AR4 reported positive decadal trends in lower and upper tropospheric water vapour based on satellite observations for the period 1988–2004. Since AR4, there has been continued evidence for increases in lower tropospheric water vapour from microwave satellite measurements of column integrated water vapour over oceans (Santer et al., 2007; Wentz et al., 2007) and globally from satellite measurements of spectrally resolved reflected solar radiation (Mieruch et al., 2008). The interannual variability and longer-term trends in column-integrated water vapour over oceans are closely tied to changes in SST at the global scale and interannual anomalies show remarkable agreement with low-level specific humidity anomalies from HadCRUH (O’Gorman et al., 2012). The rate of moistening at large spatial scales over oceans is close to that expected from the Clausius–Clapeyron relation (about 7% per degree Celsius) with invariant relative humidity (Figure 2.31). Satellite measurements also indicate that the globally averaged upper tropospheric relative humidity has changed little over the period 1979–2010 while the troposphere has warmed, implying an increase in the mean water vapour mass in the upper troposphere (Shi and Bates, 2011).
Interannual variations in temperature and upper tropospheric water vapour from IR satellite data are consistent with a constant RH behavior at large spatial scales (Dessler et al., 2008; Gettelman and Fu, 2008; Chung et al., 2010). On decadal time-scales, increased GHG concentrations reduce clear-sky outgoing long-wave radiation (Allan, 2009; Chung and Soden, 2010), thereby influencing inferred relationships between moisture and temperature. Using Meteosat IR radiances, Brogniez et al. (2009) demonstrated that interannual variations in free tropospheric humidity over subtropical dry regions are heavily influenced by meridional mixing between the deep tropics and the extra tropics. Regionally, upper tropospheric humidity changes in the tropics were shown to relate strongly to the movement of the ITCZ based upon microwave satellite data (Xavier et al., 2010). Shi and Bates (2011) found an increase in upper tropospheric humidity over the equatorial tropics from 1979 to 2008. However there was no significant trend found in tropical-mean or global-mean averages, indicating that on these time and space scales the upper troposphere has seen little change in relative humidity over the past 30 years. While microwave satellite measurements have become increasingly relied upon for studies of upper tropospheric humidity, the absence of a homogenized data set across multiple satellite platforms presents some difficulty in documenting coherent trends from these records (John et al., 2011).