One of the uncertainties in climate science is figuring out how global climate trends translate into local predictions and reconstructions, and vice versa. A classic example of this is the medieval warm period, the time when Vikings roamed the North Sea and North Atlantic in shallow-draft open boats. They settled Greenland and made periodic visits to North America to get timber, placing some short-lived settlements there.
But there's a problem. We have the evidence from the adventures of the Norse, but the change to the European climate was so subtle that not many Europeans actually noted it at the time. Apart from exceptional—in both good and bad ways—years, it has been hard to gain a sense of what the typical climate in Europe was like.
To make matters more uncertain, reconstructions of the global climate show little evidence of the medieval warm period—Europe isn't the globe, after all—and some models show a warm period, while others don't. Local indicators, such as pollen grains and tree rings, show mixed evidence for the warm period. A recent paper, published in PLoS One, focuses on calibrating climate reconstructions using European data, but one side effect of the work is a further indication that the medieval warm period was really a local event.
The actual goal of the paper was not to explore the medieval warm period, but to examine Europe's climate over as large period of modern time as possible, and, in doing so, demonstrate a new way of calibrating climate reconstructions.
Historical climate data must be derived from natural thermometers, called proxies: tree ring widths, pollen distributions, ice core data. All of these must be calibrated to get an absolute temperature. Generally, the way to do this is to extrapolate backwards from where the proxy data and instrumental record overlap. In this paper, the researchers used the similarity between tree ring data and pollen distributions as an internal calibration.
The researchers made use of ice core data and glacial records to compare how their local climate reconstructions compared with both a qualitative record—glaciers advance in cold years and retreat in warm years—and a quantitative, but more global, record. They also made use of the instrument record to provide set calibration points for the proxy data.
Understanding the medieval warm period
The end result is a remarkably thorough investigation of the European climate from 750 CE onwards. With the reconstruction complete, the researchers started validating their data.
First, they compared their data with the documented advances and retreats of a glacier. They found good agreement back to 750 CE—in fact the lack of agreement before 750 CE is why they end the reconstruction there—though there were a few years when the glaciers advanced in warm years and retreated in cold years. They also compared their reconstructions to other reconstructions and noted that although there were differences, they were not unreasonably large.
To try and understand the temperature variations, the researchers then examined correlations between their climate reconstruction and known volcanic activity and solar activity. They found at least one cold year in the following three years after major volcanic eruptions. Generally, warm years coincided with years of high solar activity, and vice versa. The one exception was at the end of the medieval warm period; the researchers speculate that deforestation in Europe may have increased the local albedo enough to compensate for the solar activity.
With all that established, what of the medieval warm period? Well, the weather was certainly warmer in Northern Europe and in the North Atlantic, but it was colder and more variable in Southern Europe. After roughly 1400 CE, the less stable weather expanded to Northern Europe and both areas cooled.
The little ice age, on the other hand, was common to most of the continent. The point is that if one used just Northern Europe's tree ring data, the medieval warm period would look very strong, while Southern European proxies would show no warm period. Only by combining data from throughout Europe can one see just how local the medieval warm period actually was.
This is pretty much what climatologists had concluded already—at least as far as the medieval warm period goes. What's important in the new way is that the temperature reconstruction drawn from the proxies resulted in what seems to be a more accurate temperature reconstruction. Because the calibration was internal, the different proxy data sets could be more accurately combined with each other and used to draw much stronger conclusions than previously.
I'm looking forward to seeing this technique applied to other regional climates.
PLoS One, 2010, DOI: 10.1371/journal.pone.0009972
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