Stratospheric water vapour changes as a possible contributor to observed stratospheric cooling

Abstract

Abstract. The observed cooling of the lower stratosphere over the last two decades has been attributed, in previous studies, largely to a combination of stratospheric ozone loss and carbon dioxide increase, and as such it is meant to provide one of the best pieces of evidence for an anthropogenic cause to climate change. This study shows how increases in stratospheric water vapour, inferred from available observations, may be capable of causing as much of the observed cooling as ozone loss does; as the reasons for the stratospheric water vapour increase are neither fully understood nor well characterized, it shows that it remains uncertain whether the cooling of the lower stratosphere can yet be fully attributable to human influences. In addition, the changes in stratospheric water vapour may have contributed, since 1980, a radiative forcing which enhances that due to carbon dioxide alone by 40%.

The representation of the TTL in a tropical channel version of the WRF model

“…Of key significance is that the coldest temperatures encountered in this region control both the occurrence of high-altitude cirrus and the amount of water entering the stratosphere. Stratospheric water is important from a radiative standpoint [Forster and Shine, 1999;Solomon et al, 2010] and in regards to ozone chemistry [Dvorstov and Solomon, 2001] and ultimately influences global climate and atmospheric chemistry. This tropical tropopause layer (TTL) [described in Fueglistaler et al, 2009] is essentially the gateway to the stratosphere, where both natural and anthropogenic species enter a region with longer lifetimes than in the troposphere.…”

Stratospheric water vapor increases over the past half‐century

“…Determining whether long-term increases in stratospheric water vapor have occurred is important given its radiative [Forster and Shine, 1999] To examine trends of-1% /yr, a long-term data set with complete seasonal coverage from a stable instrument is needed. The large amplitude seasonal cycle in stmtospheric entry-level water complicates matters in that incomplete sampling can bias trend estimates.…”

The increase in stratospheric water vapor from balloonborne, frostpoint hygrometer measurements at Washington, D.C., and Boulder, Colorado

“…Res., 2000) find roughly equal cooling from ozone and water vapor in the lower stratosphere. At the surface the warming due to the increase in stratospheric water vapor found by Forster and Shine [1999] [Mastenbrook, 1968;Mastenbrook and OItmans, 1983]. Beginning in 1980, and continuing to the present, profiles have been obtained at Boulder, Colorado.…”

“…Recent simulations [Forster and Shine, 1999] (V.L. Dvortsov and S. Solomon, Response of the stratospheric temperatures and ozone to past and future increases in stratospheric humidity, submitted to J. Geophys.…”