He's the man who used to be the next President of the USA. Now, Al Gore has reinvented himself as an eco-warrior, and is the subject of a fascinating new documentary. Miles Fielder sits down with the former vice president and tries to dig out some home truths

I'm looking into the steely blue eyes of the man who should be the president of the Unites States of America. And Al Gore, the man who didn't so much lose as begrudgingly concede to George W Bush the most contested and controversial presidential election in America's history, is looking right back at me. We're sitting in the library of Brown's Hotel in London's eminently unaffordable Mayfair district. Outside, there's a pair of PAs and a well-built man in a dark suit, who probably has a microphone in his ear and some impressive hand-to-hand skills under his (black) belt. Beyond them, the hotel foyer is filled with the kind of international business clientele that Gore would, as a party politician, have had to court. Instead, he's courting me, an arts journalist. It's all a bit surreal.

Back in the library, Gore is sipping diet coke from a glass. He's dressed in a navy double-breasted suit, the top of range tailoring of which garment camouflages the few pounds he's put on since his presidential run six years ago. He's a big, burly guy anyway, but although there's undoubtedly an impressive aura about him (which is crowned by a splendid head of well-groomed, greying hair), Gore's in no way intimidating. In conversation, he proves to be as professional an interviewee as you'd expect of a man who's been in politics for 30 years: poised, to the point, knowledgeable and not in the business of saying anything he doesn't want to say. But Gore also comes across as thoughtful, eloquent, modest, even self-effacing. Certainly, that could be a front. You don't get to run for president without knowing how to play people.

Which is exactly what Gore's doing in Europe. One month before our meeting, Gore was ascending the red carpet and waving at the crowds in Cannes, where the documentary he's made about global warming, An Inconvenient Truth, was being screened at the most famous film festival in the world. In August, he'll be in Edinburgh, to give the McTaggart keynote speech at the Television Festival and introduce An Inconvenient Truth to Film Festival audiences. Today, though, on the longest and what, with awful appropriateness, feels like the hottest day of the year, Gore is in London to promote the film in which he features so heavily.

Made by the unlikely triumvirate of Emmy Award-winning television director Davis Guggenheim (Deadwood, 24, Alias), environmental activist Laurie David and Quentin Tarantino's producer Lawrence Bender, An Inconvenient Truth is essentially a filmed version of the slide show and lecture tour Gore hit the road with after he was (according to many) cheated out of the Oval Office. It's a surprisingly engaging film. Alternately funny, troubling and inspiring, but above all illuminating, An Inconvenient Truth has enjoyed a warm reception from the critics. Released in the US at the end of May, the film grossed $1.3 million on a modest 77 cinema screens, which gives it a screen average performance almost a third higher than the then number one box office hit playing on 3000 screens, the Jennifer Aniston/Vince Vaughn's rom-com The Break-Up.

In the library of Brown's Hotel, Gore says, with a slightly apologetic, deeply resonant laugh that makes him sound like Dr Frasier Crane: 'I see it as the ultimate action movie, because it's intended to move the audience to action.'

As he takes another sip of coke I ask Gore how he has found tackling a political issue via film as opposed to party politics? 'Ah, good question,' he says, returning to his soft southern states accent. He thinks for a moment, then looks at me intently. 'I have found it fulfilling, because it reaches so many more people so quickly, and it hits many people at a deeper level. I think that political conversation in America, and perhaps in the UK as well, has suffered in recent decades. And the burden of cynicism that people are surrounded with when they hear a political speech makes it difficult to communicate essential truths. Secondly, voters have a right to hear candidates for national office give their views on a wide range of issues. So, necessarily, the focus of your communication is diverse. In my post-political life I have the luxury of focussing on a single message.'

Aside from the critical and commercial success of An Inconvenient Truth, the response of the public and the media to Gore the slide show lecturer and filmmaker has been markedly different from reactions to Gore the politician. His appearance at the Hay Festival of literature (there's a book to accompany An Inconvenient Truth) at the beginning of June prompted a UK national newspaper to liken him (in no way sarcastically) to a stand-up comedian. (In the film, Gore introduces himself to a lecture audience thus: 'I'm Al Gore, I used to be the next president of the United States.' And when the audience laughs, he says with a poker player's face, 'I don't see why that's funny,' which elicits another burst of laughter.) No more, the 'Al Bore' slogans that dogged Gore during his election campaign. Long gone are the days when this 'recovering politician' (another Gore joke) was being painted by the Republicans and the right wing press as a dull individual too starch stiff to be a man of the people.

