Braking winds over a roughening earth

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. 3, 756, 2010.)