WASHINGTON - 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 100 largest 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 today.

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," said Marilyn Brown, a professor of energy policy at the Georgia Institute of Technology and coauthor of the report.

Among metro areas, emissions of carbon dioxide are highest in the eastern United States, where people rely heavily on coal for electricity, the researchers found. They are lower in the West, where weather is more favorable and 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 Americans live. It attributed a wide disparity among the 100 cities to population density, availability of mass transit, and weather.

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

In the sprawling metropolitan area that includes Philadelphia, Camden and Wilmington, Del., the average resident emitted 2.137 tons - for a rank of 27.

When the source of that carbon dioxide is divided into transportation vs. residential use, however, the statistics are quite different: The average Philadelphia metro area resident emitted 1.023 tons from highway transportation, for a rank of sixth nationally. Subdivided further, the study found, Philadelphia metro residents emitted an average of 0.789 tons of carbon dioxide from autos (fifth place nationally) and 0.234 tons from trucks (22d place).

From residential energy use, on the other hand, the average Philadelphia resident emitted 1.114 tons of carbon dioxide, ranking the metro area 66th. The researchers broke that down further as well, determining emissions from electricity averaged 0.619 tons per resident (42d place) while those from residential fuels were 0.495 tons (78th place).

Besides the carbon footprint "snapshots" for 2005, the researchers also calculated trends over the previous five years.

Between 2000 and 2005, for example, carbon emissions from transportation and residential energy use combined increased 6.81 percent in the Philadelphia area. That compared with average increases of 7.5 percent for the largest metro areas and 9.1 percent for the entire nation.

The sharpest five-year increase in the nation was in the Trenton-Ewing metropolitan area, which the report said went up 47.84 percent, nearly all of which was due to transportation. Details were not immediately available.

Metro Rankings

Metropolitan areas with the nation's five smallest (1-5) and five largest (96-100) carbon footprints per capita in 2005, plus regional rankings:

1. Honolulu

2. Los Angeles-Long Beach-Santa Ana, Calif.

3. Portland-Vancouver- Beaverton (Ore.-Wash.)

4. New York-North Jersey-Long Island

5. Boise-Nampa, Idaho

23. Lancaster

27. Philadelphia- Camden-Wilmington

39. Allentown- Bethlehem-Easton

63. Scranton-Wilkes-Barre

64. Trenton-Ewing

92. Harrisburg-Carlisle

96. Louisville, Ky.

97. Toledo, Ohio

98. Cincinnati- Middletown

99. Indianapolis

100. Lexington-Fayette, Ky.

SOURCE: Brookings Institution

The full report, plus detailed information for each metropolitan area: http://go.philly.com/earth EndText