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Two Satellites Report: Arctic Sea Ice Declines

14 February, 2013


MessageToEagle.com - New research using combined records of ice measurements from NASA's Ice, Cloud and Land Elevation Satellite (ICESat), the European Space Agency's CryoSat-2 satellite, airborne surveys and ocean-based sensors shows Arctic sea ice volume declined 36 percent in the autumn and 9 percent in the winter over the last decade.

The work builds on previous studies using submarine and NASA satellite data and confirms computer model estimates that showed ice volume decreases over the last decade, and builds a foundation for a multi-decadal record of sea ice volume changes.

In a report published online recently in the journal Geophysical Research Letters, a large international collaboration of scientists outlined their work to calculate Arctic sea ice volume.



Click on image to enlarge

This map shows Arctic sea ice thickness, as well as the elevation of the Greenland Ice Sheet, for March 2011. The data come from the European Space Agency CryoSat-2 satellite. For the sea ice, green shades indicate thinner ice, while the yellows and oranges indicate thicker ice. Credit: NSIDC courtesy CPOM/UCL/Leeds/ESA/PVL


The satellite measurements were verified using data from NASA's Operation IceBridge, ocean-based sensors and a European airborne science expedition. This was compared with the earlier sea ice volume record from NASA's ICESat, which reached the end of its lifespan in 2009.

The researchers found that from 2003 to 2008, autumn volumes of ice averaged 11,900 cubic kilometers. But from 2010 to 2012, the average volume had dropped to 7,600 cubic kilometers – a decline of 4,300 cubic kilometers. The average ice volume in the winter from 2003 to 2008 was 16,300 cubic kilometers, dropping to 14,800 cubic kilometers between 2010 and 2012 – a difference of 1,500 cubic kilometers.

Although CryoSat-2 data show a decrease in ice volume from 2010 to 2012, two years is not a long enough time span to determine a trend.

This is where NASA's data and scientists come in.

Data from ICESat and IceBridge are freely available, but combining measurements from different sources can be challenging. Kwok said researchers spent months working out how to compare the datasets and make sure they were compatible enough to compare trends "We participated as collaborators to help interpret results from the datasets we're familiar with," said scientist Sinead Farrell at NASA's Goddard Space Flight Center in Greenbelt, Md.

CryoSat-2 and ICESat both measure sea ice freeboard, that is, the amount of ice floating above the ocean's surface. Researchers use freeboard to calculate ice thickness. This thickness measurement is then combined with ice area to come up with a figure for volume.



Click on image to enlarge

Credits: ucl.ac.uk



Click on image to enlarge

Credits: ESA


The two satellites - that gave comparable measurements - used different methods for measuring freeboard however. ICESat used a laser altimeter, which bounces a laser off the snow covering the sea ice, while CryoSat-2 uses a radar instrument that measures surface elevation closer to the ice surface.

Comparing the two datasets and ensuring their quality called for additional data. The two satellites do not cover overlapping time spans, so researchers used measurements from upward-looking sonar (ULS) moorings under the ocean's surface, located north of Alaska.

"ULS ice draft since 2003 served as the common data set for cross comparison of the ICESat and CryoSat-2 measurements," said Kwok.



Click on image to enlarge

The Polar-5 aircraft, carrying the EM instrument that was used to validate Cryosat-2 sea ice thickness measurements, flying over the validation site. Credit: NERC/UCL/Rosemary Willatt


Researchers took extra care to verify CryoSat-2's data as it is a new satellite with a new instrument. In addition to the ULS data, CryoSat-2 measurements were also verified by two airborne science campaigns: flights by an aircraft operated by the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany; and Operation IceBridge, a NASA mission tasked with monitoring changes in polar ice to bridge the gap in measurements between ICESat and its replacement, ICESat-2, scheduled to launch in 2016.

"IceBridge was used as a validation tool to understand thickness measurements from CryoSat-2," said scientist Nathan Kurtz at NASA Goddard.

After months of work, researchers had assembled a multi-year dataset that they could compare to sea ice volume predictions from the Pan-Arctic Ice-Ocean Modeling and Assimilation System (PIOMAS). Because of the short time span of previous satellite studies, researchers have used models like PIOMAS to simulate changes in sea ice volume.


Arctic Sea Ice Changes 2011-2012

Animation showing changes in monthly Arctic sea ice volume using data from ESA's CryoSat-2 (red dots) and estimates from the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) (solid line) for the 2011 and 2012 winter growth periods (October – April). Credit: CPOM/UCL/ESA/UW-APL/NSIDC/Planetary Visions


The study's observations show a larger autumn ice volume decrease than predicted, while changes in the winter are smaller than in the model simulation. "It's important to know because changes in volume indicate changes in heat exchange between the ice, ocean and atmosphere," said Kurtz.

© MessageToEagle.com

See also:
Arctic Region Becomes Warmer And Greener

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