Despite global warming, the rate of water evaporation over land surfaces has steadily declined in the past few decades. That unexpected trend, observed by farmers and climate-change scientists alike, has been linked to a decline in surface wind speeds over the same period. The challenge of quantifying the stilling-winds phenomenon on a global scale was recently taken up by Robert Vautard and his colleagues at the Climate Science and Environment Laboratory in France and the European Centre for Medium-Range Weather Forecasts in the UK. By analyzing data from more than 800 weather stations, the researchers found that 73% reported that wind speeds measured 10 meters above the ground had declined by up to 15% from 1979 to 2008. As the image shows, some regions in Europe experienced declines of as much as 5 m/s per decade. After studying climate-model simulations, the researchers attribute much of the slowdown to an increase in topographical surface roughness from a surge in vegetation growth induced by excess atmospheric carbon and also anthropogenic activities such as urbanization. Less of a slowdown, or even an increase, was seen in regions that did not experience significant vegetation growth. The analysis assigned a lesser role to reduced atmospheric circulation caused by global warming. Wind energy enthusiasts should not necessarily be worried, say the researchers, since most wind turbines operate at 50–100 m, where the analysis did not detect any noticeable trend. (R. Vautard et al., Nat. Geo., in press, doi:10.1038/ngeo979.)—Jermey N. A. Matthews
