Science 317, 233–235 (2007)

Credit: ©Getty

Global warming may result in even more rain than that currently projected by climate models. Models and observations agree that a warmer planet will have more water in the atmosphere, but exactly how much of this will fall as rain has been hard to pin down.

Frank J. Wentz of Remote Sensing Systems in Santa Rosa, California and co-workers analysed trends in water vapour, surface wind and precipitation from global satellite data between 1987 and 2006. Although climate models indicate that precipitation should have increased by only 1–3% per degree Celsius of surface warming during this period, they found an increase of 7% per degree Celsius — the same rate at which water vapour increased in the atmosphere.

The lower rainfall in the model simulations can be explained by the fact that they projected weaker global surface winds than those observed in the satellite data. Stronger surface winds increase the evaporation of moisture from the Earth's surface and transport it upwards into the atmosphere where it falls as rain. Where this additional rain will fall in the future remains a mystery: will it alleviate drought in arid areas or contribute to flooding in wet regions?