Here's a situation not many would have predicted back in 2000. Al Gore is addressing a tent packed with maybe 800 people - and the place is trembling with laughter. The famously wooden former vice-president, whose personality was so dull, according to one rival, that when "Al Gore has a fireside chat, the fire goes out", is tickling this crowd at the Hay Festival like a veteran stand-up.
"I am Al Gore and I used to be the next president of the United States of America," he says, opening the routine. When that gets a warm laugh, he scowls: "I don't happen to find that very funny."
It has become a familiar act, but people enjoy it all the same. It's a variation on a theme: how he was denied the US presidency in 2000 despite winning the popular vote. He mocks himself as a "recovering politician" who, after nearly a decade in the White House as number two to Bill Clinton, took time to get over the loss. "I flew on Air Force Two for eight years," he says, "and now I have to take off my shoes to get on an aeroplane." That gets another sustained laugh, but it's no joke. One of the more striking images of An Inconvenient Truth, the new and acclaimed documentary charting Gore's one-man campaign against climate change, is of the former VP lugging his bags, alone, through a string of anonymous airports, frisked like everybody else. (For a green, he flies a lot.)
The Hay crowd could not be more sympathetic, and it's not only because Gore gets them laughing. His style, even when kidding, is cerebral and smart. "It was an epiphany, Seamus!" he says at one point, addressing a gag to the Nobel prize-winning poet Seamus Heaney, sitting close to the front. Later he will quote Francis Bacon, discuss nanotechnology and explain the neurological "establishing reflex" - and through it all you can feel the unstated contrast that hovers in the air. Compare this with George W Bush!
Not that the Bushies would be too discomforted by that. In fact, this scene - Gore feted by bien pensant European elitists - would confirm everything they like to believe about Gore and his fellow Democrats: that they might be popular with the cheese-eating surrender monkeys of Yurp, but they're woefully out of touch at home.
Except that in Gore's case that might be changing. When he sits down for an exclusive interview with the Guardian, aide Kalee Kreider - a new hire, and ex-Greenpeace campaigner - hands him a note. The US box office figures are in for An Inconvenient Truth and it has broken two records already: the highest per-screen rating for any Memorial Day weekend opening since Jack Nicholson in The Shining - and the highest ever for a documentary. He nods with visible satisfaction, like a candidate who has just seen poll numbers moving his way.
Is that the right way to understand what Gore is up to? No one is quite sure what to make of this mission that Gore has undertaken since he was deprived of the presidency six years ago. Tirelessly, he has been touring the US and beyond, delivering a PowerPoint presentation that sets out methodically and calmly, through pictures, graphs and even cartoons, how humanity is cooking the planet. The environmental activist Laurie David (wife of comedy giant Larry) saw Gore deliver it, was "blown away" and decided the slide-show had to become a movie. She recruited the Hollywood team who have made a film that is, despite its premise - a middle-aged white man delivering a lecture about ecology - extraordinarily gripping. Those who have known the arguments for years, intellectually, suddenly find themselves moved emotionally by Gore and stirred into action. Even the trailers have been getting ovations in American cinemas.
The uncertainty comes over whether to take all this at face value. Is it a global advocacy campaign, such as Bono's RED effort against Aids, or Make Poverty History? Is Gore trying to do for the planet what Jamie Oliver did for school dinners? Or is this the first, sophisticated movement of a campaign by Gore to win the prize that slipped from his grasp in 2000, to seek the presidency once more in 2008?
Sit down face to face with Gore and the changes of the past six years are visible. He's heavier now, his jaw and jowls fuller. The hair is greyer, too. The shirt is open-necked, to signify a recovering, rather than serving, politician. And he displays a passion that many, including his own supporters, wish he had shown more often in 2000.
"The scientists are virtually screaming from the rooftops now," he says, his voice rising. "The debate is over! There's no longer any debate in the scientific community about this. But the political systems around the world have held this at arm's length because it's an inconvenient truth, because they don't want to accept that it's a moral imperative."
Patiently, and surely for the 10,000th time, he explains what's going wrong. The atmosphere is like a coat of varnish around the globe, he says. When it's thin, as it should be, heat naturally escapes. But when it gets thicker, thanks to carbon dioxide emitted by us, it traps in the heat and the world gets warmer. "It's cooking and wilting the most vulnerable parts of the eco-system, melting all the mountain glaciers, the north polar ice cap, parts of Antarctica, parts of Greenland." That molten ice-water will raise sea-levels, flooding food-producing areas that all of us rely on. Eventually it will submerge whole cities, from San Francisco to Shanghai. The site of the Twin Towers will not be a memorial garden: it will be underwater.
