U.S. emissions of carbon dioxide and other greenhouse gases decreased by 1.2 percent last year, the largest annual decline in more than a decade, according to a new government report.

Experts said the decline had nothing to do with government policy changes, but instead was the result of an economic slowdown and an unseasonably warm winter that sharply reduced demand for fossil fuels. Many scientists blame the burning of such fuels for the Earth's rising temperature.

"This is just one year's worth of data, and our projections don't indicate the beginning of a new trend," said John Cogan, a spokesman for the Energy Department's Energy Information Administration, which prepared the report. "This is perhaps an anomaly driven more by lower economic growth than anything else."

Greenhouse gas emissions totaled 1.883 billion metric tons of carbon equivalent last year, compared with a record 1.907 billion in 2000, the study showed.

The drop in emissions was mainly linked to the overall decline in the nation's economic growth to 0.3 percent in 2001, down from 3.8 percent in 2000; a 4.4 percent reduction in manufacturing output; and the warmer winter weather of 2001-02, which cut demand and combustion of coal, oil and gas.

Even so, overall emissions are 11.9 percent above the 1990 level, and the United States remains the single largest contributor to the emissions linked to global warming, which many experts say could result in long-term disruptions in weather patterns, the melting of the polar ice caps and flooding of populated coastal regions.

This year has been the second warmest in recorded history, according to NASA scientists who monitor global air temperatures. The earth's average temperature during 2002 was 58.35 degrees Fahrenheit, more than one degree warmer than the long-term average of 57.2 degrees, according to climate scientists at the NASA Goddard Institute for Space Studies.

Environmentalists and some scientists warn that dangerous signs of global warming are already evident. For example, an Antarctic ice shelf the size of Rhode Island shattered and collapsed into the sea in March, the permafrost in parts of Alaska has begun to thaw, and glaciers in the Bolivian Andes mountains are melting at an alarming pace.

The Bush administration has acknowledged that global warming poses serious problems, but senior officials speaking at a Commerce Department conference earlier this month said numerous uncertainties remain about global warming's cause and effects. They urged caution in committing the United States to long-term solutions that might hurt the economy.

Earlier this week, Canada became the 100th country to ratify the Kyoto Protocol, the 1997 agreement that would require the world's nations to reduce emissions of carbon dioxide and other heat-trapping gases by an average of 5.7 percent below 1990 levels by 2012.

Although the United States signed the agreement in November 1998, President Bush disavowed it last year, arguing that it would seriously harm the U.S. economy while exempting large, developing countries including China and India. Some economists say the United States would have to curtail its energy use by as much as 30 percent to achieve the goals spelled out in the Kyoto treaty.

Bush instead has called for more research and a series of incentive programs for industry to encourage voluntarily reductions in their emissions.

"While people can and are using energy more efficiently, a growing economy with a growing population will require more energy, not less -- which means more greenhouse gas emissions," said William O'Keefe, a former petroleum industry executive who is now president of the George C. Marshall Institute, a science policy think tank. "Right now, there is no viable alternative to coal and oil and gas, which provide close to 90 percent of our energy."

But Jennifer L. Morgan, a climate change expert with the World Wildlife Fund, said that "If the administration really rolls up its sleeves and puts together a strong plan, we could go a long ways towards tackling global warming."