How dire is the climate situation? Pretty dire, if you believe the ten leading climate scientists—including NASA's James Hansen—who just published an eye-popping article, "Target Atmospheric CO2: Where Should Humanity Aim?" (pdf) in the Open Atmospheric Science Journal. It's a staggering report, and worth discussing at length.
At the moment, atmospheric concentrations of carbon dioxide are hovering around 385 parts per million (and growing at 2 ppm per year). Up until recently, many climatologists believed that 450 ppm was a good target to shoot for—if we can stabilize emissions below that level, we'll still get some global warming, which will mess with rainfall patterns, melt some Arctic sea ice, cause a bunch of species to go extinct, but, overall, the planet will skate through more-or-less intact—most importantly, it won't embark on any dangerous feedback loops, such as large-scale melting in the Siberian tundra that would in turn belch up even more methane into the atmosphere, or having ice sheets in Greenland and Antarctica begin to slide irreversibly into the ocean, which could raise sea levels one meter or more by the end of the century (which alone would wreak havoc on many U.S. coastal cities), and after that, higher and higher…
So, yes, the old thinking went, 450 ppm was an acceptable target. It'll be tough to hit—we'd need to slash global emissions roughly 50 percent below 1990 levels by 2050—but doable. Now, though, Hansen and his co-authors are arguing that 450 ppm may actually be way too much carbon dioxide in the air—new research shows that we're already at levels that "compromise the stability of the polar ice sheets," and we need to stabilize at about 350 ppm, reducing emissions below what they are now. (Yale geophysicist Mark Pagini, one of the study's authors, offered some appropriate caveats: "How fast ice sheets and sea level will respond are still poorly understood, but given the potential size of the disaster, I think it’s best not to learn this lesson firsthand.") If we stay at 450 ppm for too long, enough climate-change feedbacks could start kicking in that we reach a point of no return, where the planet marches inexorably over the centuries toward an ice-free state and a devastating 6C rise in global average temperature may become inevitable.
What would averting this fate entail? The only hope of stabilizing atmospheric carbon at 350 ppm, the authors argue, is to phase out all coal use between 2010 and 2030, except for those power plants that can capture and safely sequester their carbon. This, and not oil, should be our short-term focus, they argue, since there may not be enough oil left underground to push the Earth's climate into the danger zone, and, anyway, oil-guzzling cars are harder to replace in the near future than dirty coal plants. I'm not sure I agree with the idea of picking and choosing exactly which fossil fuel to eliminate in the next 20 years—ideally, we would set the proper global targets for CO2 reduction and let markets decide how best to reach that level. But this is one way to conceptualize the matter.
It's worth stepping back and observing that, yes, what these ten authors recommend is an unbelievably daunting task—we're talking about emission cuts far, far, far steeper than those envisioned by Obama or any world leader right now. Phasing out dirty coal use by 2030, at a time when China's erecting a new plant every week, is a gargantuan undertaking. Not impossible, but gargantuan. Now, Joe Romm argues we might have a bit more wiggle room than the paper suggests, noting that it's still unclear how long atmospheric carbon concentrations can remain at 450 ppm before catastrophe strikes: years? decades? centuries? So we might have more time to get back down to 350, though there are enough uncertainties—and the uncertainties cut both ways—that it'd be a gamble.
Still, the main takeaway from all this is that we really need to start reducing emissions immediately—it's not something that can wait a few years (yes, a global recession in some sense buys us time, insofar as emissions won't grow as rapidly as they otherwise might, but even during an economic slump we're still adding carbon to the atmosphere, and the clock is, as they say, ticking).
P.S. Of course, if you're adamant that global warming is all a hoax, and that all these so-called "models" must be crap, the scientists at RealClimate have written up a helpful explanation of what climate models are, how they work, what their limits are, and—most importantly!—how to devise your own model to try to rebut Hansen, et. al. So if someone could get on that by, say, Friday, I'd appreciate it, because the paper discussed above is really darkening my mood. Thanks.
--Bradford Plumer