Robert Brakenridge

When a star explodes, it sends high-energy particles out in all directions. This burst of energy can travel through space for thousands of light-years, traversing solar systems and even galaxies.

In a recent paper, published in the Monthly Notices of the Royal Astronomical Society, INSTAAR senior research associate Robert Brakenridge argues that supernovae may be the key to understanding a series of abrupt climate shifts in recent geologic history. The analysis models how such radiation could collide with Earth’s atmosphere, changing its composition. Brakenridge also matches a number of known supernovae to climate shifts preserved in geologic records. 

“We have abrupt environmental changes in Earth’s history. That’s solid, we see these changes,” Brakenridge said. “So, what caused them?”

Brakenridge says that, if nearby supernovae caused such changes, further research could help scientists predict similar events in the future and prepare accordingly.

“When nearby supernovae occur in the future, the radiation could have a pretty dramatic effect on human society,” he said. “We have to find out if indeed they caused environmental changes in the past.”

Brakenridge’s recent paper is actually one of many he and others have published on the topic since the 1980s. But, in the past, the idea has rested mainly in the realm of theoretical physics. Brakenridge’s new publication is an effort to link the theory to empirical observations, both in space and here on Earth.

Telescopes and tree rings

In recent years, high-powered, orbital telescopes have offered unprecedented information about the contents and character of supernova radiation. Using these observations, Brakenridge created a more precise model of how this radiation might interact with Earth’s atmosphere than previously possible.

According to the model, a sudden influx of high energy photons from a supernova would thin the ozone layer, which shields the Earth from the Sun’s rays. Simultaneously, the radiation would degrade methane in the stratosphere, a major contributor to the greenhouse effect that keeps the Earth warm. Put together, these interactions would dampen greenhouse warming and increase the amount of ultraviolet radiation that reaches Earth from the sun. Brakenridge predicts that knock-on effects could include selective animal extinctions, increased wildfires and global cooling.

Since supernova radiation isn’t arriving on Earth today, the model can’t yet be tested in situ. Instead, Brakenridge looked to records of the past for further evidence. Specifically, he looked at tree rings. Because trees incorporate atmospheric carbon into their trunks as they grow, scientists can look to these records for a glimpse into ancient atmospheric conditions.

In the new paper, Brakenridge parses tree ring records spanning 15,000 years and identifies 11 spikes in radioactive carbon. He argues that these spikes may have been caused by 11 corresponding supernovae. 

“The events that we know of, here on earth, are at the right time and the right intensity,” Brakenridge said. 

Predicting supernovae

For now, supernovae are just one possible explanation for these phenomena — solar flares are the most prominent alternative. But, Brakenridge says the evidence is mounting behind his argument. He hopes that further efforts can refine models of environmental effects and correlate them with geologic records — from ice cores to marine sediment to tree rings.

A better understanding of supernova radiation could do more than just satiate curiosity, it could help humans prepare for  abrupt climate shifts that could arrive any day. For example, astronomers predict that Betelgeuse, a nearby red supergiant star perched on the shoulder of the Orion constellation, will meet its end in a supernova explosion sometime soon — it could be tomorrow, or any time in the next 100,000 years. 

“As we learn more about our nearby neighboring stars, the capability for prediction is actually there,” Brakenridge said. “It will take more modeling and observation from astrophysicists to fully understand Earth’s exposure to such events.”

If you have questions about this story, or would like to reach out to INSTAAR for further comment, you can contact Senior Communications Specialist Gabe Allen at gabriel.allen@colorado.edu.