By Andrew C. Revkin

http://dotearth.blogs.nytimes.com/2009/10/24/a-global-focus-on-a-hot-number/

I have a story running on the effort under way today, prompted by  Bill McKibben and building on  conclusions of NASA’s James E. Hansen, to focus the world on  350 parts per million as the long-term target for stabilizing carbon dioxide. Here’s the lede:
Campaigners against global warming have drawn on an arsenal of visually startling tactics over the years, from posing nude on a Swiss glacier to scaling smokestacks at coal-fired power plants.
As Saturday dawned, they tried something new with the goal of prodding countries to get serious about reaching an international climate accord: a synchronized burst of more than 4,300 demonstrations, from the Himalayas to the Great Barrier Reef, all centered on the number 350.

For some prominent climate scientists, that is the upper limit for heat-trapping carbon dioxide in the atmosphere, measured in parts per million. If the gas concentration exceeds that for long, they warn, the world can expect decades of disruptive climate patterns, rising sea levels, drought and famine.
The current concentration of carbon dioxide is 387 parts per million.

The story includes various views on this campaign, and the chosen target. [UPDATE, 3 pm:] In an interview Saturday morning, Bill (I’ve known him long enough that I can’t honestly put in the “Mr.” honorific) explained his focus on the long-term concentration goal, and the limits of this particular action, which he sees as a beginning — even after 20 years of working on the issue:
“When you’re trying to work around the entire globe there’s only so much you can get across,” he told me. “The thing we chose to get across is the most important number. We haven’t taken a position on trading, on technology, on anything except the fact that we have to get back to 350 parts per million. That’s the most we can actually communicate across dozens of languages and the deep divides between rich and poor. Basically it’s just a big potluck dinner.”

He added: “We’re not going to solve the problem in itself, but with any luck this’ll help point everyone in the right direction.”