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There have been five major mass extinctions in the history of complex life. They were really bad. Horribly, unthinkably bad. This might sound obvious, but it’s difficult to conceive just how bad the five worst catastrophes in earth’s history actually were. Despite the headlines, and despite the tremendous damage humanity has inflicted on the biosphere in the past few centuries, we haven’t yet consummated a modern mass extinction equal to these ancient global die-offs. This might sound reassuring, or even excusatory for humanity, but the fact that our modern destruction of the natural world is even in the same conversation as these ancient doomsdays is appalling.

The most recent major mass extinction, the one that knocked off the big dinosaurs and most other animal life 66 million years ago, was so severe that University of Chicago paleontologist David Raup calculated that, to reach similar levels of destruction today, we’d have to sterilize the entire planet except for a small circle around New Zealand. A mass extinction 200 million years earlier dwarfed even this mass killing. So why do scientists study the greatest mass extinctions of the past, besides morbid curiosity?

One of the most alarming discoveries in the past couple of decades has been the emergence of a main culprit in many of these forgotten apocalypses: carbon dioxide. This is why I wrote the book. While visions of asteroids as civilization-threatening-messengers-from-beyond captivate moviegoers and doomsday cultists, the more subtle, but no less catastrophic, prospect of ancient oceans and atmospheres derailed by carbon dioxide is emerging as one of the most dependable purveyors of mass death in the fossil record. There are lessons the rocks can teach us. The effect of CO₂ on the earth’s climate and oceans is often talked about as if it’s purely theoretical — that climate change was dreamt up in ivory-tower computer models and pushed by overzealous environmentalists. But the earth has run this experiment many times in the deep past. And as geologists have learned by studying the rocks and fossils of forgotten worlds, the results have sometimes been apocalyptic.

At the end of the Permian period 252 million years ago, the planet was populated by fantastical coral reefs in the sea and strange beasts and forests on land. A geological eye-blink later, the planet was almost vacant, the fossil record testifying to a breathtakingly impoverished world. Geologists searched in these extinction layers for the signs of an asteroid impact similar to the one found at the end of the age of dinosaurs, but they came up empty. Instead they discovered ancient mind-bending volcanoes in Siberia, dated exactly to this antediluvian Armageddon. Most concerning, in only the past few years, they’ve discovered that the magma came up through one of the largest fossil-fuel basins in the world, burning through gigatons of coal, oil, and gas in only a few thousand years. As a result of this volcanic carbon dioxide and methane, the world became excruciatingly hot. In the seas some scientists have implicated ocean acidification — the result of excess carbon dioxide reacting with seawater — as the main suspect in this crime that nearly sterilized the oceans.

Today we’re similarly burning through vast coal basins like an ancient supervolcano, injecting gigatons of carbon dioxide into the atmosphere. As a result, stark changes to the climate are barreling towards us in the coming decades, regardless of whether politicians, compromised by the fossil fuel industry, believe it or not. Our oceans are acidifying as they have only in rare geological disasters in earth's history. As a result, by midcentury, many scientists think that coral reefs are doomed.

Returning, then, to the claim that we’re not yet in a mass extinction on par with the holocausts of the ancient past, the key word is “yet.” Casual gloom about the fate of our natural world threatens to inspire inaction while there’s still much work to do. There’s still time to save the world. But, unnervingly, many biologists and paleontologists believe that the natural world, when it does begin to fail, will collapse catastrophically — as it has only a few times in the history of the planet. For a long time the living world might seem to withstand our blows with relatively good humor — that is, until unstable food webs and complex but wobbly ecosystems come crashing down all at once like a stack of Jenga blocks. These points of no return are so-called “tipping points.” Though we haven’t hit mass extinction levels yet, we might be inching up to a truly geologic catastrophe without realizing until it’s too late.

Clearly we need to stop razing forests, dynamiting coral reefs, and trawling the seafloor clean. It has been this sort of direct destruction of habitat and animal life that has caused much of the wounds in the biosphere so far during our species's short stewardship of the planet. But if ancient mass extinctions have anything to teach us, it’s that fiddling with the planet’s carbon cycle might be the most dangerous prospect of all. We have no idea whether our experiment with the oceans and atmosphere will go as haywire as it did at the end of the Permian, or during any of the other major mass extinctions in our planet’s history. And, indeed, there are reasons to think that these old catastrophes — shrouded by the fog of deep time and buried in the earth — were worse than anything humans could ever inflict on the planet. But perhaps we shouldn’t try to find out. Peter Brannen is an award-winning science journalist whose work has appeared in the New York Times, the Atlantic, Wired, the Washington Post, Slate, the Boston Globe, Aeon, and others. A graduate of Boston College, he was a 2015 journalist-in-residence at the Duke University National Evolutionary Synthesis Center and a 2011 Woods Hole Oceanographic Institution Ocean Science Journalism Fellow. The Ends of the World is his first book.