There was a time, in the not so distant past, when Hollywood was synonymous with selfish glamour - and damn proud of it. Earth mother Angelina Jolie (and the somewhat left-field hemp activism of Woody Harrelson) aside, the cream of Los Angeles was more likely to be found sipping cocktails at the Sky Bar or shimmying out of limos at Mortons on Oscar night than scrambling around in the rainforest attempting to save obscure plant life. Blatant consumerism - gas-guzzling cars, big bling jewellery and ostentatious crash pads - was seen as something to be worn as a badge of honour.

Of late, however, a growing band of A-list stars seem to have found their consciences. Indeed, Hollywood is saying yes to ethical chic faster than it takes Will Ferrell to get a bawdy comedy green-lit. In the past week alone Reese Witherspoon has launched a global United Nations campaign to end violence against women, while Tea Leoni has unveiled a new UNICEF initiative raising awareness for children with Agent Orange-caused disabilities in Vietnam.

Now Leonardo DiCaprio has gone all Al Gore on us. Next Friday will see the release of The 11th Hour, a feature-length documentary produced and narrated by DiCaprio highlighting the plight of the natural world. The film attempts to pinpoint the last moment when change is possible, exploring how humanity has found itself teetering at the edge of a precipice. A constellation of star names, including Stephen Hawking, Mikhail Gorbachev and Wangari Maathai, speak out providing visionary solutions to help save the the planet.

Playing on its green credentials, distributors Warner Brothers are releasing the film using a decidedly offbeat strategy. It will open in just one London cinema, where it will run exclusively for four weeks - the idea being to save on producing hundreds of 35mm movie prints that will ultimately end up in landfill - followed swiftly by a national DVD release and then a screening on Channel 4 within the next couple of months.

While the sceptics among us might be wont to suggest this more likely points to a reluctance to put a niche film out on general release, not so says Warner Bros. David Hall, international campaign director of The Climate Group's Together initiative, has nothing but praise for the project. "The 11th Hour is an important, hard-hitting film which picks up where An Inconvenient Truth left off," he says.

"Warner Brothers is supporting our Together Campaign which helps raise awareness of the choices people can make in their everyday life to help tackle climate change."

Another high-profile leading man eschewing Hollywood glamour for worthier activities is actor Ryan Gosling, a fervent campaigner against the genocide in Sudan. The star of films such as The Notebook, Half Nelson and the soon to be released Lars and the Real Girl can regularly be spotted sporting a T-shirt with the word "Darfur" emblazoned across the front in protest of the atrocities. Last summer he flew to Sudan to research a screenplay which he hopes will highlight the troubles in the region. Gosling is now said to have fast-tracked the project in the hope of bringing it to the big screen later this year.

Up-and-coming actress Q'orianka Kilcher, who played Pocahontas in The New World, was awarded the Brower Youth Award - one of the foremost environmental prizes in the US - last October for her work highlighting the petroleum industry's contamination of the Amazon River Basin. Kilcher starred in and co-produced a documentary on the the issue, touring impacted communities, interviewing affected individuals and meeting with employees of oil companies to request that they also tour the stricken areas.

There's also rumblings on the Los Angeles jungle drums of a Captain Planet and the Planeteers live-action movie - based on the animated environmentalist TV series - due for release next year. The show's creator, American media mogul and philanthropist Ted Turner, is reportedly in talks about bringing Captain Planet to the big screen.

Still it begs the question: when did Hollywood stars become the self-proclaimed saviours of the world? Quicker than you can say Toyota Prius (Cameron Diaz's eco-friendly, hybrid engine car of choice) everyone seems to be getting in on the act. Brad Pitt has designed environmentally friendly regeneration housing in New Orleans, Ferrell has been spotted shopping for solar panels and Cate Blanchett has given her home a green makeover.

Madonna, meanwhile, is reported to be swapping private jets for commercial flights while the likes of Jake Gyllenhaal, Sarah Michelle Gellar and Naomi Watts are forgoing limos to glide around tinsel town on bikes. Musicians including Alanis Morissette, Sheryl Crow and Norah Jones are apparently striving to make their tours greener and reduce their carbon footprints.

In fact, these days A-listers are more likely to be found with a green guru by their side than the once requisite personal trainer, stylist or new-age spiritualist. Among those most in demand is Danny Seo, an environmental lifestyle consultant and the author of Simply Green Parties. Seo started out as an activist, forming his own protest group when he was just 12 years old. Now 30, he's the go-to man for eco-conscious celebrities, advising them on everything from environmentally sound decor to sourcing "non-conflict" diamonds.

Fans of cult US TV show Heroes will be familiar with the mantra "Save the cheerleader, save the world", but it's the actress herself - Hayden Panettiere - who has been doing the heroics lately. Panettiere - in a teeny tiny bikini, natch - was involved in a protest to disrupt the annual dolphin hunt in Taiji, Japan in November. The Japanese authorities swiftly issued a warrant for her arrest, viewing the condemnation of the dolphin hunt as an attack on their culture. Unperturbed, Panettiere has since delivered a letter to the Japanese ambassador in the US calling for an end to Japan's "scientific hunt" of whales.

Where will it all end, you might wonder? As nice as the idea of ethical Hollywood seems, it's hard to dislodge the notion that it all seems a little contrived. Chances are, like finding Ferrell funny, it may just be another passing fad.