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A “More Likely Than Not” IPCC Mystery

Did the IPCC adhere to its own guidelines, or just twisted them whenever convenient?

(Most of the following text is extracted from a comment earlier today by John DeFayette)

I hope you can help me with an old, nagging questions that I have regarding the IPCC’s AR4.  To be clear, I have read the whole of the WG-I report.  There is absolutely no need to read beyond that, since WG-II and WG-III are mere science fiction once you understand the lack of conclusion in WG-I.

In my opinion the WG-I report document is well written.  For those who can read beyond the summaries the scientific evaluations are mostly honest in their admission of our ignorance regarding our climate. However, the politicians weighed in heavily even with the report’s body, and here is the question:

Who decided, and when was the decision made, to add the uncertainty category “more likely than not” to the uncertainty table (Table 4) in AR4?

Table 4 - Likelihood/Uncertainty

Table 4 - Likelihood/Uncertainty

note 12 and Table 2, mentioned in Table 4

note 12 and Table 2, mentioned in Table 4

The question is fundamental since it turns a perfectly reasonable document into a political club.

Clearly, an honest IPCC panel hammered out a reasonable likelihood scale in July 2005, published as an annex to AR4 in 2007.  Table 4 in the uncertainty guidelines document indicates the terminology “as likely as not” for the probability zone around 50% (from 33% to 66%).  This is perfectly reasonable, since a 50-50 likelihood or thereabouts has the same meaning as a coin toss.  The document further instructs the authors of AR4 to refrain from messing with these terms (note 10).

note 10

note 10

To my dismay, I find the final AR4 littered with a new term, “more likely than not” plopped right there at the 50-yard line where I would expect to find a balanced “as likely as not.”  A short search leads me to Box 1.1 of AR4 Chapter 1 as well as Box TS.1 in the Technical Summary, where I find that the AR4 authors have simply added the new term “…in order to provide a more specific assessment of aspects including attribution and radiative forcing.”

Box 1.1 Likelihood/Uncertainty table

Box 1.1 Likelihood/Uncertainty table

Box TS.1 Likelihood/Uncertainty table

Box TS.1 Likelihood/Uncertainty table

[NOTE BY MAURIZIO: Box 1.1 and Box TS.1 claim that in AR4-WGI-Chapter 2 “the basis on which the authors have determined particular levels of scientific understanding uses a combination of approaches consistent with the uncertainty guidance note as explained in detail in Section 2.9.2 and Table 2.11“.  Neither that Section nor that Table explain anything of the sort.

Box TS.1 reference to Section 2.9.2 and Table 2.11

Box TS.1 reference to Section 2.9.2 and Table 2.11

Section 2.9.2 (part 1)

Section 2.9.2 (part 1)

Section 2.9.2 (part 2)

Section 2.9.2 (part 2)

Section 2.9.2 (part 3)

Section 2.9.2 (part 3)

Table 2.11 (part 1)

Table 2.11 (part 1)

Table 2.11 (part 2)

Table 2.11 (part 2)

Table 2.11 (part 3)

Table 2.11 (part 3)

]

With this wondrous little change the AR4 is no longer a document that must admit that human activity may or may not (we don’t really know) cause an increase in hurricanes, in heavy precipitation events, in heat waves, droughts and more.  Instead it says “more likely than not” in these cases.  Obviously, the original terminology only allowed for a vague “we don’t know” whether the coin will land heads up; it sounds much better to say that the coin is more likely to land heads up than tails up.

[NOTE BY MAURIZIO: Coincidentally, the category “More likely than not” is the only one overlapping with another category, “About as likely as not“. Whoever decided not to follow the IPCC’s own guidelines, forgot to take notice that (>50%) is a subset of (33% to 66%)]

Scanning the two-year-old news I find no references to enraged citizens.  I wonder if it is possible to find the authors who are responsible for this semantic “sleight-of-hand”?

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  1. 2010/01/28 at 03:39

    Thanks for an amusing and important discussion about a little addition that tilts the whole issue…

  2. Blous79
    2010/01/14 at 01:17

    Specifically, it is feasible that black carbon emissions help create clouds which creates negative feedback on temperature. A huge chunk of the IPCC evidence on radiative forcing appears to arise from computer models, which depend on the surface temperature data. If the core data is incorrect, the whole argument comes down like a house of cards. Will be fun to watch.

  3. Blous79
    2010/01/14 at 00:55

    I have been reading parts of the details of AR4. I am curious as to how climate scientists have adopted the 90% confidence interval wheras the rest of science tends to use 95% (1.96 standard deviations).

    It seems almost engineered to create the illusion of change from a certain scientific uncertainty.

    • 2010/01/14 at 01:07

      Blous79: when you believe you are saving the world, everything that is needed to be done, must be done…

  4. Anand
    2010/01/03 at 13:48

    It is not even sleight-of-hand. It is brazenly out in the open. They have two categories with overlapping values with different meanings! One of them indicating greater certainty than the other!!

  1. 2009/12/26 at 05:16
  2. 2009/12/24 at 01:48

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