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%.