3.4.2.4 Stratospheric Water Vapour
The TAR noted an apparent increase of roughly 1% yr–1 in stratospheric water vapour content (~0.05 ppm yr–1) during the last half of the 20th century (Kley et al., 2000; Rosenlof et al., 2001). This was based on data taken at mid-latitudes, and from multiple instruments. However, the longest series of data come from just two locations in North America with no temporal overlap. The combination of measurement uncertainties and relatively large variability on time scales from months to years warrants some caution when interpreting the longer-term trends (Kley et al., 2000; Fueglistaler and Haynes, 2005). The moistening is more convincingly documented during the 1980s and most of the 1990s than earlier, due to a longer continuous record (the NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) frost-point balloon record from Boulder, Colorado; Oltmans et al., 2000) and the availability of satellite observations during much of this period. However, discrepancies between satellite- and balloon-measured variations are apparent at decadal time scales, largely over the latter half of the 1990s (Randel et al., 2004a).
An increase in stratospheric water vapour has important radiative and chemical consequences (see also Section 2.3.8). These may include a contribution to the recent observed cooling of the lower stratosphere and/or warming of the surface (Forster and Shine, 1999, 2002; Smith et al., 2001), although the exact magnitude is difficult to quantify (Oinas et al., 2001; Forster and Shine, 2002). Some efforts to reconcile observed rates of cooling in the stratosphere with those expected based on observed changes in ozone and carbon dioxide (CO2) since 1979 (Langematz et al., 2003; Shine et al., 2003) have found discrepancies in the lower stratosphere consistent with an additional cooling effect of a stratospheric water vapour increase. However, Shine et al. (2003) noted that because the water vapour observations over the period of consideration are not global in extent, significant uncertainties remain as to whether radiative effects of a water vapour change are a significant contributor to the stratospheric temperature changes. Moreover, other studies which account for uncertainties in the ozone profiles and temperature trends, and natural variability, can reconcile the observed stratospheric temperature changes without the need for sizable water vapour changes (Ramaswamy and Schwarzkopf, 2002; Schwarzkopf and Ramaswamy, 2002).
Although methane oxidation is a major source of water in the stratosphere, and has been increasing over the industrial period, the noted stratospheric trend in water vapour is too large to attribute to methane oxidation alone (Kley et al., 2000; Oltmans et al., 2000). Therefore, other contributors to an increase in stratospheric water vapour are under active investigation. It is likely that different mechanisms are affecting water vapour trends at different altitudes. Aviation emits a small but potentially significant amount of water vapour directly into the stratosphere (IPCC, 1999). Several indirect mechanisms have also been considered including: a) volcanic eruptions (Considine et al., 2001; Joshi and Shine, 2003); b) biomass-burning aerosol (Sherwood, 2002; Andreae et al., 2004); c) tropospheric sulphur dioxide (Notholt et al., 2005); and d) changes to methane oxidation rates from changes in stratospheric chlorine, ozone and the hydroxyl radical (Röckmann et al., 2004). Other proposed mechanisms relate to changes in tropopause temperatures or circulation (Stuber et al., 2001; Zhou et al., 2001; Rosenlof, 2002; Nedoluha et al., 2003; Dessler and Sherwood, 2004; Fueglistaler et al., 2004; Roscoe, 2004).
It has been assumed that temperatures near the tropical tropopause control stratospheric water vapour according to equilibrium thermodynamics, importing more water vapour into the stratosphere when temperatures are warmer. However, tropical tropopause temperatures have cooled slightly over the period of the stratospheric water vapour increase (see Section 3.4.1; Seidel et al., 2001; Zhou et al, 2001). This makes the mid-latitude lower-stratospheric increases harder to explain (Fueglistaler and Haynes, 2005). Satellite observations (Read et al., 2004) show water vapour injected above the tropical tropopause by deep convective clouds, bypassing the traditional control point. Changes in the amount of condensate sublimating in this layer may have contributed to the upward trend, but to what degree is uncertain (Sherwood, 2002). Another suggested source for temperature-independent variability is changes in the efficiency with which air is circulated through the coldest regions before entering the stratosphere (Hatsushika and Yamazaki, 2003; Bonnazola and Haynes, 2004; Dessler and Sherwood, 2004; Fueglistaler et al., 2004). However, it is not yet clear that a circulation-based mechanism can explain the observed trend (Fueglistaler and Haynes, 2005).
The TAR noted a stalling of the upward trend in water vapour during the last few years of observations available at that time. This change in behaviour has persisted, with a near-zero trend in stratospheric water vapour between 1996 and 2000 (Nedoluha et al., 2003; Randel et al., 2004a). The upward trend of methane is also smaller and is currently close to zero (see Section 2.3.2). Further, at the end of 2000 there was a dramatic drop in water vapour in the tropical lower stratosphere as observed by both satellite and CMDL balloon data (Randel et al., 2004a). Temperatures observed near the tropical tropopause also dropped, but the processes producing the tropical tropopause cooling itself are currently not fully understood. The propagation of this recent decrease through the stratosphere should ensure flat or decreasing stratospheric moisture for at least the next few years.
To summarise, water vapour in the stratosphere has shown significant long-term variability and an apparent upward trend over the last half of the 20th century but with no further increases since 1996. It does not appear that this behaviour is a straightforward consequence of known climate changes. Although ideas have been put forward, there is no consensus as to what caused either the upward trend or its recent disappearance.