Shortwave radiative forcing by stratospheric water vapor

Abstract

It is shown that most previous estimates of radiative forcing due to increased stratospheric water vapor have been overestimates because most broad-band radiation schemes underestimate the (negative) shortwave component. This paper compares line-by-line calculations of shortwave forcing with those of five broad and multiple band radiation schemes, including a new parameterization of water vapor absorption which reduces the errors to within 3%. The results of two-dimensional model experiments indicate that the magnitude of the shortwave forcing should be approximately doubled, relative to previous estimates. As a result, estimates of global average net radiative forcing due to stratospheric water vapor increase should be decreased by approximately 10%.