Past research has suggested that volcanic eruptions influence climate, but it has proved difficult to match the chronologies of annually resolved and precisely dated tree rings to the chronologies of volcanic variability recorded in ice cores. Michael Sigl et al. use a spike in atmospheric 10Be — clearly linked to a cosmic-ray anomaly that left a unique atmospheric 14C fingerprint in tree rings across Europe in the year 775 — as a means of dating a similar spike observed in ice cores from Greenland and Antarctica. In making this connection the authors establish that the ice core record should be adjusted by seven years. The data confirm that large volcanic eruptions in the tropics and high latitudes were primary drivers of temperature variability in the Northern Hemisphere during the past 2,500 years, and implicate such eruptions as catalysts in major sixth-century pandemics, famines, and socioeconomic disruptions.