Recent NASA airborne measurements and a new review of space-based
measurements of the thickness of Earth’s polar ice sheets concludes they are
changing much more rapidly than previously believed, with unknown
consequences for global sea levels and Earth’s climate.

Large sectors of ice in southeast Greenland, the Amundsen Sea
Embayment in West Antarctica and the Antarctic Peninsula are changing rapidly
by processes not yet well understood, said researchers Dr. Eric Rignot of NASA’s
Jet Propulsion Laboratory, Pasadena, Calif., and Dr. Robert Thomas of EG&G
Services at NASA’s Wallops Flight Facility, Wallops Island, Va. Their study,
published this week in the journal Science, reviews progress in measuring
changes in ice sheet thickness based upon technical advances and observations
made over the past decade.

“Earth’s polar ice sheets are changing over relatively short time scales,
that is, decades versus thousands of years,” Rignot said. Thomas added that
today’s more precise, widespread measurements tell us rapid changes are
common. “These observations run counter to much accepted wisdom about ice
sheets, which, lacking modern observational capabilities, was largely based on
‘steady-state’ assumptions,” Thomas said.

“Remote sensing is allowing researchers to look at polar processes on
continental scales and in greater detail than before,” said Dr. Waleed Abdalati,
Cryospheric Program manager, NASA Headquarters, Washington D.C. “Closer
examination using even broader advanced remote sensing techniques, including
NASA’s upcoming Ice, Cloud and Land Elevation Satellite, the Gravity Recovery
and Climate Experiment and Europe’s planned Cryosat mission–combined with
widespread interferometric synthetic aperture radar (InSAR) data, ice thickness
surveys and ground-based measurements–will enable us to estimate ice sheet
mass balance for Greenland and Antarctica even more precisely.”

Rignot said understanding how polar ice sheets evolve is vital to society.
“The Antarctic and Greenland ice sheets together hold enough ice to raise sea
level by 70 meters (230 feet),” he said. “Even a small imbalance between
snowfall and discharge of ice and melt water from ice sheets into the ocean could
be a major contributor to the current sea level rise rate of 1.8 millimeters (0.07
inches) a year and impact ocean circulation and climate. During past periods of
rapid deglaciation, ice sheet melting raised sea level orders of magnitude faster
than today. This is the real threat of the ice sheets.”

Rignot and Thomas’ review summarizes current progress for two methods
of measuring changes in ice sheet thickness: the mass budget method, which
compares losses by melting and ice discharge with total net input from snow
accumulation; and measuring elevation changes over time. These methods use
various space remote sensing resources, such as laser and radar altimetry, the
Global Positioning System and InSAR.

The review reports Greenland’s ice sheet is losing 50 cubic kilometers (12
cubic miles) of mass a year due to rapid thinning near its coasts. That’s enough to
raise sea level 0.13 millimeters (0.005 inches) annually. “Rapid coastal thinning
cannot be explained by a few warm summers and is attributed to a dynamic ice
sheet response,” Rignot said. “A possible contributor to the observed trend is
increased lubrication from additional surface melt water reaching glacier beds
through crevasses and moulins.”

Rignot says the mass balance in Antarctica is much harder to calculate
because the ice sheet is far larger, more remote and not well covered by existing
key satellites. The researchers calculated net ice gains or losses for 33 Antarctic
glaciers, including 25 of the 30 largest ice producers.

The West Antarctic ice sheet was found to be thickening in the west,
thinning rapidly in the north, and probably losing mass overall by roughly 65
cubic kilometers (roughly 15.5 cubic miles) a year, enough to raise sea level by
about 0.16 millimeters (0.006 inches) a year. InSAR observations show several
major glaciers that are accelerating and contributing to sea level rise. Radar
altimetry shows ice shelves in the Amundsen Sea Embayment are rapidly
thinning, possibly in reaction to a warmer ocean, as suggested by recent
oceanographic data. Melting of ice shelf bottoms is far larger than expected here
due to intrusion of warm water on the continental shelf, implying a larger
interplay of ice and ocean in ice sheet evolution.

Rignot said little is known about the mass balance of Antarctic Peninsula
mountain glaciers, which receive a quarter of Antarctica’s snow accumulation.
The peninsula has warmed 2 to 3 degrees Celsius (3.6 to 5.4 degrees Fahrenheit)
over the past 50 years, causing rapid thinning, enhanced melting and rapid
disintegration of its ice shelves. The peninsula is a unique laboratory to determine
whether retreating ice shelves can induce faster ice sheet flow and raise global sea
level, a hypothesis formulated decades ago but still disputed. Recent results show
large glacier acceleration in response to ice shelf collapse. If ice shelves do
buttress glaciers, the Antarctic ice sheet’s contribution to sea level rise could be
much larger in the future than previously believed.

Illustrations related to this study may be viewed at:

JPL is a division of the California Institute of Technology in Pasadena.