Carbon dioxide in US drops to 20-year low

In a surprising turnaround, the amount of carbon dioxide being released into the atmosphere in the U.S. has fallen to its lowest level in 20 years.

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.

Cautious optimism

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 technical report, the U.S. Energy Information Agency, a part of the Energy Department, said this month that energy-related U.S. carbon dioxide 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, the agency said.

Rapid shift

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 34percent in March, the lowest level since it started records nearly 40 years ago.

The International Energy Agency said the U.S. has cut carbon dioxide emissions more than any other country over the last 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 approximately 16percent.

Last year the Environmental Protection Agency issued its first rules to limit carbon dioxide 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.

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