'That,' Gore says, 'is partly due to the factors I've just mentioned - cynicism towards political speeches - but partly due to what people perceive as a change in me as a person. All of us evolve in our lives, and one of the secrets of the human condition is that we usually learn the most from the most painful experiences. The old cliché has a grain of truth: what doesn't kill you makes you stronger. So, over the last six years I have learned and grown.' Albert Arnold Gore Jr did his first learning and growing on the family farm he still owns in Carthage, Tennessee. He was born on the 31st March 1949 in Washington DC, where his late father Albert A Gore Sr worked part of the year (being a Senator for Tennessee). Al Jr left home to study literature then politics at Harvard (where he roomed with the actor Tommy Lee Jones). After graduation, he served a tour of duty as a field reporter in Vietnam (despite being opposed to the war). Following his discharge, Gore married Mary Elizabeth Aitcheson (aka 'Tipper', who he first met at a high school prom and with whom he has four children), and did a five-year stint reporting for The Tennessean newspaper before following in his father's footsteps and entering politics, first as a Congressional Representative for Tennessee (1976-84), then as Senator (84-93).

In 1993, Gore became the 45th Vice President of the United States, serving two terms alongside President Clinton. Having nearly run for office against Clinton in 1989 (he pulled out of the primary after his 6-year-old son Al was almost killed in a car accident), Gore is thought by many to have been the most active and influential (from behind the scenes) vice president in American political history. He was instrumental in getting the House of Representatives to pass the North American Free Trade Agreement, in downsizing federal government, and in furnishing American schools with internet access. And it was Gore who symbolically signed the Kyoto Treaty for climate control (which America has never ratified).

Gore is a liberal Democrat. He has supported abortion and strong environmental policy, for instance, and criticised Bush over the war in Iraq (though he supported Operation Desert Fox), the environment, civil rights and the administration's slow response to Hurricane Katrina. And although the journalist John Pilger recently pointed out that carbon emissions were higher under the Clinton-Gore administration that the current one, Gore has always worn his green credentials on his sleeve. He wrote his first book on the subject, New York Times bestseller Earth in the Balance, while his son was recovering from his accident, and he continues to count environmental scientists among his most trusted advisors. The crucial question for Gore, then, is: can the public and the politicians be convinced to change their attitudes towards the environment?

'One of the things I learned from my political career about the climate crisis,' Gore says, 'is that the resistance to change is so deep, the denial is so strong and the financial interests threatened by some of the necessary changes are so powerful, that it is foolish to expect that any political leaders will be able to make speeches and rally people to support some massive legislative action. Some changes are so difficult that they will only occur when the people themselves demand of the political leaders that they develop more backbone for the necessary work ahead. And this is one of those times.

'We have,' says Gore, really warming to his subject, 'radically changed the basic relationship between the human species and the planet on which we live. That's hard to imagine. We have quadrupled the human population in less than a century, and magnified the power of technologies we use every day. We are now capable of destroying critical aspects of the earth's ecological system, rendering it uninhabitable for human civilisation. A level of awareness among the people must be created so that the political system will respond. Tall order, that. But immanently achievable.

'I have a big ally in this effort,' Gore says. 'Reality. Hurricane Katrina, for example, was a wake up call for millions of Americans. That's the basis for my optimism.' There's another question that's been on everyone's lips, of course, and that is, will Gore run for president again? His response to such inquiries has been circumspect. In June, for instance, he told the Financial Times, 'I have no plans to run for president again.' Okay, but that's not the same as saying, 'I won't run again.' Not wishing to elicit the same line, I ask if he will continue to find non-party political ways to address political issues?

'That work will continue to evolve,' Gore says carefully. 'A lot of people have come forward recently, in the wake of the book and the movie, with some creative ideas about other ways to communicate the message. What form my efforts to communicate this will take this year and next, I can't say, but it will be whatever seems most effective in moving the world to a response.' Clearly, Gore's too experienced a politician to be caught saying something he isn't ready to.

Interview at an end, he shakes my hand and says goodbye. Already, I can see he is thinking about his next engagement (of which he says he has about 100 before he comes to Scotland). Political commentators seem unable to reach a consensus about Gore gunning for the White House again. On the face of it, he appears a strong candidate: an optimist with political experience in a world where those two things rarely go together. I can't help wondering, however, were Gore to ditch the lecture tours, books and filmmaking to re-enter the party political arena, would he not find himself making the compromises and succumbing to the cynicism that renders so many top-ranking politicians either impotent or indifferent to the world around them?

Only time will tell.

Miles Fielder is an arts journalist.

Cineworld, 27 Aug, 3.30pm.