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