U.S. emissions of the gases blamed for global warming fell 1.5 percent in 2006 on mild weather and increased use of natural gas to generate power and alternative energy, the government estimated on Wednesday.
The Energy Information Administration, the analytical arm of the Department of Energy, estimated that 2006 emissions fell to 7,075.6 million tonnes of carbon dioxide equivalent.
It was the first annual fall in U.S. emissions since 2001, when travel slowed after the airplane attacks in New York and Washington, and the third since 1990.
Output of carbon dioxide, the main greenhouse gas, was 5,934.4 million tonnes compared with 6,045 million tonnes in 2005.
"Some of the factors that led to the decrease (like weather) are variable; others (such as increased use of renewable energy for electricity generation) may indicate trends that are likely to continue," the report said.
Since 1990 U.S. greenhouse gas emissions have risen 15.1 percent, compared with 16.8 percent in 2005, the report said.
The annual report was released ahead of a meeting of delegates from 190 countries in Bali, Indonesia, next month to decide how to bind outsiders led by the United States and China into a U.N. led fight against climate change.