Past mass extinction events linked to climate change

Past mass extinction events linked to climate change
Rhett A. Butler, mongabay.com
March 29, 2006

Most mass extinctions were caused by gradual climate change rather than catastrophic asteroid impacts says Peter Ward, a paleontologist at the University of Washington in Seattle, in an upcoming article in New Scientist magazine.

Ward says that of the five major extinctions have occurred in the past 500 million years—the Ordovician, the Devonian, the Permian, the Triassic and the Cretaceous—, only one, the Cretaceous event which wiped out the dinosaurs, was likely caused by an asteroid impact. "It's such a simple idea that for 20 years we just assumed the same was true for all extinctions," says Ward. He believes that climate change has been responsible for most of the great extinctions.

As evidence, Ward cites new research that suggests the Permian extinction of 250 million years ago was caused by huge volcanic eruptions in Siberia which triggered catastrophic climate change and wiped out 95 percent of lifeforms in the world's oceans and about 75 percent of land creatures. Further, says Ward, there is evidence that the Triassic extinction involved higher temperatures. Examining carbon isotopes in rocks dating from the Triassic extinction of 200 million years ago, Ward and his colleagues found that "the amount of carbon dioxide in the atmosphere was up to 100 times what it is today, and that the levels fluctuated wildly over tens of thousands of years." This suggests a gradual increase in global temperatures doomed many of Earth's species to extinction. "The Triassic event isn't something that happened overnight," says Ward.

Ward's conclusion has significant implication in today's world where climbing carbon dioxide levels are producing higher temperatures.

Too late for frogs? Mounting evidence links the global disappearance of amphibians to changes in climate.

"We are heading down the same road, but we've traded volcanoes as the agents of destruction for SUVs," says Ward.

While Ward is well-respected in his field, not everyone agrees with his interpretation of ancient data. Many paleontologists still finger planetary impact as the primary culprit of past mass extinctions.

Regardless, it is evident that we are currently in the midst of a sixth great extinction event. One that is almost entirely of our making. Habitat destruction, introduction of alien species, and overexploitation has resulted in the documented extinction of more than 800 species since 1500, although the actual toll will never be known. Presently. extinctions are running at 100 to 1,000 times the background rate, according to the U.N.'s Millennium Ecosystem Assessment.

Past extinctions have shown it takes at least 5 million years to restore biodiversity to the level equal to that before the extinction event. Our actions today will determine whether Earth will be biologically impoverished for the 500 trillion or more humans that may inhabit the Earth during that future period.

The upcoming article appears in the April 1, 2006 edition of New Scientist. Zeeya Merali is the author.

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