According to their newest models, gamma radiation from a nearby burst would quickly deplete much of Earth's protective ozone layer, allowing increased ultraviolet radiation (UV) from the sun to reach the surface.
In the longer term, chemical reactions in the atmosphere would produce dark, nitrogen-based gases that would block the sun's heat and trigger global cooling, even as the gamma rays continued to deplete ozone and let in UV rays, the authors suggest.
Some of the pollution would fall as damaging acid rain, which can severely disrupt ecosystems.
The atmosphere might be able to recover within a decade, and a rise in DNA damage caused by increased UV exposure might pass after a few months or years, the researchers note.
But other biological impacts—such as reduced ocean productivity—could linger for an unknown length of time, Thomas said.
The Trouble With Trilobites
Bruce Lieberman, a paleontologist at the University of Kansas, helped develop the initial theory about the Ordovician die-off but did not co-author the recent papers.
The prevailing idea is that an ice age caused the extinction event, he said, but he questions the completeness of that hypothesis.
"At other times there have been ice ages without mass extinctions," he said.
Furthermore, the ice age during the Ordovician was comparatively short, lasting only about 500,000 years before the climate cycled back to a warm spell—almost as if something unusual set the icy period in motion.
So far Thomas and Melott have uncovered a pattern of higher UV radiation during the Ordovician extinction that would match cosmic bombardment over the South Pole.
And Lieberman believes the disappearance of trilobites, extinct arthropods related to horseshoe crabs, could be tied to the Ordovician event.
Although most trilobites are mud-scurrying bottom dwellers, the juveniles of some species have a life stage that sends them floating in the shallow water column, making them vulnerable to higher UV radiation.
But like NASA's Thompson, Lieberman adds that worry over a future gamma-ray burst is "not the thing that's keeping me up at night."
Instead he appreciates the new work for pointing out that Earth is a vulnerable part of the cosmos.
"It gives us a new perspective on things like natural selection and adaptation."
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