It ain't easy being green. But surprisingly, big-city dwellers have less of an environmental impact than their country cousins. According to a new report by the Brookings Institution released Thursday, residents of the 100 biggest metropolitan areas emit on average 2.47 metric tons of carbon per person per year, 14 percent less than the 2.87 ton American average. The cities with the smallest carbon footprint per capita: Honolulu, Los Angeles and metropolitan Portland.

The worst offenders?  Ohio's Cincinnati-Middletown area, which came in at No. 98, followed by Indianapolis, as well as Lexington-Fayette, Ky., ranked 99 and 100, respectively. The reason: the reliance on coal. Carbon dioxide is released from fossil fuels and is the leading greenhouse gas. [For a full list, click here.]

The cities with the smallest and biggest carbon footprints were no surprise to researchers at  Brookings—dense cities with good public transportation systems in mild climates emit less carbon than more spread out, car-dependent cities in warmer climates. A case in point: Los Angeles. According to Mark Muro, the Brookings Institution policy director in charge of the report, "California is a state that has been at the forefront of energy and building reform," he says. "This is not your parents' Los Angeles. Lot sizes are shrinking and there's organized, dense sprawl as opposed to just sprawling sprawl."

The surprise to researchers was the huge difference in per capita emissions between the top-ranked and lowest-ranked cities. In Honolulu, ranked No. 1, the average person has a carbon footprint of 1.356 metric tons per year. In Lexington-Fayette (No. 100), the average is more than two and half times higher: 3.455 metric tons per person per year.

Why such a big difference? Temperature extremes and a reliance on coal-powered electrical plants were key factors, say researchers. Residents in cities like Madison, Wis. (No. 81), where summer highs can surpass 90 degrees and winter lows hit single digits, require more electricity to heat and cool their homes than residents in cities in mild climates such as those in San Francisco (No. 8). Metros like the greater Washington, D.C. (No. 89), area come in low on the list because of their "dirty," fossil-fuel-dependent energy sources whereas areas such as Seattle (No. 6) derive the majority of their energy from alternative sources such as hydropower. Other variations include the availability and use and mass-transit systems, sprawl and city planning and public policy.

Leaders in the Lexington-Fayette metropolitan area were not surprised to learn of their last-place ranking. "It paints a picture of the challenges we face living in a community where the predominant form of energy is coal," says Cheryl Taylor, the Lexington commissioner of environmental quality. "It's disappointing to be No. 100, but we also know we have work to do." The city has already started some of that work, including creating the department of environmental quality last July. In addition, the city has increased funding for its public-transportation system, signed the Mayors Climate Protection Agreement, developed restrictions on development sprawl and joined the International Council for Local Environmental Initiatives' Cities for Climate Protection organization.

Although this study looks at data from 2005, its contribution to sustainability research and knowledge is profound, says Kent Portney, a public-policy and sustainability expert at Tufts University and the author of "Taking Sustainable Cities Seriously: Economic Development, the Environment, and Quality of Life in American Cities." "Until now we haven't had a good measure of what the carbon footprints of the majority of U.S. cities are," he says. "And it's important because if we know where the carbon is being emitted, we have a better idea of where attention needs to be paid in order to reduce emissions."

What needs to be done, Portney says, takes place at the federal, state, city and individual level: governments need to focus on increasing public transportation and creating incentives for people to drive fuel-efficient vehicles—or no vehicle at all—as well as provide residents with options to purchase electricity from renewable sources. Individuals need to take steps toward a greener life with simple changes such as changing to low-energy light bulbs—which might be enough to move cities like Lexington, Indianapolis and Cincinnati up the rankings a few spots and earn a greener reputation.

© 2008