Most scientists agree that supernova explosions have affected Earth's climate, though the details are not all clear. They likely cooled the climate several times in the last several thousand years, just as humanity was becoming established around the world. The evidence is in telescopes and tree rings.
We live in the Quaternary period which spans from 2.58 million years ago to the present. The Quaternary is characterized by climatic and environmental changes, most significantly the Ice Ages. The Quaternary also encompasses human existence and evolution from early hominids to our species.
New research in the Monthly Notices of the Royal Astronomical Society examines the role supernovae explosions played in the Quaternary climatic changes. It's titled "Late Quaternary supernovae in Earth history," and the author is Robert Brakenridge. He's a senior research associate in the Institute of Arctic and Alpine Research at the University of Colorado.
The Late Quaternary is loosely defined as the time period from 50,000 years ago to the present. It featured several abrupt, dramatic changes in Earth's climate. These include the Older Dryas and the Younger Dryas, abrupt shifts to a colder climate while the Earth was experiencing a warming trend after the last ice age, at the end of the Pleistocene epoch.
There are several ways that SN can cool Earth's climate. They can weaken or destroy Earth's ozone layer, which would allow more UV light to reach the surface. That leads to knock-on effects that can cause cooling. SN also contribute to cooling by degrading atmospheric methane, a greenhouse gas. The primary way that SN can cool Earth's climate is by increasing the cloud cover.
The idea that supernovae could be responsible for abrupt climate shifts is supported by evidence from tree rings. Trees absorb three isotopes of carbon, carbon-12 (12C), carbon-13 (13C), and carbon-14 (14C). When researchers examine ancient tree rings, they find different ratios of the carbon isotopes in different rings. 12C and 13C are stable isotopes, while 14C isn't. 14C is created continuously in the upper atmosphere all of the time.
When supernovae explode, they send high-speed energetic particles outward in all directions. When some reach Earth, they collide with nitrogen in the atmosphere and generate 14C. Because of this, the atmospheric level of 14C spikes when a nearby supernova (in astronomical terms) explodes.
Tree rings can be dated, and when scientists date tree rings with raised levels of 14C, it indicates that a supernovae explosion occurred somewhere nearby at a specific time.
It's not that cut and dried, however, and not all researchers agree that we can link tree rings with supernovae. Miyaki events also create a burst of 14C that can be identified in tree rings, but they're caused by the Sun. However, the the idea that supernovae could be responsible for 14C and climate shifts won't go away.
“We have abrupt environmental changes in Earth’s history. That’s solid, we see these changes,” study author Brakenridge said. “So, what caused them?”
The question rings out as we try to understand our own history and what the future might hold for Earth.
“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.”
Tree rings and carbon-14 are only part of the story. The other part is told by our powerful telescopes that search the heavens. When stars explode as supernovae, they don't just simply disappear. They leave behind remnants, an expanding shock wave of dead star material and swept up interstellar medium that's lit up by the explosion and depending on the type of supernova, a white dwarf.
Supernova remnants (SNR) are some of Nature's most dazzling displays. The most well-known one is probably the Crab Nebula, the remnant from the 1054 supernova. The crab nebula was the first astronomical object successfully linked with a historical supernova.
The Crab Nebula is the remnant from the 1054 AD supernova. Ancient astronomers in China and other places observed the explosion at the time and recorded it. Image Credit: By NASA, ESA, J. Hester and A. Loll (Arizona State University) - Public Domain
As our telescopes have grown in power, scientists have learned a lot about the radiation that comes from supernovae. The struggle is to understand exactly how the radiation interacted with Earth and affected its climate. There's no clear picture of how far away SN can be and still affect Earth.
"All known Late Quaternary SNRs are much further away than the solar system neighbourhood," the author writes in his research. "SNe at distances of >0.1 kpc have sometimes been considered too remote to affect Earth's biosphere and atmosphere." Brakenridge explains that this conclusion is based on catastrophic effects rather than significant effects. "Such a criterion is not appropriate for Late Quaternary time, as there is no known global extinction event comparable to, for example, those in the late Ordovician and the Cretaceous-Tertiary boundary," he explains, mentioning two of Earth's major extinctions.
"Instead, there is abundant evidence of global climate changes of lesser magnitude and also major mammalian extinction events," Brakenridge explains. "It is thus not possible with present knowledge to rule out SNe at a few 0.1 kpc as causing significant effects observable in Late Quaternary records."
In this research, Brakenridge created a new model of how SN radiation affects the planet's atmosphere.
No SN are affecting Earth right now, so Brakenridge tested the model the only way he can: with tree rings. He examined tree rings over the last 15,000 years and found 11 carbon spikes. Eventually, he determined that four supernovae could have affected Earth's climate during the Late Quaternary.
“The events that we know of, here on earth, are at the right time and the right intensity,” Brakenridge said.
The Hoinga SNR closely aligns with the Older Dryas abrupt cooling about 14,000 years ago.
The Hoinga SNR may be linked to the Earth's Older Dryas abrupt cooling period. Image Credit: Team New Horizons/Australia Telescope National Facility.
The Vela SNR is associated with the Younger Dryas cooling about 12,000 years ago.
The Vela SNR may be linked to Earth's Younger Dryas cooling period. Image Credit: By (Credit) ESO/TIMER survey - https://www.eso.org/public/news/eso2214/, CC BY 4.0
Two other C14 events, the largest of the Holocene, at about 9,000 and 7,000 years ago, are also closely aligned with nearby SNR.
The Older and Younger Dryas affected our human ancestors, who were struggling to survive. The Younger Dryas was particularly difficult for humans, who saw a significant population decrease during this time. We may be buffered from similar climatic changes by technology, but these types of rapid climate shifts could be catastrophic for civilization as we know it.
Tree rings and telescopes aren't the only evidence showing how SN could've affected Earth. There's also sediments, ice cores, and other evidence. The challenging part is to piece all of it together and understand how the past played out.
The other challenge is to look around and us and determine what nearby stars may be about to explode, and what threat they might pose to the climate. It's possible that we could be thrust back into a difficult survival situation like our ancestors were.
“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.”
