The NAS report has been chastised here and here for concluding that it is â€œplausibleâ€ that the â€œNorthern Hemisphere was warmer during the last few decades of the 20th century than during any comparable period over the preceding millenniumâ€ while at the same time conceding that every statistical criticism of MBH is correct, disowning MBH claims to statistical skill for individual decades and years, and finding little confidence in reconstructions of surface temperatures from 1600 back to A.D. 900, and very little confidence in findings on average temperatures before then.
One of the main justifications for this plausibility was the “general consistency” of other studies.
The committee noted that scientists’ reconstructions of Northern Hemisphere surface temperatures for the past thousand years are generally consistent. The reconstructions show relatively warm conditions centered around the year 1000, and a relatively cold period, or “Little Ice Age,” from roughly 1500 to 1850. (NAS press release)
I want to show to what a vacuous motherhood statement this is. Previously I have shown that virtually any data, including random data, will produce a graph similar to the existing studies if you ‘cherry pick’ proxies for correlations with temperature and then you squint your eyes a bit here, here, and here. Today I thought I would run a simple statistical test see how consistent each of the reconstructions really is.
Below I have plotted up each of the reconstructions and CRU temperatures using a lag plot. A lag plot shows the value of a time series against its successive values (lag 1). A lag plot allows easy discrimination of three main types of series:
- Random – shown as a cloud
- Autocorrelated – shown as a diagonal, and
- Periodic – shown as circles.
The data and reconstructions above are:
CRU Climate Research Unit 3 J98 Jones et al. 1998 Holocene 4 MBH99 Mann et al. 1999 Geophys Res Lett 5 MJ03 Mann and Jones 2003 6 CL00 Crowley and Lowery 2000 Ambio 7 BJ00 Briffa 2000 Quat Sci Rev 8 BJ01 Briffa et al. 2001 J Geophys Res 9 Esp02 Esper 2002 Science 10 Mob05 Moberg 2005 Science
The substantial differences between them are clear from the lag plots.
Two of the plots, MJ03 and CL00 have very strong diagonals, and look nothing like the CRU temperatures.
Two of the plots, CL00 and Mob05 have distinct circular patterns suggesting periodicity, a pattern not present in the CRU temperatures.
BJ01 appears random.
The ones that look a bit like temperatures are J98, MBH98, BJ00 and Esp02. Of these MBH98 uses a flawed methodology according to the NAS.
You have to squint very hard for the actual reconstructions of temperature to look consistent. The lag plot shows there is very little real consistency between the series in even the broad features of the underlying dynamics: random, autocorrelated and periodic. This proves what a generic motherhood statement “… generally consistent” really is.
For successful analytics you need to do more than make motherhood statements that can hardly be proved wrong, and squint your eyes very hard to see the similarity between things. You need to at least apply basic techniques and probe the underlying dynamics, carefully discriminating between things. It is no mistake that the term ” .. general consistency” is not found in the manual of scientific method. Seeking general consistency, like broad consensus, is just social constructivism, with objective fact playing a small role if any, and nicely summed up by the phrase “Hottest in Whatever Many Years”. There are hypotheses, there are tests, and there is falsification (and if you are a rabid Popperrite — corroboration — which is a form of falsification anyway).