In a study that offers a cautionary message to global policy-makers, a team of researchers from Canada, Australia and the United States has shed new light on a mysterious period of accelerated climate change that occurred 55 million years ago, long before humans appeared.
The team's analysis, published Monday in the journal Nature Geoscience, concerns the Paleocene-Eocene Thermal Maximum (PETM), a relatively brief period in geologic time when Earth's average temperature rose by an estimated five to eight degrees C. The episode is of interest to climate scientists because it offers the best-known natural analogue to human-induced global warming. It also corresponds to big changes in the distribution of animal life around the world, including the first appearance of primates and hoofed mammals in the Western Hemisphere.
The cause of the PETM is a matter of debate, but evidence suggests it is linked to a massive infusion of carbon into the ocean and atmosphere. Possible theories include volcanic eruptions or a comet impact. The only window researchers have on the climate at that time is in the relative abundance of radioactive isotopes in ocean sediments. These sediments point to a large infusion of carbon dioxide into the atmosphere during the PETM.
But the sediments also present scientists with a puzzle, because core samples from the Atlantic and the Pacific offer very different numbers on exactly how much carbon was involved in the warming. The new analysis offers a solution. Using a computer model that was developed at the University of Victoria in British Columbia, the team found that ocean currents may be responsible for the differences seen between the Atlantic and the Pacific.
The model takes into account the three-dimensional shapes of the oceans 55 million years ago. At that time, the North Atlantic was narrower and its bottom two kilometres were cut off from other oceans by the presence of underwater barriers called sills.
"It was kind of like a giant bathtub on the ocean floor," said Kaitlin Alexander, a PhD candidate in oceanography at the University of New South Wales in Sydney, Australia, and lead author on the study.
Ms. Alexander, a Winnipeg native, began working with the model last year when she went to Australia on a research exchange. She found that surface waters in the North Atlantic containing high levels of carbon dioxide from the atmosphere should have mixed with saltier water flowing westward from the Tethys Ocean (now the Mediterranean Sea). The water would then have sunk to the bottom of the Atlantic and remained isolated there, building in concentration until it spilled over its underwater barriers and began moving along the seafloor, working its way to the Pacific. "It's really dramatic," Ms. Alexander said of the computer simulation of the deep water current. "It looks like a giant slime monster in slow motion."
By the time it reached the Pacific, the water would have been more diluted, which accounts for why the sediments in the two oceans yield such different readings on how much carbon was present during the PETM.
Morgan Schaller, a geochemist at Rensselaer Polytechnic Institute in Troy, N.Y., who was not involved in the study, said the analysis "does a pretty good job" of explaining what has, until now, proved difficult to understand about what the data have to say about the period. This in turn can help scientists draw more meaningful parallels between the PETM and the present.
"The nice thing about the PETM that we don't have at our fingertips today is we can observe the way the whole event played out over thousands of years," said Timothy Bralower, a geoscientist at Pennsylvania State University and a co-author of the study.
Ms. Alexander said that the new analysis suggests the atmosphere at the time was sensitive to increases in carbon-dioxide levels as climate scientists believe it to be today, which means the world is on track to warm substantially without a concerted effort to curb emissions.
"It means we have climate sensitivity about right," she said.