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PITTSBURGH – In a surprising turnaround, the amount of carbon dioxide being released into the atmosphere in the U.S. has fallen dramatically to its lowest level in 20 years, and government officials say the biggest reason is that cheap and plentiful natural gas has led many power plant operators to switch from dirtier-burning coal.

Many of the world’s leading climate scientists didn’t see the drop coming, in large part because it happened as a result of market forces rather than direct government action against carbon dioxide, a greenhouse gas that traps heat in the atmosphere.

Michael Mann, director of the Earth System Science Center at Penn State University, said the shift away from coal is reason for “cautious optimism” about potential ways to deal with climate change. He said it demonstrates that “ultimately people follow their wallets” on global warming.

“There’s a very clear lesson here. What it shows is that if you make a cleaner energy source cheaper, you will displace dirtier sources,” said Roger Pielke Jr., a climate expert at the University of Colorado.

In a little-noticed technical report, the U.S. Energy Information Agency, a part of the Energy Department, said this month that energy related U.S. CO2 emissions for the first four months of this year fell to about 1992 levels. Energy emissions make up about 98 percent of the total. The Associated Press contacted environmental experts, scientists and utility companies and learned that virtually everyone believes the shift could have major long-term implications for U.S. energy policy.

While conservation efforts, the lagging economy and greater use of renewable energy are factors in the CO2 decline, the drop-off is due mainly to low-priced natural gas, the agency said.