Why the conventionally accepted "acceptable" level of global warming is neither safe nor possible.

At last week's G8 gathering, leaders of the world's 17 biggest greenhouse gas emitters agreed in principle on a ceiling for global temperatures. They pledged to work together to ensure that the planet won't warm more than 2 degrees Celsius (that's 3.6 degrees Fahrenheit) above pre-industrial levels. A mere two years ago, an agreement like this was impossibly ambitious-U.S. leadership was dead set against doing anything about climate change, let alone setting hard targets.The political climate has changed in those two years, but it hasn't changed enough. At the same time, the situation described by climate change science has become increasingly dire. So now, this 2 degree target reflects the vexing state of climate affairs writ large-it's going to be virtually impossible to stay below 2 degrees of warming given the political realities of the moment, while most of the latest science is telling us that 2 degrees might not actually be all that safe.That which is politically possible, in other words, doesn't come close to what is scientifically necessary. Heavy sigh.So where did this 2 degree goal come from? A whole host of studies has set 2 degrees Celsius warming over global average temperatures in 1850 as the start of a climatic danger zone, including the oft-cited 2007 Intergovernmental Panel on Climate Change assessment report that won its authors a Nobel Prize. (These 2 degrees translate roughly to a carbon dioxide concentration of 450 parts per million in our atmosphere.) It's important to note, however, that while politicians and commentators have latched on to this 2 degree target, most climate scientists warn against relying on any such hard number. (Andy Revkin collects a bunch of these hedges and caveats applied to the 2 degrees yardstick in his Dot Earth blog. "Given the drought that already afflicts Australia," say a couple of climatologists, "the crumbling of the sea ice in the Arctic, and the increasing storm damage after only 0.8°C of warming so far, calling 2°C a danger limit seems to us pretty cavalier.")It's even more important to note that the research from which the 2 degree limit emerged may be somewhat outdated. Within the past two years, researchers and observers have been bowled over by the rate at which changes to our climatic systems (arctic ice melt, spiking methane emissions, ocean acidification) were outstripping models. And last year, NASA climatologist Jim Hansen (who's been right about climate more often and for longer than anyone else) dropped a bombshell: the last time the planet was thought to be 2 degrees Celsius warmer-or, more specifically, the last time there were atmospheric CO2 concentrations of 450ppm-was the Cenozoic era, when the world was largely ice-free and sea levels were 200 feet higher. If we hope, Hansen warned, to preserve a planet "similar to the one on which civilization developed and to which life on earth is adapted," the atmosphere must return to 350ppm of carbon dioxide. Which is far lower than the 2 degree target.And now the worse news: Staying below 2 degrees Celsius is going to be really, really hard. It might be impossible. We've seen 0.8 degrees Celsius of warming from 1850 levels already, and (because it takes a few decades for emissions to settle into the atmosphere and really turn up the heat) all the carbon we've spewed lately has locked in another 0.6 degrees Celsius of change. An April poll of climate scientists found that nearly 9 out of 10 "do not believe political efforts to restrict global warming to 2 degrees will succeed." Perhaps worse, only 60 percent felt that "in theory, it was still technically and economically possible to meet the target." George Monbiot, who talked to many of the scientists surveyed, offered this foreboding take: "Quietly in public, loudly in private, climate scientists everywhere are saying the same thing: it's over. The years in which more than two degrees of global warming could have been prevented have passed, the opportunities squandered by denial and delay."So 2 degrees Celsius is probably not a very "safe" target, and we're probably going to blow right by it anyways. But not definitely. The only certainty is that keeping the planet from reaching a true "danger zone" will require an effort comparable to America's mobilization for World War II. But conducted internationally, and lasting for decades. Heading into the big Copenhagen climate talks in December, we need to demand this of our leaders-an international effort commensurate with science's demands. Because, as Bill McKibben recently wrote, "politicians can bend; physics can't."
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