Both efforts, which would set up regional emissions markets, come amid an absence of federal regulation on heat-trapping gases after US President George W. Bush withdrew from the Kyoto Protocol early in his first term.

But experts said state efforts may not lead to lower emissions nationwide. They noted that California gets some of its power from plants in Nevada that run on the fuel that produces the most carbon dioxide -- coal.

"I don't think California can ban power from Nevada," said William Pizer, an economist and senior fellow at the Resources for the Future environmental think tank in Washington, D.C.

In addition to existing Nevada plants that export coal-fired power into California, seven more are being planned in the sparsely-populated state, according to the Department of Energy.

Because a ton of CO2 produced in Nevada enters the atmosphere no differently from a ton of CO2 from anywhere else, experts worry the national effect of California's plan could be overridden by the emissions of more coal plants elsewhere.

The Nevada plants are just some of the 120 coal plants being planned in the United States -- the most in decades -- all of which are cheaper to build than nuclear plants, which emit practically no emissions.

Nobody knows how many of the Nevada plants will end up getting built, but the battle lines are forming on how much coal-fired electricity California will import in the future.

"California will try to regulate the carbon intensity of the electricity -- regardless of the source," David Victor, the director of Stanford University's Program on Energy and Sustainable Development, said in an e-mail about the Nevada plants. He said it was "unclear" whether federal laws will allow California to restrict imports of coal power.

GATED COMMUNITIES

Other carbon-intensive industries California plans to regulate, such as concrete makers, could also be tempted to cross the border into states that don't regulate CO2.

"It's like gated communities," said Jagdish Baghwati, an economics professor at Columbia University, about patchwork state regulations. "They take care of their pollution security, but you and I can get slaughtered for all they care."

"Most states will simply welcome the opportunity to say, 'Look we're going to profit from this,'" he said about the possibility of dirty industries relocating to unregulated states.

Dale Bryk, an attorney for the Natural Resources Defense Council that has advised states on how to regulate emissions, said California is working on stopping dirty power imports from unregulated states -- or "leakage". "We're confident we're going to be able to do that," she said,

The seven states in the Northeast that formed the Regional Greenhouse Gas Initiative (RGGI) also have battled "leakage." The region gets most of its current electricity from natural gas and nuclear.

But Baghwati said stopping "leakage" could be hard. "Typically you get states taking advantage (of the fact that they are not regulated), rather than states falling in line."

RGGI also hopes more states will join. Maryland is set to join in coming years but Rhode Island and Massachusetts, the region's biggest CO2 producing state, dropped out of the program last year. Pennsylvania, a big coal state, was originally interested in joining, but has no plans to do so.

Many environmentalists see the states' carbon efforts as an important first step that could lead to a national CO2 regime. But economists say the patchwork approach runs the risk of entrenching industry and politicians in highly-populated states such as Texas, Florida, Pennsylvania and Illinois to fight harder against a national plan.