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

The notion that winds may be slowing is still a speculative one, and scientists disagree whether it is happening. But a new study suggests that average and peak wind speeds have been noticeably slowing since 1973, especially in the East and Midwest.

"It's a very large effect," said coauthor 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. Wind in that region averages 10 to 12 m.p.h

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.

Measurements she has plotted on maps show declining speeds 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, show less of a decline. Some of the biggest drops in wind speeds are in Ohio, Indiana, Michigan, Illinois, Kansas, Virginia, Louisiana, Georgia, northern Maine, and western Montana.

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

Still, the study, which will be published in August in the Journal of Geophysical Research, is preliminary. There are enough questions that even the authors say it's too early to know if what they are finding adds up to a trend.

But it does raise a new side effect of global warming.

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 computer models showed no such decline.

A couple of earlier studies also found reductions in Australia and Europe, she said.

And it makes sense, Takle said, based on how weather and climate work. It is known that with global warming, temperatures at the poles are rising more and faster than the rest of the globe.

As the temperature difference between the poles and the equator shrinks, so does the difference in air pressure. Differences in barometric pressure are a main driver in strong winds. Less pressure difference means less wind.

The new study "demonstrates, rather conclusively in my mind, that average and peak wind speeds have decreased over the U.S. in recent decades," said Michael Mann, director of the Earth System Science Center at Pennsylvania State University.

Jeff Freedman, an atmospheric scientist with AWS Truewind, an Albany, N.Y., renewable-energy consulting firm, has studied the same topic but has not published in a scientific journal yet. He said his research had found no definitive trend of reduced surface wind speed.

Another naysayer is Gavin Schmidt, a NASA climate scientist in New York who said Pryor's results conflicted with climate models that show no effect from global warming.

He also doubts that any decline in the winds that might be occurring would have much of an effect on the potential of wind power.