Inside The IPCC Sausage Factory: How Does The 95% Figure Go Into The Meat?
Yesterday, a reporter asked me how the IPCC came up with the 95% number. Here is the exchange that I had with him:
Reporter: I’m hoping you can answer a question about the upcoming IPCC report. When the report states that scientists are “95 percent certain” that human activities are largely to cause for global warming, what does that mean? How is 95 percent calculated? What is the basis for it? And if the certainty rate has risen from 90 n 2007 to 95 percent now, does that mean that the likelihood of something is greater? Or that scientists are just more certain? And is there a difference?
Judith Curry: The 95% is basically expert judgment, it is a negotiated figure among the authors. The increase from 90-95% means that they are more certain. How they can justify this is beyond me.
Reporter: You mean they sit around and say, “How certain are you?” ”Oh, I feel about 95 percent certain. Michael over there at Penn State feels a little more certain. And Judy at Georgia Tech feels a little less. So, yeah, overall I’d say we’re about 95 percent certain.” Please tell me it’s more rigorous than that.
JC: Well I wasn’t in the room, but last report they said 90%, and perhaps they felt it was appropriate or politic that they show progress and up it to 95%.
Reporter: So it really is as subjective as that?
JC: As far as I know, this is what goes on. All this has never been documented.
JC conclusion: Well, I have no idea what goes on in the sausage factory. 95% – take it with a grain of salt (or a stiff whiskey). That’s their story, and they’re sticking to it. Uncertain T. Monster is not happy.