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SAN FRANCISCO, Nov. 10 (Xinhua) -- A new study has found that the amount of warm ocean water penetrating the undersides of glaciers in West Antarctica and melting them from below has increased significantly since the mid-2000s.
The study, by researchers at Stanford University and published in a recent issue of the journal Nature Communications, identifies what the researchers call stunning rates of ice loss from the glaciers' undersides in the southern continent.
The findings are based on the first direct observations of ice melt from the floating undersides of glaciers and highlights how interactions between warm ocean water and the bedrock beneath a glacier contribute to ice loss and global sea level rise.
The researchers used radar and laser altimetry instruments flown by the U.S. National Aeronautics and Space Administration (NASA) for Operation IceBridge and earlier NASA airborne campaigns to measure the thickness and height of three neighboring glaciers - Smith, Pope, and Kohler - in West Antarctica that are flowing into the Dotson and Crosson ice shelves in the Amundsen Sea Embayment, where the continent is experiencing the largest ice loss.
While most remote sensing techniques can only see the surface of glaciers, radar waves penetrate all the way to their base, allowing direct measurements of how their bottom profiles at grounding lines - at which a glacier's ice is anchored to the sea floor - changed between 2002 and 2014. Laser signals reflect off the surface, so for the floating ice shelves, laser measurements of changes in surface elevation can be used to infer changes in ice thickness.
According to Stanford School of Earth, Energy and Environmental Sciences, previous studies using other techniques estimated the average melting rates at the bottom of Dotson and Crosson ice shelves to be about 40 feet, or 12 meters, per year. Using radar measurements, the team found that the fastest-melting glacier, Smith, lost between 984 and 1,607 feet, or 300 and 490 meters, in thickness from 2002 to 2009 near its grounding line, or up to 230 feet, or 70 meters, per year.
"We knew from satellite observations of the surface of the ice sheet that the Amundsen Sea Embayment is the most rapidly changing sector in Antarctica," said study coauthor Dustin Schroeder, who heads the Radio Glaciology group at the school. "We also knew from oceanographic observations that warm ocean water is getting onto the continental shelf in this area. However, this study shows how much of the observed change is being caused by the direct melting of floating ice by that warm water."
The researchers strongly suspect that an increase in the influx of ocean heat beneath the ice shelves must have taken place. "Our observations provide a crucial piece of evidence to support that suspicion as they directly reveal the intensity of ice melting at the bottom of the glaciers during that period," study leader Ala Khazendar of NASA's Jet Propulsion Laboratory, Pasadena, California, was quoted as saying in a news release.
Khazendar said Smith's fast retreat and thinning are likely related to the shape of the underlying bedrock, which sloped downward toward the continental interior, and oceanic conditions in the cavity beneath the glacier. As the grounding line retreated, warm and dense ocean water could reach the newly uncovered deeper parts of the cavity beneath the ice shelf, causing more melting. Pope and Kohler, by contrast, are on bedrock that slopes upward toward the interior.
Currently engaged in several projects to determine whether or not other glaciers in West Antarctica will behave more like Smith Glacier or more like Pope and Kohler, Schroeder said "the behavior and evolution of the Amundsen Sea Embayment is a critical region to understand for future sea level projections."
