Why winds are slowing

Joseph Milton in Nature News: Increasing amounts of vegetation could be causing up to 60% of a slowing in wind speed across the Northern Hemisphere, according to researchers analysing three decades of wind-speed data in Nature Geoscience1 today.

The decline is a potential concern for wind-turbine efficiency. But researchers cannot tell whether the effect, an average 10% slowdown, will make much difference to wind turbines — the slowing winds measured are at 10 metres above the ground, whereas turbines operate at 50–100 metres up, where there is little global data.

Several previous regional studies looking at the United States, Australia, China and parts of Europe have shown decreasing wind speeds just above the planet's surface. Climate change, afforestation and urban development had been mooted as possible causes. But, says Robert Vautard, at the University of Versailles Saint Quentin in Yvelines, France, "people always said the data were rubbish. There was no quality-controlled global archive of data."

…Changing patterns of atmospheric circulation at high altitudes have previously been blamed for slowing wind speeds — as air temperatures and pressures in different parts of the atmosphere shift with a gradually heating Earth. But Vautard and his team say that only 10–50% of the change they observed could be explained by current understanding of how changes in high-altitude circulation affect surface wind speeds.

…Increased vegetation — resulting from ex-agricultural land becoming overgrown, afforestation and changing landscape-management practices — could account for 25–60% of the observed stilling in wind speed, says Vautard. The researchers used satellite images to estimate increases in the volume and height of vegetation, and computer climate models to see how this would affect wind speed. "We need further investigation of whether afforestation affects wind speeds," he says, "but it's almost certainly having an effect."…