How optimistic are you feeling about solving the challenge of climate change?
Earlier generations managed to resolve the big issues of their time, such as slavery or apartheid. There is nothing stopping us - other than ourselves. We're right at the end of the time when we can still do something. I'm very optimistic because of the number of people across society who are realising this and focusing all of their energies on it.
Does it all hang on Copenhagen?
The only chance is an international deal. If at Copenhagen they fail to make a deal that's as strong as the science demands, then we're up shit creek.
On 1 January, the 10:10 campaign kicks in. Whatever happens at Copenhagen, the people and businesses of Britain are going to start cutting their emissions. Understanding climate change and how close to the cliff edge we are, I couldn't work on anything else.
If you weren't doing this, what would you do?
I could never be a politician because I can't stand the process, but politicians are the ones who get to decide. I would also like to do little things, such as getting water fountains installed in Tube stations to encourage people not to buy plastic bottles. I would have liked to be a music producer. Basically, I want to have ten lives so I can do ten different things at the same time.
How do you find time to do everything?
I get bored very easily. I was always making up schemes when I was younger - let's put on a pop concert, or get all the teachers to stop driving. Also, I feel very lucky to have been born in this era. I remember telling a school careers adviser: I want to direct a play, I want to make a feature film, I want to write a book. They said, "Jesus, you can't do all that." But we don't have any limits. We are the most privileged that will ever live, because everybody who comes after will have to deal with climate change, and for everyone before us, it was a lot harder.
Why don't we all do more about climate change?
It's a mixture of society being set up wrong and laziness. The fundamental problem is the time lag - the 30 years between what we do and what happens as a result of what we do. And that's an intellectual challenge.
What made you an environmentalist?
My grandmother never threw things away. She would say there's only a certain quantity of things in the world and so we have to share them. That's the core of environmentalism. She used to shout at me for ripping all the wrapping paper off the presents. I argued back, but she was clearly right, and I guessed I should change.
Do you argue a lot?
Yes. With my best friends, I argue all the time.
Is there a plan?
There isn't a plan. You have only a short period in life when you don't have that many responsibilities and you have all your faculties. I've always felt like I don't want to waste those 20 years or so doing something pointless. I want to do really high-quality stuff, whether that's a film or a campaign or whatever.
Does activism work?
You've only to look at history to see that it does. A lot of people are looking for excuses not to change their lives, and that's one of them.
Do you vote?
Of course I vote!
Do you feel the responsibility of being seen as a leader?
The McLibel defendant Helen Steel told me it's not that she feels brilliant that she did McLibel, but she knows how bad she would have felt if she hadn't. It's kind of the same for me. I have to fight as hard as I can to prevent our species getting wiped out. And if others think that's good, then I'm glad.
Do you think about climate change all the time?
Yes, but I don't get frustrated. I'm inspired by the people who are working on it. I almost forget that 99 per cent of the population don't give a shit, or are ignoring the problem.
Will you ever feel like you have done enough?
If I got hit by a bus tomorrow, I would feel like I did well, which is a good feeling I've never had before. One of my motivations for working so hard is that I'll hate myself if, when I'm old, I think I stood by. If you stand by and let something happen, you are responsible, too.
Are we all doomed?
No. The scientists tell us - the peer-reviewed, not the oil-industry ones - that there is still time from a science point of view. But nine out of ten of them don't think that the politics is going to pull it together. That's where everybody needs to pitch in.
Read an extended version  of this interview.
1972 Born in London
1979 Writes and directs her first play at North Ealing Primary School
1997 Sets up Spanner Films. Lawyers at BBC1 and Channel 4 stop the broadcast of her first film, McLibel
2005 McLibel is released worldwide, and is later picked for the BFI series Ten Documentaries Which Changed the World
2009 Releases her climate-change film The Age of Stupid, co-written by Mark Lynas. Launches the 10:10 campaign (1010uk.org)