More than two-thirds of life on Earth died off some 252 million years ago, in the largest mass extinction event in Earth's history.
Researchers have long suspected that volcanic eruptions triggered "the Great Dying," as the end of the Permian geologic period is sometimes called, but exactly how so many creatures died has been something of a mystery.
Now scientists at the University of Washington and Stanford believe their models reveal how so many animals were killed, and they see parallels in the path our planet is on today.
Models of the effects of volcanic greenhouse gas releases showed Earth warming and oxygen disappearing from its oceans, leaving many marine animals unable to breathe, said a study published in the peer-reviewed journal Science. By the time temperatures peaked, about 80 percent of the oceans' oxygen, on average, had been depleted. Most marine animals went extinct.
The researchers tested the model's results against fossil-record patterns from the time of the extinction and found they correlated closely. Although other factors, like ocean acidification, might have contributed some to the Permian extinction, warming and oxygen loss account for the pattern of the dying, according to the research.
By this century's end, if emissions continue at their current pace, humans will have warmed the ocean about 20 percent as much as during the extinction event, the researchers say. By 2300, that figure could be as high as 50 percent.
"The ultimate, driving change that led to the mass extinction is the same driving change that humans are doing today, which is injecting greenhouse gases into the atmosphere," said Justin Penn, a UW doctoral student in oceanography and the study's lead author.
"We have no reason to think it wouldn't cause a similar type of extinction," said Curtis Deutsch, a UW associate professor of oceanography.
Earth's continents 252 million years ago were mostly one landmass. The climate, however, resembled Earth's now, and researchers believe animals would have adapted many traits, like metabolism, that were similar to creatures today. "Less than 1 percent of the Permian Ocean was a dead zone — quite similar to today's ocean," Deutsch said.
The series of volcanic events in Siberia that many scientists believe set off the mass extinction "makes super volcanoes look like the head of a pin," said Seth Burgess, a geologist and volcanologist with the United States Geological Survey. "Enough lava erupted onto the surface and intruded into the crust to cover the area of the United States … maybe a kilometer deep in lava."
Burgess said scientists believe magma rising from the earth released some extinction-causing greenhouse gases. In addition, sills of magma inside Earth heated massive deposits of coal, peat and carbonate minerals, among others, which vented even more carbon and methane into the atmosphere.
The research "takes the next step in figuring out why things died at the end of the Permian," Burgess said. "It couples what we think was happening in the climate with the fossil record."
It took a supercomputer more than six months to simulate all the changes the volcanic eruptions are suspected of causing during the Permian period. In warmer waters, animals need more oxygen to perform bodily functions. But warm waters can't contain as much dissolved oxygen, which means less was available to them. In other words, as animals' bodies demanded more oxygen, the ocean's supply dropped.
In their model, surface-temperature rise and oxygen loss were more substantial in areas farther from the equator. Extinction rates also increased at higher latitudes. Animals in the tropics were accustomed to warmer temperatures and lower oxygen levels before the volcanic eruptions shifted the climate, the research said. As the world warmed, they could move along with their habitat.
Marine creatures that favored cold waters and high oxygen levels had nowhere to go. "Those conditions completely disappear," Deutsch said.
In modern oceans, warming and oxygen loss have also been more pronounced near the poles, researchers said. "The study tells us what's at the end of the road if we let climate (change) keep going," Deutsch said. "That's frightening. The loss of species is irreversible."
