The wind, a favorite power source of the green energy movement, seems to be dying down across the United States. And the cause, ironically, may be global warming - the very problem wind power seeks to address.

The idea that winds may be slowing is still a speculative one, and scientists disagree whether that is happening. But a first-of-its-kind study suggests that average and peak wind speeds have been noticeably slowing since 1973, especially in the Midwest and the East.

"It's a very large effect," said study co-author Eugene Takle, a professor of atmospheric science at Iowa State University. In some places in the Midwest, the trend shows a 10 percent drop or more over a decade. That adds up when the average wind speed in the region is about 16 to 19 kmph.

There's been a jump in the number of low or no wind days in the Midwest, said the study's lead author, Sara Pryor, an atmospheric scientist at Indiana University.

Wind measurements plotted out on US maps by Pryor show wind speeds falling mostly along and east of the Mississippi River. Some areas that are banking on wind power, such as west Texas and parts of the Northern Plains, do not show winds slowing nearly as much. Yet, states such as Ohio, Indiana, Michigan, Illinois, Kansas, Virginia, Louisiana, Georgia, northern Maine and western Montana show some of the biggest drop in wind speeds.

"The stations bordering the Great Lakes do seem to have experienced the greatest changes," Pryor said on Tuesday. That's probably because there's less ice on the lakes and wind speeds faster across ice than it does over water, she said.

Still, the study, which will be published in August in the peer-reviewed Journal of Geophysical Research, is preliminary. There are enough questions that even the authors say it's too early to know if this is a real trend or not. But it raises a new side effect of global warming that hasn't been looked into before.

The ambiguity of the results is due to changes in wind-measuring instruments over the years, according to Pryor. And while actual measurements found diminished winds, some climate computer models did not, she said.

Earlier studies have found wind reductions in Australia and Europe, offering more comfort that the US findings are real, Pryor and Takle said.

It also makes sense based on how weather and climate work, Takle said. In global warming, the poles warm more and faster than the rest of the globe. That means the temperature difference between the poles and the equator shrinks and with it the difference in air pressure in the two regions. Differences in barometric pressure are a main driver in strong winds. Lower pressure difference means less wind.

(China Daily via Agencies June 11, 2009)