In a surprising turnaround, the amount of carbon dioxide being released into the atmosphere in the United States 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 did not 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.
A recent report by the US Energy Information Agency, a part of the Energy Department, said 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% of the total.
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.
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 US. The Energy Information Agency said that fell to 34% in mid 2012 – 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. 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.
The International Energy Agency said the US has cut carbon dioxide emissions more than any other country over the last six years. Total US 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% of the global total. The US accounted for approximately 16%.
Mann called it "ironic" that the shift from coal to gas has helped bring the US closer to meeting some of the greenhouse gas targets in the 1997 Kyoto Protocol on global warming, which the United States never ratified.
The US Environmental Protection Agency issued its first rules to limit CO2 emissions from power plants in 2011, but the standards will not take effect until 2014 and 2015.