US Energy Secretary Steven Chu says the US will not be able to cut greenhouse emissions as much as it should due to domestic political opposition.
Prof Chu told BBC News he feared the world might be heading towards a tipping point on climate change.
This meant the US had to cut emissions urgently – even if compromises were needed to get new laws approved.
Environmentalists said Prof Chu, a Nobel physicist, should be guided by science not politics.
The American political system is in the throes of a fierce battle over climate policy. President Barack Obama says he wants cuts in greenhouse gases but has left it to Congress to make the political running.
The House of Representatives is debating a climate and energy bill but even if it passes it may be rejected by senators, many of whom are funded by the energy industry.
Prof Chu is a Nobel prize-winning physicist and a world expert on clean energy. But he said it was impossible to ignore political reality.
“With each successive year the news on climate change has not been good and there’s a growing sensation that the world and the US in particular has to get moving,” he said.
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