Many scientists argue the next mass extinction isn’t a future event to predict. It has already begun. Current species are disappearing at tens to hundreds of times the natural background rate, and if nothing changes, the loss could reach the 75% threshold that defines a true mass extinction within a few centuries. The more precise question isn’t “when will it start?” but how fast it accelerates from here.
What Counts as a Mass Extinction
Earth has experienced five mass extinctions over the past 500 million years. Each one wiped out at least 75% of all species within a geologically short window, typically less than two million years. The most devastating, the end-Permian event about 252 million years ago, killed roughly 90% of all species. The most recent, 66 million years ago, ended the reign of non-avian dinosaurs.
By that strict definition, we haven’t lost 75% of species yet, so some researchers argue calling our current crisis a “sixth mass extinction” is premature. But others counter that focusing on the final body count misses the point: the trajectory matters. The rate of loss right now is so far above normal that, if sustained, the threshold could be crossed in one to a few centuries.
How Fast Species Are Disappearing Now
The natural background extinction rate for vertebrates (mammals, birds, fish, reptiles, and amphibians) would predict about nine species lost during the entire 20th century. The actual number was 390, more than 40 times higher than expected. Across all life, the Intergovernmental Science-Policy Platform on Biodiversity estimates current extinction rates are tens to hundreds of times above the average over the past 10 million years.
The IUCN Red List currently identifies more than 48,600 species as threatened with extinction, representing 28% of all species assessed. Some groups face especially steep declines: 44% of reef-building corals, 41% of amphibians, 38% of sharks and rays, and 34% of conifers are threatened. Evolution simply cannot produce new species fast enough to replace what’s being lost.
The 2100 Carbon Threshold
One of the most specific predictions comes from geophysicist Daniel Rothman at MIT, who studied the relationship between disruptions to Earth’s carbon cycle and past mass extinctions. He identified a critical threshold: when too much carbon enters the oceans too quickly, the carbon cycle destabilizes in ways that have historically preceded mass die-offs.
Rothman calculated that threshold at roughly 310 gigatons of carbon added to the world’s oceans, an amount he estimates human activities will reach by approximately 2100. Crossing it wouldn’t cause instant extinction. Instead, it would push the Earth’s carbon cycle into what he called “unknown territory,” potentially launching cascading species losses that play out over future millennia. In every scenario he modeled, the carbon cycle is either close to or well beyond the catastrophic threshold by the end of this century.
Climate Tipping Points Already Within Reach
Mass extinctions don’t happen because of a single cause. They happen when multiple Earth systems destabilize at once. A 2022 analysis published in Science identified nine global tipping points, thresholds where changes become self-reinforcing and essentially irreversible. Current warming of about 1.1°C above pre-industrial levels already falls within the lower uncertainty range of five of them.
Between 1.5°C and 2°C of warming, six tipping points become likely, with four more possible. These include the collapse of the Greenland and West Antarctic ice sheets, die-off of low-latitude coral reefs, and widespread permafrost thaw that would release enormous stores of carbon and accelerate warming further. At the 2 to 3°C of warming expected under current policies, additional tipping points come into play, including potential dieback of the Amazon rainforest.
Each of these tipping points doesn’t just cause local damage. They feed into each other. Permafrost thaw releases carbon, which accelerates warming, which destabilizes ice sheets, which raises sea levels, which floods coastal ecosystems. This kind of cascading failure is exactly the pattern that preceded past mass extinctions.
The Oceans Face the Steepest Risk
A study published in Science found that under business-as-usual warming scenarios, marine ecosystems face extinction levels comparable to the worst mass extinctions in Earth’s history, based on temperature tolerance and oxygen loss alone. That’s before accounting for ocean acidification, overfishing, pollution, or habitat destruction.
This finding is striking because documented extinctions so far have been concentrated on land, where industrial impacts started earlier. Marine species have had less time under pressure, but their vulnerability to warming is enormous. With accelerating greenhouse gas emissions, species losses from heat stress and oxygen depletion in the oceans could match the scale of current human-driven losses on land within a century.
The Debate Over Timing
Scientists broadly agree on the facts: extinction rates are far above normal, biodiversity is declining across nearly every group of organisms, and the drivers of loss are intensifying. Where they disagree is on labeling and timeline.
Critics of the “sixth mass extinction” framing point out that most documented extinctions have occurred on islands, where species are more vulnerable, and that continental species have lower extinction rates. Some argue that new species evolving or that human-caused extinctions are simply part of the natural trajectory of life on Earth. These are minority positions. The larger scientific community, drawing on data from invertebrates (which make up the vast majority of animal life), concludes that the rate of loss vastly exceeds the background rate and that we are witnessing the opening stages of a mass extinction.
The honest answer to “when is the next mass extinction?” is that the process is underway, the critical carbon and climate thresholds are likely to be crossed by 2100 under current trends, and the full extinction event would unfold over the following centuries to millennia. Whether the 75% species-loss benchmark is reached depends almost entirely on what happens in the next few decades. If emissions and habitat destruction continue at current rates, crossing that line becomes increasingly difficult to avoid. If they’re dramatically reduced, the worst outcomes can still be prevented, though significant biodiversity loss is already locked in.