"This could literally end civilisation." He smiles. "I know it sounds alarmist, as if this is hyperbole, like a man with a white beard holding a placard, saying the end of the world is near . . . but this really is a planetary emergency."
This is no recent adoption of a modish issue. Gore has been obsessed with the environment since he was taught at Harvard by Roger Revelle, the first scientist to measure levels of CO2 in the atmosphere. Elected to the House of Representatives aged just 28, he held the first congressional hearings into the subject in 1980. In 1992 he wrote Earth in the Balance, a book that many regarded as the best by a serving US politician. He can claim to be the first mainstream politician anywhere to have woken up to this danger.
Yet it's clear that most of his countrymen are asleep. The US accounts for a quarter of the world's carbon emissions, with just 5% of its population. Cheap gas is regarded as a sacred birthright; public transport is a minority activity, reserved for the poor and excluded. The result is that most of Gore's rhetorical energy goes on alerting people to the existence of the problem, rather than setting out the solutions. Pushed for steps individuals can take right now, he urges people to use cars less, to use light rail more; he recommends clock thermostats, so that people only heat their houses when they're at home; and using energy-saving lightbulbs.
Surely he could make a start by flying less? "Beginning two years ago, I made a decision to live a carbon-neutral life," he says, explaining that his family and businesses now do all they can to reduce their emissions and to "offset" the rest by giving money to carbon-reduction schemes in India and eastern Europe. He also says he has talked to Richard Branson at Virgin and British Airways about the problem with aviation fuel, one of the single biggest sources of carbon dioxide. "They're aware of it," he says. Mere awareness is hardly enough, is it? "There will have to be a day of reckoning that takes this into account," he concedes.
What does he make of nuclear power, declared by Tony Blair to be "back on the agenda with a vengeance"? "I'm sceptical about it playing a much larger role," he says. "In the eight years I served in the White House, every weapons proliferation issue we faced was linked to a civilian reactor programme." Besides, it's expensive and there are other, renewable sources that are new and efficient. "Maybe in China and in Britain it will play a role, but I don't think it's going to be a silver bullet."
There will have to be other ways, starting with a realisation that this is a shared challenge for all humanity, one that should transcend all other differences. He agrees with the scientists who say we have 10 years to act, before we cross a point of no return. "We have everything we need to solve this crisis, save perhaps political will. But in a democracy, political will is a renewable resource."
That sounds like a cue for him to renew his own career. Indeed, the emotional charge running through An Inconvenient Truth, and indeed Gore's speech at Hay, is that 2000 represented not just a personal blow to Gore but a terrible setback for the planet. The world's only superpower was within a few hanging chads of having a conviction environmentalist at the helm. Surely with the power of the White House, he would have acted to slow America's vast output of carbon emissions. You watch the movie, including the archive shots of Florida, and curse the planet's bad luck.
Gore himself works hard not to seem bitter; indeed, the humorous shtick is meant to suggest a man who can now laugh at those wretched events. He tells me he has had an exchange of letters with Ralph Nader, the environmentalist whose independent candidacy siphoned off precious votes from Gore. "Nothing substantive," he says, but a personal correspondence after both men lost their mothers.
Later I ask Gore if he's moved to the left these past six years. After all, he denounced plans for the coming war in Iraq in September 2002, long before his Democrat colleagues, and he now unashamedly attacks corporate special interests. A flash of anger: "No! If you have a renegade band of rightwing extremists who get hold of power, the whole thing goes to the right. But I haven't moved. I'm where I've always been."
To describe the Bush administration in such terms is indicative that, for all the gags, Gore's fury has not gone away. So is he gearing up for another go? "I don't expect to be a candidate." Is there some event that could change his mind? "Not that I can see," he says, with a wide grin.
But isn't there a moral obligation, given what he knows about the fate of the planet? He can't just do a slide show; surely he has a duty to run? "I resist that," he says, pausing before suddenly becoming animated once more. "Look, I don't deny that this is the most powerful position in the world, but even if you're the president you can't move if the people aren't there, and if Congress isn't there." He reminds me of the Kyoto negotiations, in which he played a key part. "I came back and sat with my staff and asked 'How many [of the 100] senators can we count on to ratify this?' We had one. We couldn't get to two."
