Global warming: 350 or Bust

Ex-New Yorker writer and now environmental campaigner Bill McKibben has a simple, but very difficult message. The concentration of carbon dioxide in the atmosphere has to be reduced back to 350 parts per million, or the planet will change irrevocably. He explains to Guardian editor Natalie Bennett how he's getting that message across.

One of the other big insights we've had is that the age of a single, huge event is over. We might have tried to organise a march on Washington back in 2007, when we organised the biggest demonstrations against global warming in American history, but to move all of those people together would have been off-message. We realised that we could work in a dispersed way and all of the small actions would add up to more than the sum of their parts.

So we had divers off Key West, hundreds of people in blue shirts forming a human tide line in Manhatten, climbers on the glaciers of the America West that won't be there for much longer. Pictures from all of these actions were uploaded to a website and we were showing them in a hall in Washington.

Far more people than we could get to Washington were involved, and we could bring, say, the Senator from Kansas into the hall and show him what his voters were doing: that had far more impact than a march on Washington in which only a few of his voters would be involved.

We were feeling very smug on April 14 2007 - then immediately afterwards the Arctic started to melt. Scientists who had been sober and concerned were suddenly panicked, and we realised that even our targets were not tough enough.

But it means that we are now getting people together around the world (and best of all in the developing world) to campaign. We've seen Chinese kids marching off with their 350 banners, Buddhist monks and nuns forming the numbers against the background of a melting glacier in Ladakh.

We remain outsiders in a sense: Al Gore has endorsed our quite radical numbers, and so has the alliance of small island states, but we are still making a statement that many are reluctant to hear: this is not a future problem for which solutions are being put in place: we have to act immediately and radically to prevent a clear, present emergency.

The rhetoric is of warding off future threats, insurance policies, but that is not what is going on now. Some scientists, like my old friend James Lovelock, say it is too late, but consensus science says we have a small window left, although it is closing rapidly.

But we are already past 350ppm, and that comes with its own set of realities. We have to move much more quickly and much more powerfully than we are at the moment. We have, for example, to stop burning coal by 2030, and quicker in the Western world.

The negotiations are not really between the US and China, industry and environmentalists, or whatever. They are between human beings and physics and chemistry, and they are poor bargainers. They won't agree to suspend the laws of nature for a decade or so, so that humans can get back on their feet.

You could argue that focusing on 350ppm of carbon is too specialised, too technical, but it is perhaps the most important number on earth. It is a boundary condition. Any value greater than 350 is not compatible with the planet on which civilization developed. What the scientists are saying is like a doctor saying to a patient: "if your cholesterol is higher than this figure, you will have a heart attack."

What our campaign is doing is trying to instill physical reality into a highly politicised regulatory process, to allow a lot more people to have a say than the experts haggling over commas.

When we talk to the White House, we keep hearing: "Make us do it. Build the kind of movement that will make us do it." For Obama to deliver a treaty he has to get 67 senators on his side, which is an extraordinarily high bar, given the realities of American politics.

The upside is that it is really extraordinary, the number and range of people who are getting involved. It is as if the antibodies are finally kicking in within our own immune system; finally, if a little late. The young people especially - almost everyone I work with is half my age - are magnificent. And we are seeing the global south take the lead, and that's about time. In international negotiations, for the first time the global south has enormous leverage.

I've just spent a week in New Zealand, where I gave 25 or 30 speeches that these kids had set up. New Zealand really is too small to make any difference with their emissions, but they are figuring out that politically they can make a difference.

I've also just been meeting with faith communities. On October 24 hundreds of churches will be ringing their bells 350 times; 350 scuba divers will be on the Great Barrier reef; in Spain a guy is going to cook 350 paellas in solar ovens: there will be pictures for editors right around the globe from the moment the sun first comes up on that date.

As all of that suggests, I've been doing a very bad job of work-life balance this year. I've been home for just three or four days. I love the place where I live, in the mountains of the northeastern US, and I love my family, but my 16-year-old daughter - she's at some level the reason why I am here. I'm pleased that she'll be campaigning in India with me next month.

And the Copenhagen talks in December are in some ways our last good bite at the apple. I feel like I'm running a marathon and I expect at the end of the Copenhagen talks I'll collapse. And I'll be able to spend time outside, and time writing, to myself that's how I define myself. To be spending time speechmaking, to be on the other side of the notebook, is a very curious business.