About 252 million years ago, a cascade of environmental changes pushed life in the oceans beyond its physiological limits. Massive volcanism in what is now Siberia loaded the atmosphere with greenhouse gases, the climate warmed sharply, seawater lost oxygen, and the most severe known mass extinction in Earth’s history followed.

Scientists have long suspected that warming and oxygen loss worked together to make much of the ocean uninhabitable. A 2026 study in the Proceedings of the National Academy of Sciences has now added direct physiological evidence for that mechanism, showing why some major groups of marine animals were far more vulnerable than others.

The result matters beyond the distant past because today’s oceans are also warming and losing oxygen. That does not mean another end-Permian extinction is imminent, and the new study did not measure modern ocean trends. It means that two linked stresses with a demonstrated capacity to reorganise marine life are once again moving in the same direction.

What the new study added

Earlier work had already built a strong case for temperature-dependent hypoxia, the combined effect of warmer water, lower oxygen supply and higher animal oxygen demand. A 2018 study in Science used climate and metabolic models to reproduce the geographic pattern of end-Permian marine extinctions recorded in fossils.

The new research tackled a harder question: why did the crisis discriminate so strongly among different kinds of animals?

Before the extinction, many seafloor communities were dominated by groups including brachiopods and crinoids, animals associated with what palaeontologists call the Paleozoic evolutionary fauna. Bivalves and gastropods, part of the assemblage that became more prominent in later seas, generally survived at higher rates.

Researchers led by J. Andres Marquez combined laboratory respirometry with a trait-based model of animals’ oxygen balance. They measured oxygen use in living representatives of major marine groups and examined how that use changed as water temperature rose. Those measurements allowed the team to estimate how much aerobic habitat each group would have lost under reconstructed end-Permian conditions.

The simulated losses closely matched the selectivity seen in the fossil record. The animals predicted to lose the most viable habitat were also the groups that suffered the highest extinction rates.

Why a slow metabolism became a liability

At ordinary temperatures, many animals resembling the dominant Paleozoic groups could tolerate relatively low oxygen levels. Their low metabolic rates appeared to give them an advantage in quiet, oxygen-poor settings.

Warming reversed that advantage.

As temperature increased, the oxygen needs of these animals rose steeply. Their capacity to take up and deliver oxygen, however, could not keep pace. Groups such as bivalves and gastropods tended to have higher oxygen requirements under normal conditions, but they also possessed more muscular bodies, more effective respiratory structures or greater capacity to increase oxygen supply when conditions changed.

This helps explain a profound transition in the history of marine ecosystems. The end-Permian event did not simply remove species at random. It disproportionately eliminated organisms built around one broad physiological strategy, opening ecological space for other lineages to expand during the recovery.

The authors argue that temperature-dependent hypoxia is the only proposed kill mechanism so far shown to explain the event’s overall severity, its geographical pattern and its selectivity among animal groups. Other stresses, including acidification and toxic chemical changes, may still have contributed. The finding is a strong mechanistic result from one study, not the final word on every cause of the extinction.

How volcanism turned the ocean hostile

The trigger was the Siberian Traps, an immense volcanic province produced by repeated eruptions and underground magma intrusions. Greenhouse gases released directly by volcanism, and probably by magma heating carbon-rich rocks, drove rapid global warming over thousands of years. The exact timing and contribution of the different carbon sources remain active areas of research.

SpaceDaily has previously examined how the Siberian eruptions and their carbon emissions coincided with the end-Permian catastrophe. The new study fills in more of the biological chain between that planetary disturbance and the pattern of deaths preserved in marine fossils.

Warm water holds less dissolved oxygen than cold water. Warming can also strengthen the layering of the ocean, reducing the mixing that carries oxygen from the surface into deeper water. At the same time, chemical reactions and biological activity can consume the oxygen that remains.

For marine animals, the problem therefore arrives from both sides. The environment supplies less oxygen while warmer bodies require more of it. Once an animal cannot obtain enough oxygen to support basic metabolism, otherwise suitable habitat becomes physiologically inaccessible.

The same combination is developing today

Modern observations show that the ocean is accumulating heat produced by human greenhouse gas emissions. They also show a measurable decline in oxygen.

The Intergovernmental Panel on Climate Change’s assessment of ocean and coastal ecosystems concludes with high confidence that deoxygenation has occurred in most open-ocean regions since the mid-20th century. It reports an estimated global mean oxygen decline of 0.5 to 3.3 per cent in the upper 1,000 metres between 1970 and 2010.

Several processes are involved. Warmer seawater can physically dissolve less oxygen. Increased stratification can weaken ventilation of deeper layers, while changes in circulation and marine productivity alter where oxygen is delivered and consumed. In coastal waters, nutrient pollution can intensify algal blooms whose decomposition creates severe local oxygen depletion.

These trends are already changing habitats and increasing the risk of hypoxic events. Continued warming is expected to drive further deoxygenation, although the scale and timing will vary sharply by region and depth.

The Permian is a warning, not a forecast

The similarities in mechanism should not erase the enormous differences between the two situations.

Estimates cited by the Stanford-led research team place end-Permian warming at roughly 8 to 12 degrees Celsius, unfolding over thousands of years. The configuration of continents, ocean circulation, atmospheric composition and marine ecosystems was unlike today’s world. The current measured decline in ocean oxygen is far smaller than the losses associated with the ancient catastrophe.

The cause is also different. Siberian volcanism drove the ancient warming, whereas the present trend is overwhelmingly caused by human emissions from fossil fuels and land-use change.

Nor does the 2026 paper itself prove that modern oceans have crossed an extinction threshold. Its experiments and model concern the physiological mechanism behind an ancient event. Evidence for current warming and deoxygenation comes from independent modern measurements and assessments.

What the comparison provides is a tested explanation for why the combination can become so destructive. Average oxygen concentration alone does not define the danger. Risk depends on temperature, species physiology, local circulation, depth and the speed at which organisms can move or adapt.

What researchers still need to know

The laboratory work covered representatives of major animal groups, but no experiment can capture the full diversity of marine metabolism. More measurements across species, life stages and regions will be needed to identify which modern communities have the narrowest safety margins.

Researchers also need to examine interacting stresses. Ocean warming and oxygen loss occur alongside acidification, marine heatwaves, pollution, fishing pressure and habitat destruction. Their combined effects may be stronger or less predictable than any stress considered alone.

The end-Permian record cannot tell us exactly what will happen this century. It can reveal a basic constraint that has not changed: marine animals need enough oxygen to meet the demands imposed by their temperature and way of life.

When warming raises those demands while the ocean’s oxygen supply falls, survival becomes a matter of physiology.