"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 ... The time we have to reverse this tide is running out."
Ban Ki-moon, the UN secretary-general, opened the climate summit of about 100 leaders by saying that failure to reach a new treaty this year on fighting global warming would be "morally inexcusable".
He called on presidents, prime ministers and other leaders "to accelerate the pace of negotations and to strengthen the ambition of what is on offer" for a deal at Copenhagen, Denmark, in December.
The Copenhagen meeting is aimed at negotiating a broader climate pact to replace the UN's Kyoto Protocol.
"Failure to reach broad agreement in Copenhagen would be morally inexcusable, economically short-sighted and politically unwise," Ban said.
"The science demands it. The world economy needs it."
Hu Jintao, China's president, announced significant carbon emissions cuts for the world's biggest emitter of greenhouse gases.
|Hu, bottom, pledged 'notable' emmissions cuts by 2020 [AFP]
The Chinese leader said carbon dioxide emissions would be cut by a "notable margin" by 2020 from their 2005 levels.
Hu added that developing nations "should not ... be asked to take on obligations that go beyond their development stage".
Obama and Hu are scheduled to meet one-on-one after the summit.
Nicolas Sarkozy, the French president, proposed that heads of state from major economies should meet in November to discuss the climate change agenda before the Copenhagen summit.
"Considering how complex this negotiation is, a new summit before Copenhagen is needed," Sarkozy said.
Sarkozy said the proposed November meeting would allow the world's biggest emitters "to make clearer commitments to ensure Copenhagen's success".
That appeal came after Jose Manuel Barroso, the European Commission president, said that climate talks were "dangerously close to deadlock" and in danger of an "acrimonious collapse".
Rich nations are likely to come under pressure at the UN summit and the G20 meeting in Pittsburgh later this week to commit to dramatic cuts in greenhouse gas emissions.
Industrialised nations have also been urged to pay for poorer nations to burn less coal and preserve their forests.
Incidences of heatwaves and droughts are on the increase and there has been an acceleration in the melting of glaciers and the recession of the Greenland ice sheet, Rajendra Pachauri, chairman of the UN Intergovernmental Panel on Climate Change (IPCC), said earlier this week.
Obama's legislative initiatives to reduce carbon emissions in the US have recently been overshadowed by his push for healthcare reform.
But in his speech on Tuesday, he said that the United States had done more over the past eight months to reduce carbon pollution than at any time in history.
"We understand the gravity of the climate threat," the US president said. "We are determined to act. And we will meet our responsibility to future generations."