As he told Grist readers earlier this year, Bill McKibben is kicking off a new campaign based around the number 350 — as in 350 parts-per-million of CO2 in the atmosphere, the level scientists like Jim Hansen now believe is the safe upper threshold. (The bad news: we’re already closing in on 390.)

The website, 350.org, officially launches in June, and in the meantime you can check out the 350 Challenge.

McKibben held a conference call with bloggers yesterday, wherein he explained the new campaign and discussed strategy. You can listen to it here:

Here’s the badge you can embed for the 350 Challenge:

Brighter Planet's 350 Challenge