WASHINGTON — Honolulu leads the nation's 100 largest metro areas in emitting the least amount of carbon dioxide per capita, according to a report from the Brookings Institution, a liberal think tank.

The study, released today, found that sprawling, car-dependent metropolitan areas with low-cost electricity emit more carbon per capita than densely populated cities where electricity is more expensive.

Honolulu emitted about 1.4 metric tons of carbon per capita. The city with the biggest carbon footprint was Lexington, Ky., at 3.4 metric tons per person. The average for the 100 cities was about 2.3.

The study compares carbon emissions from vehicle use and residential buildings. It did not include emissions from power plants, industrial, commercial sources and other forms of travel such as air, which would likely have boosted the Honolulu footprint.

Jeff Mikulina, director of the Hawai'i chapter of the Sierra Club, challenged the study, saying the numbers showed a "huge difference" from the organization's statewide studies on carbon emissions.

"Our calculations are closer to 15 to 20 tons per person," he said.

Mikulina said a missing piece from the study could be carbon emissions from aircraft, which play a huge role in the state with its tourism-dependent economy and heavy air importation of food and goods.

"As we look at the carbon footprint for Hawai'i, about a third is electricity use, about a third is ground transportation and about a third is air travel, which includes flying goods and food to Hawai'i," he said.

Laurence Lau, executive director for environmental health at the state Health Department, said other contributing factors to Honolulu's good ranking could include the relatively short commutes island residents make to work since O'ahu is a fairly small island.

He also said the state's climate generally cuts down the need for both heating and air-conditioning.

"I'm happy for the first position, but I do have a concern that it's an incomplete picture. ... I think there is more to look at," Lau said.

GETTING WARMER

Global emissions of carbon dioxide, the most abundant greenhouse gas, are contributing to warming the planet and could lead to serious economic and environmental consequences, according to leading atmospheric scientists.

Knowing more precisely what sectors of the U.S. economy contribute the most to the nation's carbon footprint can help lawmakers craft effective policies to lower emissions, according to the study co-authors.

"We hope some of this will take hold in the federal policy debate in the near future," said Marilyn Brown, a public policy professor at Georgia Institute of Technology who co-authored the study with her husband, Frank Southworth, a research scientist at Georgia Tech.

The researchers found substantial differences among metro areas.

Factors contributing to higher carbon footprints included: