As greenhouse gases increasingly warm the globe, which of America's metro areas are the "cleanest" and which are the "dirtiest" in carbon emissions? And what are the most obvious steps that could be taken to protect the planet's future?

A first-ever study of the climate footprint of America's top 100 metro regions starts to tell the story. Based on 2005 figures calculated by the Brookings Institution, each region's carbon emissions caused by cars and trucks, plus power supplied to residences, is reported -- not a complete score (industries and office buildings are omitted), but close enough for a clear picture.

The "winners" -- the most modest users, per capita -- turn out to be such regions as New York-Northern New Jersey, Portland, Ore., Seattle-Tacoma, San Francisco, Honolulu, San Diego, and a surprise performer -- Los Angeles.

The biggest carbon emitters, by contrast, include such metro areas as Lexington, Ky., Indianapolis, Knoxville, Oklahoma City, Nashville and St. Louis.

So what explains the differences?

The best performers provide a clue: high-density, compact development with new and expanded rail transit. Many of the regions with the smallest per capita carbon footprints -- among them New York, San Francisco, San Diego and Los Angeles -- fit that profile.

By contrast, some of the metros with high per capita carbon emission scores have experienced dramatic sprawling and pedestrian-hostile development, and are weaker on mass transit.

There are some exceptions: The Washington, D.C., and Atlanta regions, for example, have significant rail transit ridership, but they've also sprawled so much that they have larger-than-average carbon footprints.

And the source of power makes a real difference. The nation's capital region has a carbon footprint 10 times the Seattle region's chiefly because it is heavily dependent on coal for power, while the Pacific Northwest has major hydropower sources that don't emit carbon.

Plus there's a surprise geographic factor too: The heavy carbon footprint metros are overwhelmingly east of the Mississippi, the light carbon ones in the West. And there's a north-south divide too: the map shows a concentration of high emitters in America's heavily coal-consuming, fast-suburbanizing Southeast.

The implications are compelling: State officials, mayors and county leaders should push for protection of open lands, new transit lines that attract more compact development, and rules and incentives to get utilities to switch away from coal (the most polluting, carbon-heavy energy source of all).

But the federal government needs to play a far more constructive role -- "Metros can't 'go it alone' in solving as vast a problem as global climate change," says Mark Muro, policy director of Brookings' Metropolitan Policy Program.

And arguably, how the metros go on climate emissions, so goes America: The top 100 account for two-thirds of the country's population and almost three-quarters of its economic activity. And their carbon output, despite all their mayors' noble talk of reducing our greenhouse gas emissions, rose 7.5 percent from 2000 to 2005.

The federal government is a poor ally now, Brookings charges. It fails to tax carbon fuels enough to discourage their polluting impacts and reduce the country's massive dependence on foreign oil. While countries around the world expand their clean energy research budgets, Washington is spending just a third as much on energy research as it did in 1978. Federal transportation funding is tilted heavily toward highways, away from transit; indeed, its formulas reward states for the worst behavior -- high vehicle miles traveled, fuel use, and lane miles of travel.

Solutions offered include a targeted carbon tax or full "cap and trade" system, so that polluting energy consumption pays its full costs; dramatic increase in federal research on potentials such as wind and solar power; a minimum power share of renewable sources that states must achieve (so that some, for example, can't leave carbon-heavy coal riding high even while competitor states invest forward in more expensive renewables); and "modal neutrality" -- an even playing field between highways and rail in federal transportation funding to states and localities.

The tax code could be adjusted to give smaller houses and compact development a better break -- and some ingenious shifts in regulations. Homebuyers, for example, now benefit from the federal Real Estate Settlement Procedures Act that requires sellers to reveal hazards, impediments, detailed lending terms and the like. But why not, says Brookings, also require clear, nationally standardized energy information -- factors such as the efficiency of water heaters or furnaces or lighting that can make a big difference in a buyer's real costs?