Last year, NASA climatologist Jim Hansen explained that atmospheric carbon dioxide (CO2) needed to be reduced from 385 parts per million (ppm) “if humanity wishes to preserve a planet similar to that on which civilization developed and to which life on earth is adapted.” Scientists have identified 350 ppm as the safe upper limit of CO2 in the earth’s atmosphere, a number Bill McKibben, Noam Chomsky, Terry Tempest Williams and others have seized on in their support for the environmental activist organization 350.org.

Reigning in carbon emissions won’t be easy as, McKibben pointed out in The Nation, but “We do have one thing going for us: this new tool the web, which at least allows you to imagine something like a grassroots global effort. If the Internet was built for anything, it was built for sharing this number, for making people understand that ‘350’ stands for a kind of safety, a kind of possibility, a kind of future.” Visit 350.org to learn more about the science behind this important figure and how you can get involved before the International Day of Climate Action on October 24, 2009.

—Corbin Hiar

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