Washington ? 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,” 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 of the University of Washington now think the answer is global warming caused by volcanic activity. Their findings were 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 said.

“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.

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.