This autumn, as we speed towards Copenhagen, an almost infinite number of words will be spilled about the environment, the atmosphere, the climate, the horror. I’m pretty much all in favour of this – last year, for instance, I edited a massive doorstop of an anthology called American Earth for the Library of America. It collected America’s single greatest literature – the stories of the encounter between people and nature – from Thoreau through Muir through Rachel Carson through the amazing bloom of contemporary environmental writers: Rick Bass, Terry Tempest Williams, the incomparable Wendell Berry. I’m proud to have played some small role in that flood of words myself: The End of Nature, when it came out in 1989, was the first book for a general audience about climate change, and since then I’ve gone on to write a dozen more.

But right now –this autumn, with much on the line – I’ve put my faith not in letters but in numbers. In digits, in Arabic numerals.

We’ve been running a huge ­ campaign – it’s blown up into the first real grassroots global political protest about global warming – called 350.org. The number comes from new science that followed the shocking melt of Arctic ice in the summer of 2007. Researchers became convinced that climate change was happening faster than they had previously expected, and that they had enough data to put a real number on it. That number was 350, as in parts per million CO2 in the atmosphere. Above that level, in the powerful (and peer-reviewed) words of Nasa scientist James Hansen, we can’t have a planet “similar to the one on which civilisation developed or to which life on earth is adapted”.

So we took that number and ran with it – not with a slogan, not with a motto, but with a number. We’re building toward a giant day of global action on 24 October, that will see thousands of events from every corner of the Earth, as far-flung a political protest as the planet has ever seen. And as far as I know, there’s never been a big political campaign built around a scientific data point. But it’s worked, and for a few reasons – a few reasons that help us think about how science might become integrated a bit more easily into the work of artists of various kinds.

First, it sets a clear target, which is useful in a world of confusing and shifting political messages. When the world’s leaders come to Copehagen in December, too many journalists will be watching to see if they produce “an agreement”. Of course they’ll produce an agreement – but an agreement is not what we need. We need a solution which in this case is defined not by political fiat but by science. The negotiation isn’t really China versus the EU versus the US, it’s all of us versus physics and chemistry. And since physics and chemistry really refuse to negotiate, we better meet their bottom line, which appears to be 350 parts per million. It’s a way to make sure our leaders are dealing with the actual situation, not their particular political fortunes.

Second, Arabic numerals are the one thing that cross linguistic boundaries. I was in the hills above Bethlehem the other day, for a meeting with Israeli, Palestinian and Jordanian activists. Their plan for 24 October: on the Israeli shore of the Dead Sea (which is shrinking rapidly as the planet warms) a giant human “three”; on the Palestinian beach, a “five”; on Jordan’s strand a giant zero. We’re doing the same kind of planetary Scrabble in many of the world’s big cities; in London, for instance, a giant human “five” along the Thames near the London Eye. The resulting pictures will be as comprehensible in Beijing as in Vancouver, in Delhi as in Quito.

Best of all, in a way, the three digits are themselves devoid of significance. When we began they meant nothing to anybody – they were inscrutable. So we get to fill them with emotion, and that’s exactly what’s been happening. Dozens of musicians have written 350 songs and anthems; painters (and graffiti artists) have done beautiful renderings. Some of the world’s best writers have produced 350-word poems and stories and essays. And slowly, over time, the numbers have come to hold their own meaning: as a kind of warning, and as a kind of hope.

They’re arguably the most important numbers on earth, the boundary condition for our continued existence here. They are tough and radical, those digits – we’re already past them, at 390 parts per million CO2 in the atmosphere, and rising by 2% annually: that’s why the Arctic is melting, why Australia is burning, why the world is changing in front of our eyes.

To meet their challenge we’d need a very quick end to fossil fuel burning on this planet, and then we’d need real cooperation from the planet’s oceans and forests, which can slowly suck up the excess carbon. The challenges are immense – the call for 350 goes far beyond what most of our leaders want to hear, because the economic and political change will be impossible to disguise as business as usual. It may all be too much.

But there’s something bracing about reality, and for human beings on this earth those numbers are as close to reality as we’re going to get. They don’t cosset any of us, or let us imagine that “every little bit counts”. They make it clear that, for the future of the atmosphere, only very big steps count. We need a global agreement, and the only way to get that is to build political pressure. And the only way to get that is to build a movement. That’s what we’re doing, and the fact that the movement grows from a wonky number shows the ability of the human imagination to grapple with its fate, even if that fate is expressed numerically. At least for this autumn I’m a numbers guy, and I’m finding it unexpectedly moving.

Bill McKibben’s Deep Economy: Economics As If the World Mattered is published by Oneworld. He is founder of 350.org