WASHINGTON/COPENHAGEN - Washington took a step on Monday toward curbing U.S. greenhouse gas emissions, aiding the first day of the biggest climate talks in history where 190 nations are seeking a deal to curb global warming.

The U.S. Environmental Protection Agency ruled that greenhouse gases endanger human health, allowing it to regulate planet-warming gases without legislation from the Senate, where a bill to cut U.S. emissions by 2020 is stalled.

The ruling was welcomed at the opening day of December 7-18 talks in Copenhagen, where a record 15,000 participants are trying to work out the first new U.N. pact in 12 years to combat rising seas, desertification, floods and cyclones.

This is very significant in the sense that if...the Senate fails to adopt legislation (on emissions), then the administration will have the authority to regulate, Yvo de Boer, head of the U.N. Climate Change Secretariat, told Reuters in Copenhagen.

The United States is the number two emitter behind China and is alone among industrialized nations outside the existing Kyoto Protocol that curbs emissions until 2012. Kyoto was meant as a small step to avert heatwaves, droughts, and rising sea levels.

This is great news and shows that the administration is committed to enforcing the Clean Air Act and addressing climate change, said Keya Chatterjee of the WWF environmental group.

DEAL WITHIN REACH

Earlier, the Copenhagen talks opened with a stark U.N. warning about risks of climate change and a prediction by Denmark's Prime Minister Lars Lokke Rasmussen that a deal to combat climate change is within our reach.

He said that 110 world leaders including U.S. President Barack Obama would attend a closing summit. The planned presence of so many leaders meant an opportunity the world cannot afford to miss, he said.

Many nations say that the United States is the key to a deal in Copenhagen, seeking to break deadlock between rich and poor nations about sharing out the burden of curbs on greenhouse gas emissions. Most emissions are from burning fossil fuels.

Obama is aiming to cut U.S. emissions by 3 percent below 1990 levels by 2020 with far deeper cuts in coming decades. Many developing nations want far more from Washington by 2020.

A delegate from a small, tropical island state said the EPA decision was very positive. But let's hope that Obama will come with something even better when he comes here next week. So far it's not enough.

Politicians and scientists urged Copenhagen to agree immediate action to curb emissions and come up with billions of dollars in aid and technology to help poorer countries limit their emissions.

Rajendra Pachauri, the head of the U.N.'s panel of climate scientists, said action was needed to avoid cyclones, heatwaves, floods, and possible loss of the Greenland ice sheet, which could mean a sea level rise of 7 meters over centuries.

The evidence is now overwhelming that the world would benefit greatly from early action, he said.

And he defended the findings by his panel after leaked emails from a British university last month led skeptics to say that researchers had conspired to exaggerate the evidence. He said there were rigorous checks on all research.

The European Union said it may sharpen its carbon-cutting bid if the United States paid for more carbon cuts in poor nations, especially to curb deforestation.

Developing nations including small island states, which are most vulnerable to rising sea levels, demanded more action.

So far we have not seen any real leadership from rich nations, said Ibrahim Mirghani Ibrahim of Sudan, speaking on behalf of the Group of 77 and China.

Dessima Williams, of Grenada, speaking on behalf of small island states at risk from rising seas, said the group will not accept a made-for-television solution...We are here to save ourselves from burning and from drowning.

Outside the conference center, delegates walked past a slowly melting ice sculpture of a mermaid, modeled on the Danish fairy tale of The Little Mermaid, as a call for action.

(Writing by Alister Doyle, Editing by Dominic Evans)

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