Rainfall

With global warming and a rise in the global average surface temperature, increases in global rainfall and other forms of precipitation would be expected, due to the greater rates of evaporation of sea surface water. Unfortunately, no reliable estimates of evaporation increase exist. One problem is the effect of varying wind speed on evaporation rates, which may or may not be related to increases in temperature.

Several large-scale regional analyses of precipitation changes have been carried out. These have demonstrated that during the latter part of the 20^th century precipitation has tended to increase in the mid-latitudes, for example in the former Soviet Union, but decrease in the Northern Hemisphere subtropics. A striking rainfall decrease occurred in the African Sahel north of the Sahara Desert, between the 1960s and 1980s. This dramatic desiccation has been linked to changes in ocean circulation and tropical Atlantic sea surface temperatures. Whether such changes are linked to global warming however, remains open to analysis.

The accuracy of other precipitation records should be treated with caution. Precipitation is more difficult to monitor than temperature due to its greater geographical variability. Other uncertainties in the data set may be due to the collection efficiency of rain gauges. Consequently, the compilation of a global precipitation record can prove to be very difficult and is perhaps unjustified.