Global warming is likely to lead to overall drying of land surfaces. Global temperature increases affect the water cycle over land, but the nature of these changes remains difficult to predict. A key conceptual problem is to distinguish between droughts, which are transient regional extreme phenomena typically defined as departures from a local climatological norm that is presumed known, and the normal or background dryness itself. This background dryness depends on precipitation, but also on how fast water would evaporate. As the planet warms, global average rainfall increases, but so does evaporation. What is the likely net impact on average aridity?