When the Hydro-Québec power grid collapsed on March 13, 1989, the outage plunged the entirety of Quebec—more than six million people—into darkness for several hours. The event was triggered by a ferocious storm, but the tempest wasn’t of Earth’s making. Instead the source was the sun: our nearest star had unleashed a swarm of high-energy particles and radiation that wreaked havoc on our technological infrastructure.

Yet that event wasn’t a fluke, scientists now know. Nor was it particularly powerful. Careful analysis of evidence gathered from tree rings suggests that similar but far larger barrages have repeatedly struck Earth in the relatively recent past. Researchers have long considered our star’s sometimes extreme activity to be the culprit in these larger events, but new work incorporating insights from tree physiology and our planet’s carbon cycle challenges the idea that solar storms are responsible. 

This shift in thinking began roughly a decade ago, when Fusa Miyake, a cosmic ray physicist, began analyzing the wood of long-lived cedars felled on Japan’s Yaku Island. Miyake, then a graduate student at Nagoya University in Japan, was meticulously extracting carbon-rich cellulose from the trees’ rings, each of which typically recorded a whole year’s worth of growth in a span of less than a millimeter. Her goal was to measure the amount of carbon 14—a radioactive isotope of carbon commonly used to date archaeological artifacts

Carbon 14, also known as radiocarbon, is produced naturally on our planet when high-energy radiation and particles emitted by the sun, other stars and various cosmic cataclysms interact with atoms in Earth’s upper atmosphere, most notably nitrogen. Radiocarbon is also a by-product of human activity—the isotope’s atmospheric concentration doubled during the mid-20th century heyday of cold war–era atomic weapons testing, when the U.S. and other nations detonated hundreds of nuclear bombs in the atmosphere.

Radiocarbon accounts for only a vanishing fraction, about one part in one trillion, of the carbon cycling through our planet’s air, water and rock. But that’s still more than enough for detectable traces of the substance to accumulate within trees and other photosynthesizing plants, which suck it out of the air as radioisotope-laced carbon dioxide. This makes a tree’s annual growth rings a year-by-year record of local environmental conditions and prevailing atmospheric chemistry and allows scientists to use ancient, well-preserved wood to probe many millennia back in time.

Since the 1950s researchers have known that the concentration of radiocarbon in tree rings varies over time. But until recently, relatively large amounts of cellulose were required for radiocarbon analyses, so most measurements were based on five or even 10 years’ worth of tree rings. Tipped off by earlier research suggesting a pronounced increase in atmospheric carbon 14 some time in the late eighth century, Miyake began examining individual tree rings dating from C.E. 750 to 820. She used a sensitive technique known as accelerator mass spectrometry to detect minute amounts of radiocarbon, which she hoped would shed light on when—and why—such an event might have happened.

Many people didn’t see the point to that work, says Jesper Olsen, a physicist at Aarhus University in Denmark who was not involved in the research. “Nobody really thought that was worth it,” he says.

Miyake’s efforts paid off, however, when the data revealed an anomalously large increase in the concentration of carbon 14 precisely in the year C.E. 775. She and her colleagues reported the discovery in Nature in 2012. Since then researchers have uncovered additional “Miyake events” in other tree ring records. Six events—the oldest dating back to 7176 B.C.E.—have been particularly well studied, and Miyake events have been used to constrain the timing of various historical occurrences such as the arrival of the Vikings in the Americas. But the origins of Miyake events remain mysterious. Other than a general consensus that they’re caused by some astrophysical process, they’ve been variably attributed to solar activity, emissions from a nearby supernova or neutron star or even Earth’s close encounter with a comet.

Enormous solar flares are typically invoked to explain Miyake events, says Tim Jull, a geoscientist at the University of Arizona, who was not involved in the research. Such outpourings of electromagnetic radiation occur regularly on the sun and are often associated with bursts of high-energy particles. But this linkage between solar activity and the record of Earth’s fluctuating radiocarbon is blurred by the fact that our sun also plays defense: its magnetic field helps shield Earth from high-energy particles streaming in from beyond the solar system and potentially decreases rather than increases the amount of radiocarbon produced from cosmic sources. That shielding is particularly pronounced at the peak of the 11-year solar cycle, when the sun’s magnetic field is strongest. 

One fact is known for sure about Miyake events, says Benjamin Pope, an astronomer at the University of Queensland in Australia. “They’re a detection of a huge burst of radiation reaching the Earth,” he says. And those bursts surpass the barrage unleashed in 1989 that crippled the Hydro-Québec power grid, researchers now know. Pope and his colleagues recently analyzed radiocarbon records from more than 60 trees spread across four continents in search of the nature and likely origin of these mysterious—and worrying—signals. The team’s surprising results, which muddle the already murky understanding of Miyake events, were published late last year in Proceedings of the Royal Society A.

Plot a series of radiocarbon measurements as a function of time, and a Miyake event will look much like a cliff that’s nearly vertical on one face but much more gradually sloping on the other. Most Miyake events ramp up very quickly—often within the span of a single tree ring—and then fade away over 10 to 20 years. But that denouement isn’t because of the radioactive decay of the isotope, which occurs across millennia rather than decades. Instead it’s the outcome of Earth’s carbon, whether radioactive or not, continuously journeying through the environment, Pope says. “It gets circulated through the carbon cycle,” he adds. After a burst of radiocarbon is created in the atmosphere, it is eventually entombed in sediments at the bottom of the ocean.

