SUNSPOTS – what have they got to do with the price of bread? Quite a lot, according to a study that has found a link between the cost of wheat in medieval England and the mysterious sunspot cycle. The study, by an astrophysicist and an agricultural economist in Israel, strengthens evidence that changes in the sun over just a few years can alter the climate on Earth.
A link between sunspots and crop prices was first suggested two centuries ago. In 1801, the German-British astronomer William Herschel reported that five prolonged periods of few sunspots or “solar minima” had coincided with peaks in the price of English wheat. Herschel thought the number of sunspots must somehow influence our weather and agricultural fortunes, but other scientists scoffed at the idea.
However, recent studies suggest that the solar cycle, in which sunspot numbers rise and fall over a typical period of 11 years, does indeed subtly alter the Earth’s climate (91av, 11 July 1998, p 44). It’s not clear why. Tiny changes in the brightness of the sun over the course of a solar cycle are not big enough to explain it.
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One possible reason is that when there are fewest sunspots, the “solar wind” of particles blowing out from the sun is weakest. This in turn means that more of the energetic cosmic rays zipping around the galaxy bombard the Earth’s atmosphere. The extra cosmic rays generate showers of charged ions that possibly seed rain clouds, and the extra cloud cover might cool or warm the climate, depending on where you live.
Could that climate change be enough to alter crop yields and hence their market values? To find out, Lev Pustilnik of Tel Aviv University and Gregory Yom Din of Haifa University analysed records of the price of wheat in England from 1259 to 1702. They found that the distribution of intervals between peaks in wheat prices was very similar to the subsequent distribution of intervals between minima of the solar cycles since 1700, when astronomers started keeping good records of solar activity.
Pustilnik and Yom Din also analysed modern estimates of the numbers of cosmic rays hitting the Earth during the 17th century, from Greenland ice cores. Sure enough, the timing of high cosmic ray rates, signalling solar minima, coincided with peaks in the cost of wheat. This suggests that the solar minima caused cool spells in England, reducing wheat yields. Details of the findings are at .
The researchers now plan to look for the stamp of solar activity on other historical markets, and even modern ones. They hope their work could reveal the effect of solar activity on today’s cereal production, for example. It could also help us understand why the solar cycle alters climate at all.
Joanna Haigh, a physicist at Imperial College in London, says understanding this mechanism will be crucial if we want to know how much people are to blame for climate change. Over the past century, global temperatures have risen by 0.6 °C. Emissions of greenhouse gases from cars and industry may be the root cause, but increased solar activity could also have played a role.