Trouet et al (2009), Persistent Positive North Atlantic Oscillation Mode Dominated the Medieval Climate Anomaly, published in
Reader’s Digest Science a few days ago. Esper the non-Archiver is a co-author. New Scientist breathlessly reported :
Europe basked in unusually warm weather in medieval times, but why has been open to debate. Now the natural climate mechanism that caused the mild spell seems to have been pinpointed.
The finding is significant today because, according to Valerie Trouet at the Swiss Federal Institute for Forest, Snow, and Landscape Research in Birmensdorf, the mechanism that caused the warm spell in Europe – and meant wine could be produced in England as it is now – cannot explain current warming. It means the medieval warm period was mainly a regional phenomenon caused by altered heat distribution rather than a global phenomenon.
The finding scuppers one of the favourite arguments of climate-change deniers. If Europe had temperature increases before we started emitting large amounts of greenhouse gases, their argument goes, then maybe the current global warming isn’t caused by humans, either.
Michael Mann told the New Scientist that the new results may imply that the situation is even worse than we thought.
If one actually reads the article, the word “regional” only occurs once
The North Atlantic Oscillation (NAO) … has a substantial influence on marine and terrestrial ecosystems and regional socio-economic activity
The word “phenomenon” is not used anywhere. Nor are the words “altered” or “heat distribution”. If New Scientist is correctly reporting her statements, I did not locate anything in the article that specifically supports the reported oral summary of the conclusions. Indeed, the following sentence seems to suggest the exact opposite – that the transition from the “MCA” to the Little Ice Age was “globally contemporaneous” and, in language reminiscent of Hubert Lamb, that this was due to “a notable and persistent reorganization of large-scale oceanic and atmospheric circulation patterns”:
The relaxation from this particular ocean-atmosphere state into the LIA appears to be globally contemporaneous and suggests a notable and persistent reorganization of large-scale oceanic and atmospheric circulation patterns.
I take no position in the present post on whether the MCA was or wasn’t global – I’m merely observing that the article itself does not draw the conclusion that was reported in the aftermarket promotion. Penny mining stock press releases are not permitted to go beyond the information in their prospectuses or qualifying reports and it always surprises me to see climate scientists issue press releases that promote well beyond the four corners of what was approved for publication.
In a quick read, I spotted a number of issues that are of interest, not all of which will be covered in today’s note. On another occasion, I’ll try to connect the present analyses of NAO and the Atlantic meridional overturning circulation to what Hubert Lamb and William Gray said on these topics respectively (please save such comments for another day.)
Today I’ll post some notes on the provenance of the data. Their NAO reconstruction is based on a comparison of highly smoothed Scottish speleothem bandwidth data to a Morocco tree ring chronology by Esper the non-Archiver. They also refer to an England-Wales (EW) documentary precipitation reconstruction, which surprisingly proved to be from Lamb (1965) and an Alps temperature reconstruction by Mangini – the one that Gavin Schmidt complained about when Loehle used it. Scottish speleothem data was used in Mann 2008 as a temperature proxy, while an earlier version of the Morocco tree ring chronologies was used in MBH98, MBH99 and Juckes 2007, again as a temperature proxy.
Their Figure S1 illustrating their 4 money proxies is shown below. Given that the article is said to refute MWP concepts in some sense, it’s a little surprising that none of the underlying data in their illustration is particularly HS-shaped; on the contrary, these data sets all seem to have a medieval “anomaly” as pronounced as the modern anomaly.
Trouet Figure S1 Original Caption: Long-term winter proxy records from Europe. Comparison of the tree-ring based Morocco (S2) and speleothem based Scotland (S1) records with a documentary based estimate of September-June England-Wales precipitation (S10) and speleothem based estimate of winter temperature from the central Alps (S13) (A). Time series consist of 25-year averages, standardized over the common period (1075-1925). The Morocco PDSI record was inverted.