Gaining a better understanding of the timing and duration of Miyake events demanded a better accounting of radiocarbon moving though the global carbon cycle, Pope and his colleagues realized. The researchers turned to so-called carbon box models, which consist of systems of differential equations dictating how carbon diffuses through the atmosphere, the biosphere, the oceans and other reservoirs on Earth. Those models are all trying to answer a fundamental question, Pope says. “If you put a bit of radiocarbon into the atmosphere, where does it go and when?” 

Pope and his team began by generating a set of simulated Miyake events, each of which differed slightly in its start date, span and strength. Then they fed that simulated data through several different carbon box models and compared the output with real-world radiocarbon measurements obtained from tree rings to converge on a set of best-fit parameters for each Miyake event.

Based on this analysis, most Miyake events appeared to be consistent with short, nearly instantaneous “spikes” of radiocarbon production. Two events seemed to be prolonged in time, however. One in particular, which occurred in 663 B.C.E., lasted for roughly three years, the researchers concluded. That’s perplexing, Pope says, because solar flares, coronal mass ejections and other eruptions from the sun typically last only a few days or weeks. Such a relatively short burst of high-energy particles would presumably be captured in a single tree ring, which is assembled over the course of a year, he says. Finding evidence of a multiyear signal, then, is rather befuddling. “God knows what’s going on,” Pope quips. 

But perhaps there’s an explanation that doesn’t involve giving up on rapid outbursts from the sun, says Tamitha Skov, a heliophysicist at Millersville University, who was not involved in the research. The answer might lie in the intricacies of Earth’s magnetic field, she says. High-energy particles can become trapped there, thousands of kilometers above the bulk of the atmosphere—high enough to take several years to dribble down and form radiocarbon. That might explain why some Miyake events look so extended in time, Skov says. Perhaps “some of these longer-duration events could be a shorter-duration source,” she says.

Another explanation for prolonged Miyake events might simply be tree physiology, Jull says. When trees start to grow in the springtime, they’re in some cases relying on nutrients already stored within their cells, he says. That could cause a seemingly short-duration event such as a pulse of radiocarbon to appear smeared out over time when one looks at tree rings. “There’s some mixing between the new signal and the old signal,” Jull says. 

Of course, there’s always the possibility that multiyear Miyake events are truly extended in time, Pope and his colleagues acknowledge. In 2020 another team of researchers, including Fusa Miyake, proposed that a sequence of solar flares occurring repeatedly over several years might be responsible. 

Pope and his team were also interested in the precise timing of Miyake events relative to the solar cycle. Solar flares tend to be about four times more likely to occur around solar maximum than around solar minimum, researchers have shown, so it would make sense that Miyake events might be clustered around the peak of the solar cycle, Pope says. “What I’d hoped to do was say that these all occurred at solar maximum,” he adds. But the team found instead that none of the events synced up with the peak of the solar cycle.

Other researchers are still betting that Miyake events are linked to the sun, however. “It’s not surprising at all to me that they don’t line up with the peak of solar cycle,” says Delores Knipp, a geophysicist the University of Colorado Boulder, who was not involved in the research. After all, she notes, the sun is perfectly capable of launching high-energy particles toward Earth at times other than solar maximum. “We know that most coronal mass ejections that reach Earth—which are typically the big drivers of solar energetic particle events—tend to appear past the peak of the solar cycle,” Knipp says. 

Another idea is that Miyake events have something to do with a weaker-than-average solar cycle, Pope and his colleagues hypothesize. Scientists have spotted a decade-long rise in radiocarbon in tree rings dating to circa 5480 B.C.E. While not typically regarded to be a Miyake event, that signal might have been caused by a period of extremely weak solar activity, other researchers have proposed. The sun’s weaker-than-average magnetic field would have allowed more high-energy particles from interstellar space to reach Earth during that time. 

Tree rings aren’t the only place to look for answers about Miyake events either. Some researchers have turned to ice core records to look for beryllium 10 and chlorine 36—two isotopes that, like radiocarbon, are produced in the atmosphere by high-energy phenomena. Andrew Smith, a physicist at the Australian Nuclear Science and Technology Organization, studies such isotopes in Antarctic ice cores. Drilled from deep within a glacier or ice sheet, a few tens of centimeters from these ice cores can capture a year of past time, offering investigators more material to work with to pin down the timing of ancient events. This allows measurements on timescales of months, compared with the annual temporal resolution of tree rings. Smith and his colleagues are currently analyzing data from their ice cores with Miyake events in mind.

Miyake events and their origins are still mysterious—and might remain that way until one is finally recorded with scientific instruments. But perhaps that’s not a future we should wish for, Pope says. After all, a cosmic barrage intense enough to show up in tree rings would likely be disastrous to the thousands of satellites that encircle the planet. Their sensitive electronics would be essentially fried, Pope says, and that could have ripple effects in the spheres of navigation and communication, technologies we take for granted in modern society. If—or when—the next Miyake event occurs, he says, “good luck to telecommunications.”