"If only the US administration could flip from denial to acceptance it could save the world," he said. "If the Americans continue to do nothing then we have a big problem -- therefore they must do something."

US president George W. Bush's administration has refused to ratify the Kyoto Protocol on cutting climate-warming carbon gases from burning fossil fuels, calling it economic suicide.

Harlan Watson, Bush's top climate negotiator, ruled out joining the treaty but did not exclude the possibility that a future US President might take a different view.

However, he said whatever emerged from the current round of talks it would be radically different.

"The future regime will not look much like the current regime," he told the meeting.

Luiz Alberto Figueirdo Machado, head of the environment section of Brazil's foreign ministry, said the developed world had had a free ride for 150 years and it was payback time.

"The polluter must pay," he told the meeting at think-tank the Royal Institute for International Affairs. "It is not the wish of the developing countries to repeat the mistakes of the developed countries over the past 150 years.

ACT URGENTLY

Scientists predict average global temperatures will rise by at least two degrees Celsius this century, bringing floods, droughts and rising sea levels from melting polar ice caps.

"Whatever we do things will get worse from here, so we must act urgently," Houghton said.

Houghton said developed nations had to take a lead in combating warming because if they did not the developing world -- particularly countries like China -- would take no action.

China is building roughly one large coal-fired power station a week to fuel its booming economy.

But both Houghton and Halldor Thorgeirsson of the UN Framework Convention on Climate Change expressed optimism.

"Climate change is a solvable problem. We have the technology to deal with this," Thorgeirsson said, saying that if it had not been for Kyoto, global carbon emissions would have been at least 30 percent higher than current levels.

He said he was confident there would be a follow-up to Kyoto when the treaty expired in 2012, adding it would have to be broader than the current agreement.

"We won't have all the answers by 2012, but we need to have found a way forward by then," he said. "We have a window of 20-30 years to find a solution."