Scientists think they've figured out the falling dominoes that led to Earth's largest mass extinction and worry that human-caused climate change puts the planet on a vaguely similar path.
Some 250 million years ago, about 90 per cent of sea life and 70 per cent of land life went extinct in what is now called the Great Dying.
Scientists have long speculated that massive volcanic outbursts triggered the cataclysmic event, but how that worked was still a bit fuzzy.
It wasn't the lava itself. A new study in Thursday's journal Science used complex computer simulations to plot out what happened after the volcanoes blew: It led to ocean temperatures rising by about 11 degrees Celsius, which then starved the seawater of oxygen.
That hot oxygen-starved water caused the mass marine die-off, especially farther from the equator.
After the volcanoes blew, the level of heat-trapping carbon dioxide soared to a level more than 12 times what it is today, said study lead author Justin Penn, an Earth sciences researcher at the University of Washington.
Water loses oxygen when it warms, much like a warm can of cola goes flat, Mr Penn said.
Scientists looked at dozens of modern species to see what happens to them in warmer, oxygen-starved water and that helped them understand the past extinction.
One of the keys in the research is that more species died off away from the equator. That's because tropical species were more acclimated to low oxygen levels, Mr Penn said.
While humans aren't warming the Earth anywhere close to as much as what happened naturally 250 million years ago, "this puts our future into the category of contenders for true catastrophe," said study co-author Curtis Deutsch, an Earth scientist at the University of Washington.
The ancient die-off "shows almost exactly what lies at the end of the road we're on," Mr Deutsch said. "We're really doing the same thing to Earth's climate and oceans."
The study calculates that if heat-trapping carbon dioxide emissions continue on current levels, by the year 2300, the globe will experience 35 to 50 per cent of the extinction level seen in the Great Dying.
University of Leeds palaeontologist Paul Wignall said no current global warming scenario envisions 20 degrees of warming in the next few centuries, so it could be millennia away.
However, even an event 10 per cent as bad as the Great Dying "would be dreadful," said Wignall, who wasn't part of the study.
Other outside scientists said the study provides a scary glimpse into Earth's possible future.
"Because we are warming up the Earth at a rapid rate, results from this study could prove to very useful in understanding" what happens to life in future oceans, University of Southern California Earth scientist David Bottjer said in an email.