Missing 'Ice Arches' Contributed to 2007 Arctic Ice Loss

PASADENA, Calif. – In 2007, the Arctic lost a massive amount of thick, multiyear sea ice, contributing to that year's record-low extent of Arctic sea ice. A new NASA-led study has found that the record loss that year was due in part to the absence of "ice arches," naturally-forming, curved ice structures that span the openings between two land points. These arches block sea ice from being pushed by winds or currents through narrow passages and out of the Arctic basin.

Beginning each fall, sea ice spreads across the surface of the Arctic Ocean until it becomes confined by surrounding continents. Only a few passages -- including the Fram Strait and Nares Strait -- allow sea ice to escape.

"There are a couple of ways to lose Arctic ice: when it flows out and when it melts," said lead study researcher Ron Kwok of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We are trying to quantify how much we're losing by outflow versus melt."

Kwok and colleagues found that ice arches were missing in 2007 from the Nares Strait, a relatively narrow 30- to 40-kilometer-wide (19- to 25-mile-wide) passage west of Greenland. Without the arches, ice exited freely from the Arctic. The Fram Strait, east of Greenland, is about 400 kilometers (249 miles) wide and is the passage through which most sea ice usually exits the Arctic.

Despite Nares' narrow width, the team reports that in 2007, ice loss through Nares equaled more than 10 percent of the amount emptied on average each year through the wider Fram Strait.

"Until recently, we didn't think the small straits were important for ice loss," Kwok said. The findings were published this month in Geophysical Research Letters.

"One of our most important goals is developing predictive models of Arctic sea ice cover," said Tom Wagner, cryosphere program manager at NASA Headquarters in Washington. "Such models are important not only to understanding changes in the Arctic, but also changes in global and North American climate. Figuring out how ice is lost through the Fram and Nares straits is critical to developing those models."

To find out more about the ice motion in Nares Strait, the scientists examined a 13-year record of high-resolution radar images from the Canadian RADARSAT and European Envisat satellites. They found that 2007 was a unique year – the only one on record when arches failed to form, allowing ice to flow unobstructed through winter and spring.

The arches usually form at southern and northern points within Nares Strait when big blocks of sea ice try to flow through the strait's restricted confines, become stuck and are compressed by other ice. This grinds the flow of sea ice to a halt.

"We don't completely understand the conditions conducive to the formation of these arches," Kwok said. "We do know that they are temperature-dependent because they only form in winter. So there's concern that if climate warms, the arches could stop forming."

To quantify the impact of ice arches on Arctic Ocean ice cover, the team tracked ice motion evident in the 13-year span of satellite radar images. They calculated the area of ice passing through an imaginary line, or "gate," at the entrance to Nares Strait. Then they incorporated ice thickness data from NASA's ICESat to estimate the volume lost through Nares.

They found that in 2007, Nares Strait drained the Arctic Ocean of 88,060 square kilometers (34,000 square miles) of sea ice, or a volume of 60 cubic miles. The amount was more than twice the average amount lost through Nares each year between 1997 and 2009.

The ice lost through Nares Strait was some of the thickest and oldest in the Arctic Ocean.

"If indeed these arches are less likely to form in the future, we have to account for the annual ice loss through this narrow passage. Potentially, this could lead to an even more rapid decline in the summer ice extent of the Arctic Ocean," Kwok said.

For more information about NASA and agency programs, visit: http://www.nasa.gov .

JPL is managed for NASA by the California Institute of Technology in Pasadena.

##

Contact:

Alan Buis
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-0474
alan.buis@jpl.nasa.gov

Kathryn Hansen
NASA Goddard Space Flight Center, Greenbelt, Md.
301-352-4638
kathryn.h.hansen@nasa.gov

This text derived from:
http://www.jpl.nasa.gov/news/news.cfm?release=2010-056&icid=%27NewsFeaturesHome%27

ICESat's Notable Moments in Science

Over the last decade, NASA has launched a series of satellites to monitor the health of our planet. One such satellite -- the Ice, Cloud and land Elevation Satellite (ICESat) -- has provided a sustained, big-picture look at ice thickness at Earth's polar regions.

Now, after seven years in orbit and 15 laser-operation campaigns, ICESat has stopped collecting science data. The last of three lasers on the satellite's Geoscience Laser Altimeter System (GLAS) ceased emitting light on Oct. 11, 2009. Attempts to restart the lasers have ended, and NASA is pursing options for satellite decommissioning.

"ICESat's loss is disappointing and it comes at a critical time," said Tom Wagner, cryosphere program manager at NASA Headquarters in Washington. "But we can't lose sight of the fact that ICESat and its team of talented scientists and engineers helped us see the Earth's polar ice caps in a new way. Those observations are feeding a new generation of models to help us figure out where the planet is headed.‪‪"

As the world's first laser-altimeter satellite, ICESat has measured Earth's surface and atmosphere in "unprecedented 3-D detail," said Jay Zwally, ICESat's project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. "ICESat has been an outstanding success, despite disappointing limitations in the laser lifetimes. Scientific advances have been made in measuring changes in the mass of the Greenland and Antarctic ice sheets, polar sea ice thickness, vegetation-canopy heights, and the heights of clouds and aerosols."

In the Arctic, for example, scientists used ICESat to map Greenland's dramatic surface elevation, rising to 4,000 meters above sea level. They watched as thin, seasonal sea ice replaced thick, older sea ice as the dominant type in the Arctic Ocean. In Antarctica, scientists achieved a comprehensive inventory of lakes that actively drain or fill under the ice. At both poles, they have tracked glaciers along the coast of the Greenland and Antarctic ice sheets as they empty into the sea.

Learn more about the satellite's early days and subsequent discoveries in this Flickr image gallery.

Despite the end of ICESat's mission, NASA's observations of Earth's polar regions continue. Operation Ice Bridge began in 2009, becoming the largest airborne survey of Earth's polar ice ever flown. For the next five years, instruments on NASA aircraft will target areas of rapid change to yield an unprecedented 3-D view of Arctic and Antarctic ice sheets, ice shelves, and sea ice. The mission will bridge the gap in satellite data until the launch of ICESat-2, planned for 2015.

"Operation Ice Bridge is allowing us to get much higher resolution data over smaller, targeted regions," said Lora Koenig of NASA Goddard, and acting project scientist for the Ice Bridge mission.

Targeted information from aircraft combined with the broad and consistent coverage from satellites contribute to a more complete understanding of Earth's response to climate change, helping scientists make better predictions of what the future might hold.