This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Our sun is capable of storms far bigger than we've ever recorded. Studying the world's oldest trees suggests we could soon be due one of these gargantuan events.
The first clear sign of exceptionally high solar activity came from Japanese cedar trees. In 2012, Fusa Miyake was studying slices of ancient Japanese cedar taken from a tree felled in the 1950s. It had grown on Yakushima Island, a protected site famed for its long-lived trees, some of which have stood for millennia, laying down one ring of wood after another.
These tree rings aren't just markers of age—they are also annual records of Earth's atmosphere, and studying them could help us predict and prepare for future disruptive solar storms. While perusing these ancient tree records, Miyake stumbled upon something that gave her pause. The relative proportion of carbon-14 in each tree ring reveals how active the sun was each year.
Between AD 774 and 775, the amount of this isotope in the cedar jumped by about 12 parts per thousand—roughly 20 times larger than the change expected from ordinary variation in the sun. That was hard to explain. A spike this sharp meant Earth's atmosphere had been hit by a sudden burst of unusually energetic particles.
At first, there were reasons to be cautious: Perhaps this was a laboratory artifact or something peculiar to the Japanese cedars. Then other teams found the same signal in trees from North America and Europe. Whatever had happened in AD 774, it had marked the whole planet.
That made the list of possible culprits quite short. A nearby supernova explosion, a gamma-ray burst or a blast from a neutron star could each have been to blame—but all were ruled out because possible sources were too distant. Eventually, researchers returned to the object we know best: the sun.
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