Unfortunately, the authors failed to provide any digital data citations (see for example AGU policies on this, though AGU journals don’t require climate scientists – and perhaps others – to comply with these policies), though they provide references to dead tree literature from which the data provenance for 3 of the 4 series can be plausibly surmised. The “tree-based Morocco” data set is from Esper the non-Archiver, previously published in GRL without archiving any data. (Esper the non-Archiver flouted AGU policies requiring use of archived data – not that AGU journals bother enforcing what on paper is an excellent policy.)
The Scotland speleothem series is attributed to Proctor et al 2000, for which data is archived at NCDC. In the right panel below, I show rescaled versions of the archived Proctor precipitaiton reconstruction, including original data raw (blue), 25 year average (red) and 50 year average (navy blue). Although Trouet et al stated that “Time series consist of 25-year averages, standardized over the common period (1075-1925)”, this appears to be untrue as their Figure S1 does not match a 25-year average, but does match a 50-year average. This would obviously reduce the degrees of freedom even further from the already substantial reduction and the effect of this, if any, needs to be checked.
Figre 2. Left – Trouet Figure S1; right – Proctor ppt recon from speleothem: blue – as archived; red – 25 year average; navy blue – 50 year average,
The cyan series September-June England-Wales precipitation in Trouet Figure S1 has a very familiar looking shape. S10 is Lamb 1965; the shape of this figure may remind keen-eyed readers of the famous IPCC temperature curve (derived from Lamb) showing the MWP. See discussion at CA here which has many relevant figures. Here is Lamb 1965 Figure 4 showing the precipitation reconstruction, which is related to the temperature reconstruction (I’ll try to get to this on another occasion. For interested readers, I’ve placed Lamb 1965 online – see below – it has many comments about wind circulation relevant to the present discussion. As discussed on another occasion, IPCC AR4 sneered at Lamb’s work: “Lamb’s analyses also predate any formal statistical calibration of much of the evidence he considered.” I don’t want readers to spend any time debating the merit or lack of merit of the IPCC criticis, (which one disregarded reviewer took some exception to). What’s intriguing in the present context is that Lamb’s reconstruction is used to scale the Scottish speleothem record, perhaps a surprising calibration method under the circumstances.
For the PSR analysis in this work, the Scotland speleothem record was transformed into precipitation (as fraction of modern climatology) using Lamb’s documentary evidence-based estimate of a 13% MCA-to-LIA reduction in September-June England-Wales rainfall (S10).
The green series in Trouet Figure S1 is from Mangini’s temperature reconstruction from speleothems – again this appears to be the 50-year average, rather than the 25-year average as shown below using original data.
We’ve discussed this series a couple of times in the past. Gavin Schmidt excoriated Loehle’s use of this data here (See CA discussion here, on the basis that “no validation of this temperature record has been given” and that it had “been given a unique negative correlation to temperature.”
Unfortunately, no validation of this temperature record has been given… However, only one terrestrial d18O record is used by Loehle (#9 Spannagel), and this has been given a unique negative correlation to temperature…
Trouet calibrate the Mangini speleothem against a documentary series (I haven’t looked at this calibration yet) and arrive at the same “unique negative” orientation as the one that Schmidt complained about in Loehle’s usage. The difference in scale noted below would not affect the z-score plot shown here.
For Alps temperature, the smoothed (25-year running mean) Spannagel (S13) record was calibrated using linear regression against smoothed area-average (6-14E, 46-48N) December-February temperature from a documentary evidence-based data set (S14, S15) over the period 1525-1903. The correlation between these records is 0.69, with both time series showing four distinct peaks (see Fig. 4 in (S13)) over the calibration period. No rescaling was applied for this reconstruction. The resulting reconstruction gives an MCA-to-LIA winter temperature decrease in the Alps of approximately 0.4° C.
The article calculates many statistics and I’ll review some of these calculations if and when the relevant data is archived. The data has been hugely smoothed – a practice abhorred by most working statisticians (See Matt Briggs on this)- and I’d be surprised if the significance calculations hold up, but we’ll see.
Trouet, V, Esper J, Graham NE, Baker A, Scourse JD, Frank DC (2009) Persistent positive North Atlantic Oscillation mode dominated the Medieval Climate Anomaly. Science 324, 78-80. (pdf)
Lamb 1965 url