Using Earth’s Trees as nature's Astronomical Observatory

A little over 1,200 hundred years ago, something big went off in Space. There’s very little human record of it nor any current observations with today’s technology to indicate that something big and local occurred. Some evidence for such an event, through historical accounts of a red crucifix, appearing after sunset have been noted in the Anglo-Saxon Chronicles, and are currently the subject of lengthy discussions, but to date, scientists are still puzzled by what could have caused it - working on a unique set of clues that have most certainly been observed. 

Scientists today can narrow down the century it occurred in - somewhere between 774 - 775 before the current era (BCE), during the reign of Charlemagne, and when the world existed without the presence of the things we take for granted today, like electricity grids, real-time information from space, or complex scientific instrumentation. 

So, how do scientists know that something, anything, occurred in space at all? 

Remarkably, it wasn’t looking up at the cosmos. It was what the trees told us. 

Here on Earth, we have that precious thin blue line, our fragile atmosphere that acts as a shield against some of the harsh conditions that exist in space. And harsh they are - extreme radiation, temperature variations that swing in ranges of hundreds of degrees, a near-vacuum, high-velocity objects like meteors, and more. Our evolution developed hand-in-hand with our atmosphere’s protection.

Our local star, our Sun - whilst being the engine for all life as we know it - is also one of the highest sources of risks for humans in space, satellites and even life on Earth. And beyond our own Solar system, radiation, high-energy particles and more continue to come at us from all directions.

When these events reach Earth, the frontline of defence is our atmosphere. And whilst these defences have stood strong for billions of years, they do leave their scars on our planet in fascinating ways, which we can find hundreds of years later. 

Now, Dr Benjamin Pope from The University of Queensland’s (UQ) School of Mathematics and Physics is looking at finding out answers as to how these big bursts of radiation from space might affect astronauts, satellites and of course all the services we rely on here on Earth. 

As an astronomer, Ben (who has published journal articles on exoplanets, asteroseismology, optical technology, and radio emissions from stars to name a few) would normally be looking towards the stars for this type of data. But he’s just been granted a new project with a Fellowship from the Big Questions Institute to search for evidence of these events a little closer to home - inside trees. 

“I’m really happy to be able to tackle such an interdisciplinary project that connects astrophysics, archaeology, and climate science,” said Ben. “It’s rare to receive funding for something as unusual as this, and we’re very grateful for the BQI’s support.”

“We’re launching off the back of a UQ winter research scholarship project with two very bright UQ undergraduate students, Qingyuan Zhang & Utkarsh Sharma, who immediately took initiative on the project and got to work building the first open-source software to analyse these tree ring data.”

Whilst trees might not be the first place most people would consider looking for astronomical data, they contain a deeply-lived historical record of Earth’s environment as they breathe in and expel components of the atmosphere around them, and absorb molecules from the Earth below them. 

And by studying how trees have grown around the world, information such as climatic conditions, solar cycles, and astronomical events can be extrapolated, painting a picture of not only the conditions here on Earth but also the events that Earth encountered in space at different times.

In nature, there are three isotopes of Carbon that exist - two non-radioactive and stable, carbon-12 (¹²C) and carbon-13 (¹³C), and one that is radioactive, known as carbon-14 (¹⁴C)which has a half-life of 5,730 years. 

Carbon itself is one of the most abundant elements in the Universe by mass (fourth place) and features in the make-up of biological organisms. By mass, it also makes up about 18.5% of the human body. 

Carbon is normally generated in the hearts of older stars, once their cores have shifted away from fusing hydrogen, helium and oxygen (there are even carbon stars out there). As lower-mass stars evolve (or in some cases, when a star reaches the end of its life), a large portion of carbon becomes dispersed by the stellar winds of the ageing star (or supernovae event) to later become planets like Earth, then eventually (and in our case) part of the biosphere. 

Given its geologically short half-life, the amount of ¹⁴C in our atmosphere would have long since decayed unless it was continually replenished - with a source of this replenishment coming from astrophysical sources. 

When galactic cosmic rays or high-energy particles from our Sun strike our atmosphere they produce new ¹⁴C isotopes. This occurs through the production of neutrons as the high energy particles travel through our atmosphere, which strikes nitrogen-14 (¹⁴N) atoms, turning them into ¹⁴C. 

Carbon is an element that loves working with other elements, so shortly after production, it combines with oxygen to form carbon monoxide and carbon dioxide. Both of these heavier molecules sink down to the surface level where they are dissolved in the oceans, or absorbed by plants using photosynthesis - and from there, carbon enters the biosphere. 

