A More Robust Analysis of Marcott et al., 2013

Mar 13, 2013



This more robust analysis is in follow-up to my original analysis of Marcott et. al., 2013, published March 10, 2013 wherein I looked at the 9 late proxies of the 73 proxy study and could not find support of the huge hockey stick blade shown by the author. Marcott states that the giant uptick is due to global warming and collaborates the Michael Mann et. al. dendrochronology study that first introduces us to the hockey stick and has gained much media attention.


Marcott Hockey Stick
Note the prominent hockey stick blade on the right of the Marcott study graph

I wanted to take a more expanded look at the proxies used in the Marcott et al. paper. In the first analysis, I wasn’t particularly interested in aligning on Marcott’s reference period of 1961 to 1990. My main purpose was to see if there was any evidence of a large high resolution uptick in temperature in the late proxies as shown in the Marcott paper, which he claims is evidence of unprecedented global warming.

A commenter on my first analysis suggested that perhaps the reason I didn’t get the same results was because I limited my selection of proxies to only the nine that went beyond 1950. I was pretty certain that if there were positive signal in the proxies I would have found it in the nine selected where the signal would have occurred. Nonetheless, I can’t dismiss that there might be something in the broader set of proxies that could contribute to a more aggregate positive signal. I performed a more robust reconstruction using 24 of the late raw proxies used by Marcott.

This time, I aligned better with Marcott by adjusting the intercept for all proxy anomalies to the same 1961 to 1990 reference mean mentioned in the supplemental paper.

An issue that concerned me was the calibration period of 4,500 – 5,500 BP. In proxies with very low temporal resolution as used in the reconstruction, I don’t think there is a good representation of variability in this period. I understand why it was chosen – it was the only range common to all 73 original proxies. In my latest run of 24 proxies, I took advantage of the fact that there was significantly more range common to all of them. Thus, I selected 1,500 to 6,000 BP as my reference period. My tests between the 4,500 – 5,500 and 1,500 – 6,000 periods demonstrated that the 1,500 – 6,000 BP period balanced the proxy weighting better as I expected.

Here’s my result with 24 proxies:


Marcott Hockey Stick
The hockey stick appears to have inverted

It becomes obvious that there is a real problem from 1950 on. The nine remaining proxies, although centered on the 1961 to 1990 mean, are going quite negative whereas Marcott et al. show a strong positive signal. At this point, we have a higher ratio of Northern Hemisphere lake sediments and probably looking at a more regional representation of temperatures. Then there’s a statistical problem where dropping from 24 proxies down to only nine dramatically increases error, making the results non-robust.

The above 24-proxy graph is pretty busy. Here is a graph of just the running mean (the black line only).



Marcott Hockey Stick
Running mean only

I’m more convinced that the hockey stick in Marcott et al, 2013 is most likely a recalibration of the late proxies.

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Hank Hancock is a researcher and author to many leading peer reviewed journal studies and RFC standards, an FCC licensed RF Engineer, working mathematician, data analyst, and writes on the subjects of technology, medicine, climate, and outdoor life.



This article is a revised reprint of my original work titled "More Fishing for Hockey Sticks in Marcott et. al." at Suyts Space, an on-line science and politics venue.

[Edits]:

On March 11, Dr. Judith Curry linked to my original analysis on her web site. Subsequently, Dr. Steven McIntyre, a well recognized mathematician and expert in paleo-proxies, confirmed my results and identified that the box top proxies extending to 1950 were misdated, some of them off by over 1,000 years. Misdating would have caused the Monty Carlo simulation to produce the hockey stick blade due to proxies dropping off in a way to produce a positive artifact that's not real. This is an important finding as the original authors of the proxy datasets took great diligence in properly dating the proxies. The scientists and authors of the proxies re-dated by Marcott state that Marcott did not consult with them nor was he qualified to re-date the proxies.

On June 14, 2013, the UK MET office removed the Marcott et. al. study from their web site with the statement that the study did not have sufficient statistical power to support the claim being inferred by its title. Below is MET’s official statement. I would like to point out that MET did not change the title as suggested in their statement. MET Office not only removed the study but subsequently removed their statement.


Marcott Hockey Stick




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