But the leader of the world's biggest emitter dashed hopes that he would unveil a hard target to kickstart stalled climate talks due to be reconvened in Copenhagen, Denmark, in December aimed at negotiating a broader climate pact to replace the 1997 Kyoto Protocol.

Room to manoeuvre

Hu said only that carbon intensity would come down "by a notable margin by 2020 from the 2005 levels", which still leaves Beijing and other major emitters room to manoeuvre before the talks.

Rich nations are likely to come under further pressure at the G20 meeting in Pittsburgh later this week to commit to dramatic cuts in greenhouse gas emissions.

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Incidences of heat waves 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.

Tim Flannery, the chairman of the Copenhagen Climate Council and professor at Sydney's Macquarie University, told Al Jazeera that there are "a large number of people who are disappointed" with the lack of substantive progress at Tuesday's climate summit.

"This day really should have been a day of triumph for climate diplomacy ... we would have hoped for great progress, but on the surface at least, I think, it appears that progress has been quite limited," he said from New York.

Commenting on China's pledge, he said "it is a positive step but a 'notable margin' is not something you can measure".

Todd Stern, the US special envoy on climate change and one of the most vocal critics of China's emissions policy, said he "didn't hear new initiatives so much" in Hu's speech.

"It depends on what the number is and he didn't indicate the extent to which those reductions would be made," he said.

New target pledged

But Xie Zhenhua, China's most senior environment official, later said China would soon unveil a target, based on projections that by 2020 it will double its use of renewable energy and dramatically cut energy use per dollar of GDP.

"After further study and discussion, we should be able to announce a target soon," he said in New York.

Flannery said Hu and Barack Obama, the US president, both "offered rhetoric, they offered promise, but not substantial, documented, commitment and that's what we need at this stage".

China's climate challenge

Leaders of the world's largest emitter of greenhouse gases struggle to balance economic expansion with sustainable growth.

 China pumps nearly a quarter of the world's carbon emissions into the air

 In urban areas, 90 per cent of river water and half of underground water is polluted.

 Quarter of landmass and roughly 400 million lives affected by desertification, attributed to climate change as well as 'land mismanagement.'

 Government has pledged to plant enough forest to cover area size of Norway within decade.

 Aims to generate 15 per cent of energy needs from renewable sources, such as the sun, within decade

 Investing millions of dollars  to harness wind power, with plans to build seven mega bases by 2019.

While stressing that "we have a long way to go yet", Flannery said the UN climate summit "has been significant".

"One of the great things that's happened at this meeting I think, is the creation of a peer group of world leaders who have experienced that dialogue between each other in a frank and direct way and that, we hope, will pay off in Copenhagen."

Al Gore, the Nobel peace laureate and climate campaigner, praised China for "impressive leadership" and said Hu's goals pointed to more action.

"They are very important and we've had ... indications that in the event there is dramatic progress in this negotiation, that China will be prepared to do even more," he said.

However, Hu made clear that China had high expectations from the rest of the world, repeating a long-standing call for more support in moving away from dirty growth.

Backed by India and other developing nations, China argues that rich nations emit more per person and enjoyed an emissions-intensive industrialisation, so they have no right to demand others do differently - unless they are willing to pay for it.

"Developed countries should take up their responsibility and provide new, additional, adequate and predictable financial support to developing countries," Hu said.

Obama, in his address, urged the world to address climate change now or suffer an "irreversible catastrophe".

"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," he said in his first speech at the UN.

'Morally inexcusable'

Echoing Obama's words, Ban Ki-moon, the UN secretary-general, said 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 negotiations and to strengthen the ambition of what is on offer" for a deal at Copenhagen in December.

"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."

Smaller island nations again warned that their livelihoods would vanish if the world's major polluters could not reach a deal that stopped global temperatures from rising.

Mohamed Nasheed, president of the small Indian Ocean country of Maldives that fears being submerged by rising sea levels, said: "Once the rhetoric has settled and the delegates have drifted  away, the sympathy fades, the indignation cools and the world carries on as before.

"A few months later, we come back and repeat the charade."