Science News

Good News for Antarctica: Rising Bedrock Could Slow Melting of Ice Sheet

The earth is rising in one part of Antarctica at one of the fastest rates ever recorded, as ice rapidly disappears and weight is lifted off the bedrock, a new international study has found.

The findings, reported in the journal Science, have surprising and positive implications for the survival of the West Antarctic Ice Sheet (WAIS). The unexpectedly fast rate of the rising earth may markedly increase the stability of the ice sheet against catastrophic collapse due to ice loss, scientists say.

Scientists have been concerned that this ice sheet is particularly precarious in the face of a warming climate and ocean currents, because it is grounded hundreds to thousands of feet below sea level -- unlike the ice sheets of East Antarctica or Greenland -- and its base slopes inland. This bowl-like topography makes it susceptible to runaway destabilization and even complete collapse over centuries to thousands of years.

"We studied a surprising and important mechanism that may slow the demise of the massive WAIS by lifting up the bedrock and sediments beneath the ice sheet," explained CSU Professor Rick Aster, a co-author of the study and head of the Department of Geosciences at the university.

The entirety of West Antarctica contains enough ice that, if it were to melt, it would contribute more than ten feet of average global sea level rise. Furthermore, the ice sheet is so massive that it gravitationally attracts an ocean bulge that, if released, would lead to an additional increase of three feet or more to sea level in parts of the northern hemisphere.

The WAIS is currently contributing approximately 25 % of global melting land-based ice each year, and recent satellite-based studies have shown that this amount has increased in recent decades. This figure is equivalent to about 37 cubic miles or a cube of over three miles on each side. As the rapid rise of the earth in this area also affects gravity measurements, scientists believe that up to 10 % more ice has disappeared in this part of Antarctica than previously assumed.

EMBED 173219 webTerry Wilson, professor emeritus of earth sciences at The Ohio State University, assembles a GPS antenna on a monument fixed to bedrock. This GPS unit is at Westhaven Nunatak in the Transantarctic Mountains. CREDIT: The Polar Earth Observing Network

Modeling Ice Sheet Melt and Bedrock Uplift

Co-authors of the study based at The Ohio State University led the installation of six sensitive GPS stations (part of the POLENET-ANET array) on rock outcrops around the remote region to measure the regional uplift of the Earth in response to the thinning ice sheet. Measurements showed that the bedrock uplift rates near the coast of West Antarctica were as high as 1.6 inches per year, one of the fastest rates ever recorded in glacial areas. In contrast, places like Iceland and Alaska, which have what are considered rapid uplift rates, generally are measured rising 20 to 30 millimeters a year.

"This very rapid uplift may slow the runaway wasting and eventual collapse of the ice sheet," said Aster. "The uplift tends to stabilize the critical grounding line where the ice sheet loses contact with underlying bedrock or sediment and goes afloat." This grounding line then counteracts the process of the ice sheet collapsing.

Researchers also found that the uplift is accelerating and predicted that it will continue to do so into the next century. It is estimate that in 100 years, uplift rates at the GPS sites will be 2.5 to 3.5 times more rapid than currently observed.

"Our research indicates that recent and ongoing ice loss in the region has been underestimated by approximately 10 % in past studies, because this bedrock uplift was inadequately accounted for in satellite measurements," said Aster.

Terry Wilson, professor emeritus of Earth Sciences at The Ohio State University, said the rapid rise of the bedrock in this part of Antarctica suggests that the geological conditions beneath the ice are very different from what scientists had previously believed. While modeling studies have shown that bedrock uplift could theoretically protect WAIS from collapse, it was believed that the process would take too long to have practical effects.

"The rate of uplift we found is unusual and very surprising. It's a game changer. We previously thought uplift would occur over thousands of years at a very slow rate, not enough to have a stabilizing effect on the ice sheet. Our results suggest the stabilizing effect may only take decades," Wilson said.

