The planet's greatest extinction—the Permian mass extinction, or the "Great Dying"—wiped out around 90% of marine species and two-thirds of those on land over 60,000 years. Some 252 million years later, researchers are explaining what happened—and it may not bode well for us. Essentially, the world's oceans became too acidic to sustain life, reports the Los Angeles Times. Acidity levels were generally stable during the first 50,000 years of this phase, then accelerated in a big way when Siberian volcanoes sent incredible amounts of carbon dioxide into the atmosphere—"the final blow that drove the acidification," researcher Matthew Clarkson of New Zealand's University of Otago tells Nature. The team was able to determine ocean conditions at the time through chemical analysis of rocks in the United Arab Emirates that once sat on the ocean floor.
The fact that ocean acidification played a role in the extinction "is a worrying finding, considering that we can already see an increase in ocean acidity today that is the result of human carbon emissions," Clarkson says in a post at Eureka Alert. If emissions keep climbing, Clarkson says the "Great Dying" could represent a worst-case scenario once again. More carbon entered the atmosphere during the extinction than enters it now from fossil fuels, but it's being added at a similar rate, say the researchers. The rate of carbon injection was key to the mass die-off, notes the LA Times, because species weren't able to adapt quickly enough. Scientists next hope to discover whether Permian-era rocks elsewhere on the planet show similar evidence of acidification. (Some say another mass extinction is already under way.)