So say two unknown medical types in the Lancet magazine.
By way of reference, this is the same magazine which published the discredited claim of 600,000 civilian deaths in Iraq – a number which proved to be four times too high. The Lancet requires a subscription or registration or something (sorry, no patience for a “science” magazine that seems to award contributing authors extra points for creativity) so this excerpt is courtesy of TierneyLab, a New York Times blog:

Compared with the normal weight population, the obese population consumes 18% more food energy. Additionally, more transportation fuel energy will be used to transport the increased mass of the obese population, which will increase even further if, as is likely, the overweight people in response to their increased body mass choose to walk less and drive more.
Urban transport policies that promote walking and cycling would reduce food prices by reducing the global demand for oil, and promotion of a normal distribution of B.M.I. [Body Mass Index] would reduce the global demand for, and thus the price of, food. Decreased car use would reduce greenhouse gas emissions and thus the need for biofuels, and increased physical activity levels, would reduce injury risk and air pollution, improving population health.

If man even is causing global warming by his meager contribution of 6% of total greenhouse gas emissions, the margin represented by higher than average body mass indices is not at or near the top of the list of his impactful activities. Of greater concern is the understandable desire of less developed countries to acquire such “luxuries” as electricity, cars, heat, cooling, wider selection of food, etc, evidenced by China surpassing the United States in 2006 as the biggest producer of carbon dioxide.
While I am a great believer in exercise and doing errands on foot and bicycle as feasible, I am heartily sceptical of 1.) man’s role in global warming; 2.) man’s ability to reverse his impact, if any, without literally ending civilization as we know it; 3.) the adviseability of reversing global warming at all; and 4.) our obligation to the cute, cuddly polar bear. The propounding of this silly theory in Lancet, therefore, delights me because it vividly encapsulates the larger theory of anthropogenic global warming: it eschews scientific proof while attempting to make man feel guilty for his unproven role in a not unprecedented phenomenon.