An international study, published in Science and lead by the Technical University of Denmark, finds that the bedrock below the West Antarctic Ice Sheet is rising surprisingly fast which reveals an very fluid Earth Mantle. These findings have implications for the loss of ice at Antarctica, as the unexpectedly fast rate of the rising earth may increase the stability of the ice sheet against catastrophic collapse due to ice loss.
At the same time the rapid rise, known as uplift, also affects gravity measurements. This implies that up to 10% more ice has melted off the West Antarctica Ice Sheet (WAIS) than previously assumed.
Leading author of this new study, postdoctoral researcher Valentina R. Barletta at DTU Space, the National Space Institute at DTU, explained:
The results of our work will provide a very important contribution in the understanding of dynamics of the Earth along with the ice melting processes in Antarctica.
The study comes only some days, after a study funded by NASA and the European Space Agency published in the journal Nature, revealed that ice losses from Antarctica have tripled since 2012, increasing global sea levels by 0.12 inch (3 millimeters) in that timeframe alone. The study showed that ice losses from Antarctica are causing sea levels to rise faster today than at any time in the past 25 years.
- Up to 41 millimeters uplift per year
The Amundsen Sea Embayment (ASE) in West Antarctica alone contributes to 25% of all water estimated from melting ice on our planet. To make numbers easier to understand, in one year ASE loses enough to cover an area the size of Denmark (43.000 square km) with a 2.8 meters high layer of ice. ASE also holds enough ice to potentially raise the global sea level by 1.2 meters or to cover an area the size of Denmark with 11 km of ice.
The large amount of water stored in Antarctica has implications for the whole planet, but especially for northern Europe. Because of a combination of gravitational effects, surprisingly, the ice lost in Antarctica mostly raises the sea level here, in northern Europe. In contrast, the ice lost in Greenland has no effect here, but it raises the sea level in the southern hemisphere and further destabilizes the WAIS.
The uplift velocity in ASE was measured at up to 41 millimeters per year. Abbas Khan, one of the coauthors and associate professor at DTU Space, added:
This is one of the fastest rates of uplift ever recorded in glaciated areas. In comparison, the GPS stations installed nearby the Greenland ice streams record up to 30 millimeters per year, but we know that it is caused by an immediate elastic rebound of the earth, acting like a spring.
Participating researchers led by scientists at the Ohio State University (OSU) installed a series of GPS stations on rock outcrops around the ASE to measure its rise in response to thinning ice. Terry Wilson, one of the leaders of the study and professor emeritus of Earth Sciences at OSU, noted:
The rapid rise of the bedrock in this part of Antarctica suggests that the geology underneath the ice is different from what scientists had previously believed. The rate of uplift we found is unusual and very surprising. It’s a game changer.
- When the ice melts the earth adjusts
Under the massive weight of ice the earth subsides, explains Valentina Barletta:
When the ice melts and gets thinner, the earth readjusts, and rises immediately by a few millimeters, which depends on the ice lost. But the earth also acts a bit like a very hard memory-foam mattress. And it slowly keeps readjusting for several thousand years after the melting. In Scandinavia the bedrock is still rising about 10 millimeters per year because of the last ice age.
Scientists call this delayed readjustment Glacial Isostatic Adjustment (GIA), which can also be described as the Earth retaining memory of the ice lost in the past. How fast this readjustment takes place, depends on the properties of the mantle, the portion inside the earth between the crust and the core that is 2900 kilometers thick.
- Hidden ice loss of up to 10%
Valentina R. Barletta has run thousands of GIA simulations using different possible combinations of Earth properties and ice loss scenarios and found that the only way to produce such high uplift is for the Earth mantle to be very fluid (technically low viscosity). These advanced earth simulations are very different from usual ones. To guarantee the reliability of the results under those conditions it was necessary to dig deep into the theory, and also review the codes to prove their correctness and stability without sacrificing the efficiency, essential for processing such an unprecedentedly large variety of simulations. This work was done in close collaboration with DTU Computing Center, where all the simulations were run.
Normally we would see significant uplift happen slowly over thousands of years after the ice age, but here we see it take place in centuries or even decades. This tells us that the mantle below is very fluid and moves quickly when the weight of the ice is taken off.
And the uplift is getting faster. According to this new study, in 100 years, the uplift rates at the GPS sites will be 2.5 to 3.5 times more rapid than currently observed.
Among the direct consequences of this study there is a revised estimate of the ice loss in ASE. When a massive amount of the ice melts it reduces the local gravity and leaves fingerprints that can be measured by satellites and used to estimate the total mass lost. But the earth’s readjustment also produces a gravity change that partially compensates for this loss and hides the ice signal.
Now we know that in ASE the earth readjusts so fast that 10 percent of the ice loss was hidden, but now we can fix that.
- The earth uplift also works as a feedback
The fast earth response is potential good news for the future of the WAIS. In this area of Antarctica, most of the ice is grounded below sea level, and therefore vulnerable to melting from below by ocean water flowing in underneath the ice sheet. Here the earth uplift works as a feedback that can slow down the ice retreat in different ways.
The uplift raises the so called ‘pinning points’ – elevated features that pin the ice sheet to the bedrock, preventing the retreat of the grounded ice. At the same time the uplift changes the inland slope of the ground that becomes more effective in holding the ice from sliding away.
In addition, the massive amount of the ice lost reduces the local gravity and the gravitational pull on the water, resulting in a lower sea level at the adjacent Antarctic coast. This in turn reduces the buoyancy of the whole ice sheet, promoting the stability of the ice sheet.
Modeling studies have shown that bedrock uplift could theoretically protect WAIS from collapse, at least in case of moderate climate changes. But it was believed that the process would take too long to have practical effects. Terry Wilson added:
The mantle that we discovered under ASE is more fluid and hotter, and therefore the earth uplifts faster than in the most optimistic hypotheses used in previous studies. Under many realistic climate models, this should be enough to stabilize the ice sheet.
But, if future global warming is too extreme, according to the scientists the WAIS will most likely still collapse regardless of stabilizing feedbacks.
Apart from giving us a new picture of the earth dynamics in Antarctica, the new findings will push to improve ice models for WAIS to get a more precise picture of what will happen in the future. They also tell that we clearly need to improve our knowledge of the Earth structure under the whole Antarctic continent. To do so we use GPS in the few areas where they can be installed, and elsewhere we use data from ESA’s Earth Explorer GOCE and seismic tomography.
The research is supported by the European Space Agency, ESA, via project GOCE+Antarctica, the US National Science Foundation Office of Polar Programs Antarctic Earth Sciences Program and the US Antarctic Program.
