More than 99 percent of all species that have ever lived on Earth are now extinct. Under normal conditions, the fossil record shows a steady background extinction rate, with new species replacing those that disappear and total diversity remaining roughly stable. But there have been five moments in the last 540 million years when extinction rates accelerated far beyond that baseline. In each event, at least 75 percent of species died out. The causes differed, but the pattern they share has drawn increasing scientific attention.
The oldest of the five occurred 443 million years ago, at the end of the Ordovician period. At that time, life was almost entirely marine. The oceans held armored fish ancestors, early corals, and trilobites, which had existed for over 100 million years. Approximately 85 percent of marine species were then wiped out. The leading explanation involves a climate swing driven by silicate weathering. Silicate minerals in Earth’s crust absorb carbon dioxide, and when they pulled large amounts from the atmosphere, global temperatures dropped. Ice sheets expanded across a southern landmass, and sea levels fell as water became locked in glaciers. Shallow continental shelves, the most productive marine habitats, shrank dramatically.
After the glaciation peaked, temperatures rose sharply, sea levels climbed, and ocean chemistry shifted. Species that survived the cold then faced warming conditions they could not tolerate. The result was two pulses of extinction spaced about a million years apart, together eliminating 85 percent of species alive at the time. Reef systems that dominated Ordovician oceans largely disappeared. Recovery took millions of years, and the world that emerged was biologically different from what came before.
The late Devonian extinction occurred between roughly 383 and 359 million years ago. It was the most drawn out of the five, unfolding over 15 to 20 million years, and eliminated an estimated 70 to 80 percent of species. It arrived in at least two major pulses, known as the Kellwasser and Hangenberg events, with smaller waves in between. This was the age of fishes, with giant armored placoderms in the seas and extensive reef systems built by corals and sponges. By the end of the period, these reefs were essentially gone and did not return in significant form for another 100 million years.
Scientists point to ocean anoxia, a severe depletion of dissolved oxygen in seawater, as the primary mechanism. Deposits of black shale from this period, which form in oxygen-depleted conditions, record the event. A leading contributor may have been land plants. During the Devonian, trees evolved deep root systems for the first time. Those roots broke up bedrock, releasing nutrients into rivers and eventually the ocean. The nutrient influx triggered massive algae blooms, and when the algae died, their decay consumed oxygen in the water, creating large dead zones. Forests also pulled carbon dioxide from the atmosphere, potentially driving global cooling and further disrupting ocean circulation. Volcanic activity in what is now Siberia likely added stress to an already strained system.
The Permian-Triassic extinction, roughly 252 million years ago, was the largest of the five. Over a geologically short period, with some estimates placing the main pulse at as little as 60,000 years, about 96 percent of marine species and 70 percent of terrestrial vertebrate species went extinct. Forests were wiped out and did not recover in force for about 10 million years. Marine ecosystems took four to eight million years to begin rebuilding. The event also caused the largest mass die-off of insects on record.
The cause was a volcanic complex in what is now Siberia, known as the Siberian Traps. Over roughly two million years, eruptions covered an area about the size of Saudi Arabia with basaltic lava. The lava itself was not the main killer. The eruptions burned through coal and hydrocarbon deposits, releasing billions of tons of carbon dioxide and methane. Atmospheric carbon dioxide reached levels estimated at 2,500 to 8,000 parts per million, compared with a pre-extinction level that may have been as low as 400 parts per million. Global warming was extreme, with ocean surface temperatures at the equator reaching an estimated 104 degrees Fahrenheit. The oceans lost a large share of their dissolved oxygen. The eruptions also released sulfur dioxide and toxic metals, and the ozone layer was degraded, exposing surviving life to higher levels of ultraviolet radiation.
A 2015 MIT study dated the initial pulse of the Siberian Traps eruptions to within 300,000 years of the extinction, providing what researchers described as the strongest direct causal link available from the geological record. The rate of carbon release was much faster than in the preceding major extinction. Recovery from the great dying was the slowest of any mass extinction. The world that eventually emerged was dominated by new groups, including early ancestors of the dinosaurs.
The end-Triassic extinction occurred approximately 201 million years ago and eliminated about 76 percent of marine and terrestrial species. It opened the way for dinosaurs to become the dominant large animals on land for the next 135 million years. The cause was another large volcanic episode, called the Central Atlantic Magmatic Province, or CAMP. By area, it is the largest known large igneous province on Earth, with flood basalt spread across parts of what are now southwestern Europe, northwestern Africa, northeastern South America, and eastern North America. The eruptions were part of the process that broke apart the supercontinent Pangea and opened the Atlantic Ocean.
