PITTSBURGH – The amount of carbon dioxide being released into the atmosphere in the US 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 US Energy Information Agency, a part of the Energy Department, said this month that energy related US 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 US 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.

A frenzy of shale gas drilling in the Northeast’s Marcellus Shale and in Texas, Arkansas and Louisiana has caused the wholesale price of natural gas to plummet from US$7 or US$8 per unit to about US$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 per cent 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 CO2 levels globally are rising, not falling. Moreover, changes in the marketplace — a boom in the economy, a fall in coal prices, a rise in natural gas — could stall or even reverse the shift. For example, US emissions fell in 2008 and 2009, then rose in 2010 before falling again last year.

Also, while natural gas burns cleaner than coal, it still emits some CO2. And drilling has its own environmental consequences, which are not yet fully understood.

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