Power plants that burn coal produce more than 90 times as much sulfur dioxide, five times as much nitrogen oxide and twice as much carbon dioxide as those that run on natural gas, according to the Government Accountability Office, the investigative arm of Congress. Sulfur dioxide causes acid rain and nitrogen oxides lead to smog.
Bentek, an energy consulting firm in Colorado, said that sulfur dioxide emissions at larger power plants in 28 Eastern, Midwestern and Southern states fell 34 percent during the past two years, and nitrous oxide fell 16 percent. Natural gas has helped the power industry meet federal air pollution standards earlier than anticipated, Bentek said.
Last year the Environmental Protection Agency issued its first rules to limit CO2 emissions from power plants, but the standards don't take effect until 2014 and 2015. Experts had predicted that the rules might reduce emissions over the long term, but they didn't expect so many utilities to shift to gas so early. And they think price was the reason.
"A lot of our units are running much more gas than they ever have in the past," said Melissa McHenry, a spokeswoman for Ohio-based American Electric Power Co. "It really is a reflection of what's happened with shale gas."
"In the near term, all that you're going to build is a natural gas plant," she said. Still, she warned: "Natural gas has been very volatile historically. Whether shale gas has really changed that - the jury is still out. I don't think we know yet."
Jason Hayes, a spokesman for the American Coal Council, based in Washington, predicted cheap gas won't last.
"Coal is going to be here for a long time. Our export markets are growing. Demand is going up around the world. Even if we decide not to use it, everybody else wants it," he said. Hayes also said the industry expects new coal-fired power plants will be built as pollution-control technology advances: "The industry will meet the challenge" of the EPA regulations.
The boom in gas production has come about largely because of hydraulic fracturing, or fracking. Large volumes of water, plus sand and chemicals, are injected to break shale rock apart and free the gas.
Environmentalists say that the fluids can pollute underground drinking water supplies and that methane leaks from drilling cause serious air pollution and also contribute to global warming. The industry and many government officials say the practice is safe when done properly. But there have been cases in which faulty wells did pollute water, and there is little reliable data about the scale of methane leakage.
"The Sierra Club has serious doubts about the net benefits of natural gas," said Deborah Nardone, director of the group's Beyond Natural Gas campaign.
"Without sufficient oversight and protections, we have no way of knowing how much dangerous pollution is being released into Americans' air and water by the gas industry. For those reason, our ultimate goal is to replace coal with clean energy and energy efficiency and as little natural gas as possible."
Wind supplied less than 3 percent of the nation's electricity in 2011 according to EIA data, and solar power was far less. Estimates for this year suggest that coal will account for about 37 percent of the nation's electricity, natural gas 30 percent, and nuclear about 19 percent.
Some worry that cheap gas could hurt renewable energy efforts.
"Installation of new renewable energy facilities has now all but dried up, unable to compete on a grid now flooded with a low-cost, high-energy fuel," two experts from Colorado's Renewable and Sustainable Energy Institute said in an essay posted this week on Environment360, a Yale University website.
How much further the shift from coal to natural gas can go is unclear. Bentek says that power companies plan to retire 175 coal-fired plants over the next five years. That could bring coal's CO2 emissions down to 1980 levels. However, the EIA predicts prices of natural gas will start to rise a bit next year, and then more about eight years from now.
Despite unanswered questions about the environmental effects of drilling, the gas boom "is actually one of a number of reasons for cautious optimism," Mann said. "There's a lot of doom and gloom out there. It is important to point out that there is still time" to address global warning.
Associated Press writers Seth Borenstein in Washington and Jonathan Fahey in New York contributed to this story.