Geoffrey Lean: Al Gore, VP for Global Warming (hot air division)

Emissions shot up faster than at any other time in modern history

Bill Clinton christened himself "the comeback kid", but it is his former vice-president, Al Gore, who is earning the tag. For he has recently gone from zero to hero. For five years after losing the 2000 presidential election, he was the butt of pity and derision. But now he is being touted as the next President.

The secret of his resurrection is the same as the origin of the Conservative Party revival: global warming. Long before David Cameron seized on the issue - leading to the Tories' unfamiliar poll lead - Gore was tramping across America giving slide shows on climate change.

This week, a 96-minute feature film based on his show - An Inconvenient Truth - opens in cinemas across America. It got a standing ovation at January's Sundance Film Festival, and Gore has just been fêted in Cannes as an upmarket Michael Moore. This week, he is the star of the Hay Festival; at £35 it will cost over twice as much to hear him as it will Seamus Heaney.

His book is to be published next month, he is behind a new pressure group - the Alliance for Climate Protection - and has started a green investment fund with the legendary former Goldman Sachs CEO, David Blood (inevitably: "Blood and Gore"). Rolling Stone and Time magazines have called him a hero.

But does he deserve this accolade? This is Gore's second coming as an environmental campaigner, since Clinton chose him as his vice-president largely on his green credentials. And the really "inconvenient truth" is that he lamentably failed to deliver the first time.

Emissions of carbon dioxide - the main cause of climate change - shot up far faster during the Clinton-Gore administration than at any time in modern US history. From 1993 to 2000, they rose by nearly 15 per cent, half as much again as in the previous eight years. And they dwarfed a comparatively tiny 1.65 per cent rise in the present George Bush's first four years.

Admittedly, Gore did agree to the Kyoto Protocol, promising - after some arm-twisting by John Prescott - to reduce US emissions by 7 per cent over 1990 levels by 2012. But by the time Bush took over, they would have had to have been slashed by 20 per cent to reach the same target. The "toxic Texan" could hardly have achieved that, even had he wanted to. Thus Gore at least shares his responsibility for undermining the treaty.

In a bestselling book, Earth in the Balance - published just before he became vice-president, Gore lamented his failure to emphasise global warming in a previous run for the presidency. "I have become very impatient with my own tendency to put a finger to the political winds and proceed cautiously," he wrote.

But in his 2000 campaign - digit firmly back in the breeze - he again failed to highlight it. His caution probably cost him the presidency. Ralph Nader, running for the Green Party, took 100,000 votes in Florida, which Gore finally lost by only 536. If he had stuck to his principles, enough Nader supporters would have voted for him to have made the hanging chads irrelevant and put him in the White House.

None of this is to say that he would not have been a better president than Dubya and that his present campaign is not having an invaluable effect on American public opinion.

But a mea culpa - or even a clear explanation of why he did so poorly in office - is long overdue. If we are to have any confidence in a future bid for office, we need to know he has learnt lessons from his past failure. And those lessons could be invaluable for a new generation of politicians led by the new green white hope, David Cameron.