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| A composite image showing a tree ring and flames - UQ researchers used tree ring data to model the global carbon cycle to challenge the common theory about Miyake Events. Credit: The University of Queensland |
A University of Queensland study has shed new light on a mysterious,
unpredictable and potentially devastating kind of astrophysical event.
The history of Earth's bombardment with cosmic radiation is written in the
trees.
Specifically, when radiation slams into Earth's atmosphere, it can alter any
nitrogen atoms it slams into to produce a form of carbon, which is in turn
absorbed by plants. Linking spikes in this carbon isotope with the growth
rings in 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 around once every thousand years. However, we don't know what
causes them – and new research suggests that our leading theory, involving
giant solar flares, could be off the table.
Without an easy way to predict these potentially devastating events, we're
left with a serious problem.
"We need to know more, because if one of these happened today, it would
destroy technology including satellites, internet cables, long-distance
power lines and transformers,"
says astrophysicist Benjamin Pope
of the University of Queensland in Australia.
"The effect on global infrastructure would be unimaginable."
The history of Earth's encounters with storms of cosmic radiation is there
to decipher 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 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 the stuff raining down on the surface.
Some of it gets caught up in tree rings. Since trees add a new growth ring
every year, the radiocarbon deposition can be traced back through time,
giving a record of radiation activity over tens of millennia.
A large spike in radiocarbon found in trees around the world means an uptick
in cosmic radiation. There are several mechanisms that can cause this, and
solar flares are a big one. But there are some other possible sources of
radiation storms that haven't been conclusively ruled out. Nor have solar
flares been conclusively ruled in.
Because interpreting tree ring data necessitates a comprehensive
understanding of the global carbon cycle, a team of researchers led by
mathematician Qingyuan Zhang of the University of Queensland set about
reconstructing the global carbon cycle, based on every scrap of tree ring
radiocarbon data they could get their hands on.
"When radiation strikes 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,"
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 is inconsistent with solar flares. The spikes in radiocarbon do not
correlate with sunspot activity, which is itself linked with flare activity.
Some spikes persisted across multiple years.
And there was inconsistency in the radiocarbon profiles between regions for
the same event. For one major event, recorded in 774 CE, some trees in some
parts of the world showed sharp, sudden rises in radiocarbon for one year,
while others showed a slower spike across two to three years.
"Rather than a single instantaneous explosion or flare, what we may be
looking at is a kind of astrophysical 'storm' or outburst,"
Zhang says.
The researchers don't know, at this point, what might be causing those
outbursts, but there are a number of candidates. One of those is supernova
events, the radiation from which can blast across space. A supernova
possibly did take place
in 774 CE, and scientists have made links between radiocarbon spikes and other
possible supernova events, but we have known supernovae with no radiocarbon
spikes, and spikes with no 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's some previously unrecorded solar activity. But
the fact is, there's no simple explanation that neatly explains what causes
Miyake events.
And this, according to the researchers, is a worry. The human world has
changed dramatically
since 774 CE; a Miyake event now could cause what the scientists call an
"internet apocalypse" as infrastructure gets damaged, harm the health of air
travelers, and even deplete the ozone layer.
"Based on available data, there's roughly a one percent chance of seeing
another one within the next decade,"
Pope says.
"But we don't know how to predict it or what harms it may cause. These odds
are quite alarming, and lay the foundation for further research."