In a historic declaration, the Environmental Protection Agency said Monday that enough scientific evidence exists to classify greenhouse gasses as harmful to humans.

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."

However, a a CNN/Opinion Research Corp. poll released Monday did not give climate activists good news.

Only 45 percent of the 1,041 adults surveyed on December 2-3 believed global warming is a proven fact and mostly caused by human activity, down from 56 percent in October 2007, the survey found.

By comparison, Americans who believe global warming is caused by natural changes unrelated to man have increased to 22 percent from 20 percent two years ago, and those who believe global warming is a yet unproven theory grew to 31 percent from 23 percent.

Regarding how the United States should tackle global warming, 58 percent of those surveyed said it should cut carbon dioxide emissions unilaterally, down from 66 percent in October 2007.

Those who thought the United States should cut CO2 emissions only if other countries do so as well remained basically unchanged at 17 percent, from 16 percent two years ago.

But those who said carbon emissions should not be cut regardless of what the rest of the world does jumped to 24 percent, from 15 percent two years ago.

EPA Administrator Lisa Jackson said that the agency's revised position on greenhouse gasses makes the U.S. "obligated to make reasonable efforts to reduce greenhouse pollutants under the Clean Air Act," according to the AP.

The White House on Monday also dismissed as "silly" the notion that global warming science had been compromised by emails exposing a row between top climate scientists in the United Kingdom.

With AFP.