By ERIK STOKSTAD
Illustrations by JENNIFER CHRISTIANSEN

Geologic evidence suggests that the Earth's climate isn't as stable as it seems. Now, a previous rapid global warming has been tied to the arrival of mammals in North America.

The last time the greenhouse effect was in full swing, 55 million years ago, flamingos flocked in what is now North Dakota. Crocodiles prowled the woodland streams of Wyoming. The sudden hot spell was first identified in 1991 by scientists studying ancient ocean sediments.

To learn if our climate might overreact, scientists must look back in time and uncover past responses of ocean and atmosphere to stresses such as increased carbon dioxide. But the workings of ancient climates are a geologic puzzle with pieces scattered all over the globe. The investigation involves fossil mammals connected by carbon atoms with salty ocean currents and seeping gases.

The past may provide a compelling lesson for future change to our climate.