U.S. President Barack Obama's administration promised to push for a new United Nations climate treaty on Sunday but said Washington had no magic wand and that all countries had to help.

The United States is going to be powerfully and fully engaged, U.S. special envoy for climate change Todd Stern said at the opening of 175-nation U.N. talks in Bonn, the first since Obama took office in January speaking of a planet in peril.

But we are all going to have to do this together, we don't have a magic wand, Stern told a news conference. The March 29-April 8 meeting is working on a U.N. climate deal meant to be agreed in Copenhagen in December 2009.

In a speech, Stern won two rounds of applause, each about 20 seconds long, in stark contrast to the frosty reception given to President George W. Bush's envoys who were often accused of inaction and were even booed at U.N. talks in Bali in 2007.

Even so, Stern laid out clear limits to Obama's ambitions. He said the United States wanted to work for a treaty that was economically doable and that countries could not expect Washington to ride in on a white horse to solve the problem.

We can't, he said.

Calling for more action by all, he said the United States had a unique responsibility as the main historic emitter of greenhouse gases. And he said he was enormously impressed by actions by developing countries such as India, South Africa, Brazil, China and Mexico.

RECESSION

Some nations, racked by recession, have been waiting to hear more about U.S. policies before unveiling their own. The Bonn talks are due to consider issues including the levels of greenhouse gas cuts needed to slow global warming.

Obama wants to cut U.S. greenhouse gas emissions by about 16-17 percent from current levels to take them back to 1990 levels by 2020 and to 80 percent below by 2050.

Under Bush, the United States was isolated among industrialized nations in opposing caps on emissions under the U.N.'s existing Kyoto Protocol.

Everyone is very excited by signs of a stronger U.S. commitment, said Yvo de Boer, head of the U.N. Climate Change Secretariat. Environmentalists also welcomed the change of tone after eight years of the Bush administration.

But Stern said the United States could not make the deepest cuts in greenhouse gases advised by the U.N. Climate Panel for 2020 to avoid the worst of global warming, of between 25 and 40 percent below 1990 levels.

We should be guided by a combination of science and pragmatism, Stern said. Many developing nations, led by China and India, have said that Obama should do more.

Almost no developed nations have laid out goals within the 25-40 percent range. Among the most ambitious, the European Union plans cuts of 20 percent below 1990 levels by 2020.

The U.N. Climate Panel projects more floods, droughts, more powerful storms, heatwaves and rising sea levels from a gradual build-up of heat-trapping gases from burning fossil fuels.

Some Pacific island states fear they could be wiped off the map by rising seas. We welcome the new-look United States. We hope the rhetoric matches the reality, Ian Fry, of the Pacific island state of Tuvalu, told the meeting.

De Boer has in the past called Obama's 2020 goals an opening offer that he hopes will be toughened in Copenhagen.

Stern said that it was unclear whether Congress will manage to pass climate legislation before the Copenhagen talks. If a law is passed by then, he said, it would be unrealistic for Washington to sign up to any tougher cuts.

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