The claim that the Kyoto treaty would damage the US economy is "flat wrong", former President Bill Clinton has told the UN climate conference in Montreal.
Clinton said it was "crazy to play games with our children's future"
It was a key argument for his successor George W Bush to reject Kyoto in 2001, and the Bush administration has again found itself isolated on the issue.
Mr Clinton contrasted "creative" but underfunded new energy forms with the strong lobbying of traditional forms.
Speaking in a private capacity, he was greeted by a storm of applause.
"I liked the Kyoto Protocol. I helped to write it. And I signed it," he said.
With the new technology, there was "no telling how many jobs" could be created in the US, he added.
US delegation chief Paula Dobriansky issued a statement saying events like Mr Clinton's appearance were "useful opportunities to hear a wide range of views on global climate change".
Tony Juniper of Friends of the Earth International said Mr Clinton's speech had left the Bush administration "more isolated and fossilised than ever".
Fears of wavering
After two weeks of exhaustingly detailed work by the conference, Mr Clinton's rousing style woke up the meeting, the BBC's Liz Blunt reports from the Canadian city.
Delegates had packed the hall to hear him and his intervention came at a moment in the negotiations when all eyes were on the US, our correspondent notes.
Having refused to join the Kyoto protocol system of binding targets, its delegation is now resisting the Canadian chairman's plan for two years of wider discussions on how best to address climate change in the future.
The fear now is that other countries might start wavering on their own future commitments if they know that the world's largest source of carbon dioxide is definitively out of the project, our correspondent says.