Technology & Science

Arctic climate switched from greenhouse to icehouse, cores show

A 400-metre-long core recovered from near the North Pole shows the region was subtropical about 55 million years ago, and then cooled to icehouse conditions.

A 400-metre-longcore of sediment recoverednear the North Pole shows the region was subtropical about 55 million years ago.

Scientists consider the Arctic to be a bellwether for climate change, but until recently, they've had to rely on samples collected thousands of kilometres away from the Pole.

In fall 2004, scientists explored the Arctic Ocean with two icebreakers and drilling rigs on the Lomonosov ridge on the Arctic seabed, about 250kilometres from the North Pole.

The icebreaker allowed scientists to drill into a climatic time capsule – the multi-year iceis harder and thicker than ice accumulating in one Arctic winter.

"At times, the drill site was covered with ice two to three metres thick," recalled geologist Jan Blackman, a professor at Stockholm University and chief co-scientist on the expedition.

"It was like driving into a brick wall."

In three studies appearing in Thursday's issue of the journal Nature, researchers document how the region went from greenhouse to icehouse conditions.

First, the Arctic Ocean went through a warm period with temperatures of 23 C, like a tepid bath.

Then, about 49 million years ago, freshwater was released into the Arctic, cooling it to about 10 C.

At the time, the salinity was dilute enough for freshwater ferns to cover much of the surface in the summer, the team said.

Cooldown

The fast-growing greenery likely absorbed carbon dioxide, helping to cool the Arctic, theorized Henk Brinkhuis of Utrecht University, a co-author of one of the papers.

Pebbles carried to the middle of the Arctic basin, dropped by icebergs, suggest ice started to form about 45 million years ago, the start of the region's current "icehouse" conditions.

The findings suggest the Northern Hemisphere cooled earlier than previously thought, coinciding with when the climate was also changing in the Antarctic.

The temperature swings show the rate of change in carbon dioxide levels and warming are similar to those forecast in coming centuries, a journal commentary notes.

"In one respect future warming will be different," wrote Heather Stoll, a geoscientist at Williams College in Williamstown, Mass. "It will be amplified at high latitudes by the reduction in snow and sea ice cover."

The blanket of reflective snow and sea ice over the poles reduces the amount of sunlight absorbed by the Earth, helping to cool the planet.