In the state's biggest, immediate step toward cutting global warming pollution, Gov. Arnold Schwarzenegger this week began shifting California's 26 million cars and trucks off petroleum-based fuels and toward alternatives that emit less greenhouse gas.

The governor said his new greenhouse-gas standard for transportation fuels -- the world's first -- "leads us away from fossil fuel" and would help "in moving the entire country beyond debate, denial and inaction" on global warming.

State air-pollution regulators expect to work out the details during the next 18 months. The policy would limit the amount of greenhouse gases released for each bit of energy in fuel, then gradually shave those emissions down 10 percent by 2020.

Fuel producers could choose how to meet the new limits, at first probably by blending in more home-grown fuels such as ethanol, biodiesel or biobutanol, or purchasing credits from producers of lower-carbon fuels, such as electricity for plug-in electric-gas hybrids. In time, they could team with automakers to bolster the supply of hydrogen vehicles, electric cars and the like.

Environmentalists called the new plan revolutionary, and energy experts said it would be watched closely by policymakers from Washington to London and Berlin, where similar policies are under consideration. Greenhouse gases from cars, trucks and aircraft are a powerful contributor to global warming, but each mobile source is small and notoriously difficult to regulate. The new policy reaches past those difficulties to take effect at the refinery, the fueling station and perhaps the electrical outlet in the household garage.

Given California's appetite for oil, the policy stands to affect 10 percent of the nation's transportation fuel and ease the leading source of greenhouse gases in the world's eighth-largest economy. Those reductions would start at least two years before California launches mandatory cuts in stationary sources of greenhouse gases, such as refineries, cement plants and electrical power plants.

"This is a big deal. This is the world's first greenhouse-gas standard for transportation fuels," said Eric Heitz, president of the Energy Foundation, a San Francisco-based nonprofit group that monitors energy policy. "This policy will be noticed worldwide."

Several experts predicted a flush of capital for California-based green energy technologies and, for consumers, a dazzling menu of automotive choices from plug-in hybrids to cars running on hydrogen, natural gas and plant-derived biofuels -- to as many as 7 million more such vehicles in 2020, or 20 times more than are on the road now. "The policy will motivate new investment in California in alternatives such as biofuel, electric vehicles and hydrogen fuel and related products including electric motors and batteries," said Daniel Sperling, director of the Institute of Transportation Studies at UC Davis.

"It will stimulate the California economy and reduce oil imports," said Sperling, who is analyzing the policy's impact.