By Darren Waters
BBC News in Cannes
Former US vice-president Al Gore, who is at the Cannes Film Festival, has warned the world is facing a "planetary emergency" due to global warming.
Al Gore appears in film documentary An Inconvenient Truth
A documentary based on the politician's environmental campaigning is being screened at the festival.
Mr Gore said the world faced a stark choice between the end of civilisation and a future for its children.
He also said he was not considering running again for presidential office in 2008.
Mr Gore said global warming was a "challenge to our moral imagination to understand it and then to respond to it urgently".
The documentary An Inconvenient Truth is based on lectures Al Gore has been delivering about environmental crisis for many years.
The film shows photographs of changes to glaciers around the world, with snow disappearing from the Alps, Antarctica and the South Pole.
"People have been moved by it," Mr Gore said. "People coming out feeling a sense of urgency."
He stressed the problem was moral, not political, and said he hoped the current US government would re-think its environmental strategy and sign up to the successor to the Kyoto treaty.
"I even believe that there is a chance that within the next two years even Bush and Cheney will be forced to change their position on this crisis," he said.
An Inconvenient Truth is screening at the festival out of competition
"One can only attempt to create one's own reality for so long. Reality proper has a way of insisting itself upon you.
"Mother nature has joined this debate with a very powerful and persistent voice."
But he said he was not thinking of running for president in two years.
"I don't plan to be a candidate again for national office," he said. "There are other ways to serve."
He would not be drawn on his opinions of any potential presidential candidates, saying it was too early for such a discussion.
Mr Gore, who is donating his proceeds from the film towards a new environmental charity, said there were "some powerful polluting interests that have way too much influence in the American political system".
He said President George W Bush had missed an opportunity after 11 September terrorist attacks to declare that the US should be independent of oil and coal.
"Leadership can make a difference," he said. "What I can most valuably do is try to change the minds of the American people and elsewhere in the world about this planetary emergency."