Arctic Sea Ice Melt May Set Off Climate Change Cascade

BOULDER, Colorado, March 19, 2007 (ENS) - Arctic sea ice that has been shrinking for decades may have reached a tipping point that could trigger a cascade of climate changes reaching Earth's temperate regions, finds a new study from the University of Colorado at Boulder.

Dr. Mark Serreze, a senior research scientist at CU-Boulder's National Snow and Ice Data Center who led the study synthesizing results from recent research, said Arctic sea ice extent has been decreasing in every month since 1979, when satellite recordkeeping began.

Serreze and his team attributed the loss of ice, about 38,000 square miles annually as measured each September, to rising concentrations of greenhouse gases and strong natural variability in Arctic sea ice.

Serreze

Dr. Mark Serreze is a research professor of geography with the University of Colorado-Boulder and a senior research scientist with the National Snow and Ice Data Center. (Photo courtesy CU-Boulder)
"When the ice thins to a vulnerable state, the bottom will drop out and we may quickly move into a new, seasonally ice-free state of the Arctic," Serreze said. "I think there is some evidence that we may have reached that tipping point, and the impacts will not be confined to the Arctic region."

The review paper by Serreze and Julienne Stroeve of CU-Boulder's NSIDC and Dr. Marika Holland of the National Center for Atmospheric Research titled "Perspectives on the Arctic's Shrinking Sea Ice Cover" appears in the March 16 issue of the journal "Science," published by the American Association for the Advancement of Science.

"Given the growing agreement between models and observations, a transition to a seasonally ice-free Arctic Ocean as the system warms seems increasingly certain," the researchers wrote in "Science."

"The unresolved questions regard when this new Arctic state will be realized, how rapid the transition will be, and what will be the impacts of this new state on the Arctic and the rest of the globe," they wrote.

"We're seeing more melting of multi-year ice in the summer," said Stroeve. "We may soon reach a threshold beyond which the sea ice can no longer recover."

"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," said Holland. "These changes are surprisingly rapid."

Holland

Dr. Marika Holland is a scientist in the Oceanography Section of the National Center for Atmospheric Research. (Photo courtesy UCAR)
The potential for such rapid ice loss was highlighted in a December 2006 study by Holland and her colleagues published in the journal "Geophysical Research Letters." In one of their climate model simulations, the Arctic Ocean in September became nearly ice-free between 2040 and 2050.

Sea ice is frozen seawater that floats on the ocean surface. Covering millions of square miles, sea ice forms and melts with the polar seasons, affecting both human activity and biological habitat.

Arctic sea ice extent is defined as the total area of all regions where ice covers at least 15 percent of the ocean surface.

In the Arctic, some sea ice persists year after year, while almost all Southern Ocean or Antarctic sea ice is "seasonal ice," meaning it melts away and reforms annually. Sea ice in the Arctic appears to play a more crucial role in regulating climate, according to the NSIDC.

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Arctic sea ice summer minimum in September 2000, based on simulations produced by the Community Climate System Model. (Image UCAR)
Studies have linked Arctic sea ice loss to changes in atmospheric patterns that cause reduced rainfall in the American West or increased precipitation over western and southern Europe, said Serreze, who is a fellow with CU-Boulder's Cooperative Institute for Research in Environmental Sciences.

The decline in Arctic sea ice could impact western states like Colorado by reducing the severity of Arctic cold fronts dropping into the West and reducing snowfall, impacting the ski industry and agriculture, he said.

"Just how things will pan out is unclear, but the bottom line is that Arctic sea ice matters globally," Serreze said.

Passive microwave satellite data reveal that, since 1979, Arctic ice extent has decreased about 3.6 percent per decade, but in recent years, satellite data indicate an even greater reduction in regional ice cover.

In September 2002, sea ice in the Arctic reached a record minimum, according to a 2003 study by Serreze and his team. That year the sea ice extent was four percent lower than any previous September since 1978, and 14 percent lower than the 1979-2000 mean.

In the past, a low ice year would be followed by a rebound to near-normal conditions, but 2002 was followed by two more low-ice years, both of which almost matched the 2002 record.

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Arctic sea ice summer minimum in September 2040, based on simulations produced by the Community Climate System Model. (Image UCAR) (Photo courtesy )
Taking these three years into account, the September ice extent trend for 1979-2004 declined by 7.7 percent per decade, according to a 2005 study by a research team headed by Stroeve.

The year 2005 set a new record low for Arctic sea ice, dropping the estimated decline in end-of-summer Arctic sea ice to approximately eight percent per decade.

Because temperatures across the Arctic have risen from two degrees to seven degrees Fahrenheit in recent decades due to a buildup of atmospheric greenhouse gases, there is no end in sight to the decline in Arctic sea ice extent, said Serreze.

"While the Arctic is losing a great deal of ice in the summer months, it now seems that it also is regenerating less ice in the winter," said Serreze. "With this increasing vulnerability, a kick to the system just from natural climate fluctuations could send it into a tailspin."

In the late 1980s and early 1990s, shifting wind patterns from the North Atlantic Oscillation flushed much of the thick sea ice out of the Arctic Ocean and into the North Atlantic where it drifted south and eventually melted, he said.

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Computer models predict ice sheets like this one on Ellesmere Island, located north of Greenland, could melt faster than expected. (Photo by Shawn Marshall courtesy National Science Foundation)
The thinner layer of "young" ice that formed in its place melts out more readily in the succeeding summers, leading to more open water and more solar radiation being absorbed by the open ocean and fostering a cycle of higher temperatures and earlier ice melt, he said.

"This ice-flushing event could be a small-scale analog of the sort of kick that could invoke rapid collapse, or it could have been the kick itself," Serreze said. "At this point, I don't think we really know."

Researchers also have seen pulses of warmer water from the North Atlantic entering the Arctic Ocean beginning in the mid-1990s, which promote ice melt and discourage ice growth along the Atlantic ice margin.

Serreze said, "This is another one of those potential kicks to the system that could evoke rapid ice decline and send the Arctic into a new state."

"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."