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Monday, December 11, 2006

Ice retreat could produce ice-free arctic summers by 2040

A new NCAR (National Center for Atmospheric Research) study indicates that Arctic sea ice may soon begin retreating 4 times faster than at any time since observations began. The Arctic could become devoid of late summer sea ice by 2040, according to new research published in the December 12 issue of Geophysical Research Letters.

The model indicates the extent of this late-summer ice could begin to retreat abruptly within a few decades.

    This animation shows the year-to-year variability of Arctic sea ice. For much of the 20th century, the model accurately captures the expansion and contraction of the area covered by sea ice from one late summer to the next, based on natural climate cycles. By the end of the 20th century, however, the ice began to retreat significantly because of global warming.

    Within a few decades or sooner, the model simulations show that the ice is likely to shrink abruptly, losing about two-thirds of its area over the course of about a decade. By 2040, the Arctic may be devoid of sea ice during the late summer unless greenhouse gas emissions are significantly curtailed. (Animation ©UCAR.)
The study, by a team of scientists from NCAR, the University of Washington, and McGill University, analyzed the impact of greenhouse gas emissions on the Arctic.

"We have already witnessed major losses in sea ice, but our research suggests that the decrease over the next few decades could be far more dramatic" says NCAR scientist Marika Holland, the study's lead author. "These changes are surprisingly rapid."

Arctic sea ice has been in retreat in recent years, especially in late summer, when the extent and thickness of the ice are at a minimum. To analyze how global warming will affect the ice in coming decades, the team studied a series of seven simulations run on the NCAR-based Community Climate System Model. The scientists first tested the model by simulating fluctuations in ice cover since 1870, including a significant shrinkage of late-summer ice from 1979 to 2005.

The team then simulated future ice loss if greenhouse gases continue to build at the current rate, the prediction: Arctic's ice cover will go through alternating periods of stability followed by abrupt retreat. For example, in one model simulation, September ice shrinks from 2.3 million square miles to just 770,000 square miles in one 10-year period. By 2040, the model suggests a small amount of perennial sea ice may remain along the north coasts of Greenland and Canada, while most of the Arctic basin is ice-free by September. In addition, winter ice thins from about 12 feet thick to less than 3 feet.

Why expect abrupt change

The research team points to several reasons for the abrupt loss of ice in a gradually warming world. Open water absorbs more sunlight than does ice, meaning that the growing regions of ice-free water will accelerate the warming trend. In addition, global climate change is expected to influence ocean circulations and drive warmer ocean currents into the Arctic.

"As the ice retreats, the ocean transports more heat to the Arctic and the open water absorbs more sunlight, further accelerating the rate of warming and leading to the loss of more ice," Holland explains. "This is a positive feedback loop with dramatic implications for the entire Arctic region."

Avoiding abrupt change

The scientists also conclude that different rates of greenhouse gas emissions can affect the probability of abrupt ice loss. By examining 15 additional leading climate models, they found that if emissions of carbon dioxide and other greenhouse gases were to slow, the likelihood of rapid ice loss would decrease. Instead, summer sea ice would probably undergo a much slower retreat.

"Our research indicates that society can still minimize the impacts on Arctic ice," Holland says.

Marika Holland
NCAR scientist

Cecilia M. Bitz

Bruno Tremblay

Research funded by the National Science Foundation, NCAR's principal sponsor, and NASA.

Publication:: Geophysical Research Letters

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