US President Barack Obama says the United States is "determined" to act on global warming, but has warned of tough negotiating ahead in the bid to seal a new world climate pact this year.
Warning of a looming "catastrophe," at a major United Nations climate conference on Tuesday, Obama called on developing nations to agree to "strong measures" to boost hopes for a global deal on cutting greenhouse gas emissions.
"We understand the gravity of the climate threat. We are determined to act and we will meet our responsibility to future generations," Obama said, arguing that he was presiding over a new era of US action on global warming.
Without naming the Bush administration, which was pilloried around the world for lack of action on climate change, Obama said his government had made a "historic recognition" on the need for action on behalf of the American people.
But he warned that there would be tough talking ahead before the global climate change conference in Copenhagen at the end of the year, and said the deal that may eventually emerge would not be "perfect."
"There should be no illusions that the hardest part of our journey is in front of us," Obama said.
The president said that the drive for climate change action was coming at possibly the worst time, with the world locked in recession and nations wary of taking steps that would endanger their economies.
"All of us will face doubts and difficulties in our own capitals as we try to reach a lasting solution to the climate challenge."
But he warned, "our generation's response to this challenge will be judged by history, for if we fail to meet it boldly, swiftly and together, we risk consigning future generations to an irreversible catastrophe."
In a speech aimed at US domestic opinion as well as the rest of the world, Obama called on "rapidly-growing developing nations" that will produce nearly all growth in carbon emissions to act as well.
"Some of these nations have already made great strides with the development and deployment of clean energy," Obama said.
"Still, they will need to commit to strong measures at home and agree to stand behind those commitments just as the developed nations must stand behind their own."
Congressional skeptics of climate change legislation have warned that they will not support legislation or a world global warming bill, unless developing nations also pay a price for greenhouse gas emissions.
The US House of Representatives has already passed a bill that aims to cut US greenhouse gas emissions by 17 per cent from 2005 levels by 2020, and 83 per cent by 2050.
The bill has yet to pass the Senate however, and the timetable for action has slipped to next year, after the Copenhagen summit, and advocates fear the targets for emission cuts will be watered down.