Expectations of what can be achieved by the United States in fighting global warming are unrealistic, climate change negotiators from more than 170 countries have been told.

Hopes raised by a new willingness in the White House to take action to control climate change must be balanced by a realisation that there are limits to what the US can do, they were told.

Todd Stern, President Obama’s special envoy on climate change, moved to play down hopes as the US joined UN talks on global warming in Bonn. These are designed to smooth the path to a summit in Copenhagen in December when it is hoped that international agreement on cutting greenhouse gas emissions can be reached.

“The US is going to be powerfully and fervently engaged in this process,” Mr Stern said shortly before the talks started. “That doesn’t mean that anyone should be thinking that the US can ride in on a white horse and make it work, because it can’t. What we can do is return to the table with energy and commitment, and commitment to science and pragmatism to getting a deal that will be doable. We are all going to have to do this together. We don’t have a magic wand.”

Under President Bush, the US was reluctant to join international efforts to curb greenhouse gas emissions, but attitudes in the White House have altered dramatically since the inauguration of Mr Obama.

The Bonn talks are the first session of the UN Framework Convention on Climate Change since Mr Obama took office, and expectations of US involvement have been high.

Such was the relief at the willingness of the US to act on climate change that its delegation was welcomed with applause by negotiators from other countries.

The clapping became even more enthusiastic when Mr Stern said: “We are glad to be back, we want to make up for lost time, and we are seized with the urgency of the task before us. The science is clear and the threat is real.

The facts are outstripping the worst-case scenarios. The costs of inaction or inadequate actions are unacceptable.”

However, hopes that the US would use the opening of the talks to announce measures to cut its own emissions were dashed. Michael Zammit Cutajar, one of the chief UN negotiators, said that in talks before the session it was intimated that Mr Obama’s Administration needed more time. “My understanding is they need some more time to get up to speed,” he said.

The talks in Bonn continue until April 8 and a second session will be held in June, by which time, he said, it should be clearer how much farther the US will go.

Despite the widespread welcome for the US involvement, there remain concerns that it will refuse to make deep enough cuts in emissions.

President Obama has already promised to reduce emissions to 1990 levels by 2020 and by 80 per cent by 2050. While most delegates and scientists agree that the long-term target of 80 per cent matches requirements, they believe that the US needs to do better on the 2020 target if there is to be even a 50-50 chance of limiting temperature rises to 2C (3.6F).

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