by Judith Curry
I am currently digging into the treatment of uncertainty in the IPCC AR5, pursuant to Part I and the paper that I am writing for Climate Change.
Comparison of AR4 and AR5 treatments of uncertainty
I spotted this interesting document that compares the treatment of uncertainty in AR5 vs AR4. Some key changes:
Evidence and Agreement. The AR4 guidance (paragraph 12)2 presented calibrated language to describe the amount of evidence and degree of agreement regarding a finding in qualitative terms. The AR5 guidance (paragraph 8) extends this approach to incorporate explicit evaluation of the type, amount, quality, and consistency of evidence, with a modified set of summary terms. Author teams are instructed to make this evaluation of evidence and agreement the basis for any key finding, even those that employ other calibrated language (level of confidence, likelihood), and to provide a traceable account of this evaluation in the text of their chapters.
Confidence. The AR4 guidance (paragraph 13) presented quantitatively calibrated levels of confidence intended to characterize uncertainty based on expert judgment regarding the correctness of a model, analysis or statement. The AR5 guidance (paragraph 9) retains these terms, but no longer defines them quantitatively. Instead, levels of confidence are intended to synthesize author teams’ judgments about the validity of findings as determined through their evaluation of evidence and agreement, and to communicate their relative level of confidence qualitatively.
Likelihood. The AR4 guidance (paragraph 14) presented the quantitative likelihood scale, to be used when describing a probabilistic assessment of a variable or its change, or some well defined outcome having occurred or occurring in the future. The AR5 guidance (paragraph 10) retains this scale, more explicitly instructing authors to base likelihood assignments on quantitative analysis and noting that three additional terms were used in AR4 in limited circumstances and may be used in AR5 when appropriate. The AR5 guidance also is more explicit about the relationship and distinction between confidence and likelihood, and encourages the presentation of more precise probabilistic information (e.g., percentile ranges, probability distributions) instead of likelihood when possible.
Annex B of the doc presents their response to the IAC review committee.
For reference, the latest draft of the AR5 uncertainty guidelines that I can find are in Appendix 4 of this doc.
JC’s draft paper
Well, I think the AR5 guidance for treating uncertainty definitely takes some steps in the right direction; it remains to be seen how these guidelines are actually applied. But I am getting the impression that the issue of uncertainty is being taken more seriously by the IPCC.
My (tardy) paper for the special issue of Climate Change is still in draft form; when it is complete I will post for comments. The section headings that I have tentatively decided on are:
Reasoning About Climate Uncertainty
2. Framing of the climate change problem
3. Uncertainty, ambiguity, indeterminancy, and ignorance
4. Consensus, disagreement and justification
5. Reasoning about uncertainty
The target length is 3000 words, which is pretty tight to summarize all of my concerns. Note, that in preparing to write this paper, I went through the previous uncertainty threads, and found many useful comments. I look forward to your further ideas and input on this topic.
Moderation note: this is a technical thread, comments will be moderated for relevance.