In a sign that global warming is a reality, a new study reveals that ice shelves in western Antarctica are melting at a faster pace than previously known. Data collected by a NASA ice-watching satellite show that the ice shelves are being eaten away from below by ocean currents, which have been growing warmer even faster than the air above. Launched in January of 2003, NASA’s ICESat (Ice, Cloud and Land Elevation Satellite) studied the changing mass and thickness of Antarctica’s ice from polar orbit. An international research team used more than 4.5 million surface height measurements collected by ICESat’s GLAS (Geoscience Laser Altimeter System) instrument between October of 2005 and 2008. The conclusion was that 20 of the 54 shelves studied — nearly half — were losing thickness.
Melting of ice by ocean currents can take place when air temperature remains cold, maintaining a steady process of ice loss — and ultimately a rise in the sea level. “We can lose an awful lot of ice to the sea without ever having summers warm enough to make the snow on top of the glaciers melt,” said Hamish Pritchard of the British Antarctic Survey and the study’s lead author. “The oceans can do all the work from below.” The study also found a shift in Antarctica’s winds as a result of climate change. “This has affected the strength and direction of ocean currents,” Pritchard said. “As a result warm water is funneled beneath the floating ice. These studies and our new results suggest Antarctica’s glaciers are responding rapidly to a changing climate. We’ve looked all around the Antarctic coast and we see a clear pattern: in all the cases where ice shelves are being melted by the ocean, the inland glaciers are speeding up. It’s this glacier acceleration that’s responsible for most of the increase in ice loss from the continent and this is contributing to sea-level rise.”
Antarctica contains adequate ice to raise sea levels by approximately 187 feet, although it’s unlikely to melt for thousands of years, according to the United Nations. Some ice shelves are thinning by a few meters a year, and glaciers in response are draining billions of tons of ice into the sea, Pritchard said. “Most profound contemporary changes to the ice sheets and their contribution to sea level rise can be attributed to ocean thermal forcing that is sustained over decades and may already have triggered a period of unstable glacier retreat.”
Some ice shelves are thinning just a few feet a year, and glaciers drain billions of tons of ice into the sea as a result. “This supports the idea that ice shelves are important in slowing down the glaciers that feed them, controlling the loss of ice from the Antarctic ice sheet,” Pritchard said.
While conducting the study, the researchers measured how ice shelf height changed, using computer models to check changes in ice thickness due to natural snow accumulation. Additionally, they used a tide model that eliminated height changes due to rising tides. “This study shows very clearly why the Antarctic ice sheet is currently losing ice, which is a major advance,” said Professor David Vaughan, the leader of ice2sea. The study is significant because it shows the key to predicting how an ice sheet might change in the future. “Perhaps we should not only be looking to the skies above Antarctica, but also into the surrounding oceans,” Vaughan added.
Tom Wagner, cryosphere program scientist at NASA, said that the study demonstrates how “space-based, laser altimetry” can expand scientists understand of the earth. “Coupled with NASA’s portfolio of other ice sheet research using data from our GRACE mission, satellite radars and aircraft, we get a comprehensive view of ice sheet change that improves estimates of sea level rise.”
“When ice shelves completely collapse — and we’ve seen that before — the grounded glaciers behind them will speed up; we know that,” said co-author Helen Amanda Fricker of Scripps Institution of Oceanography at the University of California San Diego. “But what this study is showing, which is very new, is that you don’t need to lose the shelf entirely for this to happen; just a reduction in the thickness of the ice shelf is enough to allow more of the grounded ice behind it to flow off the continent.”