A 2013 MIT study established that the first CAMP eruptions coincided within a few thousand years of the earliest fossil disappearances. The carbon released disrupted the global carbon cycle, with the chemical signature visible in fossil soils and marine sediments worldwide. Global temperatures rose by 3 to 4 degrees Celsius on average. The oceans acidified and lost oxygen. The carbon cycle did not stabilize again until well into the Jurassic period. Magma that pushed underground into organic-rich sedimentary rock in Brazil heated coal and carbon deposits, amplifying the volcanic signal.
The fifth event, the Cretaceous-Paleogene extinction, happened 66 million years ago and killed approximately 76 percent of all species, including every non-bird dinosaur. It is the only one of the five where the primary trigger was an object from space. An asteroid estimated at 10 to 15 kilometers wide struck the Yucatán Peninsula of Mexico at high speed, leaving the Chicxulub crater, which measures about 180 kilometers across. The impact generated a heat pulse, large tsunamis, and an earthquake felt across the planet.
The longer-term killing mechanism was what the impact sent into the atmosphere. The rock the asteroid hit was rich in sulfate minerals, and the impact vaporized a large amount of sulfate-bearing rock. Sulfate aerosols injected into the stratosphere are effective at blocking sunlight and can persist for years. Models estimate the impact caused near total darkness for about two years, followed by a lengthy period of reduced light. Photosynthesis shut down. Plants died, then herbivores, then carnivores. More than 90 percent of marine plankton went extinct, collapsing the oceanic food chain. Every land animal weighing more than 25 kilograms died out. The mammals and birds that survived were small, could shelter underground or underwater, and could eat seeds or insects.
Within about 10 million years, mammals had diversified into the ecological niches left vacant. The line from that recovery leads directly to the primates and eventually to humans.
Four of the five mass extinctions involved a rapid and severe disruption to Earth’s carbon cycle. In the end-Ordovician, carbon burial triggered cooling. In the late-Devonian, ocean chemistry was destabilized by nutrient loading and carbon dioxide drawdown. In the Great Dying and the end-Triassic, massive volcanic systems released carbon at rates beyond what normal planetary systems could absorb. A 2019 study from MIT found that the carbon cycle behaves like a threshold system. When carbon is added below a certain rate, the system self-corrects. Cross that threshold, and the system spikes, with ocean acidification, oxygen loss, and temperature climbs that amplify the original disturbance.
A 2025 paper in Scientific Reports examined climate mechanisms during all five extinctions and found that four began with a sharp cooling phase followed by warming, and all five ended in a warming phase. Life tends to survive rapid cooling better than rapid warming, as sustained high heat is harder to escape. The trigger mattered less than the speed of the planetary response. The asteroid that ended the dinosaurs did in seconds what volcanoes took thousands of years to accomplish, and the cascade it triggered passed through many of the same mechanisms.
The current rate of species loss sits between 100 and 1,000 times the natural background extinction rate. A 2015 analysis in Science Advances found that vertebrate extinctions over the last century would, under normal background rates, have taken between 800 and 10,000 years to occur. A 2023 study in Biological Reviews found that of 70,000 monitored species, 48 percent are experiencing declining populations due to human pressure, while 3 percent are growing. The debate among scientists is not about whether species loss is accelerating, which the data show, but whether the current trajectory meets the formal definition of a mass extinction, which requires eventual loss of 75 percent of species.
The MIT researcher who studied carbon cycle thresholds noted that today’s oceans are absorbing carbon roughly 10 times faster than the worst-case scenario found anywhere in the geological record over 540 million years. Volcanic events lasted tens of thousands of years or more, while human industrial emissions have been concentrated into a few centuries. Whether that brevity protects against a threshold crossing or makes one more likely is the central open question in Earth system science.
The geological record shows that when the carbon cycle destabilizes rapidly, consequences follow in a predictable sequence and at a scale that no species alive at the time was equipped to survive. Every previous extinction left survivors, and every survivor became the seed of whatever came next. The record cannot answer which side of a threshold the current trajectory is on, and by the time geological evidence would make that answer clear, the moment to act would already be part of the fossil record.
