The decision comes on the same day of a new opinion poll that found Americans who accept the reality of climate change are in a minority for the first time in the last two years.
The EPA's decision, timed to coincide with the opening of the Copenhagen climate summit, is the first significant step U.S. officials have taken toward imposing emissions limits, the Associated Press reported.
"Environmentalists hailed the EPA announcement as a clear indication the United States will take steps to attack climate change even if Congress fails to act," the AP continued. "And they welcomed the timing of the declaration, saying it will help the Obama administration convince delegates at the international climate talks that the U.S. is serious about addressing the problem. Obama will address the conference next week."