Sunday, January 31, 2016

Statistician Briggs savages the latest Michael Mann paper

I have commented about this paper before but the Briggs comments were not out at the time I wrote.  The Mann et al. paper says that the known pattern of global temperature changes is consistent with human influences and that the pattern of temperature changes is unlikely to have happened without human influences.

Briggs takes us on a tour of statistical theory with common-sense examples to help us understand.  He shows that the Mann et al. paper makes a lot of assumptions that are just that: assumptions, and wrong assumptions at that.

I will not try to further explain or simplify what Briggs has written because he himself has probably gone as far as one can in that direction.  Very broadly, however, I will note that what Mann et al have written about is probabilities only -- and the probable does not always happen.

And, if there are sufficient uniformities in events, we can know probabilities and thus make accurate predictions from them without understanding anything about the causes of the events concerned.  Probability is not causation. So Mann et al. could in theory make accurate predictions but still be totally wrong about the causes of the events concerned.  As it happens, however, Mann & Co. have never even been able  to make accurate predictions.  So it is quite clear that they do NOT know what caused the observed temperature fluctuations.

Thinking about all that, I had a closer look at the journal abstract (reproduced again below).  And it seems their reasoning is circular.  They clearly assume some figure for human influence in doing their modelling.  But it is the extent of human influence that they have to prove!  They get the conclusion they do because they assume what they have to prove!  The usual high intellectual standards of Warmists.

The Likelihood of Recent Record Warmth

Michael E. Mann, Stefan Rahmstorf, Byron A. Steinman, Martin Tingley & Sonya K. Miller


2014 was nominally the warmest year on record for both the globe and northern hemisphere based on historical records spanning the past one and a half centuries1,2. It was the latest in a recent run of record temperatures spanning the past decade and a half. Press accounts reported odds as low as one-in-650 million that the observed run of global temperature records would be expected to occur in the absence of human-caused global warming. Press reports notwithstanding, the question of how likely observed temperature records may have have been both with and without human influence is interesting in its own right. Here we attempt to address that question using a semi-empirical approach that combines the latest (CMIP53) climate model simulations with observations of global and hemispheric mean temperature. We find that individual record years and the observed runs of record-setting temperatures were extremely unlikely to have occurred in the absence of human-caused climate change, though not nearly as unlikely as press reports have suggested. These same record temperatures were, by contrast, quite likely to have occurred in the presence of anthropogenic climate forcing.

Scientific Reports 6, Article number: 19831 (2016) doi:10.1038/srep19831

Posted by John J. Ray (M.A.; Ph.D.).

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