Former US Vice-President Al Gore made an emotional return to the US Congress to testify about global warming.
Al Gore brought 500,000 letters urging action on climate change
The winner of an Oscar for his film on climate change, An Inconvenient Truth, told a large crowd that "our world faces a true planetary emergency".
He said the US should freeze carbon dioxide emissions and push for a strong climate change treaty to replace the Kyoto Protocol which expires in 2012.
Mr Gore came under fire for his own high energy use at his Tennessee home.
He called his first appearance in Congress since losing the 2000 presidential race to George W Bush "an emotional occasion".
Mr Gore spoke first to a joint session of two House of Representative committees on energy and environmental issues and then the Senate Environment and Public Works Committee.
"This problem is burning a hole at the top of the world in the ice cover that is one of the principal ways our planet cools itself," he said.
"If it goes, it won't come back on any timescale relevant to the human species."
Mr Gore had with him boxes which he said contained more than 500,000 letters asking Congress to take action to stop global warming.
The measures he suggested included a freeze on carbon dioxide emissions; requiring corporations to disclose emissions in their reports; and a tough international treaty to limit global warming pollutants, to succeed the Kyoto Protocol.
But critics questioned the science behind global warming theory and criticised Mr Gore's personal energy use.
Senator James Inhofe challenged Mr Gore to consume no more energy at home than the average American.
Mr Gore said: "We live a carbon-neutral life, senator, and both of my businesses are carbon-neutral... we do not contribute to the problem that I am joining with others to solve."
Mr Gore wants to freeze, then lower, carbon dioxide emissions
Representative Joe Barton, the leading Republican on the House Energy and Commerce Committee, challenged global warming science as "uneven and evolving".
"You're not just off a little, you're totally wrong," he said of Mr Gore's conclusions that carbon dioxide emissions contribute to global warming.
Mr Barton said freezing carbon emissions would harm Americans and lead to "no new industry, no new people and no new cars".
Mr Gore's star has been rising lately.
His documentary won two Oscars; he has been nominated for a Nobel Peace Prize and several opinion polls in the US show him among the top potential contenders for the presidency, although he says he has no plans to run again.