Warm Ocean Causing Most Antarctic Ice Shelf Mass Loss

MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109 PHONE 818-354-5011
http://www.jpl.nasa.gov

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

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

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

News release: 2013-202 June 13, 2013

Warm Ocean Causing Most Antarctic Ice Shelf Mass Loss

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.php?release=2013-202&cid=release_2013-202

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 -- a 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 to 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 coastline. 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."

Imagery related to this release is online at:
http://www.nasa.gov/topics/earth/features/earth20130613.html

-end-

To remove yourself from this mailing, please go to http://www.kintera.org/TR.asp?a=9gIOK2PzGeJVK7NPE&s=jtK0L4NJJfLMK3OPLtH&m=etIULbONLiJVIhK

To remove yourself from all mailings from NASA Jet Propulsion Laboratory, please go to http://www.kintera.org/TR.asp?a=7eIKKWOrHdLJLJMyA&s=jtK0L4NJJfLMK3OPLtH&m=etIULbONLiJVIhK