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 total U.S. carbon-dioxide emissions for the first four months of this year fell to about 1992 levels. The Associated Press contacted environmental experts, scientists and utility companies and learned that virtually everyone thinks 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 carbon-dioxide decline, the drop-off is mainly a result of low-priced natural gas, the agency said.

A frenzy of shale-gas drilling in the Northeast’s Marcellus Shale deposit and in Texas, Arkansas and Louisiana has caused the wholesale price of natural gas to plummet from $7 or $8 per unit to about $3 over the past four years, making it cheaper to burn than coal for a given amount of energy produced. As a result, utilities are relying more than ever on gas-fired generating plants.

Both government and industry experts said the biggest surprise is how quickly the electric industry turned away from coal. In 2005, coal was used to produce about half of all the electricity generated in the U.S. The Energy Information Agency said that fell to 34 percent in March, the lowest level since it began keeping records nearly 40 years ago.

The question is whether the shift is just one bright spot in a big, gloomy picture, or a potentially larger trend.

Coal and energy use are still growing rapidly in other countries, particularly China, and carbon-dioxide levels globally are rising, not falling.

“Natural gas is not a long-term solution to the CO2 problem,” Mr. Pielke warned.

The International Energy Agency said the U.S. has cut carbon dioxide emissions more than any other country over the past six years. Total U.S. carbon emissions from energy consumption peaked at about 6 billion metric tons in 2007. Projections for this year are around 5.2 billion, and the 1990 figure was about 5 billion. China’s emissions were estimated to be about 9 billion tons in 2011, accounting for about 29 percent of the global total. The U.S. accounted for about 16 percent.

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