Scientists studying ancient ice cores extracted from Greenland and Antarctica have found further evidence of two very powerful solar storms in the last 1,300 years, raising concerns about similar events in the future. Extreme solar storms pose a threat to all forms of electronic technology.

In 2012, Japanese scientists first reported the discovery of a spike of carbon-14 in cedar trees on that island during the AD 774-775 time frame. Since then, similar spikes have been observed in tree rings from the same era obtained on other continents. And thanks to a college biochemistry student, scientists have rooted the event in history through a reference in the Anglo-Saxon Chronicle, which cites a “red crucifix” appearing in the heavens after sunset in the year 774.

It hasn't been entirely clear what caused this radioactive spike, however, and exotic theories such as a giant comet hitting the Sun or a nearby supernova have been put forward to explain it. Now researchers led by geologist Raimund Muscheler of Lund University in Sweden say they have solved the mystery of both the 774-775 AD event as well as a similar one in 993-994 AD. In addition to carbon-14, they have linked radioactive beryllium to both events, firmly establishing them as solar flares.

“In this study we have aimed to work systematically to find the cause for these events," Muscheler said. "We have now found corresponding increases for exactly the same periods in ice cores. With these new results it is possible to rule out all other suggested explanations and thereby confirm extreme solar storms as the cause of these mysterious radiocarbon increases.”

The new analysis of these past solar storms also confirms that they were several times stronger than the most intense solar storms that have been recorded on Earth. The largest solar flare ever measured came in 1859, during the so-called Carrington Event. Named for British astronomer Richard Christopher Carrington who discovered and tracked the solar outburst, the event disrupted telegraph service around the world.

In 2013, Lloyd's of London and the Atmospheric and Environmental Research Center estimated that the duration of power outages during a Carrington-like event today could last five months or longer for 20 to 40 million Americans at a total economic cost of $0.6-2.6 trillion.

Additionally, were an event the magnitude of the AD 774 or AD 993 solar storms to occur today, the study authors say it would critically disrupt satellite-based technology and means of communication. They urged a reassessment of the risks associated with very intense solar storms, which may very well occur more frequently than currently believed.

Nature Communications, 2015. DOI: 10.1038/ncomms9611 (About DOIs).