By JR on Thursday, July 07, 2011
As most people know, I am a lukewarmer -- somebody who accepts carbon dioxide's full greenhouse potential, but does not accept the much more dubious evidence for net positive feedbacks on top, and who therefore thinks that a temperatuire rise of more than 2C in this century is unlikely.
This view just got a strong boost. Nic Lewis, the indefatigable mathematical sleuth who helped expose the mistakes in a paper about Antarctic temperature trends has been looking at how the IPCC estimates climate sensitivity -- that is, the warming expected for a doubling of CO2. He finds that the one study that estimated sensitivity entirely from experimental data -- Forster and Gregory 2006 -- was distoted by the IPCC when it came to present their results.
The distortion was the imposition of a Bayesian "uniform prior" in a way that statisticians say is wholly inappropriate, because it effectively assumes a priori that strong warming is more probable than it is. Yet you don't even have to know that the use is inappropriate to know that it's inappropriate to take a published result and alter the graph from it, adding an obscure footnote to say you have done so. A published result is a published result.
The effect was to fatten the tail of the graph, making a warming of more than 2C look much more probable.
The probability peaks very sharply at about 1.5 degrees Celsius (meaning that more extreme temperature increases are very unlikely) and even the IPCC tricks couldn't change that much. They just increased other probabilities slightly -- JR
I defy you to look at that graph -- the green one -- and tell me that a temperature rise oif more than 2C is not "unlikely" according to that study. I defy you to look at the graph -- the blue one -- and not conclude that whoever drew it had better have a very good argument for fattening the tail compared with what the authors had originally published.
NIc has found that the IPCC did much the same to most of the other estimates of climate sensitivity, which rely mostly on models. This mistake is central to the IPCC's case, not peripheral. It undermines the credibility of the case for urgent action against climate change and strongly supports the argument that, other things being equal, CO2 doubling will not cause more than a mild and net beneficial warming.
Here's Nic's first paragraph:
"The IPCC Fourth Assessment Report of 2007 (AR4) contained various errors, including the well publicised overestimate of the speed at which Himalayan glaciers would melt. However, the IPCC’s defenders point out that such errors were inadvertent and inconsequential: they did not undermine the scientific basis of AR4. Here I demonstrate an error in the core scientific report (WGI) that came about through the IPCC’s alteration of a peer-reviewed result. This error is highly consequential, since it involves the only instrumental evidence that is climate-model independent cited by the IPCC as to the probability distribution of climate sensitivity, and it substantially increases the apparent risk of high warming from increases in CO2 concentration."