350 is the most important number on the planet.
Which is odd, because until about 22 months ago no one even knew it mattered.
But that's when, in December of 2007, NASA's Jim Hansen gave a slide show at the American Geophysical Union annual meeting in San Francisco. He'd been thinking about what it meant that we'd just come through a summer of very rapid ice melt in the high Arctic, and that researchers were reporting "ahead of schedule" changes in dozen other of the earth's big physical features--melting glaciers, acidifying oceans and so on.Combined with reams of paleo-climate data, his team believed they now had enough information to finally draw a red line for the planet: when atmospheric concentrations of carbon dioxide were above 350 ppm, they said, global warming would be dangerously out of control. In fact, they said in the abstract of the paper they soon published, above 350 you couldn't have a planet "similar to the one on which civilization developed and to which life on earth is adapted."
It's as if we suddenly discovered what normal body temperature was, so we'd be able to tell when we were running a fever. In that sense, it came as a great relief.
But in every other sense, it was a pretty devastating number. For one thing, we're already past it, at 390 ppm and rising two ppm annually--that's why the Arctic is melting. For another thing, it means the work nations and individuals must do to reduce their carbon footprints is much larger, and must happen much more swiftly, than we'd believed. Hansen's data shows that as a planet we'd need to get off coal by 2030 in order for the planet's forests and oceans ever to bring atmospheric levels back down below 350--that's the toughest economic and political challenge the earth has ever faced.
But it's not as if we have a choice. The most useful thing about having a number is that it forces us to grow up, to realize that the negotiations that will happen later this fall in Copenhagen aren't really about what we want to do, or what the Chinese want to do, or what Exxon Mobil wants to do. They're about what physics and chemistry want to do: the physical world has set its bottom line at 350, and it's not likely to budge.
Almost every month brings new data showing that Hansen et al were very nearly spot on with their original estimate. In early summer a British team demonstrated that coral reefs won't survive acidified waters unless we get co2 concentrations back down below 360 ppm. Last month, in a cover story in the journal Nature, a European-led team identified nine "planetary boundaries," the most important of which was probably that same 350 ppm line for carbon. Above it, they said, we would run the risk of "threatening the ecological life-support systems that have developed in the late Quaternary environment, and would severely challenge the viability of contemporary human societies." The name of the article: "A Safe Operating Space for Humanity." That's what we're talking about.