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Saturday, March 9, 2013

Hockey Stick, Scythe, Bottle Rocket, you take your pick: "Why Climate Sensitivity Is Not Policy-Relevant" by Michael Tobis

by Michael Tobis, Planet 3.0, March 8, 2013

First of all, we all owe Joe Romm thanks for being quick to draw up the natural extension to the Marcott et al. graph showing the consensus picture of the near future global temperature in the light of this new result:


After duly boggling about how horrible this looks on this particular time scale, a second thought hits.

A number of commentators are very interested in debating how sensitive the climate is to CO2 (the equilibrium climate sensitivity, usually expressed as temperature change per doubling of CO2 concentration). A great deal of talk goes toward discussion that this number may be as little as half of the consensus best estimate. This is highly unlikely, and if true it’s very interesting as a matter of science. But is it policy-relevant?

Given that on this time scale the warming pulse is essentially vertical, it’s easy to modify the plot to be a good approximation of the trajectory if the consensus is actually so badly wrong that it is more than double the real sensitivity. It looks like this:


Given that this change still takes us well outside the experience of humanity in range, and well outside the normal experience of the biosphere in rapidity, such an outcome offers no grounds for complacency. And the data themselves plainly argue against a sensitivity smaller than that.

Clearly we are already in trouble and need to stop accumulating carbon in the atmosphere as soon as possible. No plausible sensitivity argument can change this enough to cut us any slack.

No plausible sensitivity-related argument is policy relevant.

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