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The nation’s top climate scientist, NASA’s James Hansen, apparently now believes “the safe upper limit for atmospheric CO2 is no more than 350 ppm,” according to an op-ed by the great environmental writer Bill McKibben. Yet while preindustrial levels were 280, we’re now already at more than 380 and rising 2 ppm a year!

Like many people, in the 1990s I believed 550 was the target needed to avoid climate catastrophe — but now it’s clear that:

550 ppm would lead to the greatest disaster ever experienced by human civilization — returning us to temperatures last seen when sea levels were some 80 feet higher. This is especially true because … long before we hit 550, major carbon cycle feedbacks — the loss of carbon from the tundra and the Amazon, the saturation of the ocean sink (already beginning) would almost certainly kick into high gear, inevitably pushing us to much, much higher CO2 levels (see here, here, and my book).

Exactly when those feedbacks seriously kick in is the rub. No one knows for sure, but based on my review of the literature and interviews of leading climate scientists, somewhere between 400 and 500 ppm seems most likely. It could be lower, but it probably couldn’t be much higher.

So I, like the Center for American Progress and the world’s top climate scientists, now believe 450 ppm is the upper bound. That said, I have spent two decades managing, analyzing, researching, and writing about climate solutions and can state with some confidence that:

Staying below 450 ppm is technologically doable, but would be the greatest achievement in the history of the human race, by far. It would require a global effort sustained for decades, comparable to what the U.S. did for just the few years of World War II (the biggest obstacle is not technological, but political — conservatives currently would never let progressives and moderates pursue such a strategy). If 350 ppm is needed (and I’m not at all sure it is) then the deniers and delayers have won, since such a target is hopeless.

In 2008, I will devote a fair amount of ink bits to laying out the solution (there really is only one), but to understand why 450 is so hard, and 350 all but inconceivable, let’s look at the odd way McKibben describes the solution:

And we’re already past 350. Does that mean we’re doomed? Not quite. Not any more than your doctor telling you that your cholesterol is way too high means the game is over. Much like the way your body will thin its blood if you give up cheese fries, so the Earth naturally gets rid of some of its CO2each year. We just need to stop putting more in and, over time, the number will fall, perhaps fast enough to avert the worst damage.

Not a great analogy. Yes, CO2 concentrations will probably start dropping once we cut emissions 80 percent from current levels. But you can change your entire diet — cut cholestorol intake or carbohydrates 80 percent or more — tomorrow. Humanity cannot, however, cut its hydrocarbon diet 80 percent tomorrow or even, realistically, in 10 years. That would require replacing the world’s entire energy infrastructure — power plants, cars, planes, factories, fueling infrastructure, large parts of homes and commercial buildings — while simultaneously deploying a hydrocarbon-free energy system in the rapidly-growing developing world.

McKibben certainly understands some of the difficulty:

That “just,” of course, hides the biggest political and economic task we’ve ever faced: weaning ourselves from coal, gas and oil. The difference between 550 and 350 is that the weaning has to happen now, and everywhere. No more passing the buck. The gentle measures bandied about at Bali, themselves way too much for the Bush administration, don’t come close. Hansen called for an immediate ban on new coal-fired power plants that don’t capture carbon, the phaseout of old coal-fired generators, and a tax on carbon high enough to make sure that we leave tar sands and oil shale in the ground. To use the medical analogy, we’re not talking statins to drop your cholesterol; we’re talking huge changes in every aspect of your daily life.

A better analogy might be stomach stapling, but even that doesn’t do justice to what we would need to do to get to 350. Hansen’s three proposals are a drop in the bucket. Dealing with electricity is trivial compared to dealing with transportation.

Suppose we could get global carbon emissions to peak in 2020 at 10 billion tons, level off for a few years, and then decline 3 percent per year afterwards. No easy feat since emissions are currently at 8 billion and rising over 3 percent per year. China and India, for instance, would have to agree to a hard emissions cap in 2020. Rich countries would need to start slashing emissions immediately. CO2 concentrations in 2020 would be about 410 ppm (and rising over 2 ppm a year).

Around 2050, we’d be at 5 billion tons and very likely over 450 ppm, rising over 1 ppm a year. But remember, we need to average 5 billion tons a year for the entire century just to stabilize at 450 ppm (according to the IPCC — and that is probably a best-case scenario)!

So the scenario I laid out won’t get us to below 450 (I have a long discussion in the book about why beating 500 ppm is so hard if we try to do it the traditional (i.e., slow) way). That’s why I say 450 needs a World War II-scale effort starting in the next decade. I think 350 ppm is simply beyond serious practical and political consideration. You might as well tell people we need to develop a time machine to go back 20 years and warn the world that we need to start cutting emissions then … then again, who would listen.? (And whom would we send back, anyway? That’s an interesting parlor game all by itself.) McKibben ends:

But at least we’re homing in on the right number. Three hundred and fifty is the number every person needs to know.

I part company with him here. I haven’t talked to Hansen yet and I’ll reserve further judgment until I see a paper or PPT by him.

Since beating 450 ppm is doable and certainly necessary, that’s where I draw the line. One advantage of pursuing 450 is that if we do get some sort of unexpected breakthrough — a cheap and practical way to draw CO2 out of the air (that doesn’t use a lot of land, water, or energy) and stick it someplace permanent — then we would have a system in place to deploy it fast enough to perhaps get to below 400 ppm. And even if turns out 450 doesn’t avert catastrophe, it will surely slow down the impacts enough to make adaptation more viable.

So I’m sticking with 450. Implausible? Yes. Impossible? No. Less costly than inaction? By far.