Former US vice-president Al Gore owned up to failing to get his climate change message across as a politician when he appeared at the Hay Festival.
Gore said it was not too late to stop the catastrophe
In his first UK speech on the subject, Mr Gore promised to devote himself to the task of warning people about the impending "planetary emergency".
He appealed to the audience to act to halt the growing crisis.
"I will own up to shortcomings in my ability to communicate," said Mr Gore, who ran against President Bush in 2000.
"But I'm not through with this yet and I am devoting myself to it".
Mr Gore was the key note speaker of the 19th Hay Festival on the mid-Wales border.
Five years ago Bill Clinton spoke at Hay on his many roles in conflict resolution.
But Mr Gore, fresh from an appearance at the Cannes film festival, delivered a starker message that the world was now facing a "danger which could bring the end of civilisation."
A documentary which premiered at Cannes, An Inconvenient Truth, is based on lectures Mr Gore has been delivering about environmental crisis for many years.
He was asked by a member of the Hay Festival audience to run for president again.
But Mr Gore replied: "I honestly believe that the role I can most usefully play is to try to change the minds of the American people...about what this crisis is about."
He said addressing the issues around climate control were "on the agenda in 2000 but was never seen and heard as an issue worthy of the top rank of consideration".
Mr Gore said global warming was seen as an "arcane" issue with more than half the US media denying there was any problem and his opponent "pledged to regulate CO2 - a pledge not broken until after the inauguration".
Gore said he used to be "the next president of the US"
In a passionate speech, Mr Gore said: "We face a challenge in the conversation of democracy that we must be up to in order to save the climate balance on which our civilisation depends."
He said he believed scientists who said that there may be 10 years remaining to avoid "crossing the point of no return".
"Then does that change you? It should, it's happening on our watch," said the former vice-president.
He said he was "carbon neutral" himself and he tried to offset any plane flight or car journey by "purchasing verifiable reductions in CO2 elsewhere".
He said the only way to bring about the change was "a sea change in the public's understanding and opinion".
"The only way that political leaders of all parties will find the courage for the bold actions that are needed."
Mr Gore was given a standing ovation by the audience whom he begged "to make the changes in your own life to make your part of the solution (to the problem)".
He said: "There are more than enough people here to really change the world.
"I hope that many of you will accept and act on that - so much is at stake."