Crossing environmental thresholds determines the risk of a sixth mass extinction

The story of life on Earth is punctuated by moments of profound loss. Over the last half-billion years, the planet has endured five great catastrophes where the majority of species vanished in a geologic blink. These events were not slow, gentle declines but sudden collapses triggered when the environment pushed past a breaking point. Scientists call these breaking points thresholds. Think of a threshold like the edge of a cliff: you can walk along the flat ground for a long time without falling, but once you step over that edge, the outcome changes instantly and dramatically. Today, as human activity reshapes the planet, a critical question hangs over the future of biodiversity: are we merely walking along the flat ground, or have we already stepped toward the edge?

A new study by Kunio Kaiho at Tohoku University tackles this question by looking at the specific forces driving species toward extinction. The research moves beyond simple guesses about how many animals might disappear. Instead, it asks whether the stressors we create—such as warming temperatures, toxic pollution, the destruction of forests, and the over-harvesting of wildlife—behave like those ancient catastrophes. The study suggests that these modern pressures do not just add up in a straight line. Rather, they act like switches that, once flipped, cause extinction rates to spike. By combining data on past mass extinctions with projections of human population and carbon emissions extending far into the future, the research maps out exactly where these dangerous edges lie and what happens if we cross them.

The investigation began by examining the timing of past disasters. For a long time, scientists believed these ancient mass extinctions played out over thousands or millions of years. However, by looking at high-resolution records of rock and fossils, this study found that the most intense killing phases actually happened much faster. The end-Permian extinction, the most severe event in Earth's history, likely saw its worst pulse of loss occur within just a thousand years. The asteroid impact that ended the age of dinosaurs caused a massive die-off in just two or three years. This realization is crucial because it means the speed of today's human-driven changes is comparable to the speed of those ancient catastrophes. The planet is not reacting slowly; it is reacting with the same urgency as it did when life nearly ended.

To understand what might happen next, the researcher built a model that tracks five major drivers of extinction: climate warming, pollution, habitat loss, overexploitation, and the risk of nuclear conflict. The model does not assume that more pollution or more warming always leads to a proportionally higher number of extinctions. Instead, it tests for those critical thresholds. The study identifies specific tipping points for each driver. For instance, if global temperatures rise by about nine degrees Celsius, or if deforestation reaches eighty percent of the world's forests, the risk of a mass extinction becomes very high. Similarly, if pollution levels, measured by mercury in the environment, rise four to five times above natural background levels, the system tips.

The results show a stark difference between what is likely to happen and what could happen. Under current, plausible scenarios where human populations eventually stabilize and emissions peak, the study projects that we will lose a significant but manageable number of species. For marine life, the loss might range from three to seven percent, while for land animals, it could be between eight and fifteen percent. This is a serious decline, representing a low-level mass extinction, but it falls short of the total collapse seen in Earth's history. The research indicates that in the absence of nuclear conflict, a sixth mass extinction is unlikely. However, if the other four drivers exceed their thresholds, a low-level mass extinction may occur. If we manage to keep pollution, habitat destruction, and overhunting below their critical thresholds, the worst-case scenario of total ecosystem collapse can be avoided.

However, the study also highlights a terrifying "what if" scenario involving nuclear conflict. While the other drivers are ongoing and gradual, a large-scale nuclear war would act as a sudden, massive shock. The soot from burning cities would block the sun, causing a rapid global cooling known as a nuclear winter. This event would crash the food supply for almost all life. The model shows that if such a conflict were to occur on top of the existing environmental stress, the extinction rate would not just rise slightly; it would explode. In this worst-case scenario, the loss of marine species could jump to nearly forty-six percent, and land animals could see losses as high as seventy percent. This level of destruction would match the severity of the five great mass extinctions of the past, confirming that a sixth mass extinction is possible, but only if we cross the threshold of nuclear war.

A key insight from the study is how these different threats interact. They do not just sit side by side; they amplify each other. When populations of animals are already shrinking due to habitat loss or warming, they become much more vulnerable to other stressors. The research found that these combined effects can increase the risk of extinction by about forty percent compared to looking at each threat in isolation. Furthermore, the study emphasizes that the size of a population matters. Even if a species is not immediately wiped out, if its numbers drop significantly, it becomes far more likely to go extinct later due to a lack of mates or the inability to recover from small accidents. This "population-size effect" means that every species we lose today makes the remaining ones more fragile for tomorrow.

Looking far into the future, the model projects that human population and carbon emissions will likely peak around the middle of this century and then begin to decline. As the human footprint shrinks, the pressure on the environment is expected to ease, allowing the most vulnerable species to stabilize. The study suggests that the window for preventing a mass extinction is closing, but it is not yet shut. The path forward depends entirely on whether we can keep pollution, overexploitation, and the risk of nuclear conflict below their specific tipping points. If we succeed, the Earth may suffer a significant loss of biodiversity, but the system will remain intact. If we fail to respect these limits, particularly by allowing pollution to spiral or by engaging in nuclear conflict, the result could be a permanent scar on the history of life, indistinguishable from the great die-offs of the past. The choice, the study concludes, is not between a perfect world and a broken one, but between a world that recovers and one that does not.