We’ve frequently observed that the reduction of data to standard deviation units (z-scores) is often associated with seemingly opportunistic orientation of the data sets. Often this is buried in the multivariate methodology. Principal components and RegEM can both function to opportunistically provide orientations to “proxies”.
In Mann 2008, we saw pretty examples of proxies being oriented upside-down from the sense of the original analysis (Tiljander) and even examples of the same “proxy” being oriented both up and down depending on whether there was “late” or “early” calibration. (Not that anyone in the “community” cares – even when, as in the Mann 2008 case, Ross and I went to the trouble of publishing a PNAS comment reporting this.)
MBH98 had some pretty and under-appreciated examples (dating from pre-blog days) in which actual gridcell temperature data sets (“proxies” ?) were inverted in the Mannian meatgrinder. And whatever the rationale for using z-scores, the idea of opportunistically inverting the orientation of temperature data (or for that matter, proxies oriented as direct temperature proxies by the original authors) is something that I find offensive.
Unfortunately, Trouet et al fall into this all too common Team practice in the graphic below (An excerpt from Trouet Figure 4 with original caption), which they use in their arm-waving discussion of global circulation patterns in the MWP.
Trouet Fig. 4. .. . Geographical location and time series (1050–2000) of proxy records of .. North Atlantic conditions [(B), black squares], tropical Pacific conditions [(C), black circles]… Records … were smoothed with a 60-year spline and normalized over the full period. References for all records are provided in table S1.
Exceptionally alert readers may recognize a few of these proxies as ones that have been previously discussed at CA. (The inventory of such discussions makes this not totally unsurprising).
The “Palmyra” proxy in the Pacific (intermittent black lines in the lower panel) is Kim Cobb’s coral dO18 interpreted as SST, previously discussed here. Cobb interpreted this proxy as evidence of a “cool medieval Pacific” – an interpretation which has been very influential in anti-MWP circles. As I observed in my earlier post, it seemed to me that the evidence was also coherent with a slight northward movement of the ITCZ in the MWP:
Cobb’s recent sampling and interpretation of dO18 values at Christmas Island at 2N makes me wonder whether these results can’t be readily interpreted as evidence of a more northerly ITCZ in the 10th century, which would be a very interesting result and easily consistent with medieval NH warmth.
I’ve added a note to my earlier post, adding a graphic from a Sept 2008 presentation by Cobb, showing the present ITCZ to the north of the coral islands under study and linking the dO18 gradient to rainout. My prior interpretation seems just as/even more plausible now than then.
Be that as it may, Trouet et al adopted the “natural” orientation of SST (up!) – which for this proxy shows a “low” MWP.
Cape Ghir Alkenones
Cape Ghir alkenone were also discussed previously at CA here. Cape Ghir has some additional interest in this case because it is located very near Esper’s Morocco tree rings and, indeed, it is too bad that there is not a direct discussion of the inter-relationship of the two proxies.
Alkenones are a popular SST proxy with a well-defined orientation. In this case, the Cape Ghir alkenone SST estimates show a remarkable decline of 20th century SST, which is interpreted as increased upwelling. Here is a plot of original data (which happily was archived by McGregor et al).
Re-plot of McGregor et al alkenone SST estimates.
However, unlike the Palmyra coral SST, Trouet inverted the orientation of Cape Ghir, plotting SST upside-down! Indeed, vsually, the upside-down Cape Ghir series provides the most vivid proxy illustration of 20th century “warming”, even though it actually goes down.
We also encontered this situation with Moberg’s G. Bulloides series from the Arabian Sea. As previously observed at CA, this is an indicator of cold water (upwelling), every bit as much as the Cape Ghir alkenones. Moberg also inverted this proxy and used it as evidence of 20th century warming.
Having said that, both Cape Ghir alkenones and Arabian Sea G Bulloides provide evidence of 20th century upwelling being outside the MWP range, for whatever that’s worth – “unprecedented” since earlier in the Holocene, I guess.