It has been a while since I have posted a review of a paper purporting to find a correlation between a proxy and solar activity. This is not for lack of interest — plans are developing for a meta-analysis, but some preliminary work is still in progress.
Meanwhile, I’d like to take a look at El Bilali et al (2013). El Bilali et al reconstruct eastern Canadian climate over the last few thousand years using the δ18O of cellulose of Sphagnum moss from an ombrotrophic bog (a bog that receives water and nutrients from precipitation only rather than groundwater or run-off and is consequentially very nutrient poor).
El Bilali et al claim that the oxygen isotopic composition of the Sphagnum in their record is dependent on temperature and that precipitation and evaporation effects are small.
I’m interested in the section correlating the δ18Ocel record with solar activity. This is the section of text of interest:
The Mer Bleue Bog temporal δ18Ocel record generally correlates with the Beryllium (10Be) isotope anomaly, sunspot number, and solar variation events records. The Maunder minima and maximum cooling is less pronounced in the δ18Ocel data than in the Northern Hemisphere reconstructed paleotemperature record (Moberg et al., 2005), which may be due to less pronounced cooling in eastern Ontario. The δ18Ocel record from Mer Bleue Bog shows a good correlation with the smoothed 10Be-record (Figure 6a, b). The low δ18Ocel values at ~AD 1810–1820 are verified by repeated and high-resolution sampling (Table 2) and may be related to the lower solar activity during the Dalton Minimum (Figure 6), to the cooling influence of the Tambora volcanic eruption for the summer of AD 1816 and subsequent years, or both.
I showed this figure to the students at the INTIMATE training school, and asked them whether the correlation was positive or negative. Not one of them was prepared to raise their hand in agreement with either possibility. It is clear from the text that El Bilali et al believe the correlation between the inverted 10Be and δ18Ocel to be positive. I can only see a relationship in the period after AD 1700 in the unsmoothed 10Be, and none at all in the smoothed record, which is supposed to have a good correlation.
Curiously, the 10Be-record from Bard et al (2000) goes back to AD 843, but the figure is truncated at AD 1400 even though the δ18Ocel extends for thousands of years. Normally if half the data were not shown, I would suspect that the correlation breaks down in the portion unseen, but here I don’t think the correlation could get much worse.
Whenever I am presented with a “good” correlation between proxy records I want to ask what the correlation coefficient is (almost guaranteed bluster if you do this at a conference). Like many papers reporting a “good correlation”, El Bilali et al do not attempt to put numbers on the strength of their correlation even though this is not difficult to do. As El Bilali et al provide the data in their paper, the INTIMATE class could estimate the correlation coefficient for these records, using linear interpolation to put the dates on the same time scale. The Pearson correlation is less than 0.1 (note however that the age-depth model the INTIMATE class generated using Bacon probably differs from the original): not exactly what I would call good. Allowing for chronological uncertainty didn’t improve matters.
El Bilali, H., Patterson, R.T., Prokoph, A., 2013. A Holocene paleoclimate reconstruction for eastern Canada based on δ18O cellulose of Sphagnum mosses from Mer Bleue Bog. The Holocene. 23, 1260–1271.