So now the focus is on moving public opinion, so that the politicians are pressured into, and then have the room, to do the right thing. "I honestly believe that the role I can most usefully play is to change the minds of the American people."
That says he's not running. And there's plenty of supporting evidence. First, if he was, there would be signs of an embryonic campaign in the key, early voting states of Iowa and New Hampshire: "There is no trace of that," says one Democratic strategist. Nor is he collecting the early commitments of support that Hillary Clinton has been gathering from crucial party officials.
The Hillary factor is a problem in its own right. "If he's against Hillary in a primary, Bill gets involved and not on Gore's side - that's weird," says the Democrat. Besides, they'd be drawing staff, supporters and donors from "the same gene pool".
What's more, Gore admits that he doesn't much like being an active politician. "There's lots about politics I don't feel comfortable with. To talk about the politics of future ideas is impossible in soundbite form." Campaigns consist of "the repetition of emotion-laden drivel." Did he do that himself in 2000? "Of course . . . but what can you do in 27 seconds?"
Those around him echo the point. He has a "great private life now," says one colleague. He has homes in San Francisco and Nashville; his marriage to Tipper is considered one of the strongest in US politics. Their children, now grown up, are thriving. He advises both Google and Apple and has successful businesses, including a green investment company in London founded with David Blood ("I wanted to call it Blood and Gore . . .") and the Current cable TV company. Above all, he has found his true voice doing what he does now, serving as a kind of educator and preacher. "We want him to be happy," says one aide. "Doesn't he look happy?"
There could be a political calculation here, too. Gore's global warming speech would be "robbed of its moral authority the moment you added those three little words, 'Vote for me'," says one leading Democrat. If he wants to drive home his message on climate change, he needs to be the non-politician, with no self-serving motive. On the other hand, it makes sense to keep the possibility of a 2008 candidacy alive, just to pique the interest of the press and others. Let's face it, Walter Mondale wouldn't have packed out Hay. But if people think Gore might run again, they'll queue up to listen to him. I put that theory to Gore, who smiles: "In US-China relations, one of the guiding principles is constructive ambiguity."
There are indeed plenty of signals pointing in the opposite direction, towards a run. Gore is fond of quoting Churchill and his warnings of "the gathering storm" of fascism. And everyone knows that Churchill came out of the political wilderness to lead the battle against the storm. It's telling too that when it comes to the hard choices people will have to make to curb carbon emissions, Gore is reluctant to be specific, preferring to suggest that we in the west can both lead comfortable, luxurious lives and be good to the environment. Under pressure, he admits there will have to be hard choices, but even then he sounds more like a politician wary of alienating voters than an activist telling the unvarnished truth.
Ask his staff whether they want him to run and they do their best not to smile. The Clinton-era Democratic consultant James Carville says that wanting to be president is not an itch that you scratch and then it's gone. Gore has run four national campaigns and was all but groomed for the top job by his senator father since boyhood. Can that urge really have gone away?
I can see the US electorate giving him a second look. For one thing, they love nothing more than a heroic story of comebacks and second chances, and a Gore presidency would have the added merit of poetic justice. And maybe he would like to redeem himself. In an intriguing moment he told the Hay audience of his regret that he has not managed to convey the truth of the environmental crisis facing the world. "I feel as if I've failed over 30 years - but I'm not done yet".
From activism to energy-efficiency: Al Gore's top tips for saving the planet
1. Insist that politicians in all parties make this planetary emergency their top priority; this is crucial
2. Think about conservation and efficiency in the way you use resources in your own life, eg:
· drive less (every mile not driven saves one pound of CO2)
· fly less
· recycle more
· keep your tyres properly inflated
· use less hot water
· use a clothesline instead of a tumble-dryer
· insulate your home
· avoid products with a lot of packaging
· turn down the thermostat by 2C
· plant a tree (one tree absorbs one tonne of CO2 over its lifetime)
· turn off electronic devices when they are not in use
3. Be a conscious consumer with regard to everything you buy, both in the marketplace and in the investments you make, eg:
· switch to green power
· buy locally grown food, and organic where possible
· buy fresh food over frozen
· buy less meat
· offset your impact on the planet's resources by investing in renewable energy projects For more information go to climatecrisis.net.
Jonathan Freedland's interview with Al Gore will be aired on More4 on Saturday June 3 at 4.55pm.
