"We can't allow the presence of one single head of state to stymie the world's affairs," Nicolas Sarkozy, the French president, told reporters at the Commonwealth talks in Trinidad last week.

"The decisive moment is December 17 and 18. If some come at the beginning and others at the end, when will we be able to take decisions?" he asked.

However, Obama agreed to attend the last leg of the two-week conference if his appearance would help clinch a deal.

The move means Obama will be at the summit on December 18, considered a crucial period when at least 100 heads of state will be in attendance, as opposed to his scheduled stop in Denmark on Wednesday on his way to Nobel Peace Prize events in Oslo, the capital of Norway.

Country goals

India pledged on Thursday to cut the ratio of greenhouse gases pollution to production by 20 to 25 per cent from 2005 levels by 2020 but would not agree to hard limit on the amount of heat-trapping gases it could release.

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India's pledge is a cut in carbon intensity, which means that emissions can keep rising as their developing economies grow but they would do so more slowly.

China also pledged weeks ago to commit to a 40 to 45 per cent reduction in carbon intensity from 2005 levels over the next decade, meaning emissions would grow at half the rate they would otherwise.

By contrast, the US will propose a cut in emissions over the same time period in the range of 17 per cent, regardless of the growth of its economy.

However, the US congress will have to pass legislation to curb greenhouse gases blamed for global warming for the US to achieve the target it proposes.

The Senate has said it will not take up the measure until next year and even if it does, a 17 per cent reduction by 2020 is lower than what scientists say is needed to avert the dangerous consequences of climate change.

Scientists say the industrial countries must slash carbon emissions by 25 to 40 per cent by 2020 below 1990 levels to prevent the Earth from warming two degrees centigrade, the maximum to reduce the worst risks.

Obama's proposal translates to a four to five per cent reduction from 1990 levels.

"This is a very significant development, which in substance and symbolism greatly enhances the prospects of a satisfactory agreement at Copenhagen," Rajendra Pachauri, whose Intergovernmental Panel on Climate Change shared the Nobel Peace Prize with Al Gore, the former US vice president in 2007, said about Obama's plans.

None of the three countries, which are among the top five emitters of carbon dioxide in the world, were subject to limits put in place by the 1997 Kyoto Protocol, the treaty that negotiations in Denmark seeks to replace.

Financing the goals

Beyond carbon emissions, another hurdle at the climate negotiations is money.

After talks this week with European leaders, the White House said that Obama has come up with an "emerging consensus" on how much money polluting rich countries should pay by 2012 to poorer countries, which are more often the victim of global warming – a total of $10 billion a year.

"This is a very significant development, which in substance and symbolism greatly enhances the prospects of a satisfactory agreement at Copenhagen."

Rajendra Pachauri, 2007 Nobel Prize winner

Yvo de Boer, the UN climate chief, says $10 billion to $12 billion a year is needed from developed countries through 2012 to "kick-start" the issue.

Gibbs said the US would pay its "fair share" of the $10 billion amount but did not identify what that was or from where it would come.

Overall, such money would be used to help those countries adapt to warming already under way by building flood control, changing agriculture, raising buildings and getting better water supplies.

The money also would be used to help those poorer countries cut their own greenhouse gas emissions.

The Copenhagen conference on climate change is scheduled to end on December 18.