You're a Hollywood heart-throb with the world at your feet, a model on your arm and Oscar nominations in your bag. So what on earth do you have to be worried about?

Well, the Earth actually.

Titanic star Leonardo Di Caprio is turning his attention to the melting ice caps and insists this is not about jumping on any trendy bandwagon.

"This documentary is something I have been trying to do for many years," he says. "Ever since I started environmental work, I've wanted to do a documentary that really encompasses every major environmental issue in the world and speaks to all the greatest experts."

But this isn't just a film about global warming or carbon-dioxide emissions.

Instead it is a film about humanity's relationship with the planet, about how we treat it as both an unlimited resource and a rubbish tip and how this desperately needs to change or our very survival hangs in the balance.

Director Nadia Conners says: "Global warming is only part of the problem.

We have overshot the limits of this planet and therefore we, as a species and civilization, are hanging in a precarious imbalance with our future.

"If we don't fully recognise our environmental problems as a product of the way we are thinking, as a product of our very consciousness then I don't think we have much hope of turning things around."

It is heavy stuff, but The 11th Hour is far from the fear factory of its Al Gore counterpart, An Inconvenient Truth.

One incredibly excited commentleaver on the environment website treehugger.com enthused wildly after seeing the film, explaining how the environmental movement should come to be seen as a human rights movement, saying: "Perhaps your reading of this review sees someone a little over-enthused on the subject.

I contend that watching this movie will give you exactly this empowered sense."

The documentary, aside from talking to scientist Stephen Hawking and former Soviet president Mikhail Gorbachev as it explores man's impact on the planet, looks at what we can do to change it.

The tag line of the movie "consume less, live more" will come as a blissful relief to environmental campaigners who have been extolling the virtues of a more simple life for years.

This message - the idea of changing not only the way we live but what we buy and how we entertain ourselves - has been of increasing concern to Leo, one of the documentary's producers, as well as its narrator.

"I was tired of watching news programmes and listening to scientists or environmentalists who have devoted their lives to an issue such as global warming, having to sit there and get wrangled into some sort of argument about whether this is really happening or not," says the 33-year-old star.

"We wanted to make a homemade movie where we got the greatest minds possible all speaking uninterrupted, without any corporate backing or studio involvement from the outset."

Documentary film in Hollywood is on the rise, with the success of Michael Moore films such as Fahrenheit 911 and Sicko and, of course, An Inconvenient Truth. This has had a profound impact on the already environmentally conscious Leo.

"There is tremendous capability in the world of documentary. I look at films such as Fahrenheit 911 and numerous other films which have changed the political climate. There is a tremendous role to be played in that respect. The message has been put out there in a much more profound way specifically because of Al Gore's movie.

"If it weren't for An Inconvenient Truth and him having put out the science of global warming and its impact on a global scale, a movie like this wouldn't be possible," he says.

"This film has a broader spectrum in the sense we talk about personal responsibility - and about the role of governments and corporations along with solutions.

"It really highlights the fact we have solutions out there today which can reduce the human carbon footprint by 90 per cent.

"It's about implementing these things into our daily lives," he says.

But that isn't the only impact onetime US presidential hopeful Al Gore has had on Leo.

"About ten years ago, Al Gore explained to me what climate change and global warming was, the science behind it and the decades of research he'd done on the subject," he says.

"It really propelled me to want to be more vocal about the issue because it seemed to me the change of weather we'd been having - the flooding, the hurricanes, all these things - there wasn't enough of a connection being made in the mainstream media."

Nowadays he drives a hybrid car, has solar panels on his house, uses filtered water and buys organic produce.

"I built my house to be green but that isn't necessarily feasible for everyone out there," he concedes.

His passion for the environment is obviously a big part of his life but Leo insists he won't be joining Arnold Schwarzenegger in turning to politics: "I have no political aspirations whatsoever."

However, he realises it's his star appeal which will entice people into cinemas to see a low-budget movie about global warming.

"If I wasn't an actor, I don't think a film like this would be possible in the same sense," he admits.

"I'm very committed to being an environmentalist but one hand sort of feeds the other here. I know, with the number of people who have seen past works of mine, the younger generation will possibly go to see this movie because I'm in it - and that's the role I've played in this film.

"Ultimately, I'm going to continue to be an actor and hopefully do more work like this."

And he's prepared to campaign on green issues for the long haul, starting with his own, environmentally suspect, country.

"We need to be the ones to set an example for the rest of the world," he says. "We are the leading consumers, the biggest producers of waste around the world and unless we're the ones to set an example for less industrialised countries, how is the rest of the world going to follow? If you ask any environmentalist about George Bush's policies on the environment, he gets close to an F grade.

"It's interesting because all this inevitably boils down to a publicity game for the planet and what's good for the place we live in," he continues.

"There's an important statement in the movie. Not only do we have to vote at the voting booth but we have to be very conscious about what we buy.

We're voting every time we pay for something and we're advocating the way that company does business - whether they're good to the environment or whether they're not.

"It would be wonderful to live in a world where we wouldn't have to think about these things but it's a market-driven society."

  • The 11th Hour is released in the UK next Friday