"Global Cooling" Wiped Out North America's Reptiles, Amphibians, Study Finds

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Land snails became smaller, indicating a change from moist to dry conditions. Similarly, aquatic turtles and crocodilians became less diverse. Terrestrial turtles also showed a major decrease in size.

In contrast, mammals were almost unaffected by the cooling.

"The vast majority—62 out of 70—of mammalian lineages persisted through the climatic event with no response," Zanazzi said.

"This is because mammals can regulate their body temperature so that they can better survive abrupt episodes of environmental changes."

Antarctica Puzzle

The team's findings in North America may also shed light on the origins of Antarctica's ice sheets.

Some scientists believe that tectonic shifts that caused Antarctica to separate from South America and Australia isolated the continent from warm equatorial currents, allowing the buildup of ice.

But computer models suggest that such a change in ocean circulation would warm, rather than cool, the northern continents.

"Because we found a major cooling in [North America] we think that carbon dioxide played a bigger role in this transition," Zanazzi said.

Gabriel Bowen, an expert in ancient climates at Purdue University, explained that if ocean currents were responsible for the change, the currents would have carried Antarctica's lost heat to North America and other continents.

"The new paleoclimate records [from Zanazzi's team] are telling us that during the glaciation of Antarctica, cooling was also occurring in North America in places that were expected to receive the excess heat … robbed from Antarctica," said Bowen, who wrote an accompanying article in Nature.

"This doesn't add up and [instead] points to a triggering mechanism that affected temperatures in a similar way across the Earth's surfaces, like dropping greenhouse gas levels."

While the increase in carbon dioxide levels today is blamed primarily on humans burning fossil fuels, scientists are not sure what caused the carbon dioxide levels to fall 34 million years ago.

According to one theory, the oceans became more productive, with a larger number of small organisms sucking carbon dioxide from the atmosphere.

Another theory is related to the uplift of the Tibetan plateau. The creation of huge mountain chains, which happened around this time, would have exposed more rocks to weathering, which consumes atmospheric carbon dioxide.

Future Model

The freeze 34 million years ago may provide important clues for scientists trying to understand how the climate responds to various changes in the environment.

(See National Geographic magazine's "Global Warning: Signs From Earth.")

"Climate models are very sensitive to certain parameters that we cannot predict very accurately," Zanuzzi said.

"So it is important to study the major episodes of climate change that happened on Earth throughout its history."

Bowen says the freeze demonstrates how critical greenhouse gases are in shaping the global climate, and how shifts in the concentration of those gases can affect global temperatures.

"Earth's surface temperatures are now the warmest in several centuries, if not many millennia, and current carbon dioxide levels have not likely been seen in millions of years," he said.

"Although it has been 34 million years since Antarctica was last ice-free, we seem to be reaching climate milestones at an accelerated pace, and perhaps such dramatic changes are not beyond the scope of what humans will see during their time on Earth."

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