WASHINGTON (AP) — Global warming's milder winters will likely nudge Americans off the couch more in the future, a rare, small benefit of climate change, a new study finds.

With less chilly winters, Americans will be more likely to get outdoors, increasing their physical activity by as much as 2.5 percent by the end of the century, according to a new study in Monday's edition of the journal Nature Human Behaviour . Places like North Dakota, Minnesota and Maine are likely to see the most dramatic increases, usually the result of more walking.

But that good global warming side effect is not likely to extend to the deep south and especially the desert southwest because hotter summer days may keep people inside. Arizona, southern Nevada and southeastern California are likely to see activity drop off the most by the year 2099, the study found.

"It's a small little tiny silver lining amid a series of very bad, very unfortunate events that are likely to occur," said study lead author Nick Obradovich , who studies the social impacts of climate change at both Harvard's Kennedy School of Government and MIT. Global warming "almost certainly will be very costly on net for humanity."

Any overall benefit for Americans as a whole will probably be far outweighed by many other ways that climate change hurts health, said both Obradovich and outside health experts. For example, deaths from heat waves are expected to increase, allergies are likely to worsen and infectious diseases will be more easily spread, said Dr. Howard Frumkin, a University of Washington environmental health professor.

Obradovich said he got the idea to look about what climate change will do to people's activities a few Octobers ago when he was living in San Diego and running regularly in the afternoon. There was a heat wave, temperatures broke 100, and he stayed home.

Obradovich looked at government surveys about health activity habits, daily weather data from when they were interviewed and simulations of future climate conditions. The warmer it gets, the more people go outside, which he said makes sense. Until it gets too hot. At about 82 to 84 degrees (28 to 29 degrees Celsius) people start to go out less.

For most of America for most of the year, the daily high does not hit 84, so the net effect nationwide is more exercise.

But the affect varies by month and location. Nearly all the country is likely to be less physically active in July, August and September by the end of the century, but a similar majority would also likely exercise more in November, December, January, February, March and even April in the year 2099, the study finds.

Dr. Jonathan Patz, director of the Global Health Institute at the University of Wisconsin-Madison, faulted the study for not taking into account people who have jobs that require lots of physical activity nor the growing popularity of winter sports.

Other outside experts said the study made sense, but the bigger picture is more important.

"While milder winters will permit more exercise — a good thing — it's important to put the results of the paper in that broader context," Frumkin said in an email, emphasizing "climate change threatens far more than it benefits."

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Follow Seth Borenstein at http://twitter.com/borenbears and his work can be found at http://tinyurl.com/sethap .