A pickle
Aug 29, 2013
Bishop Hill in Climate: Models, Climate: Statistics, Climate: Surface, Climate: sensitivity

The must-read post this morning is Judith Curry's coverage of a new paper by Kosaka and Xie in Nature. The paper received some attention yesterday, the BBC reporting that it explained the 21st century temperature plateau, saying it was due to...

natural cooling in part of the Pacific ocean.

Although they cover just 8% of the Earth, these colder waters counteracted some of the effect of increased carbon dioxide say the researchers.

But temperatures will rise again when the Pacific swings back to a warmer state, they argue.

However, as Judith Curry notes, if the cause of the pause is the Pacific Decadal Oscillation then size of the rise in the last decades of the twentieth century must have been heavily influenced by it too. And, unmentioned in the text of the Kosaka and Xie paper, she notes that when the paper's authors ran their climate models without greenhouse gas forcings they still got a considerable rise at the end of the twentieth century: in her (eyeball) estimate, more than half the magnitude of the rise when greenhouse gases were included.

In other words the majority of the 20th-century warming may be natural rather than anthropogenic.

Which looks like a bit of a problem for the IPCC's imminent announcement that it's all down to us.

Footnote: I'm going to get into terrible trouble with Doug Keenan if I don't mention this paper by Gerald Roe of the University of Washington. It's entitled "Feedbacks, Timescales and Seeing Red" and looks at applications of feedback analysis to geophysical systems, including the PDO. Roe concludes that "By these statistical measures, the PDO should be characterized neither as decadal nor as an oscillation (but it is in the Pacific)", although this statement comes with caveats that are worth reading. I surmise that we are still far from understanding what is going on here. It's all a bit of a pickle.

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