An ancient version of global warming may have been to blame for the greatest mass extinction in Earth's history.
In an event known as the "Great Dying (search)," some 250 million years ago, 90 percent of all marine life and nearly three-quarters of land-based plants and animals went extinct.
Scientists have long debated the cause of this calamity — which occurred before the era of dinosaurs — with possibilities including such disasters as meteor impacts.
Researchers led by Peter Ward (search) of the University of Washington now think the answer is global warming caused by volcanic activity. Their findings are reported in Thursday's online edition of the journal Science.
They studied the Karoo Basin of South Africa, using chemical, biological and other evidence to relate layers of sediment there to similar layers in China that previous research has tied to the marine extinction at the same period.
Studying a 1,000-foot thick section of exposed sediment, Ward's team found evidence of a gradual extinction over about 10 million years followed by a sharp increase in extinction rate that lasted another 5 million years.
Ward's team believes the extinctions were caused by global warming and oxygen deprivation over long periods of time.
Massive volcanic flows in what is now Siberia brought on the warming while, at the same time, geologic action caused global sea levels to drop, Ward explained in a telephone interview.
"Once you expose a huge amount of underwater sediment to the atmosphere, two very bad things happen — a huge amount of carbon in the sediments is released and also methane. Once (methane) hits the atmosphere it's the most efficient greenhouse gas on the planet," he said.
That provided a one-two punch of warming and a decline in oxygen levels, he said.
"Some of us have been toying with the idea that dinosaurs evolved to be a low-oxygen adaptation," resulting from this era, Ward said. "We know birds can live at much lower oxygen concentrations than we do, and we and think there were similar lung adaptations in dinosaurs."
Currently the atmosphere consists of about 21 percent oxygen, but the addition of gases at that time could have lowered levels to 16 percent or less, Ward said.
"If you didn't live on the sea level you didn't live," he commented, reflecting the fact that oxygen concentrations decline with altitude. The result would have been to eliminate half the living space on the planet, said Ward.
The more recent mass extinction that killed the dinosaurs — 65 million years ago — has been linked to an impact by a large asteroid or comet that struck in an area off the coast of what is now Mexico and left a distinctive layer of dust worldwide.
Some researchers have argued that the Great Dying might also have resulted from such an impact, but Ward's team said it could find no evidence for such an event.
That doesn't mean there wasn't one, argues Luann Becker of the University of California at Santa Barbara, commenting that "the absence of evidence isn't evidence for absence."
Becker, who was not part of Ward's research team, said "they did a nice job of presenting the paleontological data and the stratigraphy, which seem to show some indication of an evolutionary change going on for a prolonged period of time." However, she added, she doesn't believe that addresses the subject of cause and effect.
"I think that this is an ongoing discussion," said Becker, who previously reported on a crater off the northwest coast of Australia that shows evidence of a large meteor impact at about the time of the early extinction.
Ward's research was funded by the NASA Aerobiology Institute, the National Science Foundation and the National Research Foundation of South Africa.