Old Rocks
Diamond Member
Do you understand what the residence time in the atmosphere of the various GHGs mean? Water vapor stays in the atmosphere 10 days or less, then is rained out. CO2 has a residence time measured in centuries. So water vapor is a feedback the other GHGs, primarily CO2 and CH4. Yes, there is more water vapor in the atmosphere now than there was 100 years ago. Due entirely to the increase in GHGs.
3.4.2.4 Stratospheric Water Vapour - AR4 WGI Chapter 3: Observations: Surface and Atmospheric Climate Change
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).
