Policy Documents

Precipitation Measurements and Trends in the Twentieth Century

Mark New, Martin Todd, Mike Hulme, Phil Jones –
January 28, 2002


Concern about anthropogenic climate change has heightened the need for accurate information about spatial and temporal variations in precipitation at the Earth's surface. Large-scale precipitation estimates can be derived from either surface gauge measurements or by satellite remote sensing, both of which have shortcomings. Gauge measurements provide information about trends and variability of monthly precipitation throughout the entire twentieth century, but because of the lack of data from most ocean regions, this information is representative of only about 25–30% of the Earth's surface. In contrast, satellite (especially multi-platform) measurements provide spatially complete coverage at monthly to subdaily resolution, but do not extend back beyond 1974. Merged gauge–satellite datasets maximize (and minimize) the relative benefits (and shortcomings) of each source type. While these merged products only extend back to 1979, their importance will grow as we move into the new century.

Precipitation gauge data indicate that global land precipitation (excluding Antarctica) has increased by about 9 mm over the twentieth century (a trend of 0.89 mm/decade), which is relatively small compared with interannual and multi-decadal variability. Within this century-long trend, global precipitation exhibits considerable variability on decadal time-scales, with departures of up to ±40 mm from the century mean of about 950 mm. Regionally, precipitation has increased over most land areas, with the exception of tropical North Africa, and parts of southern Africa, Amazonia and western South America. The dominant mode of interannual variability in global and hemispheric land precipitation is related to the El Niño–Southern Oscillation (ENSO), which explains about 38% of the interannual variance in globally averaged land precipitation and about 8% of the space–time variability of global precipitation. In the mid- and high latitudes, the Arctic and Antarctic oscillation (AO and AAO) are the dominant modes of interannual climate variability. The AO explains 48 and 35% of area-averaged winter precipitation variability over land in the latitude bands 60–80°N and 40–60°N, respectively. The North Atlantic Oscillation is not an important modulator of global precipitation, but it does explain 8% of annual (more in winter) variability in spatially averaged northern mid-latitude precipitation. Analyses of precipitation over land and ocean (spatially complete) since 1980 indicate that only a few regions show marked trends in precipitation over this short period, but there is a suggestion that there has been a shift in zonal precipitation. There are coherent regions where the tropical ENSO signal extends in a northeast/southeast direction into the subtropics, especially in the Pacific-Indonesian region, but also over the Atlantic-African and Indian Ocean domains.

Data from a number of countries provide evidence of increased intensity of daily precipitation, generally manifested through increased frequency of wet days and an increased proportion of total precipitation occurring during the heaviest events. Over most land areas there has also been an increase in the persistence of wet spells.