It seemed like a curse. The summer of 821 was wet, cold and yielded a poor harvest. Then winter came. Temperatures plunged. Blizzards smothered towns and villages. The Danube, the Rhine and the Seine – rivers that never froze – froze so hard that the ice covering them could be crossed not just on foot but by horse and cart. Nor did spring bring respite. Terrible hailstorms followed the snow. Plague and famine followed the storms. The next few winters were worse. Fear stalked the land. Paschasius Radbertus, a monk of Corbie, in what is now northern France, wrote that God Himself was angry. Yet it was not God that wrought this destruction, according to Ulf Büntgen of the University of Cambridge, but rather a volcano now called Katla, on what was then an unknown island, now called Iceland.

   At the moment Katla, one of Iceland’s largest volcanoes, located near the island’s southern tip, sleeps beneath 700 metres of ice. It has so slept, albeit fitfully, for almost 100 years. Its last eruption big enough to break through the ice was in 1918. A score of such ice-breaking awakenings have been recorded by Icelanders since the first Norsemen settled there in 870. In 821, however, Iceland was not on the Norsemen’s horizon. They were concentrating their activities on the lootable monasteries and villages of coastal Europe. There is thus no man-made record of what Katla was up to then. But Dr. Büntgen thinks he has found a natural one. A memorandum of an eruption that coincides with the events described by Radbertus is, he believes, written in a prehistoric forest.

   Large volcanic eruptions can affect the weather. In particular they eject sulphur dioxide, which reacts with atmospheric gases to form sulphate aerosols that reflect sunlight back into the space, cooling the air beneath. That is well known. So the suspicion that what happened in the early 820s was precipitated by such an eruption has been around for a long time.