IPCC don´t even know how to quantify uncertainty!

I struggle to understand the way Intergovernmental Panel on Climate Change express uncertainty about the equilibrium climate sensitivity:

“The equilibrium climate sensitivity quantifies the response of the climate system to constant radiative forcing on multi-century time scales. It is defined as the change in global mean surface temperature at equilibrium that is caused by a doubling of the atmospheric CO2 concentration. Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence)16. The lower temperature limit of the assessed likely range is thus less than the 2°C in the AR4, but the upper limit is the same. This assessment reflects improved understanding, the extended temperature record in the atmosphere and ocean, and new estimates of radiative forcing. {TS TFE.6, Figure 1; Box 12.2}» Ref the (Contribution from Working Group I; on the Physical Science basis; to Assessment Report 5 by IPCC).

There actually exists an international guideline on expression of uncertainty – Guide to the expression of uncertainty in measurement. This is the only broadly recognized guideline on the expression of uncertainty. The following seven organizations* supported the development of this Guide, which is published in their name:
BIPM: Bureau International des Poids et Measures, IEC: International Electrotechnical Commission, IFCC: International Federation of Clinical Chemistry **, ISO: International Organization for Standardization, IUPAC: International Union of Pure and Applied Chemistry, IUPAP: International Union of Pure and Applied Physics, OlML: International Organization of Legal Metrology ”.

Intergovernmental Panel on Climate Change failed to identify or recognize this guideline. They made up their own in a hasty way. Their guideline is a largely a joke called:
Guidance Note for Lead Authors of the IPCC Fifth Assessment Report on Consistent Treatment of Uncertainties.
(I will not touch into their qualitative terms of “confidence” and “agreement” in this post. As those terms are more suitable as subjects for phycological inquiry than scientific research, Ref. The logic of scientific Discovery, by Karl Popper, page 24 )

 

 

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Simply put, by Guide to the expression of uncertainty in measurement (Ref. section 7.2.3), the result of an estimate should be reported by:
– giving a full description of how the measurand Y is defined
– stating the result of the measurement as Y = y ± U and give the units of y and U
– giving the approximate level of confidence associated with the interval y ± U and state how it was determined;

Further, “In statistics, the so-called 68–95–99.7 rule is a shorthand used to remember the percentage of values that lie within a band around the mean in a normal distribution with a width of one, two and three standard deviations, respectively; more accurately, 68.27%, 95.45% and 99.73% of the values lie within one, two and three standard deviations of the mean, respectively.» (Wikipedia; 68–95–99.7_rule)

A correct way of expressing equilibrium climate sensitivity could then have been:
Equilibrium climate sensitivity is 3 °C +/- 1,5 °C at 68 % confidence level.
Which is the same as to say that:
Equilibrium climate sensitivity is 3 °C +/- 3 °C at 95 % confidence level.
or:
Equilibrium climate sensitivity is 3 °C +/- 4,5 °C at 99,7 % confidence level.
or even:
Equilibrium climate sensitivity is 3 °C +/- 6 °C at 99,99 % confidence level.

All the statement above are equivalent statements about uncertainty.
(95% confidence level is commonly used for expression of uncertainty.)

So what is Intergovernmental Panel on Climate Change actually saying? Lets look at the statement:
“Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C»
«Likely» is defined as : 66% – 100 % Probability (Ref WGI;AR5; Box TS.1 | Treatment of Uncertainty)

I would interpret their statement to something like this:
The central estimate of equilibrium climate sensitivity is 3 °C.
The uncertainty is 1,5 °C, at a confidence level in the range 68% to 99,99 %.

Which is basically the same as to say that they have no clue about the standard uncertainty.

At least, in my profession, that is what I would tell a supplier if he tried to sell me something having these specifications.