Thwaites Glacier, the large, rapidly changing outlet of the West Antarctic Ice Sheet, is not only being eroded by the warming ocean, it’s also being melted from below by geothermal heat, researchers report.
The glacier is on the way to collapsing, but more data and computer modeling are needed to determine when the collapse will begin in earnest and at what rate the sea level will increase as it proceeds, scientists say.
Using radar techniques to map how water flows under ice sheets, researchers were able to estimate ice melting rates and then identify that significant sources of geothermal heat under the glacier are distributed over a wider area—and are much hotter than previously assumed.
The geothermal heat contributes significantly to melting of the underside of the glacier, and might be a key factor in allowing the ice sheet to slide, affecting its stability and its contribution to future sea level rise.
The cause of the variable distribution of heat beneath the glacier is thought to be the movement of magma and associated volcanic activity arising from the rifting of the Earth’s crust beneath the West Antarctic Ice Sheet.
MOST COMPLEX THERMAL ENVIRONMENT
Knowing the pattern of heat distribution beneath the glacier is crucial information that enables ice sheet modelers to more accurately predict the response of the glacier to the presence of a warming ocean.
Until now, scientists have been unable to measure the strength or location of heat flow under the glacier. Current ice sheet models have assumed that heat flow under the glacier is uniform like a pancake griddle with even heat distribution across the bottom of the ice.
But the new findings, published in the Proceedings of the National Academy of Sciences, show that the glacier sits on something more like a multi-burner stovetop with burners putting out heat at different levels at different locations.
“It’s the most complex thermal environment you might imagine,” says coauthor Don Blankenship, a senior research scientist at University of Texas Institute of Geophysics. “And then you plop the most critical dynamically unstable ice sheet on planet Earth in the middle of this thing, and then you try to model it. It’s virtually impossible.”
RISE IN GLOBAL SEA LEVELS
That’s why, getting a handle on the distribution of geothermal heat flow under the ice sheet has been considered essential for understanding it, he says.
Gathering knowledge about Thwaites Glacier is crucial to understanding what might happen to the West Antarctic Ice Sheet. An outlet glacier the size of Florida located in the Amundsen Sea Embayment, the glacier is up to 4,000 meters thick and is considered a key question mark in making projections of global sea level rise.
The glacier is retreating in the face of the warming ocean and is thought to be unstable because its interior lies more than two kilometers below sea level while, at the coast, the bottom of the glacier is quite shallow.
Because its interior connects to the vast portion of the West Antarctic Ice Sheet that lies deeply below sea level, the glacier is considered a gateway to the majority of West Antarctica’s potential sea level contribution.
The collapse of the Thwaites Glacier would cause an increase of global sea level of between 1 and 2 meters, with the potential for more than twice that from the entire West Antarctic Ice Sheet.
100 MILLIWATTS PER SQUARE METER
Researchers had previously used ice-penetrating airborne radar sounding data to image two vast interacting subglacial water systems under Thwaites Glacier. The results from this earlier research on water systems formed the foundation for the new work, which used the distribution of water beneath the glacier to determine the levels and locations of heat flow.
In each case, lead author Dusty Schroeder, who received his PhD in May, used techniques he had developed to pull information out of data collected by the radar.
The findings show that the minimum average geothermal heat flow beneath Thwaites Glacier is about 100 milliwatts per square meter, with hotspots over 200 milliwatts per square meter. For comparison, the average heat flow of the Earth’s continents is less than 65 milliwatts per square meter.
The presence of water and heat present significant challenges, Schroeder says.
“The combination of variable subglacial geothermal heat flow and the interacting subglacial water system could threaten the stability of Thwaites Glacier in ways that we never before imagined.”
《Evidence for elevated and spatially variable geothermal flux beneath the West Antarctic Ice Sheet》, Published on Journal 《PNAS》in May 8, 2014.