In the first
comprehensive satellite study of its kind, a University of Colorado at
Boulder-led team used NASA data to calculate how much Earth's melting
land ice is adding to global sea level rise.
Using satellite measurements from the NASA/German Aerospace Center
Gravity Recovery and Climate Experiment (GRACE), the researchers
measured ice loss in all of Earth's land ice between 2003 and 2010, with
particular emphasis on glaciers and ice caps outside of Greenland and
Antarctica.
The total global ice mass lost from Greenland, Antarctica and Earth's
glaciers and ice caps during the study period was about 4.3 trillion
tons (1,000 cubic miles), adding about 0.5 inches (12 millimeters) to
global sea level. That's enough ice to cover the United States 1.5 feet
(0.5 meters) deep.
"Earth is losing a huge amount of ice to the ocean annually, and
these new results will help us answer important questions in terms of
both sea rise and how the planet's cold regions are responding to global
change," said University of Colorado Boulder physics professor John
Wahr, who helped lead the study. "The strength of GRACE is it sees all
the mass in the system, even though its resolution is not high enough to
allow us to determine separate contributions from each individual
glacier."
About a quarter of the average annual ice loss came from glaciers and
ice caps outside of Greenland and Antarctica (roughly 148 billion tons,
or 39 cubic miles). Ice loss from Greenland and Antarctica and their
peripheral ice caps and glaciers averaged 385 billion tons (100 cubic
miles) a year. Results of the study will be published online Feb. 8 in
the journal Nature.
Traditional estimates of Earth's ice caps and glaciers have been made
using ground measurements from relatively few glaciers to infer what
all the world's unmonitored glaciers were doing. Only a few hundred of
the roughly 200,000 glaciers worldwide have been monitored for longer
than a decade.
One unexpected study result from GRACE was that the estimated ice
loss from high Asian mountain ranges like the Himalaya, the Pamir and
the Tien Shan was only about 4 billion tons of ice annually. Some
previous ground-based estimates of ice loss in these high Asian
mountains have ranged up to 50 billion tons annually.
"The GRACE results in this region really were a surprise," said Wahr,
who is also a fellow at the University of Colorado-headquartered
Cooperative Institute for Research in Environmental Sciences. "One
possible explanation is that previous estimates were based on
measurements taken primarily from some of the lower, more accessible
glaciers in Asia and extrapolated to infer the behavior of higher
glaciers. But unlike the lower glaciers, most of the high glaciers are
located in very cold environments and require greater amounts of
atmospheric warming before local temperatures rise enough to cause
significant melting. This makes it difficult to use low-elevation,
ground-based measurements to estimate results from the entire system."
"This study finds that the world's small glaciers and ice caps in
places like Alaska, South America and the Himalayas contribute about
0.02 inches per year to sea level rise," said Tom Wagner, cryosphere
program scientist at NASA Headquarters in Washington. "While this is
lower than previous estimates, it confirms that ice is being lost from
around the globe, with just a few areas in precarious balance. The
results sharpen our view of land-ice melting, which poses the biggest,
most threatening factor in future sea level rise."
The twin GRACE satellites track changes in Earth's gravity field by
noting minute changes in gravitational pull caused by regional
variations in Earth's mass, which for periods of months to years is
typically because of movements of water on Earth's surface. It does this
by measuring changes in the distance between its two identical
spacecraft to one-hundredth the width of a human hair.
The GRACE spacecraft, developed by NASA's Jet Propulsion Laboratory,
Pasadena, Calif., and launched in 2002, are in the same orbit
approximately 137 miles (220 kilometers) apart. The California
Institute of Technology manages JPL for NASA.
