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  National Geographic 12 Dec 06
Arctic Summers Ice-Free by 2040, Study Predicts
John Roach for National Geographic News

Summers in the Arctic Ocean may be ice-free by 2040—decades earlier than previously expected, according to a new study of the effects of global warming on sea ice.

The scenario is predicted by computer models that assume greenhouse gas emissions will continue unabated. Gases such as carbon dioxide spewed into the atmosphere by coal-fired power plants and automobiles are considered major drivers of global warming.

According to computer models, if the gases continue to build up in the atmosphere at the current rate, sea ice will steadily decline for decades and then abruptly disappear.

"There are tipping points in the system," said Bruno Tremblay, an assistant professor of atmospheric and ocean sciences at McGill University in Montreal, Canada. "When we reach them, things accelerate in a nonlinear way."

Tremblay is a co-author of the research, published today in the journal Geophysical Research Letters and presented on Monday at the fall meeting of the American Geophysical Union in San Francisco, California.

Feedback Loop

In the Arctic, the summer melt reduces ice cover to its minimum by September, when the arrival of winter usually refreezes the sea ice. In one model simulation, September sea ice coverage will shrink from about 2.3 million square miles (6 million square kilometers) to 770,000 square miles (2 million square kilometers) in a span of ten years.

By 2040 only a small amount of sea ice will remain along the north coasts of Greenland and Canada, while most of the rest of the ocean basin will remain ice-free through the summer. Winter ice will thin from about 12 feet (3.7 meters) to 3 feet (1 meter) thick.

Tremblay explains that the Arctic sea ice is like a giant mirror. It reflects the sun's energy back into the atmosphere and prevents much of it from being absorbed by the ocean.

But as warmer average temperatures melt the ice, the mirror shrinks. A smaller mirror means that the ocean absorbs more of the sun's energy, which creates further warming. This warming causes more ice to shrink, which causes more heat absorption.

"It goes into a positive feedback loop—a very efficient way of getting rid of the ice cover," Tremblay said.

In addition, climate models suggest that global warming will alter ocean circulation patterns and drive warmer currents into the Arctic. "That is a positive feedback as well," he added. "It enhances the melting of the ice."

Dire Consequences

According to Tremblay, a summer with particularly thin ice cover combined with a pulse of warm water from a modified circulation pattern might be the tipping point.

"For us to say it could happen by 2020 or 2030 is not unrealistic," he said. "We are already seeing very strong signs in the rate of sea ice change."

Loss of Arctic sea ice would likely take a lethal toll on animals such as polar bears that rely on the ice as a hunting platform. Local indigenous people would also be unable to fish from the ice, forcing them to adapt. "That's going to be a big strain on their mode of living," he said.

What's more, the melting ice could open up new shipping lanes through the Arctic and spark a race to exploit newly exposed resources.
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