WHAT DOES 95% CONFIDENCE MEAN?

Below is an excerpt from Judith Curry's blog, "Climate Etc" 

 

Reporter:  I’m hoping you can answer a question about the upcoming IPCC report. When the report states that scientists are “95%  certain” that human activities are largely the cause of global warming, what does that mean? How is 95% calculated? What is the basis for it? And if the certainty rate has risen from 90% in 2007 to 95% now, does that mean that the likelihood of something is greater? Or that scientists are just more certain? And is there a difference?

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JC:  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.

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Reporter:  You mean they sit around and say, “How certain are you?” ”Oh, I feel about 95% 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% certain.”  Please tell me it’s more rigorous than that.

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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%.

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Reporter:  So it really is as subjective as that?

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JC:  As far as I know, this is what goes on.  All this has never been documented.