Since the early 1990s, estimates of column integrated water vapour have been obtained from ground-based Global Positioning System (GPS) receivers. An international network started with about 100 stations in 1997 and has currently been expanded to more than 500 (primarily land-based) stations. Several studies have compiled GPS water vapour data sets for climate studies (Jin et al., 2007; Wang et al., 2007; Wang and Zhang, 2008, 2009). Using such data, Mears et al. (2010) demonstrated general agreement of the interannual anomalies between ocean-based satellite and land-based GPS column integrated water vapour data. The interannual water vapour anomalies are closely tied to the atmospheric temperature changes in a manner consistent with that expected from the Clausius–Clapeyron relation. Jin et al. (2007) found an average column integrated water vapour trend of about 2 kg m–2 per decade during 1994–2006 for 150 (primarily land-based) stations over the globe, with positive trends at most NH stations and negative trends in the SH. However, given the short length (about 10 years) of the GPS records, the estimated trends are very sensitive to the start and end years and the analyzed time period (Box 2.2).