By Dr. Robert Thorson

Meet the Pliocene. Getting to know this recent warm geological epoch would be a giant step forward in the politics of climate change because it would decouple the certain reality of a warmer world from the finger-pointing about who is responsible for causing it.

The Pliocene contained a protracted interval of warmth between 3 million and 3.5 million years ago, when global temperatures averaged 4 to 5 degrees Fahrenheit above those of today.

This interval came just before the cool-down toward major mid-latitude glaciations, and just before the apes who would someday evolve into human beings invented their first hand ax. Clearly, our modern species of humans did not cause the Pliocene warming.

Yet, at the same time, planet Earth was fairly similar in the geography of its continents and oceans – something that cannot be said about earlier periods of geologic history such as the Cretaceous, when the dinosaurs were prowling the Earth; or the Silurian, when shallow limy seas covered much of the globe.

With the continents in nearly their present configurations, the basic patterns of oceanic circulation were similar to those of today. The amount of sunlight striking the Earth was nearly identical. Many of the plants and animals in ecosystems today were extant and doing quite well. The fossil record for these organisms – from familiar maples to oceanic plankton and opossums – is abundant and diverse, allowing the distribution of modern organisms to be used as climate proxies for the past. And, most important, the amount of CO(-2) in the atmosphere fluctuated within a range of values comparable to those of today, though on the high side of that range.

There were some global differences between then and now. Glaciers were more restricted. Florida was under water. The Great Lakes had not yet been carved. Global sea level was much higher than today, about 25 meters higher, not far above the estimate for a complete melting scenario for today’s Greenland and West Antarctic ice sheets. These facts are more completely reported in a recent article from EOS, the Transactions of the American Geophysical Union.

But the biggest difference between the world of 3 million years ago and that of today was the much greater warmth and moisture at subarctic latitudes. Though equatorial temperatures may have been only a few degrees higher than those of the late 20th century, those above the latitude of Scotland, Russia and Quebec were as much as 20 to 30 degrees Fahrenheit above present values. Pack ice was completely absent from Arctic Ocean, similar to the projections for the late 21st century.

I got to meet the Pliocene epoch early in my career. Between 1973 and 1979, I trekked all over the foothills of the North Alaska Range, many of which are composed of a tilted sheet of brown gravel associated with the birth of the range. The brown color comes from a deep, penetrating rust caused by prolonged soaking under the warmer conditions of the high-latitude Pliocene epoch. The fossils were there as well.

One candidate cause for Pliocene warming was a concentration of atmospheric carbon dioxide not far above the projections for Earth at the end of this century. Another candidate was the greater absorption of solar energy by the expanded sea; or by the darker land surfaces with more forest than prairie, and very little ice. Certainly, such elevated temperatures at high latitudes point to a greater south-to-north transfer of heat within in the oceans.

Paleoclimate scientists aren’t really sure what caused the Pliocene climate warming. But they are sure that humans had nothing to do with it. They are also sure that feedbacks within the climate system were strong, if not volatile.

Geologists are fond of saying “The present is the key to the past.” In the case of the Pliocene warming, the past may instead be the key to the present. Meeting the Pliocene may be meeting our future.