In a little-noticed scientific paper in 2005, Paul Higgins, a scientist and policy fellow with the American Meteorological Society, calculated specific savings from adopting federal government recommendations for half an hour a day of exercise instead of driving.

The average person walking half an hour a day would lose about 13 pounds a year. And if everyone did that instead of driving the same distance, the nation would burn a total of 10.5 trillion calories, according to the scientist, formerly with the University of California at Berkeley. At the same time, that would cut carbon dioxide emissions by about the same amount New Mexico produces, he said.

"The real bang for the buck in reducing greenhouse gas emissions was from the avoided health expenses of a sedentary lifestyle,'' said Higgins.

But it's not just getting out of the car that's needed, said Dr. Robert Lawrence of the Johns Hopkins School of Public Health. A diet shift away from heavy meat consumption would also go far, he said, because it takes much more energy and land to produce meat than fruits, vegetables and grains.

Recent studies support that argument. Last year the United Nations Food and Agriculture Organization reported that the meat sector of the global economy is responsible for 18 percent of the world's greenhouse gas emissions. Much of that is indirect, including the fertilizer needed to grow massive amounts of feed for livestock, energy use in the whole growing process, methane released from fertilizer and animal manure, and transportation of the cattle and meat products.

As for fighting obesity and global warming by walking and cycling, don't expect people to do it easily, said Kristie Ebi. She's a Virginia public health consultant and one of the lead authors of the Intergovernmental Panel on Climate Change report.