Around the world, surface winds are slowing down, a new study says. Strangely enough, the alleged culprits aren't new buildings but new trees.

The easing breezes—if also detected higher up—could affect movements of air pollution but may not necessarily give the wind power industry a case of the doldrums, experts say.

For the new study, published Sunday by the journal Nature Geoscience, scientists analyzed nearly 30 years' worth of wind speed data collected from more than 800 land-based weather stations, mostly in the Northern Hemisphere, where long-term wind-data collection has been most reliable.

The average annual surface wind speed in countries in mid-northern latitudes—including the United States, China, and Russia—had dropped by as much as 15 percent, from about 10.3 miles (17 kilometers) an hour to about 9 miles (14 kilometers) an hour, the study found.

Wind speeds in the Southern Hemisphere are probably slowing too, the team speculates. Winds in different parts of the world are coupled, so average wind speed can't decrease at one latitude but increase in another—a hypothesis perhaps underscored by Australian data also showing mysteriously slowing surface winds.

If wind speeds higher in the air are also slowing down, that could affect the spread of atmospheric pollutants or even the dispersal of windblown seeds, said study leader Robert Vautard, a researcher at the Climate and Environment Science Laboratory in France. "We are not saying it's a good or bad thing," he added. "We are just observing it and trying to explain it."

Wind-Speed Puzzle Far From Finished

The regions with the most wind stalling also saw the greatest increase in plant growth during the three-decade period—suggesting trees and shrubs in these regions are acting as windbreaks, the study team says.

In North America and Europe, deliberate reforestation drove the plant surge. In Russia and other parts of Eurasia, the tree boom is likely due to abandoned farmland reverting back to wilderness after the collapse of the Soviet Union, the scientists say.

But reforestation can explain only about 60 percent of the wind speed reductions, the study says. Changes in air circulation due to global warming may be responsible for the rest, but more studies are needed to be sure, according to Vautard.

"So far we only have pieces of the puzzle," he added. "We have not completely attributed the phenomenon."

Sara Pryor, an atmospheric scientist at Indiana University, cautioned against prematurely linking changes in wind speeds to climate change.

"We should be very careful in the way we interpret studies on wind climates," Pryor, who wasn't involved in the study, wrote in an email.

"This type of research is really nascent, and it may be many years before we can say something concrete about the ways in which wind climates may alter in a warmer world."

Also unclear is whether surface wind speeds will continue to decrease in the coming decades. That will depend on what the major driving factor turns out to be.

"If the major driver is cropland abandonment in Eurasia, then I do not expect much more future decay," because the great post-Soviet farm abandonments are over, study leader Vautard said.

"But if the driving factor is climate change, then it's a different story."

Wind Industry Above It All?

If winds at higher altitudes are also found to be decreasing, that may have implications for the generation of electricity through wind power, since most wind turbines are located high off the ground, the researchers say.

But for now, the new wind speed findings aren't ruffling Michael Goggin, manager of transmission policy at American Wind Energy Association, a trade group. (Related: "First U.S. Offshore Wind Power Project Approved.")

"The reassuring consensus of wind resource assessment experts is that wind speeds at the height of modern wind turbines, several hundred feet above the Earth's surface, have not declined," Goggin said in an email.

"As a result, there has been no significant change in wind energy output, and one is not expected going forward."

Goggin added that the U.S. regions that have seen the most wind-energy development, such as West Texas and the Great Plains, are unlikely to become forested any time soon.

"In the unlikely event that there is an impact on hub-height winds in some areas, the impact will be extremely localized to the small area just downwind of the area that has been reforested," he said.