Much to researchers surprise, sea-bottom sediments reveal that the Arctic was warm and ice-free 55 million years ago.

H. Brinkhuis / Utrecht

Balmy Arctic Stymies Climate Modelers

Fifty million years ago, with nary an iceberg in sight, the Arctic Ocean was just right for a warm swim, according to a study of sediments extracted from the seafloor. That conclusion has climate modelers stumped, and it also provides support for the idea that a boost in hurricane activity may exacerbate warming.

When it comes to predicting climate change, the Arctic Ocean is one big question mark. The enormous polar ice cap helps cool the planet by reflecting sunlight back into space. But as global warming melts the ice and exposes the heat-absorbing ocean water, the warming could accelerate in a runaway process. To predict how the Arctic will respond to a changing climate in the future, scientists have tried to piece together its past. But the effort has been hampered by a lack of hard data.

Now, an international team has given geologists their best glimpse yet of the Arctic's climate history. Using ice-breaker ships and a floating drill rig during a 2004 expedition, the researchers extracted sediments down to 400 meters beneath the Arctic seafloor, deep enough to track the Arctic climate back 56 million years. Three papers detailing the analysis of the cores appear this week in Nature.

The biggest shock: There wasn't any ice in the Arctic about 55 million years ago. At that time, the planet heated up dramatically--possibly because of changes in the atmosphere caused by volcanic eruptions or deep sea methane release--and researchers have assumed that the melting Arctic ice contributed to the warming. But by analyzing oil molecules in fossilized plankton, a team led by Appy Sluijs, a paleoceanographer at Utrecht University in the Netherlands, calculates that the Arctic ocean was already an ice-free, 18 degrees Celsius before the warming started and then warmed up to 23 degrees Celsius. That will send climate modelers back to the drawing board, says Sluijs, because some other mechanism is now needed to explain the additional temperature increase. Ferocious hurricanes may have contributed by pushing hot water around the globe, the authors suggest.

The findings pose "quite a challenge" to climate science, says Gabe Bowen, an earth scientist at Purdue University in West Lafayette, Indiana. "Amplification of hurricane intensity as a mechanism for warming the poles is a really interesting and provocative hypothesis," he says, "and what remains now is for scientists to scour the geological record for evidence of super-hurricanes during these ancient warm climate intervals."

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