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Smithsonian National Museum of Natural History

Global Warming 55 million years ago - Wyoming



The Paleocene-Eocene Thermal Maximum (PETM) was an abrupt global warming event that occurred at the beginning of the Eocene Epoch, about 55.8 million years ago. At the start of the event something like 5-10,000 gigatons of carbon were released into the ocean and atmosphere in less than 10,000 years. The source is still not firmly determined. As a result of the carbon release, temperatures rose 5-9˚C globally. The PETM is widely recognized by scientists as the best geological analog for the human-induced global warming that is happening now.

The PETM was the warmest period of the last 65 million years. This graph shows global ocean temperature reconstructed from the chemical composition of the shells of unicellular marine organisms called foraminifera (inset photos). The fossil foraminifera are collected from cores taken from the ocean bottom.


The PETM had widespread effects on the earth’s plants and animals. My work has focused on how continental climates changed during the event, and the effects of climate change on plants. I have done a lot of field work in the Bighorn Basin of northwestern Wyoming because the area has beautiful badland exposures of sediments that were deposited during the PETM.

Badlands of the PETM in the southeastern Bighorn Basin.


Over the years we have collected thousands of plant fossils from the PETM in Wyoming. First we find a place where the rocks have the right color and grain size to preserve plants (these are usually sediments that were deposited in a pond). Then we make an excavation, split open the rocks, and inspect them for fossils.

Fossil quarry site with field crew.

Fossil leaves on split rock surfaces.

Our work has shown that the climate warmed by about 5 ˚C, and we also think that rainfall dropped or became more seasonal. The types of plants that grew in Wyoming during the PETM were quite different from those that came before or after. Fossils of plants in the bean family, Fabaceae, are unusually abundant and diverse during the PETM. Vegetation before and after the PETM in this area might have looked like that in south Georgia or north Florida today. During the PETM it probably looked more like dry, tropical Mexico.

Plants changed rapidly during the PETM in Wyoming. The red line here represents rising temperature, and the plants under the high part of the curve are those that lived in this part of the world only during the PETM.



A short video on my work with colleagues on the PETM (including field footage!):

A very good popular description of the PETM by award-winning science journalist Madeleine Nash:

A short news piece about plant migration during the PETM:

Learn how we infer temperature from fossil leaves:

A lesson plan for middle school children about how to reconstruct paleotemperature from fossil leaves:

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