Washington Post - Chelsea Harvey
Greenland ice loss has recently contributed to twice as much sea-level rise than in the preceding two decades.
(Reuters)
It’s no news that Greenland is in serious trouble — but now, new research has helped quantify just how bad its problems are. A satellite study,
published last week in the journal Geophysical Research Letters,
suggests that the Greenland ice sheet lost a whopping 1 trillion tons of
ice between the years 2011 and 2014 alone. And a big portion of it came
from just five glaciers, about which scientists now have more cause to
worry than ever.
It’s
the latest story in a long series of increasingly worrisome studies on
ice loss in Greenland. Research already suggests that the ice sheet has
lost at least 9 trillion tons of ice
in the past century and that the rate of loss has increased over time.
Climate scientists are keeping a close eye on the region because of its
potentially huge contributions to future sea-level rise (around 20 feet
if the whole thing were to melt) — not to mention the damage it’s
already done. Ice loss from Greenland may have contributed as much as a
full inch of sea-level rise in the last 100 years and up to 10 percent
of all the sea-level rise that’s been documented since the 1990s.
The new study takes a detailed look at ice loss in Greenland between 2011 and 2014 using measurements from the CryoSat-2,
an environmental research satellite launched by the European Space
Agency in 2010. It relied on a type of measurement known as altimetry
— basically, measuring how the surface of Greenland’s altitude changed
over time in response to ice gains or losses.
“Simplistically, if the ice sheet’s going up, we can find that as evidence that the ice sheet is growing,” said lead author Malcolm McMillan,
a research fellow at the University of Leeds. “And where we see that
the ice sheet surface is lowering, we can find that the ice sheet is
losing ice.”
But he
cautioned that this is something of a simplification. The researchers
also had to consider how other factors such as snowfall — which would be
difficult to differentiate by satellite — might be affecting changes on
the surface of the ice sheet.
“Snow
and ice are at different densities, so they’re associated with a
different amount of mass loss,” McMillan explained. “We used a regional
climate model and a model of the surface of the ice sheet to really
inform us and tell us about the nature of the changes that we’re
seeing.”
Using this method
— combining the satellite observations with modeling — the researchers
found that the Greenland ice sheet lost mass at an average rate of about
269 billion tons per year from January 2011 through December 2014.
Altogether, this comes to about 1 trillion tons of ice loss over the
four-year period.
That
said, there were some major fluctuations from one year to the next — an
observation that University of Sheffield climate expert Edward Hanna
(who was not involved in the new study) said is one of the paper’s most
notable findings. The biggest losses were observed in 2012, when an
unusually warm summer helped bring about a loss of more than 400 billion
tons of ice. The next year, 2013, saw a comparatively modest loss of
just over 100 billion tons.
“There’s
not so many studies that do these sort of trend analyses or time
studies for the latest few years,” Hanna noted. “So it’s really trying
to assess how the ice sheet is responding to ongoing climate variability
or change.”
Overall, the
ice loss was particularly prevalent in the southwest, but the scientists
noted that there were also losses observed in the cooler, northern
parts of the ice sheet. Notably, the researchers also found that a solid
12 percent of all the ice loss came from just a handful of glaciers
composing less than 1 percent of the ice sheet’s total area.
Each
of these five glaciers flows outward into the sea, so that a
combination of both rising air temperatures and ocean temperatures
likely play a part in their ongoing retreat. Among these was the iconic
Jakobshavn glacier, a well-studied location now famous for its recent
massive ice losses. It’s been known to calve blocks of ice boasting
several square miles in surface area, as measured from above.
Scientists
were already fairly well aware of the massive losses being suffered by
these glaciers, McMillan acknowledged. But the finding helps reinforce
previous observations and drive home their disproportionate role in the
ice sheet’s contributions to sea-level rise. “Also … it means that going
forward, we’re able to kind of develop long-term and systematic records
that we can [use to] regularly monitor these glaciers and see how
they’re changing into the future,” he said.
In
fact, the study’s results match up reasonably well with measurements
taken by certain other satellites. After doing some comparisons, the
researchers found that data from NASA’s GRACE satellites,
for instance, suggest that Greenland is losing ice at a rate of about
287 billions tons per year. And according to Hanna, the results stand
well with scientists’ overall estimates of recent ice loss in Greenland,
which he says are consistently suggested to be around 250 billion tons
annually for the past few years.
In
this way, the study reinforces many beliefs that were already widely
held about Greenland’s precarious condition. But the techniques used to
do so may strengthen future measurements, which will be used to inform
the climate models that help scientists make predictions about how the
ice sheet will behave in the future — a crucial step in determining the
amount of sea-level rise we might expect over any given time period.
“I
guess the most significant or the most novel aspect of the study is
really the resolution or the detail that we’re able to measure,”
McMillan said. “Although satellite techniques give us a holistic view of
how the ice sheet as a whole is changing, what we’re able to do by
using this specific technique is identify specific regions that are
changing. And that’s really important because it kind of gives us more
of an idea of the processes that are causing the changes.”
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