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Climate: How fast are West Antarctic glaciers melting?

New study once again tabs oceans as main cause for acceleration of Pine Island, Thwaite glaciers meltdown

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What happens in Antarctica doesn’t stay in Antarctica. bberwyn photo.

By Summit Voice

FRISCO — After a long and complex research mission, scientists say they have more evidence that the rapid melting of Antarctica’s Pine Island Glacier is primarily fueled by relatively warm ocean water flowing beneath the floating 37-mile tongue of the glacier all the way to the grounding line. In some places, the ice is melting at the rate of more than 2 inches per year, a jackrabbit pace by geological standards.

Previous research has shown that the current rate of melting is nearly unprecedented, at least in the past 10,000 years. The speed with which the West Antarctic Ice Sheet dwindles has significant implications for global sea level rise.

The international team, led by NASA’s emeritus glaciologist Robert Bindschadler, spent five years just trying to figure out how to adequately explore the glacier, including aborted attempts in 2007 and 2011. In December 2012 the researchers were finally able to place their instruments. The project was funded by NASA and the National Science Foundation and the findings were published Sept. 13 in the journal Science.

The measurements have helped determining the rate at which warm sea water is eating away the ice from underneath the floating portion of the glacier.

“This is the first observation of the actual melt rate underneath the ice shelf,” said Timothy Stanton, an oceanographer at the Naval Postgraduate School in Monterey, Calif., and lead author of the paper. “We have observations using remote sensing of various kinds, but these are actual in situ measurements.”

The measurements also detected differences in melt rates across the channel system that runs underneath the ice shelf, Stanton said. Such features are important for adjusting models so they can accurately predict ice melt and its contribution to sea level rise.

“Our direct measurements are consistent with the larger scale averages that remote sensing data have provided, but our data capture an enormous fine scale variability of the basal melting rate that remote sensing can’t resolve,” Bindschadler said. “Using only the average melt rates would not lead to a correct understanding of the actual ocean-ice interaction processes taking place in the boundary layer.”

Ice shelves buttress seaward glaciers, slowing the speed at which these rivers of ice dump their contents into the sea. If an ice shelf is weakened at its grounding line, the point where the glacier loses its grip on the land and starts floating, it allows the ice to flow faster, which impacts sea level. Pine Island Glacier and its neighbor, Thwaites Glacier, drain a large fraction of the West Antarctic ice shelf and are of great importance to its stability.

More research is planned during the upcoming Austral summer, when researchers with the British Antarctic Survey will take another detailed look at the two glaciers.

The first group of scientists will spend 10 weeks traveling 600 miles across the ice sheet by tractor-traverse, using ground-based radar and seismic technologies to map the bed beneath Pine Island Glacier. The research may help show how  glacier bed conditions affect the flow and thickness of ice all the way from the floating shelf up into its inland tributaries. Satellite remote sensing technology will allow the science team to measure the changing configuration of the glacier in areas that are inaccessible from the ground.

In January 2014, a team will sail into the Amundsen Sea aboard the RRS James Clark Ross to spend 30 days putting a range of instruments and devices into the ocean near Pine Island Glacier to discover when, where, and how warm ocean water gets close to the ice. Ocean measurements and observations are essential for improving a wide variety of computer models used by the international scientific community to forecast future climate and sea level.

Ocean currents

Research shows that melting of the underside of Antarctic ice shelves is ultimately driven by changes in the southernmost atmospheric circulation. Strong westerly winds push the frigid top water layer of the Southern Ocean away from land, which allows deeper, warmer water to raise and spill over the border of the Antarctic continental shelf.

Since the weight of land ice tilts the continental shelf inland, streams of warm water can travel all the way to the ice shelf’s grounding line, where they melt the ice. The resulting warm, fresh melt water rises against the underside of the ice shelf along the length of the ice shelf and carves melt channels that look like inverted river valleys.

Placing instruments at various locations on the glaciers helped the NASA team study the melt rates within these channels and observe the ocean cavity beneath the ice shelf. At their research sites, they drilled through the ice with hot-water drills, then lowered oceanographic instruments to measure the seawater beneath.

On the surface, the scientists left high-resolution radars at different sites for 24 hours, and measured how the sea-ice interface, or the point where water touches the ice shelf’s underbelly, moved as the ice melted.

The radar and oceanographic measurements translated into very similar melt rates: 2.36 inches (6 centimeters) per day, or about 72 feet (22 meters) per year in the middle of the channels, and almost non-existent at their flanks. The authors calculate that melting at the grounding line possibly doubles that higher rate. This would agree with previous estimates of basal melt made by a team led by Eric Rignot, jointly of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and the University of California, Irvine. In 2002, Rignot’s group used satellite radar data and calculated that the warm marine waters were melting Pine Island Glacier’s ice shelf at around 144 feet (44 meters) per year at its grounding line.

For decades, Pine Island Glacier was considered too dangerous and remote to explore, despite its scientific interest. But a careful study of satellite imagery by Bindschadler identified an area where planes could land safely.

“The success of this project shows the strength of marrying satellite data with field data,” Bindschadler said. “The satellite data told us where to go, helped guide us and it told us in broad brush strokes that this part of West Antarctica was changing a lot. But field work was the only way to get these measurements underneath the ice shelf; satellites couldn’t do that for us.”

“In my 35 years doing fairly large oceanographic projects, the Pine Island Glacier one tops it in terms of its complexity and challenge,” Stanton said. “But it’s clear that it’s very important to understand how these massive ice shelves are influenced by changes in the ocean. These observations will provide the basis for improving global climate models.”

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