Researchers have also deployed a large network of sensitive seismographs across Antarctica to produce seismic tomographic images -- analogous to a gigantic CAT scan -- of the deep Earth below Antarctica. This work assisted in the interpretation and modeling of the GPS uplift data by providing detailed mapping of a vast region of Earth's mantle up to 400 miles below West Antarctica.

These new measurements of Glacial Isostatic Adjustment (GIA), the scientific term for uplift due to ice sheet unloading, are an important part of a wider story about the fate of the Antarctic ice sheets, said Doug Kowalewski, the Antarctic Earth Sciences program director in the National Science Foundation's Office of Polar Programs (OPP).

"The observed GIA response captured by the POLENET array is an order of magnitude greater than previously thought. The upcoming challenge is to couple the GIA observations with ice-sheet models," Kowalewski said. "These data will be of great value to the modeling community who examine the complex relationships between GIA, sub-ice shelf ocean circulation, and ultimately, ice sheet stability."

"These results provide an important contribution to our understanding of the dynamics of the Earth's bedrock, along with the thinning of ice in Antarctica. The large amount of water stored in Antarctica has implications for the whole planet," said lead study author Valentina R. Barletta, who started this work at Ohio State and now is a postdoctoral researcher at the National Space Institute (DTU Space) at the Technical University of Denmark. "The new findings raise the need to improve ice models to get a more precise picture of what will happen in the future."

Rare Good News for Antarctica

Underneath the solid upper layer of Earth is a hotter and more fluid layer of rock called the mantle. Exactly how hot and fluid the mantle is varies across the planet.

The rapid uplift around the Amundsen Sea Embayment suggests that the mantle in this area is hotter and more fluid (or it has lower viscosity) than expected, according to Barletta. That would mean the bedrock reacts more quickly when the weight of ice is removed.

The biggest practical effect of the uplift may be a rare bit of good news for what is happening in this part of Antarctica as a result of climate change. Some scientists suggest that WAIS may have passed a tipping point in which ice loss can no longer be stopped, which could be catastrophic, Wilson said.

As much of this area of Antarctica is below sea level, relatively warm ocean water has flowed in underneath the bottom of the ice sheet, causing thinning and moving the grounding line - where the water, ice and solid earth meet - further inland.

The process seemed unstoppable, explains Wilson. "But we found feedbacks that could slow or even stop the process."

One important feedback involves "pinning points" - elevated features of the earth rising from the surface below the grounding line that pin the ice sheet to solid earth. These pinning points are going up in response to the uplift of the earth and could prevent further retreat of the ice sheet.

Another feedback is lowering sea levels. Massive ice sheets along the ocean have their own gravitational pull and raise the sea level near them. But as the ice thins and retreats, the gravitational pull lessens and the sea level near the coast goes down.

"The lowering of the sea level, the rising of pinning points and the decrease of the inland slope due to the uplift of the bedrock are all feedbacks that can stabilize the ice sheet," Wilson said.

Other researchers had estimated how much the earth would have to rise to protect WAIS given a range of future climate warming scenarios. Results of this study estimate that the bedrock at the Pine Island Glacier grounding line (which is part of WAIS) will have risen about 8 meters in 100 years. That is about three times higher than values shown by others to reduce run-away retreat in this area.

"Under many realistic climate models, this should be enough to stabilize the ice sheet," Wilson said. Adding that while this study delivers some potentially good news for the Amundsen Sea Embayment, that doesn't mean all is well in Antarctica. "The physical geography of Antarctica is very complex. We found some potentially positive feedbacks in this area, but other areas could be different and have negative feedbacks instead."

Aster concludes that while the research does provide room for a positive outcome, if future global warming is extreme, the entire WAIS will still melt. "To keep global sea levels from rising more than a few feet during this century and beyond, we must still limit greenhouse gas concentrations in the atmosphere, which can only occur through international cooperation and innovation," he said. 

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