Boing Boing links to an interview with environmentalist James Lovelock which is a curious mixture of pessimism and optimism but makes for interesting reading:

Do you think we will survive?

I'm an optimistic pessimist. I think it's wrong to assume we'll survive 2 °C of warming: there are already too many people on Earth. At 4 °C we could not survive with even one-tenth of our current population. The reason is we would not find enough food, unless we synthesised it. Because of this, the cull during this century is going to be huge, up to 90 per cent. The number of people remaining at the end of the century will probably be a billion or less. It has happened before: between the ice ages there were bottlenecks when there were only 2000 people left. It's happening again.

I don't think humans react fast enough or are clever enough to handle what's coming up. Kyoto was 11 years ago. Virtually nothing's been done except endless talk and meetings.

It's a depressing outlook.

Not necessarily. I don't think 9 billion is better than 1 billion. I see humans as rather like the first photosynthesisers, which when they first appeared on the planet caused enormous damage by releasing oxygen - a nasty, poisonous gas. It took a long time, but it turned out in the end to be of enormous benefit. I look on humans in much the same light. For the first time in its 3.5 billion years of existence, the planet has an intelligent, communicating species that can consider the whole system and even do things about it. They are not yet bright enough, they have still to evolve quite a way, but they could become a very positive contributor to planetary welfare.

Lovelock came up with the controversial Gaia Hypothesis in the 1970s. It's a theory which suggests that our bio-sphere, that is every living thing on Earth, acts like one giant organism to regulate the planet's climate to maintain ideal conditions for life. The theory gathered a lot of attention from science fiction authors and from counter-culture types but has been strongly criticized by scientists. It's hard to know what to think when dealing with ideas which as far as I know are pretty far out of the scientific mainsteam. But sometimes it is interesting to keep an open mind and look at new ideas.