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From hot to cold in the Arctic

For the first time, scientists have recovered direct evidence of what life in the Arctic has been like for the past 56 million years. A new 400-meter-long sediment core is revealing that all in the Arctic has not always been as it seems, with times in the past far warmer and ice buildup beginning earlier than previously surmised. Such a varied history may have implications for Earth’s future as the climate changes, the researchers say.

In 2004, a science team, led by Kathryn Moran of the University of Rhode Island and Jan Backman of Stockholm University, drilled the middle of the Arctic Ocean for the first time, collecting a core that records the history of the Arctic back 56 million years. Photograph is by D. McInroy, copyright of Integrated Ocean Drilling Program.

“One of the most significant things we found was evidence of ice-rafted debris at 42 million years ago,” says Kathryn Moran, a paleoceanographer at the University of Rhode Island and co-leader of the Arctic Coring Expedition. The debris from the icebergs found in the core reveals that the Arctic was cold more than 30 million years earlier than most scientists had predicted, says Moran, who with her team (supported by the Integrated Ocean Drilling Program), published a suite of papers about the analysis of the core in the June 1 Nature.

The core is revealing much about the history of the Arctic, Moran says. Starting 56 million years ago, the Arctic was enveloped in a “greenhouse” world with no ice. The Arctic Ocean was a balmy 18 degrees Celsius (64.4 degrees Fahrenheit), “much warmer than anyone thought possible,” says Mark Pagani of Yale University and a co-author on one of the Nature papers. Then, for a brief period about 55 million years ago, the Arctic warmed further during the Paleocene-Eocene Thermal Maximum (PETM). The ocean’s surface temperatures reached 23 degrees Celsius, he says.

Soon after, the world began to cool, shifting toward an “icehouse” around 49 million years ago, Moran says. At that time, sea-surface temperatures averaged closer to 10 degrees Celsius, and the surface of the Arctic Ocean became freshwater. Then, about 42 million years ago, the core reveals evidence of the water turning salty again as ice began to spread out.

By 14 million years ago, she says, it appears that much of the Arctic was icy, in sync with the growth of the East Antarctic Ice Sheet. The core also shows another abrupt expansion of ice around 3.2 million years ago, which corresponds with the expansion of the Greenland Ice Sheet.

That the onset of ice in the Arctic was synchronous with cooling in Antarctica eliminates the need for “alternative explanations” as to how Antarctica cooled, says Heather Stoll, a paleoclimatologist at Williams College in Massachusetts. Previously, researchers had attributed glaciation in Antarctica to a separate set of processes involving changing ocean currents and tectonics (see Geotimes, June 2006). The bipolar symmetry in climate change, however, indicates that the major driving force on the planet was probably carbon dioxide or other greenhouse gases, she says, which has implications for future climate changes.

Not only does the study revise scientists’ views of the frozen Arctic, it also sheds new light on the Arctic’s periods of greenhouse conditions. The core showed that temperatures in the Arctic Ocean during the PETM were a good 15 degrees Celsius warmer than any model had predicted, Pagani says.

The PETM was a 100,000- to 150,000-year period of widespread, extreme climatic warming associated with a massive increase in atmospheric greenhouse gases. The time period has long been thought the geologic record’s closest analog to today’s world, Pagani says, and “is probably the best geologic example of carbon-dioxide-induced global warming.”

If the new PETM temperatures are correct, researchers will have to reconfigure paleoclimate models, says Gavin Schmidt, a researcher with NASA Goddard Institute for Space Studies in New York City. The problem, says Appy Sluijs, a researcher at Utrecht University in the Netherlands and lead author on one of the Nature papers, is that no climate model currently can produce high enough Arctic temperatures to match the new evidence for the PETM, given what researchers previously thought were the amounts of carbon dioxide and other greenhouse gases in the atmosphere at the time (see Geotimes, November 2004).

Furthermore, Sluijs says, no climate model can explain the reduced temperature gradient from the equator to the poles that this record reveals. Scientists have generally thought that the mean annual temperature at the equator averages about 40 degrees Celsius warmer than the poles, but this record reveals they were only a few degrees different during the PETM, Pagani says. “There must be some other mechanism that either cools the tropics or warms the poles under extreme greenhouse conditions.”

Or, Schmidt says, the temperatures the team recorded are inaccurate. The method the researchers used to measure the Arctic temperatures is relatively new, he says, so all of the bugs might not have been worked out yet. Also, he notes, the authors recognize that the temperatures could have been a summer maximum rather than an annual average, and if that were true, the temperatures would not be so far off from what models predict. “If not, however, and the numbers stand up, this is a huge mystery and the ball goes straight to the modelers’ court.”

It seems, Schmidt says, that the origin of the PETM “has become more shrouded in mystery with every new piece of data,” noting that the new core results are “very preliminary.” Indeed, Stoll says, “this core lays out a decade of next steps and further studies.”

Megan Sever

"Fish teeth bite into Antarctic formation," Geotimes, June 2006
"Methane Hydrate and Abrupt Climate Change," Geotimes, November 2004
"Planet warms, plants move in," Geotimes, January 2006
Integrated Ocean Drilling Program information on Arctic coring expedition

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