Experts credit the drop to power plant operators switching from coal to natural gas.
A crew works on a drilling rig at a well site for shale based natural gas in Zelienople, Pa. 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.
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.
Low-priced natural gas
In a little-noticed report, the U.S. Energy Information Agency, a part of the Energy Department, said this month that total U.S. CO2 emissions for the first four months of this year fell to about 1992 levels. While conservation efforts, the lagging economy and greater use of renewable energy are factors in the decline, the drop-off is due mainly to low-priced natural gas, it 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 $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 United States. 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 in other countries, particularly China, and CO2 levels globally are rising. 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.
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.
'It's been a real surprise'
The International Energy Agency said the United States 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; it was about 5 billion in 1990. It accounted for about 16 percent of the global total.
China's emissions were estimated to be about 9 billion tons in 2011, accounting for about 29 percent of the global total.
"The trend is good," said Janice Nolen, an American Lung Association spokeswoman. "It's been a real surprise to see this."
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, said the Government Accountability Office, the investigative arm of Congress.
Jason Hayes, a spokesman for the American Coal Council, predicted cheap gas won't last. He said, "Coal is going to be here for a long time. ... Even if we decide not to use it, everybody else wants it."