The study, "Clearing California's Coal Shadow from the American West", calls on California to insist that coal power plants that supply it with electricity burn cleaner. It also urges California to carry through with Gov. Arnold Schwarzenegger's ambitious plans to reduce greenhouse gases by 2050 to 80 percent of 1990 levels.

The study says existing coal power plants that feed California electricity each year release 67 million tons of carbon dioxide, as much as 11 million automobiles. And they release 200 times the mercury than all power plants in California.

California may soon require that any new coal-fired power plant from outside the state meet strict clean-burning requirements if its owners want to sell electricity into California.

"Right now, we are at a confluence," said Vicki Patton, attorney for the Denver-based Environmental Defense.

"Two great legacies of California are colliding - the long history of California having led the nation to clear-air solutions ... with this long history of California's reliance on high-polluting coal plants in distant Western states."

More than 30 new coal-fired plants are on the drawing board, with a sharp eye to serving California, said White.

The power generated by the new plants would be between 14,000 megawatts and 20,000 megawatts, said John Barth of the umbrella environmental group Western Clean Energy Campaign.

But if they are built as currently designed, Barth said, emissions from the plants "would cancel out all global warming reductions that have been made in California".

But states such as Wyoming that want to build the coal plants have options other than selling power into California, said Steve Waddington, executive director of the Wyoming Infrastructure Authority, which was established by the Wyoming legislature last year to push for development of Wyoming's energy resources.

"Markets other than California are just as hungry and growing," Waddington said.

He points to an effort announced several weeks ago by Arizona Public Service, the biggest utility in that state, to build transmission lines from Wyoming to Arizona capable of running up to 3,000 megawatts of electricity.

And the stricter requirements won't work, Waddington said, because coal from many areas, including Wyoming, can't be used in plants that meet the same standards as combined-cycle natural gas plants.

White said that California's economic power can hold sway over power plants built in other states.

"California can send the signal to the marketplace that it will only buy power from new coal plants with the very best technology," said White.

The marketplace may speak otherwise, said Waddington, when Californians find that their electricity bills keep rising without the benefit of relatively low-cost power generated from coal power plants.

A proposed policy that may be adopted by the California Public Utilities Commission (CPUC) would require that investor-owned utilities such as Southern California Edison and Pacific Gas & Electric only enter into long-term contracts for new coal-fired power plants if they are as efficient as modern natural gas-fired power plants.

Municipal utilities such as the Los Angeles Department of Water and Power do not come under the CPUC's jurisdiction. But Eric Tharp, spokesman for the LADWP, said the utility seeks to increase the amount of power it uses from renewable sources.

While 21 percent of California's electricity is met by coal-fired power plants, the LADWP imports between 40 percent and 50 percent of its power needs from coal-fired plants.