Changes in solar activity sent wheat prices soaring in medieval England.
The belief in the Middle Ages that the heavens govern fate and fortune might not have been as ludicrous as it sounds.
Two researchers in Israel have found a statistical link between the activity of the Sun and the price of wheat in seventeenth-century England. At the point in the solar cycle when sunspots were least likely, wheat prices tended to be high, report Lev Pustilnik of Tel Aviv University and Gregory Yom Din of the Golan Research Institute in Kazrin1.
For a medieval peasant, this could mean the difference between life and death: bad harvests and famine were common; high grain prices sometimes triggered riots in Europe.
The new findings will help to burnish some tarnished reputations. When the British astronomer William Herschel suggested a link between sunspots and wheat prices in 1801, he met such ridicule that he had to cancel lectures on the topic.
And towards the end of the nineteenth century William Stanley Jevons, one of the main architects of conventional economic theory, proposed the same idea as an explanation for the recurring booms and slumps in the economy.
Jevons claimed that high prices and stock-market panic seemed to recur on the same 10-11-year timescale as sunspots. He reasoned that fluctuating solar activity could affect weather and therefore harvests.
It is now clear that the business cycle - which is so irregular that it hardly merits being called a cycle - cannot be explained in this way. But modern research offers a sound scientific basis for Herschel's and Jevons' ideas.
The Sun's interior is laced with magnetic fields caused by convective motion of charged particles within its hot gases. Periodically, these fields become more agitated, rising into the Sun's outer layers.
Sunspots are darker, cooler regions formed at the points where magnetic field lines puncture this bright skin. They are often accompanied by gigantic explosions called solar flares. In such a state, the Sun is said to be more active.
Changes in solar activity alter the strength of the solar wind, the stream of charged particles that flows from the Sun throughout the solar system. When the solar wind is strong, it is more difficult for charged particles from deep space to penetrate Earth's atmosphere. Once in the atmosphere, these cosmic rays collide with molecules in the air to produce ions, which help cloud droplets to form.
So in periods of high solar activity the skies are less cloudy. Over the past few years, satellite observations have confirmed this link.
Grain of truth
The effect of all this on grain yields, say Pustilnik and Yom Din, is hard to predict. Heavy rain or no rain can equally ruin a harvest. In the medieval market, supply could barely meet demand at the best of times, so even small weather fluctuations had a big impact on prices.
The researchers suppose that in medieval England, high cloudiness - and therefore harsh winters and lots of rain - posed more risk than low cloudiness, as drought was rare. So they expect to see unstable wheat prices during periods of low solar activity. They looked at grain price records and at the amount of beryllium-10 in Greenland ice sheets. This isotope is produced by cosmic rays in the atmosphere and so is a record of solar activity.
For all ten solar cycles between 1600 and 1700, high wheat prices coincided with low activity, and vice versa. The probability of this happening by chance is less than 1 in 500, the researchers say.
Globalization of international food markets probably eliminates this effect in most of the modern world. But countries where agricultural production remains precarious and which cannot afford to import food in times of shortage could still be at risk of such a solar influence.
Pustilnik, L. A. & Yom Din, G. Influence of solar activity on state of wheat market in medieval England. Preprint, http://xxx.lanl.gov/abs/astro-ph/0312244, (2003).
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Ball, P. Sun set food prices in the Middle Ages. Nature (2003). https://doi.org/10.1038/news031215-12