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Are America's Winds Taking a Breather?

Wind speeds, expected to help propel the advance of renewable energy, may be slowing, a cautious new study suggests. Then again, they may be blowing even harder.

It's easy to take the wind for granted. We count on it for all kinds of things, like propelling sailboats, turning giant power-producing turbine blades and keeping kites aloft. But what if the wind doesn't blow as hard as it used to?

That possibility presents itself in a new, cautiously worded report suggesting that wind speeds may have been declining in the Midwest and northeastern states since the 1970s by 0.5 to 1 percent a year.

The finding is based on data collected from wind-measuring anemometers placed at weather stations throughout the United States. But it is only one component of the study, "Wind speed trends over the contiguous United States," recently published in the Journal of Geophysical Research. The team, led by Indiana University's Sara C. Pryor, also looked at a half dozen other data sets and climate models, which generally showed that wind speeds are not declining, and if anything may be on the increase.

That prompted some scientific head scratching: Are wind speeds really declining in North America? If so, what could be causing it? Is climate change to blame or simply natural variation in weather cycles?

The implications are sobering. If the winds actually are blowing less forcefully than in the past, it could affect everything from agriculture to wind-energy-generation schemes being contemplated as a green alterative to fossil fuel power.

"One of the things we're trying to express in the paper is that there is this discrepancy," Pryor says. "It's important to say the in situ [weather station] observations reflect wind speeds that may not be reasonably representative. These are stations largely at airports. These data are being collected largely for aviation safety and to some extent weather forecasting, so they're not really designed to be used for climatological analyses."

The wind speed data was collected between 1973 and 2005 at hundreds of sites around the country, said Eugene S. Takle, an Iowa State University climatologist who collaborated on the study.

"There are a lot of pitfalls in trying to understand and analyze these data," Takle said. For one thing, there have not always been uniform data collection procedures, and anemometer designs have changed through the years. "Now it's more standardized than it was in the past."

Changing land use is another complicating factor. Extensive urban development in recent decades may have led to new buildings and trees near the measurement sites. "That has a tendency to create friction to the wind, so that will slow it down," he said. A related theory is that as agriculture has declined in the eastern U.S., widespread natural reforestation has gradually reshaped the landscape.

Climate change is a more controversial possibility, Takle said. With warming temperatures, "we know that the period of ice on the Great Lakes has been declining," he said. Because open water is rougher than smooth ice, that could also cause a friction-related drop in wind speeds, he said.

"There has been a precedent for models showing wind speed decline from climate change," Takle said, noting that one study suggested that increasing levels of atmospheric carbon dioxide could cause wind speeds to drop.

Simulations run on various computer-driven climate models yield results that are "all over the map," Takle said. Like Pryor, he's puzzled by the discrepancies.

"Our view is it could have been a fluke. We would like to work together with people who have competing models. We're hoping this launches a new investigation of wind trends so we can get this sorted out."

In Pryor's view, wind speed researchers have historically been the "poor relations," compared to climate scientists who focus on temperature and precipitation. "Very few people have studied wind speeds," she said. "It picks up the difficulty in getting quality data."

While the weather station observations clearly point to a decline in northern U.S. wind speeds over three decades, Pryor is wary of leaping to conclusions about what is going on.

"I'm certainly not saying wind speeds haven't declined," she said, "but I think it's important to say that we have seen — certainly in Europe, where there are longer time series of up to 150 years — trends of similar magnitude over 30-year periods in the historical record.

"One of the points we make in the article is the declines don't have to be continued. We see quite natural variability that is of this magnitude. It doesn't have to be a climate change signal."

Some reasonably well-understood climactic cycles could also be involved, Pryor said. "We know that there are phenomena, such as ENSO (the El Nino-Southern Oscillation), that have time cycles of years or decades," she said. "The same with sun spot cycles of 22 years."

She added, "One of the things that can be true of our analysis in the U.S. is that we simply captured a period where the natural system was moving toward perhaps lower storm intensities, and that may in fact reverse in the future. I think we're a long way from seeing any kind of climate change signal in wind speeds, even if one exists."

If average wind speeds really were to decline, it could affect urban areas that rely on the wind to disperse air pollution. Farmers would also see less ventilation of their crops, which could cause added heat stress on the plants. It would also mean that estimates of wind power potential would have to be scaled back (one recent study found that an extensive network of land-based wind farms potentially could supply more than 16 times the current U.S. power consumption).

Pryor hopes to shed more light on this question. "There is another data set that records wind speeds not right at the surface but close to the surface, and so I'm analyzing those data," she said. "They should be relatively insensitive to land use change. What do we see in that data in terms of trends?"

She also wants to study how well regional and global climate models predict wind speeds. "Those models have been largely evaluated and validated on temperature and precipitation and they do a pretty good job," she said. "But now we need to expand the variables that we're evaluating. If we do that, obviously we'll better understand their performance."

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