Parting company with McKibben and, maybe, Hansen

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 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

1. 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….
2. 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 in to high gear, inevitably pushing us to much, much higher CO2 levels (see here and 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:

1. 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).
2. 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:

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