Assessing the climate impact of trends in stratospheric water vapor

It is now apparent that observed increases in stratospheric water vapor may have contributed significantly to both stratospheric cooling and tropospheric warming over the last few decades. However, a recent study has suggested that our initial estimate of the climate impact may have overestimated both the radiative forcing and stratospheric cooling from these changes. We show that differences between the various estimates are not due to inherent problems with broadband and narrow‐band radiation schemes but rather due to the different experimental setups, particularly the altitude of the water vapor change relative to the tropopause used in the radiative calculations. Furthermore, we show that if recent estimates for the observed water vapor trends are valid globally they could have contributed a radiative forcing of up to 0.29 Wm −2 and a lower‐stratospheric cooling of more than 0.8 K over the past 20 years, with these values more than doubling if, as has been suggested, the trend has persisted for the last 40 years. This 40 year radiative forcing is roughly 75% of that due to carbon dioxide alone but, despite its high value, we find that the addition of this forcing into a simple model of climate change still gives global mean surface temperature trends which are consistent with observations.