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While I was on vacation last week I took a side trip to New Haven to visit Jeff Park, an old high school friend who’s now a geology professor at Yale. We ate some pizza at Frank Pepe, walked around the campus a bit, and then dropped by his office, where he had a stack of reprints of his latest journal article. Take one, he said. Maybe it’ll be good fodder for the blog.

The title is a mouthful: “Geologic constraints on the glacial amplification of Phanerozoic climate sensitivity,” coauthored with Dana Royer. (The Phanerozoic, in case it’s slipped your mind, is the geologic eon spanning approximately the last 500 million years.) Roughly speaking, the article is an updated look at a computer model that estimates how much climate reacts to a doubling of CO 2 in the atmosphere.

The model originally concluded that a doubling of CO 2 produces a temperature increase just under three degrees Celsius, an estimate that’s in pretty good agreement with other models. So far, so good. But 500 million years is a long time, and several researchers have proposed that climate sensitivity might vary over that period depending on whether or not the earth is in an ice age. So in the new paper, the authors modeled glacial and non-glacial eras separately. And the best fit with the data suggests that climate sensitivity does indeed change depending on glaciation. In fact, during an ice age, the most probable climate sensitivity is six to eight degrees Celsius for a doubling of CO 2 , more than twice the previous estimate.

Why do we care? As the authors drily put it, “Because the human species lives in a glacial interval of Earth history, this modeling result has more than academic interest.” You see, the most recent ice age in human history is the one that started about 30 million years ago and continues to the present day. We’re living through a glacial interval right now, and that means that a doubling of CO 2 in the atmosphere might produce a temperature increase of six to eight degrees Celsius, not the mere three degrees Celsius most commonly estimated.