The United States, the world's top carbon dioxide emitter, in 2001 pulled out of the pact that requires developing countries to cut emissions by an average of 5 percent below 1990 levels by 2012. China, the world's second-largest emitter, was not required, as a developing country, to limit emissions in the first round of the international agreement.

Global carbon dioxide emissions will hit 42.88 billion tonnes in 2030, up from 26.9 billion tonnes in 2004, and 21.2 billion in 1990, the US Energy Information Administration said in its annual International Energy Outlook.

The forecast was down slightly from last year's prediction of 43.7 billion tonnes by 2030 on signs that concerns about global global warming have begun to change the world's fuel mix.

The trim in expected emissions did not represent the type of deep cuts of about 50 percent below 1990 levels in CO2 and other heat-trapping gases that scientists say will be necessary to cut risks of deadly storms, heat waves, droughts and floods that climate change could bring.

CHINA

The percentage of total CO2 emissions from plants that burn coal, the dirtiest fossil fuel, will rise from 39 percent in 2004 to 43 percent by 2030.

By 2010, CO2 output in rapidly growing China, which is rapidly building coal plants and highways, will edge out emissions from the United States, by 6.49 billion tonnes to 6.21 billion tonnes, the EIA said. That confirmed a report this spring from the Paris-based International Energy Agency that said China would overtake the United States as the world's biggest CO2 emitter either this year or next.

By 2030 Chinese emissions will be 11.2 billion tonnes annually, while US output will be 8.0 billion tonnes, the EIA said.

Chinese officials point to their country's relatively low per-capita emission of greenhouse gases, saying that historically, the main culprits of the emissions buildup in the atmosphere are developed nations, which have no right to deny economic growth to others.

In 2003, US individuals were far bigger emitters, at 20 tonnes per capita against China's 3.2 tonnes per capita and a world average of 3.7, according to the UN

Many environmentalists say China is working hard to cut emissions. "In terms of absolute emissions China may overtake us, but they are much larger in terms of population ... and have actually put in some real policies in place to reduce emissions," said Gary Cook, director of the US Climate Action Network, a coalition of nongovernmental organizations.

China's auto efficiency standards, for instance, are higher than those in the United States.

A mainstream energy source could get a boost as the debate on climate change goes on. Concerns about global warming, high fossil fuel prices, and domestic energy supplies could boost electricity generation from low-emissions nuclear power 7.7 percent annually in China and 9.1 percent a year in India from 2004 to 2030, the EIA forecast.