How bad can a solar storm be? Just ask a tree. Unlike human records, which go back hundreds of years, trees can remember solar storms for millennia.

Nagoya University doctoral student Fusa Miyake made the discovery in 2012 while studying rings in the stump of a 1900-year-old Japanese cedar. One ring, in particular, drew her attention. Grown in the year 774–75 AD, it contained a 12% jump in radioactive carbon-14 (14C), about 20 times greater than ordinary fluctuations from cosmic radiation. Other teams confirmed the spike in wood from Germany, Russia, the United States, Finland, and New Zealand. Whatever happened, trees all over the world experienced it.