for National Geographic News
Today the world faces global warming, but 34 million years ago a distinctly chillier menace was sweeping our planet.
Average temperatures around the world plunged nearly 15 degrees Fahrenheit (8.2 degrees Celsius) during a span of hundreds of thousands of years, according to a new study.
The cooling was so severe that it likely led to the extinction of many of North America's reptiles and amphibians.
The cause of the temperature shift was a change in the level of greenhouse gases—specifically carbon dioxide—the study shows.
Researchers believe an increase in carbon dioxide is mainly responsible for the global warming occurring today. (Get the facts about global warming.)
Thirty-four million years ago, "we probably had a change in the opposite direction—that is, a drop in atmospheric carbon dioxide that could have caused the global cooling," said Alessandro Zanazzi, a doctoral candidate in geological sciences at the University of South Carolina.
His team's findings are reported in tomorrow's issue of the journal Nature.
Before the cooling took place, the climate was relatively warm everywhere on Earth. North America had tropical forests, and the poles were completely ice-free.
Studying the chemical isotopes of fossil teeth and bones found in Wyoming, South Dakota, and Nebraska, Zanazzi's team was able to measure the temperature changes that took place in North America (see U.S. map).
They found a large drop in mean annual temperatures of 14.8 degrees Fahrenheit (8.2 degrees Celsius).
The climate change had a major impact on cold-blooded animals—reptiles, amphibians, and snails—with many of them going extinct, the study team reports.
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