America's Environmental Protection Agency formally declared that greenhouse gases endanger human health – a decision which will give President Obama the power to regulate emissions with or without the backing of Congress.
There had been fears Mr Obama would arrive at the conference almost empty-handed after Congress held up attempts to pass new legislation, but the EPA's watershed decision will inject fresh optimism that Copenhagen can now set meaningful climate control targets which will include the US.
Environmental campaigners hailed the news as a sign that the US, second only to China among the world's biggest polluters, was "serious" about signing up to stringent targets on cutting carbon emissions.
Mr Obama, who plans to attend the conference late next week, has made energy reform one of his top priorities but has until now been held back by strong opposition from Republicans and some Democrats, who argue that the proposed "cap and trade" system for carbon emissions will be too costly for industry.
Although he will still hope to win the backing of Congress for his plans to cut emissions by 17 per cent by 2020 and about 80 per cent by the middle of the century, the EPA's so-called endangerment finding is likely to act as a spur to legislators and will enable Mr Obama to bypass Congress if necessary.
Meanwhile the Copenhagen conference's head negotiator warned that the summit's target of limiting global warming to 2C is not enough to save whole nations from "disappearing" beneath rising sea levels.
Yvo de Boer, executive secretary of the UN climate change negotiations, said an agreement to allow world temperatures to rise by 2C would amount to a "suicide pact" for small island states, leaving millions of people homeless.
Mr de Boer challenged world leaders to find a way of limiting global warming to 1.5C rather than the 2C limit which is currently on the table.
He said: "What the small island nations are telling us here is anything over 1.5 C increase will be a suicide pact for them because it means their nations disappearing."
Dr Rajendra Pachauri, Chairman of the Intergovernmental Committee on Climate Change, also said vulnerable people will suffer if temperatures rise by even a small amount.
"Some even question the goal of 2C as a ceiling because that would lead to sea level rise on account of thermal expansion alone of 0.4 to 1.4 metres," he said. "This increase added to the effect melting of snow and ice across the globe, could submerge several small island states and Bangladesh."
Speaking on behalf of the small island states Dessima Williams, the chief negotiator for Grenada, said the group would not accept a "made for TV" solution.
She called for a legal treaty that would commit rich countries to cutting their carbon emissions by between 25 to 40 per cent.
"We are here to save ourselves from burning and drowning," she said. "We are here to work towards an ambitious outcome."
In a separate development, Britain's independent Committee on Climate Change warned the Government that the era of cheap flights must end if the UK is to meet its carbon reduction target.
A report by the CCC called for higher taxes on plane fares to price passengers out of the skies.
"Air tickets are going to get a lot more expensive, but people will become a lot richer over the next 40 years. If you're going to constrain demand growth, you do need rising prices," said David Kennedy, the Committee's chief executive.
The Committee has said that the number of people flying to and from British airports should increase by no more than 60 per cent by 2050 for the Government to meet its target of ensuring that aviation's carbon emissions are no higher then than they were in 2005.
But the latest projections show that Britain is on course for a rise of more than 200 per cent in passenger traffic, with numbers rising from 230 million in 2005 to 695 million in 2050.