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Margaret Harris: February 2013 Archives

Of physics and famine

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The Harvesters

Pieter Bruegel the Elder’s painting The Harvesters (1565) shows a scene of plenty, but
people like the peasants depicted in it would have been all too familiar with famine.
(Courtesy: The Metropolitan Museum of Art)

By Margaret Harris

Physics and medieval history don’t overlap that often. I should know: I got an undergraduate minor in medieval and renaissance studies in part because I wanted a break from doing physics. So the fact that this arXiv paper and this documentary have both come out in the past 10 days is about as unusual as – well, finding a medieval king buried in a car park.

Fascinating as the discovery of Richard III’s skeleton is, though, I’m going to write instead about the arXiv paper, which proposes something even more remarkable: a possible link between space weather and episodes of famine in late medieval Europe.

The paper focuses on the years 1590–1702, a period during which Europe’s population suffered repeatedly from famine. Over the same period, the Sun was experiencing a decades-long lull in activity, known as the Maunder minimum. Might there be a connection?

To answer this question, the paper’s authors – physicist Lev Pustilnik and economist Gregory Yom Din – begin by summarizing the evidence for a connection between space weather and local weather. Overall, this appears fairly convincing, if a bit circumstantial. For example, a 1997 study found a link between cosmic rays and cloud cover, while a 2004 paper demonstrated a similar correlation between global atmospheric circulation and level of activity in the Earth’s magnetosphere.

With the principle of a connection thus established, Pustilnik and Yom Din go on to suggest three conditions under which space weather could lead to famine:

• Local weather has to be in a “threshold state” such that it is sensitive to space weather. For example, if there is no water vapour present, clouds won’t form even if space weather is “seeding” the Earth’s atmosphere with lots of extra ions.

• Harvests must be sensitive to weather anomalies. This is more likely in areas of so-called “risk farming”, where conditions are marginal enough that a few days of bad weather can completely wipe out a crop.

• The area has to be economically isolated, such that local shortages cannot be ameliorated by buying grain from elsewhere.

To test these hypotheses, Pustilnik and Yom Din begin by comparing levels of solar activity with grain prices in 17th century England. Between 1590 and 1700, the price of grain in England and the abundance of 10Be isotopes (a proxy for solar activity) in Greenland ice cores exhibit an almost exactly inverse relationship. High prices correspond to periods of low solar activity and vice versa. Several other European markets that the authors studied also showed strong correlations between grain prices and solar activity, but in southern Europe, where crops are more likely to suffer from drought than from excess rain, prices tended to spike during solar maxima rather than minima.

Things get a bit shakier when the authors turn their attention to 19th century Iceland. In this case, famines seem to correlate with both minima and maxima in solar activity. Pustilnik and Yom Din claim this is what they expected to see, but don’t really say why; in particular, they don’t explain why the Icelandic pattern should differ so markedly from the English one.

Still, it’s an interesting study, and reading it stirred up some memories from my brief foray into medieval studies. In particular, I thought of a book called Lost Worlds whose author, a Swiss historian called Arthur Imhof, makes unusually good use of hard data in analysing what life was like for an ordinary person in early modern Europe. Might his book have something to add to the famine/space weather debate?

I skimmed my copy of Lost Worlds a couple of times before I located the bit where Imhof writes about famine. Tree-ring data and written sources from the 16th and 17th centuries, he notes, indicate a long series of harsh winters and summers with too much rain, resulting in exceptionally bad growing conditions. As a result, he adds, “our ancestors had more reason to beg for their daily bread between 1550 and 1700” than they did at almost any point before or since.

This is, of course, almost exactly the same period that Pustilnik and Yom Din studied, and it’s nice to see that Imhof’s sources corroborate their grain-price data. But Imhof wasn’t interested in climate for climate’s sake. Instead, he was trying to demonstrate that populations in areas prone to famine, plague and war became traumatized by their repeated misfortunes. You’d have to read the book to appreciate Imhof’s argument in full, but among other things, he suggests that people in these “unlucky” areas developed fatalistic attitudes to life, death and birth. These attitudes show up not only in religious beliefs, but also in data on infant and maternal mortality. For example, even in peaceful, plague-free years, more than one-third of babies born in the plague-prone and war-torn German village of Gabelbach died in infancy. In “luckier” villages, the comparable figure was one in eight.

Where does this leave us regarding space weather? Well, if we add Imhof’s conclusions to Pustilnik and Yom Din’s, it seems that the behaviour of heavenly bodies could have influenced not only the viability of medieval grain crops, but also the habits and attitudes of the people who tended them – perhaps even to the extent of determining whether their children were likely to live or die. That might not be very surprising to the peasants of 17th century Gabelbach, who lived in a more religious age (and, according to Imhof, believed fervently in astrology). But to me, it’s absolutely mind-blowing – and a whole lot more interesting than England’s “Tricky Dick” turning up in a car park.