U.S. emissions of carbon dioxide blamed for climate change fell in 2011 and have slipped to a 20-year low this year as the world's largest economy uses more natural gas and less coal, data shows.

The surprise drop from the world's second biggest emitter comes despite the lack of legislation on climate change but it was unclear if the change marked a trend or would be enough to meet goals on fighting global warming.

Official data showed that energy-related U.S. carbon emissions fell by 2.4% in 2011 from the previous year.

The decline did not fully track the broader economy as the United States posted growth in 2011, whereas the last time emissions declined was in 2009 during a contraction in the economy.

In the first three months of 2012, U.S. carbon emissions from energy use were down by almost 8% from the same period last year, marking the lowest level for the quarter since 1992, the Energy Information Administration said.

The government agency said that the drop in emissions in the first quarter was due in part to the warmer winter, which decreased the need for heating during the season that traditionally sees the most energy demand.

The quarter saw the lowest level since 1983 in carbon emissions generated by coal, which is the dirtiest major source of energy. In 2011, coal accounted for 43% of U.S. power generation, down from 51% in 2005.

"The decline in coal-related emissions is due mainly to utilities using less coal for electricity generation as they burned more low-priced natural gas," said the report issued this month.