The frequency of heavy precipitation events has increased over most land areas. Significantly increased precipitation has been observed in eastern parts of North and South America, Northern Europe and Northern and Central Asia. There is also observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970.

Drying has also been observed over large regions, including the Sahel, the Mediterranean, Southern Africa and parts of Southern Asia. In Africa's large water catchments of Niger, Lake Chad and Senegal, total available water has decreased by 40 to 60 per cent, and desertification has been worsened by lower average annual rainfall, runoff and soil moisture, especially in Southern, Northern and Western Africa.

Precipitation can be expected to continue to decrease in most subtropical land regions, but to increase in the high latitudes. The IPCC (2007a) found with 'high confidence that many semi-arid areas (e.g. Mediterranean Basin, western US, Southern Africa and north-east Brazil) will suffer a decrease in water resources due to climate change'. By the 2050s, it is projected that there will be less annual river runoff and water availability in dry regions in the mid latitudes and tropics, but an increase in high latitude regions and in some tropical wet areas.

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