The most devastating mass extinction in planetary history is called the end-Permian mass extinction event, or "the Great Dying." It happened 252 million years ago, prior to the dawn of the dinosaurs.
During the Great Dying, roughly 90% of the Earth's species were wiped out; less than 5% of marine species survived, and only a third of land animal species made it, according to National Geographic. The event far eclipsed the cataclysm that killed the last of the dinosaurs some 187 million years later.
But the Great Dying didn't come out of left field.
Scientists think the mass extinction was caused by a large-scale and rapid release of greenhouse gases into the atmosphere by Siberian volcanoes, which quickly warmed the planet. So there were warning signs. In fact, a 2018 study noted that those early signs appeared as much as 700,000 years ahead of the extinction event.
"There is much evidence of severe global warming, ocean acidification, and a lack of oxygen," the study's lead author, Wolfgang Kießling, said in a release.
Today's changes are similar but less severe — so far.