Posted on behalf of Rick Lovett.

A mysterious spike in atmospheric carbon-14 levels 12 centuries ago might be a sign the Sun is capable of producing solar storms dozens of times worse than anything we’ve ever seen, a team of physicists calculates in a paper published this week in Nature.

Carbon-14 (14C) is created when high-energy radiation strikes the Earth’s upper atmosphere, converting nitrogen-14 into 14C, which eventually makes its way into plants via photosynthesis.

Earlier this year, a team of Japanese physicists discovered a spike in 14C in tree rings of Japanese cedars dating from the 774–75 growing season. But they were unable to explain where that 14C might have come from because all possible explanations appeared unlikely.

But Adrian Melott, a physicist at the University of Kansas in Lawrence, who is the lead author of the new study, says that the Japanese team made a miscalculation in ruling out one of these possibilities — a giant solar storm.

The problem, Melott says, is that the Japanese team treated solar storms as if they shone like light bulbs, radiating energy uniformly in all directions. But actually, they produce ‘blobs’ of energetic plasma that explode outwards unevenly. Adjusting for that, he says, reduces the size of the solar storm needed to produce the observed 14C spike from 1,000 times larger than anything known, to only 10–20 times larger — meaning that a giant solar storm is suddenly back on the table as a reasonable explanation.

Furthermore, observations by NASA’s Kepler space telescope have found that Sun-like stars are capable of generating superstorms of this type every few hundred to 1,000 years. This doesn’t mean the Sun does the same, “but it suggests it’s reasonable”, Melott says.

Other possible explanations for the spike seem unlikely. Radiation from a supernova explosion has enough power, but the supernova would have to have been within about 100 light years, Melott says. “Such an event would have been blindingly bright in the sky, much brighter than a full Moon. It would have been bright like that for months and could not have failed to be noted by every civilization on Earth.”

Another possibility is a gamma-ray burst from a more distant supernova. But such bursts are rare and produce searchlight-like beams of radiation unlikely to hit us. “I don’t think it’s likely,” Melott says.

If the 774–75 event was indeed a flare, it’s a disturbing find. Such a flare would be about 60 times more powerful than the 1989 solar storm that knocked out power to much of Quebec for nine hours on a cold winter night. Multiply that by 60 and add two decades of increased technological vulnerability, and the effects might be disastrous. “A lot of people could die,” Melott says. “You could have power out for months or longer — no refrigerated food, no food being transported to all the people who live in big cities.”