By knowing the half-life of ¹⁴C, scientists can use this information as a dating technique - with the older the sample being studied, the less amount of the isotope it should have. This is known as radiocarbon dating and can be applied to the fossil record and living organisms. 

Because of the ongoing production of ¹⁴C from astrophysical sources, the baseline has been studied and measured to be a constant background value. However, if something big happens astrophysically - say a nearby supernova that sends a shower of high-energy particles smacking into our atmosphere, then the global radiocarbon record should see a spike to represent this event. 

In 2012, a paper by Miyake et al. appeared in Nature, studying the traces of carbon-14 from the annual rings of Japanese cedar trees dating across the period 750 to 820 BCE. They discovered a sudden spike of carbon-14 in 774 AD, and further studies in 2013 and  2014 showed the same signature in tree rings from Germany, New Zealand, Finland and the United States - the hallmark of a global event. 

What the data reflected was a 12% rapid increase of the value of ¹⁴C in the year from 774 - 775 AD - a value that was 20 times larger than the ordinary background astrophysical generation of the isotope. The relative amount of radioactive ¹⁴C to ordinary ¹²C occurred in that one year period and then decreased at a steady rate over the coming several years. But what could have caused such an event?

The first clue was that it was on a global scale - not a local issue, so it had to be either evidenced by a strong record of large-scale Earth-based events (i.e. many different witness accounts of it) or something of astrophysical nature. The latter being most likely because of the role that events beyond our atmosphere play in the production of the radioactive isotope. 

At the time, researchers looked at two potential scenarios to explain what could have caused such a sharp increase. The first scenario was a supernovae explosion which would have produced a high flux of high-energy particles (like gamma-rays) which would have slammed into our atmosphere, resulting in the sudden production of lots of ¹⁴C.

This scenario was ruled out though, because two other supernovae (SN1006 and SN1054), which were visually observed by witnesses across the world, produced no increase in the value of ¹⁴C for their periods. Additionally, when a supernova is not visually observed, it can be detected in other bands such as X-rays and radio (the younger the brighter), but no detection has been made of any supernovae events for this period so far. 

The second event scientists considered was something more local, coming from our own host star and known as a solar proton event (SPE). The Sun is always sending out high-energy particles that collide with the Earth’s magnetosphere. This magnetic field also acts as a shield protecting us (and our atmosphere) from this constant radiation. 

When the particles are electrons, they get caught in our magnetic field, exciting the atoms in our upper atmospheres to produce the famous auroras at the poles. However, when the particles are protons - they normally don’t have as much energy to penetrate our magnetic field.

Unless of course, the solar event is exceptionally powerful - in which the proton particles can penetrate the magnetic field and cause our ionosphere to ionize, as well as a suite of other issues. 

In this scenario, some additional data was also accounted for - though the authors are cautious - in that ice core samples of beryllium-10 also increased between the years 755 - 785. An SPE event with the right energy spectrum therefore could potentially explain the increase of both these isotopes. 

Whilst the 2012 research by Miyake et al. left this question open, most scientists now believe it was in fact an SPE, which would be more than 40 times larger than any flare directly observed from the Sun since modern technological observations. These superflare events have been observed on other stars by astronomers, but none ever from our own. 

“I think the jury is still out on whether the Sun can cause these - we are not even 100% sure that stars like our Sun when it was young and active can produce superflares, let alone what our Sun can do in its middle age, and we don’t have definitive evidence that the timing or spectrum are what you would expect.” 

“In this project what we hope to do is to really nail down the timing, duration, and size of these Miyake events, and where they occurred in the solar activity cycle, in order to put this on a solid quantitative footing.”

In 2019, a study even looked at another type of star altogether that might have caused the sudden spike of ¹⁴C across four recorded significant events. Instead of our Sun or a supernova, Wang et al. described the sudden increase in the isotope in Earth’s atmosphere as a result of radiation flares from pulsars. 

Pulsars are rapidly rotating high-mass neutron stars that are intensely magnetised. The most magnetic of these are called magnetars, and their twisted magnetic fields carry enormous energy. These sometimes twist into a different shape, and the built-up magnetic energy is unleashed, producing a giant flare of radiation that propagates through space. Such an event was witnessed in 2004 when a magnetar located 50,000 light-years away on the other side of the Milky Way core set off a cosmic blast that caused notable vibrations in Earth’s magnetic field, and several satellites to become temporarily non-functional. What Wang et al. suggest is that flares from neutron stars, similar to those observed on magnetars, could generate the Miyake events.

