From Wyoming to Current-Day Tropics: How Prehistoric Crocodiles Survived

Today, crocodiles live in what are now the tropics. Fifty million years ago, crocodiles' range included southwestern Wyoming to southern Canada, which were warm and muggy. While this is evident from fossil records, it has been unclear to scientists how this area stayed so temperate during Cretaceous and Eocene winter months. New research suggests that rising temperatures in the Arctic and low-lying clouds could explain this past and future phenomenon.

"High-latitude clouds have a strong heating effect at the surface," Timothy Cronin, a National Oceanic and Atmospheric Administration (NOAA) Climate and Global Change postdoctoral fellow at Harvard University's Center for the Environment (HUCE), said in a news release. "We've all seen how a cloudy night cools off slower than a clear night. The same process is in effect here. Increased low cloud cover with warming would slow the formation of Arctic air. If the Pacific Ocean were very warm, low clouds could help these air masses to make it all the way across North America without ever dropping below freezing at the surface."

According to the researchers' findings as noted in the release, these days, cold Arctic air masses form as cool, temperate air moves over the sunless Arctic. and over high-latitude oceans during winter months. The heat radiated from these air masses cools rapidly over the ice and snow-covered surface, and then generally continues south. This pattern made researchers question whether the Arctic air masses would form more or less rapidly if high-latitude oceans were warmer.

Cronin worked alongside Eli Tziperman, a Jr. Professor of Oceanography and Applied Physics at Harvard's John A. Paulson School of Engineering and Applied Science (SEAS) and Department of Earth and Planetary Sciences. Together they developed a model of a single air column to answer this question.

They simulated the temperature that the Cretaceous and Eocene crocodiles would have experienced by raising the initial ocean surface temperature to 20 degrees Celsius. This is compared to today's ocean temperatures of zero degrees Celsius. From this, they found that a thick layer of clouds and fog formed as this air column moved over the Arctic and cooled. The air masses also cooled more slowly because the clouds acted as insulators. Even though there was only an initial 20 degree increase, the simulation resulted in a 40 degree higher surface air temperature. 

Cronin explained that this could potentially prevent cold Arctic air from forming. So, warmer high-latitude oceans could explain how the crocodiles lived in Wyoming. When researchers tested this model with ranging temperatures, results remained consistent. 

"If we take into account factors related to climate change, such as reduced winter sea ice and snow cover, we may find ever stronger suppression of Arctic air formation in the future," Cronin said in the release.

Their study was recently published in the journal Proceedings of the National Academy of Sciences (PNAS).

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