What can the Eocene epoch teach us about today's global warming?

By delving into the mysteries of Earth's climate 50 million years ago, scientists hope to understand how our planet may cope with global warming, providing independent insight into today's climate models.

Richard Bizley and Paul Pearson/Courtesy of Cardiff University
Reconstruction of the 40 million year old planktonic foraminifer Acarinina mcgowrani.

Ancient climates can help us understand the dynamics of future climate change, the impacts of global warming, and the accuracy of current climate models, say scientists.

In a recent study, published Monday in the journal Nature, researchers delved into the character of Earth's climate during the "Eocene epoch," the first part of which was the warmest period our planet has seen in the past 65 million years.

By examining the shells of marine animals alive at the time, they were able to draw conclusions about carbon dioxide levels, and in comparing those data with temperature records, their study has important implications for our understanding of where the climate of today may be heading.

"Looking at the early Eocene gives us a picture of how Earth works in a warm climate state," says coauthor Gavin Foster, of Ocean and Earth Science at the University of Southampton, England, in a telephone interview with The Christian Science Monitor.

The importance of gleaning such insight, explains Dr. Foster, lies in the fact that the climate models we use at the moment to consider future scenarios are based on our understanding of current climate systems.

But what happens if temperatures soar by 14 degrees Celsius, which would put them on a par with the early Eocene? Or even by 3 degrees? Maybe the climate would act differently, perhaps there would be "unknown unknowns, as well as the known unknowns."

Many climate models, as used by the Intergovernmental Panel on Climate Change (IPCC), consider "climate sensitivity," generally defined as the "global mean surface temperature change following a doubling of atmospheric CO2 concentration."

So Foster and his team found a way to measure carbon dioxide levels in the Eocene: by examining the chemical composition of marine fossils – foraminifera – found in deep sea sediments, they could determine the ocean’s pH. From there, they could assess the atmospheric carbon dioxide concentrations.

"It's not as precise as looking at ice core records in Antarctica," Foster tells the Monitor, "where bubbles of ancient atmosphere are actually trapped in the ice; but those only go back 800,000 years. So beyond that, we have to use indirect methods."

Scientists are also able to establish likely temperatures in such ancient eras, using a plethora of "proxy" variables.

For example, we know there were palm trees and no ice in Antarctica, and crocodiles roamed the Arctic. Again, scientists can use the fossil record to examine shell composition, look at the shapes of leaves and the types of trees.

"The phrase 'standing on the shoulders of giants' is very apt here," says Foster. "There is a whole army of people trying to generate these temperature records."

So, armed with temperature records and their new data on carbon dioxide levels, what did the team discover?

They found that the Eocene represented one of the great natural oscillations of Earth's climate, whereby every 300-400 million years, our planet switches between a warm phase (greenhouse) and a cold one (icehouse).

The start of the Eocene was a greenhouse period, and then it transitioned to the icehouse. And that shift was accompanied by a halving of carbon dioxide levels.

Foster admits that "it's hard to nail down causality," but he and his team feel certain that the drop in carbon dioxide was the direct cause of the plummeting temperatures. Both the general warmth that characterized much of the Eocene, and then the cooling period that followed, are entirely consistent with sensitivity of the climate as predicted by the IPCC climate models and the levels of carbon dioxide reconstructed by the researchers.

The question that begs to be asked next, however, is what caused the drop in carbon dioxide levels? Foster thinks it likely that a major shift in the activity of supercontinents had a prominent role: Fifty million years ago, India was colliding with Asia, snuffing out volcanic activity that would have been spewing carbon dioxide into the atmosphere, buckling the Earth's crust, and throwing up the Himalayas.

Such epic mountain formation sparks a chain of activity that feeds the ocean with nutrients, boosts organic activity, and sucks carbon dioxide from the skies, eventually locking it away in the Earth's mantle.

Thus begins a supercycle of continental activity, which, as described in a recent paper by McKenzie et al., may well lie behind the icehouse-greenhouse variability.

"The geological record is a rich archive of Earth's functioning in different climate states," says Foster. "It's the only way of testing our climate models."

You've read  of  free articles. Subscribe to continue.

Dear Reader,

About a year ago, I happened upon this statement about the Monitor in the Harvard Business Review – under the charming heading of “do things that don’t interest you”:

“Many things that end up” being meaningful, writes social scientist Joseph Grenny, “have come from conference workshops, articles, or online videos that began as a chore and ended with an insight. My work in Kenya, for example, was heavily influenced by a Christian Science Monitor article I had forced myself to read 10 years earlier. Sometimes, we call things ‘boring’ simply because they lie outside the box we are currently in.”

If you were to come up with a punchline to a joke about the Monitor, that would probably be it. We’re seen as being global, fair, insightful, and perhaps a bit too earnest. We’re the bran muffin of journalism.

But you know what? We change lives. And I’m going to argue that we change lives precisely because we force open that too-small box that most human beings think they live in.

The Monitor is a peculiar little publication that’s hard for the world to figure out. We’re run by a church, but we’re not only for church members and we’re not about converting people. We’re known as being fair even as the world becomes as polarized as at any time since the newspaper’s founding in 1908.

We have a mission beyond circulation, we want to bridge divides. We’re about kicking down the door of thought everywhere and saying, “You are bigger and more capable than you realize. And we can prove it.”

If you’re looking for bran muffin journalism, you can subscribe to the Monitor for $15. You’ll get the Monitor Weekly magazine, the Monitor Daily email, and unlimited access to CSMonitor.com.