By Anne McIlroy

19-10-03 It sounds like a rude way for civilization as we know it to end -- in a giant belch. But that's just what a US scientist thinks was responsible for one of the largest and more mysterious mass extinctions in the earth's history.
Not only does Gregory Ryskin, a chemical engineer at Northwestern University, believe that a deadly burp of methane from deep in the ocean wiped out much of life on the planet. He thinks it could happen again. One day, he says, humans could face the same gaseous fate as the mammal-like reptiles that roamed the planet 250 mm years ago.

We know what these ancient creatures looked like only by their fossils. Almost all of the species on land or water were killed in an environmental catastrophe even worse than the one that wiped out the dinosaurs less than 200 mm years later. A comet or asteroid hitting the earth is the widely accepted explanation for the death of the dinosaurs 65 mm years ago, but evidence that a heavenly body also was to blame for themuch earlier mass extinction is not as clear-cut.
This has left more room for alternative theories, including Ryskin's. In the journal Geology, he argues that an extremely fast release of gas from the ocean could explode with a force greater than that created by detonating all the planet's nuclear weapons at once.

Just such a gas event, although on a much smaller scale, happened in Africa much more recently. In 1986, an eruption of carbon dioxide in Cameroon's Lake Nyos killed 1,700 people. It was a silent catastrophe. Because the deadly gas is denser than air, it travelled quickly along the ground, suffocating anyone in its way.
The oceans contain more of such explosive gases, including frozen deposits of methane deep beneath the ocean floor off Canada's Vancouver Island. Experts believe that the methane hydrate, also found off the coast of Japan, the United States and other continental shelves, may contain more energy than all the world's oil and natural gas combined. Methane is produced in the deep by decaying bacteria.
"It can be calculated that 1 % of ocean volume can contain so much methane that its explosive nuclear force exceeds the total nuclear arsenal of the world by 10,000 times," Ryskin said.

His theory is that, deep in the ocean, methane dissolves in water and is trapped there, just as the carbon dioxide was trapped in Lake Nyos. A disturbance, perhaps an earthquake, forces the water laden with methane closer to the surface where, under reduced pressure, the gas starts to bubble out. It quickly moves upward and leaves the water in a giant flammable burp. Large amounts of methane and other gases are ejected into the air, and vast areas of land are flooded.
Pure methane is lighter than air, but methane loaded with water droplets is heavier, and spreads across the land until it encounters a flash of lightning. Kaboom.
"Explosions and conflagrations destroy most of terrestrial life," Ryskin says, "and also produce great amounts of smoke and carbon dioxide."

It sounds like the plot of a Hollywood thriller, but University of Toronto physicist Nigel Edwards, who studies methane hydrate, says: "It is not madness by any means. It is possible."
Ryskin believes it is a possibility worth studying that fits with theories about asteroids and other possible natural or human causes that could bring another great extinction.