While cities are hot spots for global warming, people living in them turn out to be greener than their country cousins.

Each resident of the largest 100 metropolitan areas is responsible on average for 2.47 tons of carbon dioxide in energy consumption each year, 14 percent below the 2.87-ton U.S. average, researchers at the Brookings Institution say in a report being released Thursday.

Those 100 cities still account for 56 percent of the nation's carbon dioxide pollution. But their greater use of mass transit and population density reduce the per person average.

"It was a surprise the extent to which emissions per capita are lower," Marilyn Brown, a professor of energy policy at the Georgia Institute of Technology and co-author of the report, said in an interview.

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Metropolitan area emissions of carbon dioxide are highest in the eastern U.S., where people rely heavily on coal for electricity, the researchers found. They are lower in the West, where weather is more favorable and where electricity and motor fuel prices have been higher.

The study examined sources and use of residential electricity, home heating and cooling, and transportation in 2005 in the largest 100 metropolitan areas where two-thirds of the people in the U.S. live. It attributed a wide disparity among the 100 cities to population density, availability of mass transit and weather.

Lexington, Ky., had the biggest per capita carbon footprint: Each resident on average accounted for 3.81 tons of carbon dioxide in their energy usage. At the other end of the scale was Honolulu, at 1.5 tons per person.

Carbon dioxide is released from burning fossil fuels and is the leading "greenhouse gas." It drifts into the atmosphere and forms a blanket that traps the Earth's warmth. About 6.6 billion tons of carbon dioxide are released into air annually in the United States.

From 2000 to 2005, carbon dioxide from transportation, electricity use and residential heating in the largest metropolitan areas increased 7.5 percent. For the entire nation, it rose 9.1 percent. The average per capita footprint in those 100 cities rose at an annual rate of 1.1 percent a year, half the average yearly increase of 2.2 percent nationwide.

In explaining differences among cities, the researchers cited weather, the type of fuel used for heating and cooling, the development of rail transportation, the amount of urban sprawl and the cost of energy.

Cities with the largest carbon footprints are mostly in the eastern half of the country from Indiana to western Pennsylvania — areas that rely heavily on coal for electricity production and natural gas for heating.

The smallest carbon footprint was in cities in the West and New England.

Half of the dozen cities with the stingiest carbon output were in California, where electricity prices and motor fuels are expensive. Also cited was the Seattle-Portland, Ore., region, which relies heavily on hydropower.

Cities in Kentucky, Ohio and Indiana dominated the bottom tier of high carbon emitters.

These urban areas are "kind of a poster child of what high carbon intensive growth looks like," said Brown. She noted their reliance on coal for electricity and natural gas for heating, a shortage of mass transit, and often older, energy-inefficient buildings.