Greenland didn't shed about 2,500
gigatons of ice between 2003 and 2013, as satellite
measurements had indicated, but almost 2,7000 gigatons
instead(Credit:
Ohio State
Today, an ultra-hot column of partial molten rock
lies beneath Iceland and feeds the country's
dramatic volcanic landscape, but it wasn't always
this way. Due to the slow-shifting of Earth's crust in
the region, this same hotspot sat beneath Greenland
millions of years ago. And when it did, a new study has
found, it softened the mantle rock in a way that has
recently come to fool scientists trying to gauge ice
loss in the area. So much so, that it has been losing
around 20 gigatons per year more than we previously
thought.
Because gravity is related to mass, scientists are
able to
use satellites to map the Earth's gravitational
field and make assessments on the overall structure of
the planet. As some surface features contain more mass
than others, the resulting variations in gravity can
allow them to measure things like water stored below and
above the ground, mass within the Earth and melting ice
sheets.
But the thing is, this method doesn't quite paint a
complete picture. It will tell scientists if there is a
loss in mass but not whether that mass was made up of
land, ice, or a combination of the two. So when making a
judgement on the scale of ice melt, they rely on
modeling of rock flow within the Earth to fill in the
gaps.
But new research carried out by an international team
of scientists has revealed that the standard models used
may be a little off, give or take a few billion metric
tons.
The reason for this is that when Greenland passed
over this hotspot 40 million years ago, it softened and
lowered the viscosity of the rock in the mantle beneath
the country's east coast. In the last ice age when
Greenland's ice sheet was a lot bigger than it is now,
it pressed the crust into the softened mantle
underneath.
Then when the Ice Age came to an end, around 11,500
years ago, the decreasing weight of the ice sheet on top
allowed the crust to rebound and mantle rock to flow
inwards and upwards underneath Greenland. This is still
happening today, and as it turns out, at faster rate
than scientists had thought.
By installing 50 GPS units along the coastline, the
team behind the new study set out to measure the lifting
crust and they found that two particular locations were
rising a lot quicker than models had predicted.
"We did not expect to see the anomalous uplift rates
at the two stations that sit on the 'track' of the
Iceland hot spot," says study co-author Michael Bevis.
"We were shocked when we first saw them. Only afterwards
did we make the connection."
The upshot of this discovery is that what researchers
once thought was ice is really rock, so there's less ice
around than previously thought. The new calculations
show that Greenland didn't shed about 2,500 gigatons of
ice between 2003 and 2013, as satellite measurements had
indicated, but almost 2,700 gigatons instead, a
difference of 7.6 percent. That sounds like a lot, but
Bevis puts it into perspective.
"It's a fairly modest correction," he says. "It
doesn't change our estimates of the total mass loss all
over Greenland by that much, but it brings a more
significant change to our understanding of where within
the ice sheet that loss has happened, and where it is
happening now."
More important than the 7.6 percent underestimation,
the team says, is that they now have a clear picture of
which parts of the ice sheet are most feeling the brunt
of climate change, and in turn, how this will influence
rising sea levels. It says that the findings will have
ramifications for how ice loss is measured elsewhere in
the world.
"This result is a detail, but it is an important
detail," Bevis says. "By refining the spatial pattern of
mass loss in the world's second largest — and most
unstable — ice sheet, and learning how that pattern has
evolved, we are steadily increasing our understanding of
ice- loss processes, which will lead to better-informed
projections of sea level rise."