5 climate models exhibit a range in global SWA of ~10 Wm-2 and as a class underestimate our present best observed estimate of SWA by ~5 Wm-2. While a significant part of this range may result from differing assumptions about cloud and aerosol, it is also possible that there is a sizable contribution from poor representation of SWA by radiative gases in GCM radiation codes. Part of the problem stems from that until recently there was considerable uncertainty about the SWA of gases. However, improved measurements of the line and continua spectra of gases in the past decade have considerably narrowed this uncertainty range. In this work we derive a best and uncertainty estimate of the SWA by each important greenhouse gas. This provides benchmark values that can be used to perform a robust test of the quality of SWA by gases in GCM radiation codes.