Global warming – not a cataclysmic collision with an asteroid – may have nearly snuffed out life on Earth 250 million years ago, two recent reports suggest.

The biggest mass extinction in the planet's history, known as the "Great Dying," eliminated 90 per cent of marine life and nearly three-quarters of all plants and animals on land.

Recently uncovered evidence led many scientists to speculate that a giant meteor crashed to earth and triggered the catastrophe, just like the one believed to have wiped out the dinosaurs millions of years later.

Dinosaurs disappeared very quickly after a huge meteorite hit Earth 65 million years ago.

But two studies reported in Thursday's online edition of the journal Science point to a much slower-acting culprit – global warming.

Fossils point to long, slow extinction

Scientists, led by Peter Ward of the University of Washington, studied fossils and analyzed the chemistry of sediment layers in the Karoo Basin of South Africa.

The new studies suggest global warming caused by volcanic activity triggered the Great Dying.

They found evidence that reptiles, amphibians and other vertebrates declined gradually over about 10 million years, then disappeared in a final pulse of extinction that lasted another 10,000 years.

The finding suggests the Great Dying wasn't the result of a meteorite impact, Ward told the CBC in a telephone interview from Seattle.

"We know pretty much what an asteroid-impact extinction looks like," said Ward, a paleontologist at the university and lead author of the paper.

"The dinosaurs disappeared really in a heartbeat, over a matter of months or weeks. When you investigate their fossils, you see they go up to a certain line in the sand, which is really a level of stratum, and then they're gone."

That's not what Ward's team found while investigating mammal-like reptiles in South Africa.

"Their fossils stretch out. The extinction isn't a sudden disappearance: it's a rather long, slow agonizing disappearance."

Global warming 'very dangerous thing'

The team believes that massive volcanic eruptions in Siberia filled the air with greenhouse gases and dust that would have trapped heat in the atmosphere and raised temperatures.

Methane gases frozen at the bottom of the ocean would have been released as the planet got warmer, pushing the thermometer higher in a vicious cycle known as the "runaway greenhouse effect."

"It certainly looks like global warming can be a very dangerous thing," Ward said.

Similar pattern found in water

A second study published in the same issue of Science found a similar pattern in the chemistry of ancient sea sediments from the time and also points the finger at global warming.

Researchers, led by Kliti Grice of the Curtin University of Technology in Perth, Australia, analyzed the chemistry of ancient sea sediments drilled off the coasts of Australia and China.

Their study suggests life suffocated to death slowly over millions of years in stagnant seas of hydrogen sulfide as the ecosystem collapsed.

Grice, a geochemist, found the oceans became very scarce in oxygen and dominated by a type of bacteria that thrives on hydrogen sulfide – forming an environment that would have been toxic to fish and other marine organisms.

The researchers found the numbers of bacteria gradually increased during the extinction period, followed by one major peak.

The finding could be the result of an atmosphere that was low in oxygen and poisoned by sulfurous volcanic emissions, the researchers conclude.