President Barack Obama's chief climate-change negotiator said Sunday that the U.S. would be "powerfully, fervently engaged" in global talks to reduce carbon emissions but warned of a difficult path ahead.

Todd Stern, a Washington lawyer and former Clinton White House official, said at a UN conference in Bonn, Germany, that despite high expectations, Obama did not have a magic solution for fashioning a global climate-change treaty by year's end.

"We all have to do this together. We don't have a magic wand," he told reporters on the sidelines of the conference. "I don't think anybody should be thinking that the U.S. can ride in on a white horse and make it all work."

Stern did not offer specific policy proposals and acknowledged that Obama's negotiating team would be constricted by the domestic political challenge of winning approval from Congress. But his speech was greeted with sustained applause as many UN delegates and environmental groups celebrated the exit of the Bush administration, which had resisted proposals for binding reductions on carbon emissions.

"This is a new start for the U.S. delegation and the start of a new hope to solve the problem of climate change," Matthias Machnig, Germany's deputy minister of the environment, said in a speech at the conference.

UN delegates are trying to negotiate a global accord on the reduction of greenhouse gases in time for a December summit in Copenhagen. The accord would replace the 1997 Kyoto Protocol, which called for many industrial nations to cut gas emissions but was rejected by the United States and a handful of other countries.

Many UN delegates want major cuts in greenhouse-gas production—25 percent to 40 percent below 1990 levels—by 2020.

Those cuts would be deeper than Obama's recently announced goal of reducing U.S. carbon dioxide emissions during that time frame by 16 percent from present levels. But Stern said more ambitious targets might not be politically or economically feasible.

"Let me speak frankly here: It is in no one's interest to repeat the experience of Kyoto by delivering an agreement that won't gain sufficient support at home," Stern told the delegates.

Stern also said that any global treaty would require deeper concessions from rising economic powers such as Brazil, India and China, the world's leading greenhouse-gas producer.