Numbers have no meaning until we attach it: 365 days, the 747, 1-20-09. So far, I wouldn't say 350 has been tagged, but the environmental writer — and, increasingly, activist — Bill McKibben and some compatriots intend to change that.

To them, the significance is in parts per million, as in, the maximum safe level of carbon dioxide in the atmosphere. Saturday, during an address to a rapt capacity audience in the main gathering space of the Down:2:Earth consumer expo at the Hynes Convention Center, McKibben explained that before the Industrial Revolution, the level was fairly constant at 275 ppm, but it has steadily risen since, and the most recent measurements put the earth at 385 ppm and gaining about 2 ppm per year.

Yes, we're already over the safe limit. Any efforts you hear about reducing carbon in the atmosphere, anywhere on the planet, will harken back to that number. The further over it we go, the more tenuous is our hold on life as we know it. The sooner we get back to 350, the sooner we will have halted the climate crisis.

To draw attention to that goal, McKibben's group has established a website, 350.org, which he said is still in beta, and indeed, there's not much there yet beyond a sign-up sheet. Saying he hasn't been much of an activist before now, "but I'm trying," he incorporated e-mail sign-up sheets into his talk, asking volunteers to come forward and circulate them through the audience as he spoke, and said that when they get started, we'll be contacted.

"What we need is ideas. We need your music. We need your art," he said. "We're going to see if we can figure out together something to actually give us some kind of future."

Another goal of the site, he said, "is to take that number and drive it into every head on the planet. People who don't know a lot will at least know that that's the number to give us safety."

McKibben and six Vermont college students were behind stepitup2007.org, which spawned 1,400 local global-warming demonstrations on April 14, 2007, when they had hoped to get perhaps a hundred of them, he said. He said that some people had suggested a march on Washington, for a massive show of concern, but apart from the disconnect of hundreds of thousands of people expending fossil fuel to gather in Washington to protest global warming, he felt that local expressions of concern were more appropriate, since the problem exists everywhere, and won't be solved just in Washington.

McKibben said that working on his latest book, "Deep Economy," quickly showed him that it wouldn't be predominantly about the environment. He said that people's level of happiness, as charted in surveys, has been declining since the mid-'50s, which is when suburban sprawl and shopping in ever larger, more impersonal markets had begun to take hold.

"What we're really missing is connection and community," he said, adding that the sort of localization that is enlivening farmers' markets and local music scenes at the expense of top-down structures as supermarkets and record labels is needed in politics.

"I'm not an insane optimist by nature," he said, pointing out that "I wrote a book called 'The End of Nature.'" But 350.org is trying to inspire that localization. He likened the effort to the civil rights movement of the '60s, with one important caveat:

"They had the knowledge that they were going to win," citing Martin Luther King's oft-stated observation that they broad sweep of history was toward freedom. "But we don't know that."

What we do know, he said, is that "if we don't rise up in the next couple of years, then we will lose it. The world will be infinitely degraded if we don't act."