3.4.2.2 Upper-Tropospheric Water Vapour
Water vapour in the middle and upper troposphere accounts for a large part of the atmospheric greenhouse effect and is believed to be an important amplifier of climate change (Held and Soden, 2000). Changes in upper-tropospheric water vapour in response to a warming climate have been the subject of significant debate.
Due to instrumental limitations, long-term changes in water vapour in the upper troposphere are difficult to assess. Wang et al. (2001) found an increasing trend of 1 to 5% per decade in relative humidity during 1976 to 1995, with the largest increases in the upper troposphere, using 17 radiosonde stations in the tropical west Pacific. Conversely, a combination of Microwave Limb Sounder (MLS) and Halogen Occultation Experiment (HALOE) measurements at 215 hPa suggested increases in water vapour with increasing temperature (Minschwaner and Dessler, 2004) on interannual time scales, but at a rate smaller than expected from constant relative humidity.
Maistrova et al. (2003) reported an increase in specific humidity at 850 hPa and a decrease from 700 to 300 hPa for 1959 to 2000 in the Arctic, based on data from ships and temporary stations as well as permanent stations. In general, the radiosonde trends are highly suspect owing to the poor quality of, and changes over time in, the humidity sensors (e.g., Wang et al., 2002a). Comparisons of water vapour sensors during recent intensive field campaigns have produced a renewed appreciation of random and systematic errors in radiosonde measurements of upper-tropospheric water vapour and of the difficulty in developing accurate corrections for these measurements (Guichard et al., 2000; Revercombe et al., 2003; Turner et al., 2003; Wang et al., 2003; Miloshevich et al., 2004; Soden et al., 2004).
Information on the decadal variability of upper-tropospheric relative humidity is now provided by 6.7 µm thermal radiance measurements from Meteosat (Picon et al., 2003) and the High-resolution Infrared Radiation Sounder (HIRS) series of instruments flying on NOAA operational polar-orbiting satellites (Bates and Jackson, 2001; Soden et al., 2005). These products rely on the merging of many different satellites to ensure uniform calibration. The HIRS channel 12 (T12) data have been most extensively analysed for variability and show linear trends in relative humidity of order ±1% per decade at various latitudes (Bates and Jackson, 2001), but these trends are difficult to separate from larger interannual fluctuations due to ENSO (McCarthy and Toumi, 2004) and are negligible when averaging over the tropical oceans (Allan et al., 2003).
In the absence of large changes in relative humidity, the observed warming of the troposphere (see Section 3.4.1) implies that the specific humidity in the upper troposphere should have increased. As the upper troposphere moistens, the emission level for T12 increases due to the increasing opacity of water vapour along the satellite line of sight. In contrast, the emission level for the MSU T2 remains constant because it depends primarily on the concentration of oxygen, which does not vary by any appreciable amount. Therefore, if the atmosphere moistens, the brightness temperature difference (T2 − T12) will increase over time due to the divergence of their emission levels (Soden et al., 2005). This radiative signature of upper-tropospheric moistening is evident in the positive trends of T2 − T12 for the period 1982 to 2004 (Figure 3.21). If the specific humidity in the upper troposphere had not increased over this period, the emission level for T12 would have remained unchanged and T2 − T12 would show little trend over this period (dashed line in Figure 3.21).
Clear-sky outgoing longwave radiation (OLR) is also highly sensitive to upper-tropospheric water vapour and a number of scanning instruments have made well-calibrated but non-overlapping measurements since 1985 (see Section 3.4.3). Over this period, the small changes in clear-sky OLR can be explained by the observed temperature changes while maintaining a constant relative humidity (Wong et al., 2000; Allan and Slingo, 2002) and changes in well-mixed greenhouse gases (Allan et al., 2003). This again implies a positive relationship between specific humidity and temperature in the upper troposphere.
To summarise, the available data do not indicate a detectable trend in upper-tropospheric relative humidity. However, there is now evidence for global increases in upper-tropospheric specific humidity over the past two decades, which is consistent with the observed increases in tropospheric temperatures and the absence of any change in relative humidity.