During the last ice age, Greenland's ice sheet was much larger than
now, and its enormous weight caused Greenland's crust to slowly sink
into the softened mantle rock below. When large parts of the ice sheet
melted at the end of the ice age, the weight of the ice sheet decreased,
and the crust began to rebound. It is still rising, as mantle rock
continues to flow inwards and upwards beneath Greenland.
The same hotspot in Earth's mantle that feeds Iceland's active
volcanoes has been playing a trick on the scientists who are trying to
measure how much ice is melting on nearby Greenland.
According to a new study in the journal Science Advances
hotspot softened the mantle rock beneath Greenland in a way that
ultimately distorted their calculations for ice loss in the Greenland
ice sheet. This caused them to underestimate the melting by about 20
gigatons (20 billion metric tons) per year.
‘The hotspot in Earth's mantle that feeds Iceland's active volcanoes has ultimately distorted calculations for ice loss in the Greenland ice sheet.’
That means Greenland did not lose about 2,500 gigatons of ice from
2003-2013 as scientists previously thought, but nearly 2,700 gigatons
instead - a 7.6% difference, said study co-author Michael Bevis of
The Ohio State University.
"It's a fairly modest correction," said Bevis, the Ohio Eminent
Scholar in Geodynamics, professor of earth sciences at Ohio State and
leader of GNET, the Greenland GPS Network.
"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."
The Earth's crust in that part of the world is slowly moving
northwest, he explained, and 40 million years ago, parts of Greenland
passed over an especially hot column of partially molten rock that now
lies beneath Iceland. The hotspot softened the rock in its wake,
lowering the viscosity of the mantle rocks along a path running deep
below the surface of Greenland's east coast.
The existence of mantle flow beneath Greenland is not a surprise in
itself, Bevis said. When the Gravity Recovery and Climate Experiment
(GRACE) satellites began measuring gravity signals around the world in
2002, scientists knew they would have to separate mass flow beneath the
earth's crust from changes in the mass of the overlying ice sheet.
"GRACE measures mass, period. It cannot tell the difference between
ice mass and rock mass. So, inferring the ice mass change from the total
mass change requires a model of all the mass flows within the earth. If
that model is wrong, so is the ice mass change inferred from GRACE," he
Models of this rock flow depend on what researchers can glean about
the viscosity of the mantle. The original models assumed a fairly
typical mantle viscosity, but Greenland's close encounter with the
Iceland hot spot greatly changed the picture.
To the GNET team, the 7.6% discrepancy in overall ice loss is
overshadowed by the fact that it concealed which parts of the ice sheet
are most being affected by climate change. The new results reveal that
the pattern of modern ice loss is similar to that which has prevailed
since the end of the last ice age.
"This result is a detail, but it is an important detail," Bevis
continued. "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
Computer models can give a good estimate of mantle flow and crustal
uplift, he said, and GNET's mission is to make those models better by
providing direct observations of present-day crustal motion. That's why
the GNET team includes GRACE scientists and earth modelers as well as
GPS experts and glaciologists.
The team used GPS to measure uplift in the crust all along
Greenland's coast. That's when they discovered that two neighboring
stations on the east coast were uplifting far more rapidly than standard
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," Bevis said.
"We were shocked when we first saw them. Only afterwards did we make the
He added that the discovery holds big implications for measuring ice loss elsewhere in the world.
For instance, GNET has a sister network, ANET, that spans West
Antarctica. It employs roughly similar numbers of GPS stations, but
spread out over a vastly larger area. Unless more stations are added to
ANET, anomalous rates of uplift may go undetected, Bevis cautioned, and
analyses of GRACE data will lead to inaccurate estimates of ice loss in