The history of the Earth’s bombardment with cosmic radiation is written in the trees.
Specifically, when radiation hits the Earth’s atmosphere, it can change any nitrogen atoms it hits to produce a form of carbon, which is in turn absorbed by plants. Linking peaks in this carbon isotope with the growth rings of trees can give us a reliable record of radiation storms going back thousands of years.
This record shows us that the most colossal of these events, known as Miyake events (after the scientist who discovered them), occur about once every thousand years. But we don’t know what causes them – and new research suggests that our leading theory, involving giant solar flares, may be off the table.
Without an easy way to predict these potentially devastating events, we have a serious problem.
“We need to know more, because if one of these occurred today, it would destroy technology including satellites, internet cables, long-distance power lines and transformers,” said astrophysicist Benjamin Pope of the University of Queensland in Australia.
“The impact on global infrastructure would be unimaginable.”
The history of Earth’s encounters with cosmic ray storms is there to tell if you know what to look like. The most important clue is a radioactive isotope of carbon called carbon-14, often called radiocarbon. Compared to other naturally occurring isotopes of carbon on Earth, radiocarbon is relatively scarce. It forms only in the upper atmosphere, when cosmic rays collide with nitrogen atoms, triggering a nuclear reaction that creates radiocarbon.
Because cosmic rays are constantly colliding with our atmosphere, we have a constant but very small supply of what rains down on the surface. Some of it gets stuck in tree rings. As trees add a new growth ring each year, the radiocarbon fallout can be traced back through time, providing a record of radioactive activity over tens of thousands of years.
A large spike in radiocarbon found in trees around the world indicates an increase in cosmic radiation. There are several mechanisms that can cause this, and solar flares are big ones. But there are some other possible sources of radiation storms that have not been definitively ruled out. Nor have solar flares been finally ruled out.
Because interpreting tree-ring data requires a comprehensive understanding of the global carbon cycle, a team of researchers led by mathematician Qingyuan Zhang at the University of Queensland set out to reconstruct the global carbon cycle, based on every bit of tree-ring radiocarbon data they could get their hands on.
“When radiation hits the atmosphere, it produces radioactive carbon-14, which filters through the air, oceans, plants and animals, producing an annual record of radiation in tree rings,” Zhang explains.
“We modeled the global carbon cycle to reconstruct the process over a 10,000-year period, to gain insight into the scale and nature of the Miyake events.”
The results of this modeling gave the team an extremely detailed picture of a number of radiation events – enough to conclude that the timing and profile are not consistent with solar flares. The peaks in radiocarbon do not correlate with sunspot activity, which itself is linked to flare activity. Some peaks persisted for several years.
And there was inconsistency in the radiocarbon profiles between regions for the same event. For one major event, recorded in AD 774, some trees in some parts of the world showed sharp, sudden increases in radiocarbon over a year, while others showed a slower peak over two to three years.
“Instead of a single instantaneous explosion or flare, what we can be looking at is a kind of astrophysical ‘storm’ or eruption,” Zhang says.
Scientists currently do not know what could be causing these outbreaks, but there are a number of candidates. One of these is supernova events, whose radiation can be blasted across space. A supernova possibly took place in AD 774, and scientists have made links between radiocarbon peaks and other possible supernova events, but we have known supernovae without radiocarbon peaks and spikes without linked supernovae.
Other potential causes include solar superflares, but a flare powerful enough to produce the 774 CE radiocarbon spike is unlikely to have erupted from our sun. Perhaps there is some previously unrecorded solar activity. But the fact is that there is no simple explanation that neatly explains what causes Miyake events.
And this, according to the researchers, is a concern. The human world has changed dramatically since 774 CE; a Miyake event now could cause what scientists call an “internet apocalypse” as infrastructure is damaged, harming the health of air travelers and even depleting the ozone layer.
“Based on the available data, there’s about a one percent chance of seeing another within the next decade,” says Pope.
“But we don’t know how to predict it or what damage it might cause. These odds are quite alarming and lay the groundwork for further research.”
The research has been published in Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences.
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