The Second Version

11/10/08

The Data Grinder

Blogger Jeff Id is an aeronautical engineer who did some investigation into the statistical methods used by Mann to produce his Hockey Stick temperature graph.

And his work has uncovered what probably is a fundamental flaw not only of Mann's temperature reconstruction, but of paleoclimatology at large.

Past temperatures are measured through proxies such as tree ring width, which is (or at least, supposed to be) correlated with temperature of the environment where the tree grew.

These time series, however, are highly noisy and uncalibrated against temperature. Calibration is done by calculating the correlation of the proxy series with instrumental temperature record (that covers only recent times); Mann's method then rejects series having a correlation value lower than a certain threshold. And here is where the problems begin.

The chosen data series are then scaled to match the slope of temperature in the calibration period (or its mean/standard deviation), and all these manipulations have a curious effect: they introduce a rising temperature signal in recent times (obviously, because only series in agreement with this assumption are retained), and compress and offset the signal in the past. Jeff used random noise superimposed to temperature signal that is flat except for a hump in the past to obtain these findings.

He then used actual Mann's data, and not only confirmed his findings, but also that the scaling process gives a disproportionate weight, in the reconstruction, to the series with lowest correlation to instrumental temperature.

But the best comes last. The high noise of proxy series allows for a lot of room in correlating them to a temperature trend, so Jeff was able to fit the same proxies used by Mann to all sorts of temperature trends: linear rise, linear fall, cyclic variations... in a final twist of irony, the reconstruction getting the best score is one with a falling temperature trend in recent times.

Mann's method of temperature reconstruction is fatally flawed: it is only useful to find only what one is looking for. Paleoclimatology must find better methods for the task of reconstructing past temperatures.

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