Under this theory, the outburst would collide with the interstellar medium generating a shock that would accelerate electrons to extreme energies (trillions of electron volts) which would strike the Earth’s atmosphere and produce ¹⁴C.

Our daily lives have become extremely dependant on data, as it ebbs and flows back and forth between our terrestrial stations and the heavens -  communications, navigation, positioning, financial transactions, agricultural technology, military operations, the entire internet - all of this information is routing through space at some point or another. It’s the result of a deeply connected world that processes data in as real-time as possible. 

Additionally, humans have started to leave the confines and safety of Earth’s protective and blanketing atmosphere - with human habitation now continually occupying Low-Earth Orbit for several decades. As the barriers of entry of access to space continue to become smaller, we can safely assume that in the future more humans will be spending time in space. 

While this reliance has provided our daily activities with hyper-efficiencies (e.g., using Google Maps to get from one place to another through our own mobile devices), these services and applications are not existing in a vacuum of zero risks.

Additionally, humans have started to leave the confines and safety of Earth’s protective and blanketing atmosphere - with human habitation now continually occupying Low-Earth Orbit for several decades. As the barriers of entry of access to space continue to become smaller, we can safely assume that in the future more humans will be spending time in space. 

And herein lies the importance of continually monitoring events that occur in space - be that the Sun, any distant supernovae or other phenomena. These events present a clear risk with a high magnitude to disrupt our very lives both on the surface and in space. This level of disruption could range from minor and inconvenient, to catastrophic. 

Atmospheric particles that collide with these high-energy protons ionize the ionosphere (esp. Between 50 - 80 kilometres in altitude), which can have the effect of blocking ionospheric radio communications around the poles. 

In severe cases, high-energy proton events can cause Earth’s magnetic field to fluctuate, thereby triggering widespread disruption to electrical grids. 

Issues related to biological health relative to these events are also well-known, with probability and impact increasing as one’s altitude increases from the surface. Even NASA caps the amount of time any astronauts can spend in space due to their exposure to standard radiation that is emitted from the Sun, let alone any superflare event. 

And it’s not just people in space. When flying on aircraft, as millions do on a daily basis, higher levels of radiation are experienced as we travel above the thicker layers of our atmosphere. Transpolar aircraft have measured rapid increases in incoming radiation during energetic proton events. 

And here is where Ben and his team come into the picture. The new research will see the development of open-source statistical software that models astrophysical-based events through history, by studying growth rings of trees over thousands of years. 

What the team aims to achieve is to infer the astrophysical radiocarbon production rate from these tree-ring observations, computing the entire carbon cycle from the production of the isotope generated by astrophysical sources through to how it filters down into the biosphere. 

“In order to take tree ring data and infer radiocarbon production rates, you actually have to model the whole global carbon cycle - how it flows between the atmosphere and oceans, forests and peat bogs and everything in between,” said Ben.

“Climate scientists and archaeologists use ‘carbon box models’ for this - where you identify each type of reservoir (peat bogs, oceans, sediments, etc.) and how they are connected, and simulate what happens.”

“We’re building the first, and hopefully very fast, open-source carbon box models connected to modern statistical inference algorithms, so that we can look at all the available tree ring data, run our carbon box models millions of times, and statistically determine when these events happened, how big they were, and so forth.”

The results of this unique research will likely have implications on a number of interdisciplinary fields - starting from astrophysics and planetary sciences, running through to climate modelling, and risk mitigation. 

Whilst many of us hear about future missions that extend the reach of humans out into the Solar system, starting with the Moon, then Mars - there still remains a large amount of knowledge in which we need to resolve, as a matter of safety, before sending people on long-duration journeys. 

“If these Miyake events happen once every thousand years or so, that translates to a 1% chance per decade. But for such a remote risk, the consequences could be catastrophic - the biggest known solar flares could leave satellites ruined and astronauts poisoned, and these may be even more serious than that.” 

“In my view, it is essential to find out what causes Miyake events so that we can understand the risks and take steps to mitigate them.”

Currently, there are methods to perform this research using existing pipelines of data related to climate models, but unfortunately, these are not open-sourced nor apply modern statistical methods to them. Often, a third party is required to help with the data analysis. 

However, Ben and his team are planning to make their library an open-source tool (Python library) that will replicate existing pipelines, allowing other scientists to also utilise these data. This new research will generate open-source, reproducible, high-performance computing models for the wider scientific community.

This carbon box model will in turn trace the carbon cycle around the oceans, atmosphere, through the forests and more, in particular looking into how the different isotopes of carbon (¹²C and ¹⁴C) move around the planet. The long term perspective of this research is to improve climate models by better matching radiocarbon data to them.