DENVER - Environmental groups warned this week that plans for more coal-fired power plants in the Southwest will worsen the threat of global warming and undermine the states' conservation efforts.

A report by Environmental Defense and Colorado-based Western Resource Advocates says the greenhouse gas emissions from the roughly dozen coal-fired plants planned or recently built will be the equivalent of 12.5 million cars driving around the Southwest for a year.

Spokesmen for two companies that have built or will build plants counter that there's a huge demand for energy in the fast-growing Southwest that renewable energy and increased efficiency can't meet alone.

The report comes as states throughout the West have adopted or are considering laws requiring utilities to get more of their power from renewable energy sources. Colorado and other states are also debating first-ever caps on mercury emissions from coal-burning power plants.

The new Democratic-controlled Congress is holding hearings on human-caused global warming, which most scientists agree results from the burning of fossil fuels.

"This is a call to leaders across the American Southwest to continue carrying the ball down the field and build on momentum already started to ensure that we have a future for our children that is safe and secure and not saddling them with the very serious threat of global warming," said Vickie Patton, a lawyer with the Boulder office of Environmental Defense.

The report singles out about a dozen coal-fired power plants already built or being considered in Colorado, Arizona, New Mexico, Nevada and Utah.

The report says all, except one proposed by Xcel Energy in Colorado, would be conventional operations, in which coal is pulverized and burned in a boiler to produce electricity. Emissions are caught and filtered at the back end of the process.

Xcel Energy is exploring developing a "clean-coal" plant based on technology that converts coal to a gas, which is burned in a turbine to produce electricity. Pollutants are removed before the fuel is burned.

In Arizona, lawmakers are looking at incentives for new energy-efficient homes and tougher conservation standards for public buildings. Legislation in New Mexico would create a renewable energy production tax credit. A bill in the Colorado Legislature would double the amount of power utilities must get from renewable energy sources from 10 percent by 2015 to 20 percent by 2020.

Increasing energy efficiency can eliminate the need to build some or all of the power plants, environmentalists said. Mark Ruzzin of the Boulder-based Southwest Energy Efficiency Project said a 2002 study by his group showed that cutting electricity use by 2.6 percent to less than 1 percent a year through 2020 would save the equivalent of the energy produced by 34, 500-megawatt power plants.

One megawatt generates enough energy for about 750 homes a year.

Spokesmen for two companies, however, disputed the report's findings, saying wind and solar power and conservation alone won't bridge the gap between demand and supply.

"There's a huge need for energy in the Four Corners region," said Frank Maisano, spokesman for Houston-based Sithe Global Power, which wants to build coal-fired power plants in New Mexico and Nevada.

Maisano said the 1,500-megwatt Desert Rock facility planned on the Navajo Indian Reservation in northwestern New Mexico will be 15 percent to 20 percent more efficient than other coal plants and will use air rather than water in the cooling towers.

Of the power Colorado-based Tri-State Generation and Transmission sells, 14 percent is hydroelectric, spokesman Jim Van Someren said. He said Tri-State is also looking at tying in wind farms on the eastern Colorado plains to a transmission line proposed to link coal plants in western Kansas to Colorado.

But Tri-State sees coal as the most reliable, economic energy source to meet its rising demands, Van Someren said.

"We've grown at 4 percent a year. That's 100 megawatts," Van Someren said. "We're in a deficient situation right now."

Tri-State, which supplies wholesale power to 44 electric cooperatives in Colorado, Nebraska, Wyoming and New Mexico, was the subject of another report by Western Resource Advocates last year. The environmental group contended the company is overestimating the power it needs.