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The U.S. Environmental Protection Agency has declared greenhouse gases endanger human health, allowing the agency to regulate six gases that contribute to global warming, including the main one, carbon dioxide.

The ruling came as the biggest and most important climate summit opened in Copenhagen.

Addressing delegates from nearly 200 countries, the head of the UN's panel of climate scientists, Rajendra Pachuari, said action was needed to avoid cyclones, heatwaves, flood,s and possible loss of the Greenland ice sheet.

"The evidence is now overwhelming that the world would benefit greatly from early action and that delay would only lead to costs in economic and human terms that would become progressively high," Pachauri said.

Denmark's Prime Minister Lars Lokke Rasmussen opened the talks -- which run till December 18 -- by saying a deal to combat climate change is "within our reach."

China, Brazil, South Africa, and India -- the four major emerging economies responsible for 30 percent of global carbon emissions -- want a global climate treaty wrapped up by June 2010, according to a joint draft document, prepared for the Copenhagen talks and obtained by Reuters.

compiled from agency reports