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But extinctions were likely happening anyway as temperatures were increasing, Mayhew said. Massive volcanic activity, which releases large amounts of carbon dioxide, have also been blamed for the dinosaur extinction.

The author of the second study, which focuses on carbon dioxide, said he does see a cause-and-effect between warmer seas and extinctions.

Peter Ward, a University of Washington biology and paleontology professor, said natural increases in carbon dioxide warmed the air and ocean. The warmer water had less oxygen and spawned more microbes, which in turn spewed toxic hydrogen sulfide into the air and water, killing species.

Ward examined 13 major and minor extinctions in the past and found a common link: rising carbon dioxide levels in the air and falling oxygen levels. Ward's study will be presented Sunday at the Geological Society of America's annual convention in Denver.

Mayhew also found increasing carbon dioxide levels in the air coinciding with die-offs, but concluded that temperatures better predicted biodiversity.

Those higher temperatures that coincided with mass extinctions are about the same level forecast for a century from now if the world continues its growing emissions of greenhouse gases, according to the Nobel Prize-winning Intergovernmental Panel on Climate Change.

In April, the same climate panel of thousands of scientists warned that "20 to 30 percent of animal species assessed so far are likely to be at increased risk of extinction" if temperatures increase by about 3 to 4 degrees Fahrenheit.

"Since we're already seeing threshold changes in ecosystems with the relatively small amount of climate change already taking place, one could expect there's going to be severe transformations," said biologist Thomas Lovejoy, president of the H. John Heinz Center for Science, Economics and the Environment in Washington.

University of Texas biologist Camille Parmesan, who studies how existing species are changing with global warming but wasn't part of either team, said she was "blown away" by the Mayhew study and called it "very convincing."

"This will give scant comfort to anyone who says that the world has often been warmer than recently so we're just going back to a better world," Pennsylvania State University geological sciences professor Richard Alley said.