The one-day summit gathered 150 nations, some 80 of them represented by heads of state or government, making it the highest level meeting on climate change ever hosted by the United Nations.

Secretary General Ban Ki-moon declared no-one could now question the evidence for climate change and warned history would condemn those who did nothing.

"The time for doubt has passed," he said. "What we do about it will define us, our era, and ultimately the global legacy we leave for future generations."

Ban demanded that nations should push for "a real breakthrough" at a key conference taking place in Bali, Indonesia, from December 3-14.

That meeting is tasked with setting a roadmap for negotiations to deepen action against climate change after the first phase of the UN's Kyoto Protocol runs out at the end of 2012.

British Environment Minister Hilary Benn said the Bali talks should agree to reduce global emissions to "at least 50 percent" below 1990 levels by 2050.

Agreement in Bali "will have to be ambitious," he told a breakfast meeting with journalists.

"There is no point in negotiating a deal that will not take us towards (greenhouse-gas) stabilization and so avoid dangerous climate change," he added.

Benn also called on the United States to end its opposition to making binding pledges on cutting its emissions.

These commitments, which only apply to industrialized countries that are Kyoto parties, were a key argument for President George W. Bush's decision to abandon the landmark UN treaty in 2001.

"All of us, including the largest economy in the world -- the United States -- (have to be) taking on binding reduction targets. It is inconceivable that dangerous climate change can be avoided without this happening," said Benn.

The European Union (EU), Canada and Japan all favour the 50-percent reduction by 2050 with the aim of pegging global warming to 2 degrees C (3.6 degrees F) above pre-industrial times.

In addition, the EU this year unilaterally decided to cut its greenhouse-gas emissions by 20 percent by 2020 and by 30 percent if other industrialized partners follow suit.

"All the developed countries and the major emitting countries must commit to the objective of reducing emissions by at least 50 percent between now and 2050," French President Nicolas Sarkozy said.

"Collective action is imperative. The fate of each is linked with that of all. Solidarity is imperative. The poor would be the first victims of our selfishness."

Portuguese Prime Minister Jose Socrates, speaking on behalf of the 27-nation EU and nine countries on the EU's rim, said developed countries had to agree a new roster of binding targets, using the global carbon market and Kyoto's Clean Development Mechanism (CDM) to reduce costs.

"Emissions from deforestation and from international aviation and maritime transport must also be addressed," said Socrates.

US Secretary of State Condoleezza Rice, in her speech, said the United States viewed climate change seriously, and spelt out US initiatives to foster cleaner technology.

The chairman of the UN's Intergovernmental Panel on Climate Change (IPCC), Rajendra Pachauri, summarized the IPCC's 4th assessment report, issued earlier this year.

Glaciers and Arctic sea ice were retreating rapidly and "major precipitation changes" -- droughts and floods -- were occurring.

Hundreds of millions of people faced worsening water scarcity as a result of glacier loss in the Himalayas, which fed key rivers in China and South Asia. Water scarcity would affect the growing of key crops.

"Climate change is accelerating," Pachauri said.

Tackling the problem swiftly though would keep the bill to a manageable level, but the cost will rise in line with the global temperature, he said.

In a lone dissenting note, Czech President Vaclav Klaus said: "The hypothetical threat connected with future global warming depends exclusively upon very speculative forecasts, not upon undeniable past experience, and upon its trends and tendencies."

Thursday and Friday, Rice will host a meeting in Washington of the world's 16 biggest polluters.

That conference, which will be addressed by Bush, has been viewed with suspicion by Kyoto's champions, who view it as an attempt to hijack the UN process.

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