The recent retreat of Arctic sea ice is likely to accelerate so rapidly that
the Arctic Ocean could become nearly devoid of ice during summertime as early as
2040, according to new research published in the December 12 issue of
Geophysical Research Letters.
The study, by a team of scientists from the National Center for Atmospheric
Research (NCAR), the University of Washington, and McGill University, analyzes
the impact of greenhouse gas emissions on the Arctic. Scenarios run on
supercomputers show that the extent of sea ice each September could be reduced
so abruptly that, within about 20 years, it may begin retreating four times
faster than at any time in the observed record.
"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 than
anything that has happened so far," says NCAR scientist Marika Holland, the
study's lead author. "These changes are surprisingly rapid."
The research was funded by the National Science Foundation, NCAR's principal
sponsor, as well as by NASA.
Arctic sea ice has retreated in recent years, especially in the late summer,
when ice thickness and area 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, one of the world's leading
tools for studying climate change. 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 simulations closely matched
observations, a sign that the model was accurately capturing the present-day
climate variability in the Arctic.
The team then simulated future ice loss. The model results indicate that, if
greenhouse gases continue to build up in the atmosphere at the current rate, the
Arctic's future ice cover will go through periods of relative stability followed
by abrupt retreat. For example, in one model simulation, the September ice
shrinks from about 2.3 million to 770,000 square miles in a 10-year period. By
2040, only a small amount of perennial sea ice remains along the north coasts of
Greenland and Canada, while most of the Arctic basin is ice-free in September.
The winter ice also 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.
The study drew on extensive and sophisticated computer modeling recently carried
out for the Intergovernmental Panel on Climate Change. The IPCC's next
assessment report will be released early in 2007.