Environment

Ice in Greenland Melts 7 Percent Faster Than Previously Thought

Ice-Melting

Scientists are trying to measure how much ice is melting on Greenland due to the volcano activity in Iceland. The hotspot in Earth’s mantle that feeds Iceland’s active volcanoes has unexpected side effects that has distorted their calculations for ice loss in the Greenland ice sheet.

In a study published in the journal Science Advances, co-author Michael Bevis of The Ohio State University explains that the hotspot softened the mantle rock beneath Greenland in such a way that scientists underestimated the melting by about 20 billion metric tons (20 gigatons) per year.

Looking at the calculations made for the 10-year period from 2003 to 2013, this adds up to a 7.6 percent difference, with Greenland losing nearly 2,700 gigatons of ice instead of the about 2,500 gigatons previously calculated.

Bevis, an Ohio Eminent Scholar in Geodynamics, professor of earth sciences at Ohio State and leader of GNET, the Greenland GPS Network, described this as a fairly modest correction and adds that the estimates of the total mass loss all over Greenland has not changed by much. What is however more significant is that scientists’ understanding of where within the ice sheet that loss has happened, and where it is happening now, has changed.

This is a GPS unit (one of more than 50) that researchers have planted in the Greenland bedrock to measure ice loss. Image credits: Ohio State University
This is a GPS unit (one of more than 50) that researchers have planted in the Greenland bedrock to measure ice loss. Image credits: Ohio State University

Earth’s crust in that geographic area is moving northwest slowly, and areas of Greenland moved over an especially hot column of partially molten rock 40 million years ago. That hot column is now beneath Iceland and it softened the rock in its wake. This had the result of reducing the viscosity of the mantle rocks along a path that runs deep below the surface of Greenland’s east coast.

Greenland’s ice sheet was much bigger during the last ice age than now, and its massive weight caused Greenland’s crust to sink into the pliable mantle rock below slowly. The weight of the ice sheet became less when huge parts of the ice sheet melted at the end of the ice age. This caused the crust to recover. As mantle rock continues to flow inwards and upwards beneath Greenland, the crust continues to rise.

Bevis explained that the existence of mantle flow underneath Greenland is not a surprise in itself. In 2002, the Gravity Recovery and Climate Experiment (GRACE) satellites began measuring gravity signals around the world. Scientists already knew at that stage that they would have to separate changes in the mass of the overlying ice sheet from the mass flow beneath Earth’s crust.

GRACE cannot tell the difference between ice and rock mass as it only measures mass. A model of all the mass flows within Earth is therefore required to infer the ice mass change from the total mass change. The ice mass change inferred from GRACE will be wrong if the overall inference model is wrong.

Researchers use information about the viscosity of the mantle to create models of this rock flow. Greenland’s close proximity to the Iceland hot spot greatly changed the picture and the original models assuming a typical mantle viscosity no longer hold true.

For the GNET team, the fact that climate change concealed which parts of the ice sheet are most being affected, dwarfs the 7.6 percent discrepancy in overall ice loss. The latest results reveal that the pattern of modern ice loss is nearly the same as what is has been since the end of the last ice age.

Bevis adds that this result is an important detail. Scientists are gradually increasing their understanding of ice loss processes by refining the spatial pattern of mass loss in the world’s most unstable and second largest ice sheet. Learning how that pattern has evolved will lead to projections of sea level rise that are more accurate.

GNET’s mission is to improve computer models that can give a good estimate of mantle flow and crustal uplift. This is done by providing direct observations of present day crustal motion. The GNET team includes GPS experts and glaciologists as well as GRACE scientists and earth modelers for this reason.

The same hotspot that feeds Iceland’s active volcanoes has been causing them to undervalue ice loss on Greenland. Image credits: Zachariae Isbrae /Anders A Bjork / The Ohio State University.
The same hotspot that feeds Iceland’s active volcanoes has been causing them to undervalue ice loss on Greenland. Image credits: Zachariae Isbrae /Anders A Bjork / The Ohio State University.

GPS is used to measure uplift in the crust all along Greenland’s coast. This process resulted in the discovery that two neighboring stations on the east coast were rising faster than standard models had predicted.

Bevis notes that the team was shocked when they first saw the anomalous uplift rates at the two stations that sit on the ‘track’ of the Iceland hot spot. The connection made to the hotspot softening the mantle rock beneath Greenland was only made afterwards.

He also believes that this discovery holds big implications for measuring ice loss elsewhere in the world. GNET’s sister network, ANET, spans West Antarctica and it uses a similar number of GPS stations, but these are spread out over a substantially larger area. Bevis warns that unless more stations are added to ANET, anomalous rates of uplift may not be detected and analyses of GRACE data will result in estimates of ice loss in Antarctica not being accurate.

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