Montreal - Former US president Bill Clinton took to the podium at the UN climate talks here to ram home a grim message about global warming and demand the United States move quickly away from the fossil fuels causing the problem.

In a show-stealing appearance rumoured to have ired the US delegation, Clinton defended the UN's Kyoto Protocol on greenhouse gases that was ditched by his successor, President George W. Bush, and said the switch to cleaner energy would create millions of jobs for the American economy.

"There's no longer any serious doubt that climate change is real, accelerating, and caused by human activities," Clinton said.

The Earth, he said, was "literally a biological miracle ... it's crazy for us to play games with our children's future."

He pointed to an array of gloomy scientific studies published in past weeks, including evidence that carbon dioxide levels are at their highest in 650,000 years, that glaciers in the Himalayas and Arctic sea ice were melting and the warm Atlantic currents that bathe northwestern Europe were slowing down.

The United States "is the worst offender" for the problem, said Clinton.

America has just four percent of the world's population, but accounts for nearly a quarter of global greenhouse gas emissions.

Clinton stoutly defended the Kyoto Protocol, whose framework was approved by his administration in 1997, but which was ditched by Bush in March 2001, in one of his first acts in office.

Bush said the evidence for global warming was uncertain and Kyoto's format of binding industrialised countries to making cuts in greenhouse gases was too costly for the US economy, and unfair because big developing countries did not have a similar obligations.

To loud cheers from an audience of thousands of delegates and green activists, Clinton said: "I liked the Kyoto Protocol. I helped to write it. And I signed it."

Clinton acknowledged, though, that the protocol "was not a perfect agreement" and there were some big criticisms of it at the time.

Conservatives argued -- and still do -- that the move to cleaner energy sources and tougher fuel efficiency standards would cost jobs and weaken the economy, he said.

In fact, cleaner technology "would strengthen, not weaken our economy," said Clinton, "... in America, there's no telling how many jobs we could create."

He spelt out a long list of things that could be done, including improving energy efficiency in US power plants, cars and buildings, switching to solar, wind and biofuels and even simple initiatives for ordinary citizens, such as using high-efficiency lightbulbs.

Clinton was invited by the Canadian branch of the Sierra Club environment group to speak at the final day of the UN Framework Convention on Climate Change (UNFCCC), the treaty that oversees Kyoto.

Because it was not an official UNFCCC event, all UN logos and backdrops were carefully removed from the podium.

Negotiations were going to the wire on Friday on how to further greenhouse gas cuts beyond Kyoto's present commitment, which runs out in 2012.

Rumours spread among the conference that the US delegation was angry that Clinton had come, and that it had even tried to block his appearance.

The delegation issued a statement, though, saying it encouraged "stakeholder presentations" of the kind symbolised by the Clinton visit.