The most severe biological event in Earth's history happened at the boundary between the Permian and the Triassic, 252 million years ago. The rock record across that boundary — best preserved in marine sediments at Meishan, China — shows the loss of perhaps 96% of marine animal species and around 70% of terrestrial vertebrate species over a geological interval of less than 100,000 years. Forests collapsed; coral reefs vanished and did not return for ten million years; the ancestor of every modern fish, amphibian, reptile, and mammal squeezed through what paleontologists now call The Great Dying. The cause was the Siberian Traps: a volcanic province erupting an estimated four million cubic kilometers of basalt over roughly a million years, intruding through coal-rich sedimentary rock and producing a CO₂ release that warmed the climate, acidified the oceans, and depleted seawater oxygen on a global scale. Earth has done something like this five times in the last 540 million years.
Three of the Big Five — the End-Permian (252 Mya), the End-Triassic (201 Mya), and the End-Cretaceous (66 Mya) — coincided with massive flood basalt eruptions: the Siberian Traps, the Central Atlantic Magmatic Province at the breakup of Pangaea, and the Deccan Traps in what is now India. Each injected carbon dioxide into the atmosphere on a geologically rapid scale, and the kill mechanism that follows is the same in each case. Warming shifts climate regimes; ocean acidification dissolves carbonate shells from the bottom up; anoxia spreads from warm surface waters as oxygen solubility drops and biological productivity collapses; ecosystem failure follows on the continents. The chemistry, on the timescale of an eruption, is roughly that of rapidly burning a continent's worth of fossil carbon. The other two events — the Late Devonian (370 Mya), driven by ocean anoxia probably aggravated by the spread of land plants pulling nutrients into the sea, and the End-Cretaceous, finalized by the Chicxulub asteroid impact and the global cooling its ejecta produced — confirm by exception: when CO₂ release is not the kill mechanism, the geological signature is identifiably different.
Standing diversity returns to pre-extinction levels over five to ten million years, but the taxonomic composition is reset. Brachiopods dominated marine benthos before the End-Permian; bivalves dominated after. Dinosaurs radiated into vacated niches after the End-Triassic; mammals radiated after the End-Cretaceous. Reefs disappear after each major event and reappear only after a reef gap of several million years, often built by entirely different organisms — rugose and tabulate corals before the End-Permian, scleractinian corals after. The post-extinction world is qualitatively different because the survivors are not a representative sample of what was there before; they are whichever lineages happened to be lucky, and their subsequent radiations are contingent on which small populations of unrelated taxa happened to make it through. Mass extinctions, in this view, are evolutionary regime changes — and the most consequential structural events in the history of life.
The end-Permian event is the closest geological analogue to anthropogenic CO₂ release the rock record contains. The Siberian Traps put out perhaps a gigaton of carbon per year over hundreds of thousands of years; humans presently emit roughly 10 GtC per year. The Permian release ran for a million years; ours has run for two centuries. But the resulting climate disruption, ocean acidification, and ocean oxygen loss are chemically the same processes preserved in the rock record 252 million years ago. Modern paleoclimate research treats the end-Permian as a calibration point for forecasting where unchecked emissions go, and the comparison is sometimes used to dramatize but is empirically the closest analogue the deep-time record holds.