Warm Ocean, Not Icebergs, Causing Most of Antarctic Ice Shelves' Mass Loss

June 13, 2013

J.D. Harrington
Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov

Maria-Jose Vinas Garcia
Goddard Space Flight Center, Greenbelt, Md.
301-614-5883
maria-jose.vinasgarcia@nasa.gov

Whitney Clavin
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-4673
whitney.clavin@jpl.nasa.gov

RELEASE: 13-183

WARM OCEAN, NOT ICEBERGS, CAUSING MOST OF ANTARCTIC ICE SHELVES' MASS LOSS

PASADENA, Calif. -- Ocean waters melting the undersides of Antarctic
ice shelves are responsible for most of the continent's ice shelf
mass loss, a new study by NASA and university researchers has found.

Scientists have studied the rates of basal melt, or the melting of the
ice shelves from underneath, of individual ice shelves, the floating
extensions of glaciers that empty into the sea. But this is the first
comprehensive survey of all Antarctic ice shelves. The study found
basal melt accounted for 55 percent of all Antarctic ice shelf mass
loss from 2003 to 2008, an amount much higher than previously
thought.

Antarctica holds about 60 percent of the planet's fresh water locked
into its massive ice sheet. Ice shelves buttress the glaciers behind
them, modulating the speed at which these rivers of ice flow into the
ocean. Determining how ice shelves melt will help scientists improve
projections of how the Antarctic ice sheet will respond to a warming
ocean and contribute to sea level rise. It also will improve global
models of ocean circulation by providing a better estimate of the
amount of fresh water ice shelf melting adds to Antarctic coastal
waters.

The study uses reconstructions of ice accumulation, satellite and
aircraft readings of ice thickness, and changes in elevation and ice
velocity to determine how fast ice shelves melt and compare the mass
lost with the amount released by the calving, or splitting, of
icebergs.

"The traditional view on Antarctic mass loss is it is almost entirely
controlled by iceberg calving," said Eric Rignot of NASA's Jet
Propulsion Laboratory in Pasadena, Calif., and the University of
California, Irvine. Rignot is lead author of the study to be
published in the June 14 issue of the journal Science. "Our study
shows melting from below by the ocean waters is larger, and this
should change our perspective on the evolution of the ice sheet in a
warming climate."

Ice shelves grow through a combination of land ice flowing to the sea
and snow accumulating on their surface. To determine how much ice and
snowfall enters a specific ice shelf and how much makes it to an
iceberg, where it may split off, the research team used a regional
climate model for snow accumulation and combined the results with ice
velocity data from satellites, ice shelf thickness measurements from
NASA's Operation IceBridge -- an continuing aerial survey of Earth's
poles -- and a new map of Antarctica's bedrock.
Using this information, Rignot and colleagues were able to deduce
whether the ice shelf was losing mass through basal melting or
gaining it through the basal freezing of seawater.

In some places, basal melt exceeds iceberg calving. In other places,
the opposite is true. But in total, Antarctic ice shelves lost 2,921
trillion pounds (1,325 trillion kilograms) of ice per year in
2003-2008 through basal melt, while iceberg formation accounted for
2,400 trillion pounds (1,089 trillion kilograms) of mass loss each
year.

Basal melt can have a greater impact on ocean circulation than glacier
calving. Icebergs slowly release melt water as they drift away from
the continent. But strong melting near deep grounding lines, where
glaciers lose their grip on the seafloor and start floating as ice
shelves, discharges large quantities of fresher, lighter water near
the Antarctic coast line. This lower-density water does not mix and
sink as readily as colder, saltier water, and may be changing the
rate of bottom water renewal.

"Changes in basal melting are helping to change the properties of
Antarctic bottom water, which is one component of the ocean's
overturning circulation," said author Stan Jacobs, an oceanographer
at Columbia University's Lamont-Doherty Earth Observatory in
Palisades, N.Y. "In some areas it also impacts ecosystems by driving
coastal upwelling, which brings up micronutrients like iron that fuel
persistent plankton blooms in the summer."

The study found basal melting is distributed unevenly around the
continent. The three giant ice shelves of Ross, Filchner and Ronne,
which make up two-thirds of the total Antarctic ice shelf area,
accounted for only 15 percent of basal melting. Meanwhile, fewer than
a dozen small ice shelves floating on "warm" waters (seawater only a
few degrees above the freezing point) produced half of the total melt
water during the same period. The scientists detected a similar high
rate of basal melting under six small ice shelves along East
Antarctica, a region not as well known because of a scarcity of
measurements.

The researchers also compared the rates at which the ice shelves are
shedding ice to the speed at which the continent itself is losing
mass and found that, on average, ice shelves lost mass twice as fast
as the Antarctic ice sheet did during the study period.

"Ice shelf melt doesn't necessarily mean an ice shelf is decaying; it
can be compensated by the ice flow from the continent," Rignot said.
"But in a number of places around Antarctica, ice shelves are melting
too fast, and a consequence of that is glaciers and the entire
continent are changing as well."

For images related to this release, please visit:

http://go.nasa.gov/175OAkF


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