Tree rings tell of a mysterious cosmic storm that strikes every thousand years

Tree rings tell of a mysterious cosmic storm that strikes every thousand years

The story of the bombardment of the Earth by cosmic radiation is written in the trees.

Specifically, when radiation hits the Earth’s atmosphere, it can alter any nitrogen atoms it hits to produce a form of carbon, which is in turn taken up by plants. Linking peaks of this carbon isotope to tree rings can give us a reliable record of radiation storms dating back thousands of years.

This record shows us that the most colossal of these events, known as the Miyake events (named after the scientist who discovered them), occur approximately once every thousand years. However, we don’t know what causes them – and new research suggests that our leading theory, involving giant solar flares, may be ruled out.

Without an easy way to predict these potentially devastating events, we are left with a serious problem.

“We need to know more because if any of them happened today, it would destroy technology, including satellites, internet cables, long-distance power lines and transformers,” says the astrophysicist Benjamin Pope from the University of Queensland in Australia.

“The effect on global infrastructure would be unimaginable.”

The story of Earth’s encounters with cosmic radiation storms is there to be deciphered if you know how to look. The main clue is a radioactive isotope of carbon called carbon-14, often referred to as radiocarbon. Compared to other natural carbon isotopes on Earth, radiocarbon is relatively rare. It only forms 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 amount of stuff raining down on the surface. Some of it is caught in the tree rings. Since trees add a new ring of growth each year, the deposition of radiocarbon can be traced back in time, giving a record of radiation activity over tens of millennia.

A large radiocarbon spike found in trees around the world signifies an increase in cosmic radiation. Several mechanisms can be the cause, and solar flares are an important part of it. But there are other possible sources of radiation storms that have not been definitively ruled out. Solar flares have also not been definitively 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 of the University of Queensland set out to reconstruct the global carbon cycle, based on every piece 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, and produces an annual record of radiation in tree rings,” says Zhang.

“We modeled the global carbon cycle to reconstruct the process over a period of 10,000 years, to better understand the magnitude 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 timing and pattern are inconsistent with solar flares. Radiocarbon spikes do not correlate with sunspot activity, which itself is related to flare activity. Some peaks have persisted for several years.

And there were inconsistencies in radiocarbon profiles between regions for the same event. For a major event, recorded in 774 CE, 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.

“Rather than a single instantaneous explosion or eruption, what we can observe is a kind of ‘storm’ or astrophysical explosion,” Zhang said.

Researchers don’t know at this point what could be causing these explosions, but there are a number of candidates. One of them is supernova events, the radiation from which can explode into space. A supernova may have occurred in 774 CE, and scientists have linked radiocarbon spikes to other possible supernova events, but we have known supernovae without radiocarbon spikes and spikes without supernovae linked.

Other potential causes include super solar flares, but a flare powerful enough to produce the 774 CE radiocarbon peak is unlikely to have erupted from our Sun. Perhaps there is previously unrecorded solar activity. But the thing is, there is no simple explanation that clearly spells out what is causing the events in Miyake.

And that, according to the researchers, is a concern. The human world has changed dramatically since 774 CE; a Miyake event could now 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 data available, there’s about a 1% chance of seeing another one in the next decade,” Pope said.

“But we don’t know how to predict it or what damage it may cause. These probabilities 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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