Published in the Proceedings of the National Academy of Sciences’: ‘Some of Eastern Europe’s greatest wars and plagues over the last millennium coincided with cold periods. Ulf Büntgen of the Swiss Federal Institute for Forest, Snow and Landscape Research in Birmensdorf and his colleagues reconstructed temperatures using tree rings from 282 living larch trees and 263 construction timbers from historical buildings in northern Slovakia. Because the width of tree rings varies with the temperature during the growing season, the researchers could estimate changes in May and June temperatures from 1040 to 2011. The Black Death in the mid-14th century, the Thirty Years’ War in the early 17th century, the French invasion of Russia in the early 19th century and other social upheavals occurred during cold spells. The team suggests food shortages could explain the timing of some of these events.