The worst mass extinction in Earth’s history is the Permian-Triassic extinction, sometimes called the “Great Dying.” It occurred roughly 252 million years ago and wiped out 80 to 90% of all species in the ocean and about 70% of land-based vertebrate families. Some analyses push the marine species loss as high as 96%, though more recent statistical work places it closer to 81%. Either way, nothing else in the fossil record comes close.
How It Compares to the Other Mass Extinctions
Earth has experienced five major mass extinctions over the past 540 million years, collectively known as the “Big Five.” The one most people know is the end-Cretaceous extinction 66 million years ago, which killed off the non-bird dinosaurs after an asteroid impact. That event was devastating, but the Permian-Triassic was far larger in scale. In every analysis of ecological and biogeographic impact, the end-Permian ranks highest. Some paleontologists have argued that if you’re being strict about the data, there’s really only the “Big One,” with the end-Permian standing so far above the rest that calling it one of five understates the gap.
At the genus level (the taxonomic rank just above species), the end-Permian erased between 56 and 69% of all marine genera. The end-Cretaceous, by comparison, killed roughly 40 to 47% of genera. The difference grows even wider at the species level, where smaller, less widespread organisms were hit hardest.
What Actually Caused It
The trigger was not an asteroid. It was volcanism on a scale that’s hard to visualize. A massive eruption system in what is now Siberia, known as the Siberian Traps, poured out lava across an area roughly the size of Western Europe. Over thousands of years, these eruptions released an estimated 199,000 gigatons of carbon dioxide into the atmosphere. For context, extinction-causing volcanic events all released CO2 on the order of 100,000 gigatons or more, and the Siberian Traps were the most prolific.
That CO2 set off a cascade of environmental changes. Atmospheric greenhouse gas concentrations may have climbed from around 500 to 4,000 parts per million before the extinction to roughly 8,000 ppm at its peak. Tropical sea surface temperatures, which had ranged from about 22 to 25°C, rose to around 30°C. That 5 to 8 degree jump in ocean temperature doesn’t sound dramatic until you consider what it did to ocean chemistry.
The Oceans Lost Their Oxygen
Warmer water holds less dissolved oxygen. Climate simulations matching the geochemical evidence show that marine oxygen levels dropped by almost 80% globally. Vast stretches of the ocean became effectively suffocating for most complex life. The combination of warming and oxygen loss was especially lethal in the tropics, where temperatures were already high and organisms were already living near their thermal limits.
On top of the oxygen crash, the enormous pulse of CO2 acidified seawater. Organisms that built shells or skeletons from calcium carbonate, like corals and certain plankton, were hit from two directions at once: less oxygen to breathe and water that actively dissolved their protective structures.
What Disappeared Forever
The extinction did not strike evenly. Research on fossil beds in South China reveals that the crisis unfolded in two distinct pulses. The first pulse, in the latest Permian, was the more selective of the two. It completely eliminated several ancient groups that had thrived for hundreds of millions of years:
- Trilobites, the iconic armored arthropods that had survived every previous crisis for over 270 million years, finally went extinct.
- Rugose corals, the dominant reef-builders of the Paleozoic, vanished entirely and were never replaced by close relatives.
- Fusulinids, a group of large single-celled organisms with elaborate shells, disappeared along with calcareous algae and radiolarians (microscopic plankton with glassy skeletons).
A second pulse near the Permian-Triassic boundary then hammered groups that had survived the first wave, including brachiopods, ammonoids, snails, clams, and small foraminifers. These groups weren’t wiped out entirely, but their diversity collapsed. Brachiopods, which had been the dominant shelled animals on the seafloor for over 200 million years, never recovered their former ecological role. Clams and snails eventually took their place, reshaping marine ecosystems into something closer to what we see today.
How Long the Extinction Took
High-precision dating of volcanic ash layers in Chinese rock sequences pins the main extinction interval to a remarkably narrow window. Mercury enrichment linked to volcanic activity appeared just before 252.07 million years ago, with the most intense phase occurring between about 251.8 and 251.6 million years ago. That means the worst of the killing played out over roughly 200,000 to 400,000 years. In geological terms, this is almost instantaneous. In human terms, it was a slow catastrophe, unfolding over thousands of generations of every living species on the planet.
How Long Recovery Took
The aftermath was almost as remarkable as the extinction itself. Global biodiversity took at least 5 million years to return to pre-extinction levels. Recovery wasn’t a smooth climb. It followed a slow, stepwise pattern, with simple ecosystems of primary producers (algae, bacteria) re-establishing first, followed gradually by herbivores, then predators, until full ecological complexity returned by the Middle Triassic.
For millions of years after the extinction, the fossil record shows “disaster taxa,” a handful of hardy, fast-reproducing species that dominated nearly every environment. Reefs essentially disappeared from the planet for several million years. The eventual recovery, though, set the stage for the rise of the dinosaurs and the modern structure of marine ecosystems.
Parallels to Modern Carbon Emissions
The Permian-Triassic extinction is sometimes invoked in discussions about current climate change, and the comparison is not entirely academic. The killing mechanism was carbon dioxide, not an asteroid. The pathway from CO2 to extinction ran through warming, ocean oxygen loss, and acidification, the same processes measurably underway in today’s oceans.
The rates are different, though, and that matters in both directions. The Siberian Traps released their carbon over tens of thousands of years. Modern anthropogenic emissions are compressing a comparable chemical shock into centuries rather than millennia. Research comparing volcanic carbon pulses to human output found that a single eruptive pulse during the related end-Triassic extinction released roughly as much CO2 as humanity is projected to emit over the entire 21st century. Current emission rates are, by geological standards, extraordinarily fast. The end-Cretaceous asteroid impact is the only event in the past half-billion years that changed the biosphere faster than what’s happening now.