Are surface winds around the world really slowing down? That's the suggestion of a new study in Nature Geoscience. The authors built on previous studies indicating such a trend by analyzing surface wind data from 822 wind stations in Europe, Asia and North America. The study concludes that the widespread "atmospheric stilling" has more to do with what's happening on the ground than it does changes in general atmospheric circulation—specifically, increased "surface roughness" due to land-use changes may be responsible for the slow-down.

Recent research indicates surface winds are slowing in China, The Netherlands, the United States and the Czech Republic. But this data have been confined to localities, says lead author Robert Vautard of the Laboratoire des Sciences du Climat et de l'Environnement in France, due to reasons stemming from the inherent difficulty of collecting surface wind data. Many of the weather stations have large breaks in their data, and in other cases, stations have changed locations or switched instruments, compromising data quality. This has led research groups to limit analysis to data they could vouch for locally. "In the past, people have been very careful, but they have not used a global data set because didn't trust the data of other countries," says Vautard.

His group examined the data of more than 5,000 stations that used anemometers to measure wind speed, eliminating stations with large gaps and changes in location and instruments. They subtracted 85 percent of the stations, leaving data from 822, collected during the period between 1979 and 2008. They found that surface wind speed decreased over the 30-year period at 73 percent of the stations, and that surface wind speeds have decreased by -0.09, -0.6, -0.12, -0.07 meters per second per decade in Europe, Central Asia, Eastern Asia and North America, respectively. 

Why is this happening? "That's where the thing gets more complicated," says Vautard, "There may be several causes." 

First, wind data is difficult to collect, and the trend could be an artifact of anemometer degradation or changes in observation methods. But the authors conclude the trend was too pronounced to be due solely to the way the data were collected. "It is difficult to believe that all the anemometers have the same problem all over the world," says Vautard, adding, "In previous papers, there are some discussions of the quality of these measurements, but nobody came to the conclusion that these phenomena could come from the observations themselves." 

Next, the authors explored whether or not the slow-down was due to changes in general atmospheric circulation—in particular, changes in the speed of upper winds. Specifically, they note, if these changes are to blame for surface wind slowing, a similar slowing trend should be seen in the "reanalysis" of 3-D wind models from the National Centers for Environmental Prediction/National Center of Atmospheric Research and European Center for Medium-range Weather Forecast. "The reanalysis do have some trends in some areas, but not everywhere, not with the same pattern, and are not fully correlated" to the observed surface wind changes. Thus, the authors concluded that changes in general circulation account for only 10-50 percent of the changes at the surface.

Finally, the researchers analyzed changes in land use that may have led to increased "surface roughness," which can slow winds. Indeed, says Vautard, surface roughness has been increasing due in part to abandonment of cropland, which is being replaced with trees and shrubs, and to previously deforested land in North America and Europe becoming reforested. The growth of cities could also be contributing. The authors estimated that these factors were responsible 20-65 percent of the surface wind slowdown.

Do slower winds mean we should temper our optimism for the global wind power industry? We don't know enough yet, says Vautard. Wind energy is not collected at the surface, and requires a specific range of wind speed (between five and 25 meters per second). "We have to really look at whether and in which regions the decline is really in this range of wind, and we have to take into account that wind power is taken at 80 meters," he says, "We need a model to understand that."

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