A new study based on GPS measurements of the Earth's crust suggests the Greenland ice sheet is melting about seven per cent faster than previously believed and may contribute more to future sea level rise than predicted.
"We've underestimated the rate of ice loss by about 7.6 per cent," said Michael Bevis of The Ohio State University, one of the study's co-authors.
The research found that Greenland did not lose about 2,500 gigatons of ice from 2003-2013 as scientists previously believed, but closer to 2,700 gigatons — a difference Bevis calculates at 7.6 percent.
The study, published in the journal Science Advances, is an international effort that started in 2007, with contributions from the U.S., Denmark and Luxembourg.
Over the past two decades the Greenland ice sheet has been shrinking — partly due to accelerated glacier flow and partly because of surface melt.
However, scientists have not been able to pinpoint exactly how much the melting ice sheet is contributing to global sea level rise — information key to making predictions about future sea rise levels.
Part of the challenge has been a lack of on-site data.
Quantifying the uplift rate
For this study, teams of scientists spent years installing GPS devices around the entire perimeter of the Greenland ice sheet.
The researchers wanted to separate two different kinds of movement. The first is elastic motion caused by an uplift or bouncing of the bedrock underneath the ice sheet due to recent ice loss.
"The Earth is a big elastic, like a spring: you put something heavy on it, it bends down. If that load decreases because it's melting away, then this upper spring rises," said Bevis.
The other movement comes from a delayed response to the ice losses since the Last Glacial Maximum — 20,000 years ago — in a process called viscous rebound.
"This is the first time we've been able to separate which part is elastic and which part is this viscous rebound," said Bevis.
Hotspots distorting data
"What we found was really surprising," said Bevis.
The team discovered that the hotspot in the Earth's mantle that feeds Iceland's active volcanoes has been distorting data.
Forty million years ago, parts of Greenland passed over a particularly hot column of partially molten rock that now lies beneath Iceland — a result of the Earth's crust in that region slowly moving northwest.
The hotspot softened the rock, lowering the viscosity of the mantle along a path running deep below the surface of Greenland's east coast.
During the last ice age, the massive weight from Greenland's ice sheet caused the crust underneath to slowly sink into the softened mantle rock.
As that ice age came to an end, the crust began to lift. That uplift motion is still continuing as the mantle rock flows (slowly) inwards and upwards beneath Greenland.
This kind of phenomenon has been seen before, explains Bevis, in places, like Patagonia in South America, that are tectonically active.
"This one is a bit more of a surprise because it was 40 million years ago that that hotspot passed under the eastern margin of Greenland and it's still got this anomalous structure," said Bevis.
The findings, he said, show that scientists must pay much more attention to the movement of rock masses in order to prevent confusing rock mass changes with ice mass changes.
Projections for the future
William Colgan, from York University's Lassonde School of Engineering, studies the Greenland ice melt and its effects on the ocean.
"We care about how much water Greenland added to the ocean coming out of the last glaciation," he said.
"We like to assess our projections for the future based against what we know about the past behaviour of the ice sheet."
The new study estimates the Greenland ice sheet was almost 44 per cent larger during the last glaciation than previously believed.
"That's a big difference," said Colgan.
"It means instead of Greenland contributing something like 3.5 metres of sea level to the world's oceans since the peak of the last glaciation, it may have in fact contributed more like 4.6 metres of global sea level rise."