Get back to 350 ppm or risk an ice-free planet

Here is the draft [PDF] of the long-awaited defense of why we need an ultimate target of 350 ppm for atmospheric carbon dioxide, by NASA's James Hansen et al., titled "Target Atmospheric CO2: Where Should Humanity Aim?" (Yes, they know we're already at 385 ppm and rising 2 ppm a year.)

The paper does suffer from one analytical weakness that makes it a tad less dire than it appears -- and some people believe the core element of this analysis is wrong (see very end of post), although I don't.

This paper is really just a continuation of Hansen's earlier analysis arguing that the real-world or long-term climate sensitivity of the planet to doubled CO2 [550 ppm] is 6 degrees C -- twice the short-term or fast-feedback-only climate sensitivity used by the IPCC. (You might want to read this post first, as it is a bit clearer on the difference between the two sensitivities.)

The key paleoclimate finding of the article:

We infer from the Cenozoic data that CO2 was the dominant Cenozoic forcing, that CO2 was only ~450 ppm when Antarctica glaciated, and that glaciation is reversible.

That is, if we stabilize at 450 ppm or higher, we risk returning the planet to conditions when it was largely ice-free, when sea levels were higher by more than 200 feet!

Why a carbon price beats technology breakthroughs

The decarbonization data makes clear that if you want to beat 450 ppm and avoid catastrophic climate impacts, a significant price for carbon (plus aggressive technology deployment) is much more important than technology breakthroughs.

That is a central point of this post. That is what I learned in the mid-1990s when I helped to run the billion-dollar office at DOE in charge of federal clean energy technology breakthroughs and deployment -- and had the chance to work with the top scientists and technology modelers at the national labs to figure out how we can cut emissions most quickly and cost-effectively.

The pursuit of the Holy Grail of multiple technology breakthroughs is, in fact, a side show -- and for many, like Bush/Luntz/Gingrich/Lomborg, that pursuit is meant as a complete rhetorical distraction to the public so we can continue to avoid action, as I have repeatedly blogged. It was specifically designed by conservative strategist Frank Luntz as a core delaying strategy.

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