Editor's Note: This guest post was written by Bill McKibben, who wrote the first book for a general audience on climate change, The End of Nature, in 1989. A scholar in residence at Middlebury College, he's the co-founder of 350.org

For years people working on climate change have worried that it's too complicated for "ordinary people" to understand, and often as a result they've paid too little attention to the science and too much to slogans.

So it's good news--sort of--to be able to say with conviction: don't worry. People can handle it. For the last 18 months they've been spreading one number in particular: 350. As in parts per million carbon dioxide. Two years ago it was an obscure scientific data point, which a team of American scientists calculated was the maximum safe amount of co2 the atmosphere could contain. Anything more, they said, was not compatible with "the planet on which civilization developed and to which life is adapted." It's technical, it's scary (because we're already past it, at 390 parts per million, hence all those melting ice caps). You would think it would be the last thing on earth people would choose as a rallying cry.

And yet they have. For more than a year "ordinary people" all over the world have done something extraordinary. They've pushed those three digits into the middle of the climate debate. Churches have rung their bells 350 times, and in mosques people have recited 350 verses from the Koran; yogis have made 350 sun salutations, and rabbis have blown the shofar horn 350 times. Great athletes have led 350-kilometer bike rides, and planted the 350 flag on top of the world's highest peaks; entire government cabinets have gone underwater to pass 350 resolutions from the edge of dying coral reefs. In late October, at 5,200 demonstrations in 181 nations, people came together around that number--CNN called it the 'most widespread day of action in the planet's history.'

Once the people led, others followed. The UN's chief climate scientist, Rajendra Pachauri, said that 350 needed to be the world's target. Al Gore--who has been a staunch ally since day one--said just last week that whatever is negotiated at Copenhagen will be at best a beginning, because the world must go to 350. 92 nations have endorsed the target--but they're the poorest, most vulnerable nations on earth.

I don't know how it all will end in Copenhagen. For the moment, 350 is still in the treaty, as part of a global 'shared vision.' That's better than nothing, even if it doesn't have much teeth. But we're going to find out how committed to the science our leaders really are. And no matter what we're not giving up.