Ancient Warming Has Disturbing Implications for Our Future

By Michael D. Lemonick

Seafloor sediments from Spitzbergen, on the edge of the Arctic Ocean, tell a tale of ancient warming. Credit: Michael Lemonick.

People who want to cast doubt on the idea that humans can change the climate sometimes point to the distant past. Climate change was happening long before anyone invented cars or coal-burning power plants, they argue, so the current episode of climate change must be natural as well. It’s kind of like arguing that since wildfires existed for millions of years before matches were invented, they can’t be started by people.

Climate scientists think of ancient climate change in a very different way. They see it as something that might help us understand what might be in store for us in the future. One period that especially interests them is something called the Paleocene-Eocene Thermal Maximum (PETM), a time about 60 million years ago, when levels of carbon dioxide (CO2) — the primary greenhouse gas — shot up relatively fast, stayed high for about 170,000 years, and then dropped again. Global temperatures rose by about 9°F on average — at the upper end of what’s projected for the coming century.

There’s plenty that scientists don’t understand about the PETM, including what, precisely caused it. It may have been triggered initially by orbital changes in the Earth’s path around the Sun, much as the warm periods during ice ages have been over the past couple of million years. It certainly involved huge releases of carbon, but it’s not certain whether this came from the massive burning of biomass or coal (naturally ignited, of course), or by the release of undersea methane which oxidized into CO2, or both.

But researchers are learning more all the time, as a new paper in the journal Nature Geoscience makes clear. By delving into an unusually rich and well-preserved set of ancient marine sediments from Spitzbergen Island on the edge of the Arctic Ocean, an international team of researchers has come up with an estimate of how quickly carbon entered the atmosphere at the start of the PETM.

It’s a key question because climate change is really only a major problem if it happens relatively quickly. If it plays out over many centuries, ecosystems will have time to adapt, and the vast infrastructure that underlies the developed world — cities, transportation systems, industrial facilities and more—can be moved, rebuilt, or modified at a reasonable pace and cost. If, as is currently the case, climate change unfolds rapidly, the changes can be enormously destructive.

Based on their analysis of carbon levels in ancient sea creatures dredged from those ancient sediments, the news is not good.

“Our major conclusion,” says lead author Ying Cui, a graduate student in geosciences at Penn State, “is that we’re pumping CO2 into the atmosphere at about ten times the average rate during the PETM.”

There weren’t any cities or highways to destroy back then, but many species of sea-dwelling organisms went extinct. Other species, including many mammals, did better during the PETM than they had before — but that would have forced ecosystem readjustments as well.

That’s at a time when carbon levels were climbing a tenth as fast as they are today, which suggests that the biological world, at least, is poised to go through a far more wrenching set of changes (and just to compound the situation, there weren’t any icecaps before the PETM, so that period of warming didn’t raise sea levels as much as they’re likely to go up during our own episode of climate change).

If these numbers are confirmed by future studies, at least one mystery of the Paleocene-Eocene Thermal Maximum will have been solved — and not in a way that inspires comfort.