"But didn't Al Gore already make the climate change documentary?" has been a common question over the five years we've been making The Age of Stupid. It never fails to raise a weary smile. Casablanca had already done love, so why bother with Brokeback Mountain? Apocalypse Now did war. What's the point of Three Kings?

Love and war will soon become minor concerns, as the full horrors of climate change begin to unfold.

When I started my first documentary, McLibel, I never for a moment thought it would have any effect on that immovable corporate mountain called McDonald's. I just found the story of two people daring to stand up to the Big Mac enormously inspiring - and felt that others would too. But only 10 years later - thanks also to Fast Food Nation, Jamie's School Dinners and Super Size Me - has there been a sea-change in public awareness about healthy eating. McDonald's UK profits have since collapsed and advertising junk food to children is now banned.

Someone recently called independent cinema documentaries: "the new rock'n'roll". Forget writing books, singing songs, taking photographs, or even building websites. If you have a burning idea you need to communicate, uncensored, with maximum possible emotional punch and a potential audience of tens of millions, a doc's the way to go.

So in my not very humble opinion we need more, not fewer, films about every aspect of the climate crisis and how we might yet solve it. Inconvenient Truth did the science. Fantastic. 11th Hour investigated climate change alongside its non-identical twin, peak oil. No Impact Man gets on to practical solutions from an individual's perspective and The Power of Community does the same at the community level. Our film, The Age of Stupid, focuses on the big moral human stuff.

Which is all good. But even the most powerful film in the history of cinema is never going to change anything if nobody sees it. McLibel eventually managed to amass 25m viewers, with no distribution budget whatsoever and just me on the team. For The Age of Stupid we now have more than 1,000 volunteers working from every corner of the planet and a small (but dwindling) pot of cash. So together we're aiming for ten times McLibel's viewers: 250m.

It kicks off next Monday, September 21 at the Global Premiere in New York. Movie stars, politicians and climate thinkers will arrive at our solar-powered cinema tent by sailing boat, bike, rickshaw, skateboard or low-carbon transport of their choice, before braving the photographers on the green carpet. Following the screening of The Age of Stupid, we will be joined live by scientists on a melting glacier in the Himalayas and in a rainforest in Indonesia. Radiohead's Thom Yorke will wrap the evening with a little live music. All of which will be broadcast live by satellite to 440 theatres across America and then to 52 countries, from Argentina and Austria to Papua New Guinea and Peru.

And if we do reach 250m people, and the majority of them do agree with the film's key thesis - that unless we move very, very fast we will make the planet uninhabitable - then so what? What influence could 250m angry, inspired, motivated citizens possibly have in 2009, the year of the Copenhagen climate summit, when the governments of the world will come together in December to finalise the successor to the Kyoto treaty?

• Franny Armstrong is the director of The Age of Stupid and the founder of the 10:10 climate change campaign. You can buy tickets for The Age of Stupid Global Premiere on Sept 21 - one night only - at www.ageofstupid.net. And you can enter the Guardian's competition to win tickets here.