Water vapour is a key climate variable. It is the most important gaseous source of infrared opacity in the atmosphere, accounting for about 60% of the natural greenhouse effect for clear skies, and provides the largest positive feedback in model projections of climate change. Time series analyses indicate (i) increases in moisture in the atmosphere near the surface, (ii) an upward trend in the lower-tropospheric water vapour over the global oceans and some northern hemisphere land areas, (iii) no detectable trend in uppertropospheric relative humidity, and (iv) an apparent upward trend in the water vapour in the stratosphere over the last half of the 20th century, but with no further increases since 1996 (IPCC2007).