Scientists and policy makers have been trying to hash out for years what an acceptable level of climate change might be. Even if we stopped all greenhouse gas emissions tomorrow, the planet will continue warming for decades because it has a lot of inertia—like an avalanche, once it’s started you can’t simply stop it dead.
Many have argued that we should limit our greenhouse gas emissions to that the level of carbon dioxide—or CO2—doesn’t climb above 550 parts per million. (We’re at about 390 ppm now, compared with 280 in pre-industrial times—a 40% jump.)
But at 550, the planet would likely warm up some 3 degrees C above pre-industrial levels—an amount of warming generally regarded as dangerous. Eminent climatologist James Hansen has supported a target of 350 parts per million of carbon dioxide in the atmosphere, and journalist and activist Bill McKibben took up this call with his group 350.org that’s galvanized people around the world.
This is a big set-up to explain a single number—but as McKibben says, this is “the most important number in the world.”
So it’s important to listen when people like Harvard geologist Daniel Schrag of say something like this (from a talk in early 2009, posted on YouTube):
CO2 is higher today than it has been for at least—from direct observations—in the past 650,000 years. And over next 40, 50, 60 years, it will get to about 500 parts per million. It is almost unthinkable that it will not get to 500 parts per million. The discussion, among we who work on energy technologies and climate solutions—is about how to keep it from going higher than 500 parts per million. And that’s what a lot of people don’t understand about the climate discussion. It’s not whether there will be climate change. It’s how bad it’s going to get.
… This is really a remarkable thing that’s happening to the atmosphere. Today, atmospheric CO2 is higher than what any human—indeed, any hominid species—has ever seen. So there’s going to be all sorts of surprises.
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