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It is famous for being one of the first true planetary emergencies when sea temperatures rose more than 5C, the oceans turned to acid, and thousands of species became extinct.
Now scientists believe they know why the world nearly died 55 million years ago.
A series of huge volcanic eruptions - a million times bigger than anything in modern human times - sent so much carbon dioxide and methane into the atmosphere that the planet experienced a rapid and intense period of global warming.
The phenomenon, known as the paleocene-eocene thermal maximum, or PETM, and it lasted about 220,000 years. Until now no one could be sure what caused it and theories ranged from a collision with a huge asteroid to the sudden release of huge stores of frozen methane from the ocean floor.
However, a study by scientists from Denmark and America appears to have solved the mystery by linking this rapid period of global warming with a series of immense volcanic eruptions that pushed Greenland away from northwest Europe to create the North Atlantic Ocean.
Precise geological dating has found that the volcanoes exploded at the same time that the seas began to heat up. Temperatures overall rose by 5C but in the Arctic they increased by 8C to 10C over a relatively short period of a few thousand years.
Over the past couple of centuries volcanic eruptions account for only about 2 per cent of the greenhouse gases emitted, but scientists believe that the latest findings indicate how dangerous the world can get during a period of rapid climate change.
"That prehistoric volcanic activity released more than 2000 gigatonnes of carbon into the oceans and atmosphere in the form of methane and carbon dioxide - two potent greenhouse gases," says Dr Michael Storey of Roskilde University, Denmark, who led the study published in the journal Science.
"The carbon probably came from the heating of earlier deposits of decayed organic matter, similar to the deposits in the Atlantic and North Sea we tap for oil and gas."
The oceans today are getting more acidic as levels of man-made carbon dioxide in the atmosphere continue to rise, but 55 million years ago the seas became so acid that they dissolved the shells of many marine species, which became extinct.
More recent volcanic eruptions, such as Mt Pinatubo in the Philippines or Mt St Helens in Washington havehad a slight cooling effect on the global climate because they emit sulphate particles that tend to reflect sunlight, says David Rothery of the Open University.
"There has been no eruption either large enough or of the right kind to account for the current global warming trend, for which we must look for a non-volcanic explanation - most likely carbon dioxide released by human activities." Dr Rothery said.
Ellie Highwood, a climate physicist at the University of Reading, says: "The type of volcano eruption described here has not been seen over the past 1000 years, and therefore volcanoes are unlikely to have contributed significantly to recent climate change."
- Independent
