As a young environmentalist, I fought global warming with words, writing what’s often called the first general-interest volume on climate change. It became an international bestseller, published in 24 languages. But it flopped as a piece of social activism, doing virtually nothing to slow the heating of the -planet. So, two decades later, I’m promoting something new: the number 350.

That’s the upper limit for carbon dioxide in the atmosphere (measured in parts per million). Anything above it, say some NASA scientists, is incompatible with “the planet on which civilization developed.” When I first heard this, in 2008, I figured the idea of heat-trapping particles would be too obscure for most people—until I remembered my cholesterol number. I don’t need to understand the lipid system to know that if my cholesterol count gets too high, I’ve got to lay off the chocolate cake. Today our atmosphere is at 392, which helps explain the hottest summer in history, the fires in Russia, and the floods in Pakistan. Clearly, it’s time to change our habits.

Bill McKibben in Newsweek. On Sunday (10/10/10), 350.org is organizing the Global Work Party and I hope you can join me on a bike ride to Heron Pond Farms for Farm Potluck for the Climate.