The cause of one of the world's biggest-ever extinctions might have been found - and it's something that could happen again if climate change isn't addressed, scientists say.
Around 359 million years ago, three-quarters of the world's species were wiped out - including 97 percent of vertebrate species.
But what caused the Late Devonian extinction has remained a mystery until now, as there's no evidence of a massive volcanic event or an asteroid strike like the one that ended the dinosaur age 66 million years ago.
But researchers at the University of Southampton have now uncovered evidence the ozone layer briefly broke down, allowing the sun to bathe the planet in harmful ultraviolet (UV) light.
They collected rock samples from around the world, including Greenland, Bolivia and South America. In the lab they extracted ancient plant spores, which "had bizarrely formed spines on their surface" thanks to damaged DNA and "dark pigmented walls, thought to be a kind of protective 'tan'" - both evidence of damage from UV rays.
The ozone layer protects us from UV rays. Anyone who grew up in the 1980s and '90s would remember the ozone hole that used to exist above Antarctica, caused by chlorofluorocarbons found in aerosol sprays, packing materials and refrigerants. The hole has begun to shrink in recent years after they were phased out.
The collapse of the entire layer 359 million years ago clearly wasn't caused by manmade aerosols - it was rising temperatures, following a protracted ice age - just like the situation we're in now, except now we're adding fuel to the fire by burning carbon-rich fossil fuels.
The warming atmosphere became more humid, carrying more water vapour up into the stratosphere than before.
"This increased high-altitude water content then passes the threshold for an increased production of catalytically active [chlorine oxide] with ensuing increased rate of ozone loss," the study said.
"The recognition that a known extinction kill mechanism, the loss of the ozone layer, occurred not only during [a large volcanic event] but at times of high global temperature identifies a new mechanism for mass extinctions.
"However, unlike a LIP or [asteroid] impact, higher temperatures are a certainty in the immediate future with implications for a similar collapse of the ozone layer."
Study lead author Prof John Marshall says we're on track for temperatures similar to those found in the Late Devonian.
"Current estimates suggest we will reach similar global temperatures to those of 360 million years ago, with the possibility that a similar collapse of the ozone layer could occur again, exposing surface and shallow sea life to deadly radiation. This would move us from the current state of climate change, to a climate emergency."
The Late Devonian extinction was the second of five to hit Earth. Scientists in recent years have suggested we could be in the sixth right now. A UN report last year concluded a million species or more will go extinct in the next decade unless urgent action is taken.
The University of Southampton research was published in journal Science Advances on Thursday (NZ time).