Greenland ice melting 7 per cent faster than thought: study

Washington: Greenland lost nearly 2,700 gigatonnes of ice from 2003-2013, 7.6 per cent more than previously thought, a new study has found.

According to researchers from Ohio State University in the US, the hotspot that feeds Iceland's active volcanoes has 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
billion metric tonnes or 20 gigatonnes per year.

That means Greenland did not lose about 2,500 gigatonnes of ice from 2003-2013 as scientists previously thought, but nearly 2,700 gigatonnes instead which is 7.6 per cent more, said Michael Bevis, professor st the Ohio State University. "It is a fairly modest correction, which does not 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," said Bevis.

The Earth's crust in that part of the world is slowly moving northwest, and 40 million years ago, parts of Greenland passed over an especially hot column of partially molten rock that now lies beneath Iceland, he said. 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.

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 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 said. 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. The 7.6 per cent 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, researchers said. The study appears in the journal Science Advances.