Carbon-14 spike of 774 CE

In 2012 a team led by Fusa Miyake at Nagoya University measured carbon-14 in Japanese cedar tree rings and found that the level jumped by about 1.2 percent in a single year, between 774 and 775 CE. That is far larger than the normal variation from the solar cycle. Carbon-14 forms when cosmic particles hit the upper atmosphere, so something delivered a vast pulse of energetic particles to Earth. Later studies found the same jump in tree rings in Europe, Russia, North America and New Zealand, and beryllium-10 and chlorine-36 spikes in Antarctic ice cores.¹²

No one wrote down an explanation, and no contemporary record fits cleanly. Some researchers connect it to the Anglo-Saxon Chronicle, which for the year it dates 773 reports a red crucifix in the sky after sunset, perhaps an aurora. The cause is debated: candidates include an extreme solar storm far stronger than the Carrington Event, a short gamma-ray burst, or other cosmic sources. Similar spikes have since been found around 993 CE and 660 BCE. The event matters because a solar storm of that size would badly damage modern power grids and satellites.²³