The average value of an ensemble of climate models is often used as an argument in the debate. What does it mean? The following is a quote from the contribution from Working group I to the fifth assessment report by the Intergovernmental Panel on Climate Change.:
Box 12.1 | Methods to Quantify Model Agreement in Maps
“The climate change projections in this report are based on ensembles of climate models. The ensemble mean is a useful quantity to characterize the average response to external forcings, but does not convey any information on the robustness of this response across models, its uncertainty and/or likelihood or its magnitude relative to unforced climate variability.”
This can be regarded as a glimpse of realism. Except from the logical fallacy expressed in the same section. Let us rephrase the section:
The ensemble mean does not convey any information on:
– the robustness of this response across models
– its uncertainty
– its magnitude relative to unforced climate variability
but it is a useful quantity to characterize the average response to external forcing.
That is a quite silly thing to say – isn´t it?
How can it be useful when you do not know
– if it is robust
– its uncertainty
– its likelihood
– its magnitude relative to unforced climate variability?
Exactly what is the ensemble mean then supposed to be useful for?
Later in the same section it is stated:
“There is some debate in the literature on how the multi-model ensembles should be interpreted statistically. This and past IPCC reports treat the model spread as some measure of uncertainty, irrespective of the number of models, which implies an ‘indistinguishable’ interpretation.”
I think this section speaks for it self. What ” implies an indistinguishable interpretation” is supposed to mean – I have no idea – I don’t even find it worth analyzing the argument. If you have read this far – I think you agree with me that it is totally meaningless.
The panel should have concluded that the models are not reliable. The models have not fulfilled the requirements for an quantitative theoretical model to be regarded as reliable. See: What does it take for an quantitative theoretical model to be reliable?