Through enhanced evaporation, irrigation cools the Earth's surface and provides a counterbalance to global warming in some parts of the world. But additional warming could throw that balance off kilter, suggests a study.

A team led by climate scientist Benjamin Cook, of NASA's Goddard Institute for Space Studies in New York City, compared the irrigation-induced cooling generated during the year 2000 with that expected in 2050 if atmospheric concentrations of carbon dioxide rise to about 40 per cent higher than they are now1. In today's climate, the regions that benefit most from irrigation-induced cooling are North America, India, the Middle East and eastern Asia. But evaporative cooling in those regions won't be equally influenced by further warming, the study finds.

In North America, for example, if precipitation declines in areas downwind of irrigated regions, as anticipated, a reduction in the surface water available to soak up energy during evaporation will diminish the cooling effect. In China, however, where humidity limits today's evaporative cooling, climate change is expected to lead to a drier atmosphere, which would boost evaporation and intensify the cooling from irrigation.