About 252 million years ago, Earth almost died.
In the oceans, 96% of all species disappeared. It is more difficult to determine how many land species disappeared, but the loss was comparable.
This mass extinction, at the end of the Peruvian period, was the worst in the history of the planet, and this happened for several thousand years – the moment of the geological eye.
On Thursday, a team of scientists provided detailed accounts of how marine life was wiped out during the mass extinction of Perm-Trias. Global warming robbed the oceans with oxygen, they say, putting many species under such stress that they died.
And we can repeat this process, scientists warn. If so, then climate change is "firmly in the category of a catastrophic event of extinction," said Curtis Deutsch, a land scientist at Washington University and co-author of the new study published in the journal Science.
Researchers have long known the general outlines of the Peruvian-Triassic cataclysm. Just before the disappearance, the volcanoes in the current Siberia exploded on a massive scale. The magma and the lava, which they burn, produce huge amounts of carbon dioxide.
Once in the atmosphere, the gas catches heat. Researchers estimate that the surface of the ocean is warming by about 18 degrees Fahrenheit. Some researchers claim that only heat has killed many species.
Others believe that heat reduces oxygen in the ocean and stifles the species that live there. The cliffs of mass extinction seem to have formed when at least part of the ocean lacked oxygen.
In previous studies, Dr. Deutsch investigates how animals adapt to the temperature and oxygen levels in the seas. For example, fast-moving animals need a lot of oxygen, so they can not live in parts of the ocean where oxygen falls below a certain threshold.
Hot water makes the challenge even harder. Hotter water can not hold as much dissolved oxygen as cold water. Worse, hot water can also increase the metabolism of the animal, which means that it requires more oxygen just to stay alive.
The chicken, for example, is not below latitude, which roughly drives from New England to Spain. South of this line, heat and low oxygen are too big for the species.
Dr. Deutsch and Justin Penn, a graduate student, recreated the world at the end of the Peruvian period with massive computer simulation, full of warmth and circulating ocean.
As Siberian volcanoes flooded the virtual atmosphere with carbon dioxide, the atmosphere warmed. The ocean also warmed up – and the model began to lose oxygen.
Some parts lost more than others. On the surface, for example, with photosynthetic algae, fresh oxygen is produced. But as the ocean warmed, its circulatory currents also slowed down, demonstrating the pattern.
Slow oxygen with oxygen settled to the bottom of the oceans, and before that the deep draft.
Rising temperatures and oxygen have made huge oceans undisturbed. Some species survived here and there. But most of them disappeared completely.
"Everything has lost many habitats, creating a risk of extinction," said Dr. Deutsch. "But the risk was actually higher in places that were cold. It was a little surprising.
You can expect animals near the Equator to be at greater risk because the water is warm to get started. But Dr. Deutsch's model offers us a very different kind of apocalypse.
Animals with oxygen-rich cold water can not cope with the sudden drop while those in the tropical waters have already been adapted to bad oxygen. Cold species can not find shelter elsewhere.
To test their simulation, the researchers united with Jonathan Payne and Eric Sperling, paleontologists at Stanford University. They are buried in a huge online fossil database to mark the risks of extinction in different latitudes during the crash.
When they finished their analysis, they sent their chart to Seattle. Dr. Deutsch and Mr Penn compare it to the prognosis of their computer model.
They match. "It was the most exciting moment in my scientific life," said Dr. Deutsch.
Michael Benton, a paleontologist at the University of Bristol, England, who did not participate in the study, said he decided the role of heat and oxygen as the cause of mass extinction. "This is a clear case, of course, that both are related," he said.
The new study offers an important warning to people over the next few centuries.
Siberian volcanoes eventually deliver much more carbon dioxide into the atmosphere than we ever emit from the burning of fossil fuels. But our annual carbon footprint is actually higher.
The carbon we have released over the last two centuries has made the atmosphere warmer and the ocean has absorbed much of that heat. And now, just as during the Peruvian-Triasian extinction, the ocean loses oxygen. Over the last fifty years, oxygen levels have fallen by 2%.
"The way the Earth system is reacting now to the build-up of CO2 is exactly the way we've seen it react in the past," said Dr. Kumpe.
How warmer the planet will be, it depends on us. This will take a tremendous international effort to keep the increase below 4 degrees Fahrenheit.
If we continue to use all of the fossil fuels on Earth, it may warm up to 17 degrees Fahrenheit by 2300.
As the oceans get warm, oxygen levels will continue to decline. If ancient history is a guide, the consequences for life – especially marine life in the cooler parts of the ocean – will be catastrophic.
"Leave unchecked, warming the climate makes our future on the same scale as some of the worst events in geological history," said Dr. Deutsch.