Abstract
Various studies have documented the effects of modernâday irrigation on regional and global climate, but none, to date, have considered the timeâvarying impact of steadily increasing irrigation rates on climate during the 20th century. We investigate the impacts of observed irrigation changes over this century with two ensemble simulations using an atmosphere general circulation model. Both ensembles are forced with transient climate forcings and observed sea surface temperatures from 1902 to 2000; one ensemble includes irrigation specified by a timeâvarying data set of irrigation water withdrawals. Early in the century, irrigation is primarily localized over southern and eastern Asia, leading to significant cooling in boreal summer (JuneâAugust) over these regions. This cooling spreads and intensifies by century's end, following the rapid expansion of irrigation over North America, Europe, and Asia. Irrigation also leads to boreal winter (DecemberâFebruary) warming over parts of North America and Asia in the latter part of the century, due to enhanced downward longwave fluxes from increased nearâsurface humidity. Precipitation increases occur primarily downwind of the major irrigation areas, although precipitation in parts of India decreases due to a weaker summer monsoon. Irrigation begins to significantly reduce temperatures and temperature trends during boreal summer over the Northern Hemisphere midlatitudes and tropics beginning around 1950; significant increases in precipitation occur in these same latitude bands. These trends reveal the varying importance of irrigationâclimate interactions and suggest that future climate studies should account for irrigation, especially in regions with unsustainable irrigation resources.
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