Researchers have found a way to use global positioning system (GPS) to
measure short-term changes in the rate of ice loss on Greenland—and reveal a
surprising link between the ice and the atmosphere above it.
The study, published online in the Proceedings of the National Academy
of Sciences, hints at the potential for GPS to detect many consequences of
climate change, including ice loss, the uplift of bedrock, changes in air
pressure—and perhaps even sea level rise.
The team, led by earth scientists at Ohio State University, pinpointed a
period in 2010 when high temperatures caused the natural ice flow out to sea to
suddenly accelerate, and 100 billion tons of ice melted away from the continent
in only 6 months.
They were able to make the measurement because the Earth compresses or
expands like a spring depending on the weight above it, letting them use the
Greenland bedrock like a giant bathroom scale to weigh the ice atop it. As ice
accumulates, the bedrock sinks, and as the ice melts away, the bedrock rises.
Measurements revealed that Greenland sank by about 6 mm over the winter of
2010, and the researchers determined that half of the sinking (3 mm) was
actually due to high air pressure above the ice, and the other half was due to
Further, they determined that the bedrock lifted 11 mm over the course the
summer. Air pressure appeared to affect the bedrock less during this time, so
that the bounce back appears to be mostly due to ice loss.
This method has been used to study ice loss before, in Antarctica as well as
Greenland. But previously, GPS could only detect changes over a period of
several years, said project leader Michael Bevis, Ohio Eminent Scholar in
Geodynamics and professor in the School of Earth Sciences at Ohio State.
While shortening the detection time to six months is a substantial advance,
Bevis thinks his team will soon do even better.
“Within the next year or so, we should be able to process the GPS data
within a month of its being collected,” he said, “and then we can
monitor abrupt changes in ice mass only a month or two after they occur.”
The key to the improvement is the network of GPS stations that the
researchers stationed around the Greenland ice sheet. More than 50 transmitters
are planted close enough together to detect changes in uplift along most of the
Greenland coast. These GPS antennas are supported on poles anchored into bare
rock, and so they record the rise of the bedrock itself.
The network is called the Greenland GPS Network (GNET).
GNET’s measurements were so detailed that the researchers were able to
determine what portion of bedrock motion was due to the ice melting away and
what portion was due to seasonal swings in air pressure above the ice.
It’s startling to think that changes in the weight of the air could push
down on the ice with enough force to compress the bedrock below, Bevis said.
Though researchers strongly suspected that this was happening, GNET has
provided the first chance for them to isolate the air pressure signal from the
overall motion of the bedrock.
The “bathroom scale” movement of the bedrock is thus reacting to
the weight of the ice and the weight of the air.
“It surprises most meteorologists that there is such a strong seasonal
signal in surface pressure in Greenland. But it amazes almost everyone to learn
that seasonal changes in air mass push on the bedrock just as strongly as
seasonal changes in ice mass. It is highly counterintuitive, but true!”
They compared GNET measurements to eight years worth of air pressure data,
and were able to see patterns in the rise and fall of the bedrock.
The changes due to the ice and the air aren’t exactly in sync—the air
pressure rises steadily over the spring and drops off slowly over the summer
and fall, while the weight of the ice grows through the spring, drops off
quickly over the summer, and begins to recover in the fall.
The seasonal cycle of bedrock displacement is due to the interplay of those
Now that researchers can isolate the air pressure signal, they can make more
accurate measurements of ice mass. The idea is to calibrate GNET as an
‘ice-weighing machine’ by correlating daily displacements of the GPS stations
with daily changes in surface pressure fields produced by numerical weather
Although this study revealed a dramatic six-month period of melting in
Greenland in 2010, that short-term ice loss isn’t necessarily a sign of a
long-term trend, Bevis cautioned.
“It is dangerous to assume that rates observed over even two or three
years reflect a long-term trend. Rates are known to change. So, it would be
even more dangerous to assume that the record breaking summer of 2010 is the
“That being said, the summer of 2011 was also very hot. And this summer
is starting off hot, too. So, I do expect to see a sustained increase in uplift
rates when we compare 2010-2012 to 2007-2009,” he added.
The researchers are continuing to monitor Greenland. In the meantime, they
are investigating the possibility of detecting changes in sea level rise via
GPS units planted at coastlines and in small ocean islands. Not all mechanisms
of sea level rise produce variations in seafloor pressure, Bevis explained, but
some of them do.
Source: Ohio State University