redOrbit Staff & Wire Reports – Your Universe Online
Pine Island Glacier, one of the largest routes for ice to flow from Antarctica into the sea, is far more susceptible to climatic and ocean variability than previously believed, according to research published Thursday in the advanced online version of the journal Science.
Observations from Dr. Pierre Dutrieux of the British Antarctic Survey (BAS) and his colleagues showed sizable fluctuations in ocean heat in Pine Island Bay. They reported that oceanic melting of the ice shelf into which the glacier flows decreased by 50 percent between the years of 2010 and 2012, and that a La Niña weather event may have been to blame.
“Understanding the processes driving ice shelf thinning and the glacier’s response is key to assessing how much it will contribute to rising sea levels,” the British Antarctic Survey explained in a statement. “It’s now known that much of the thinning is due to a deep oceanic inflow of Circumpolar Deep Water (CDW) on the continental shelf neighboring the glacier. This warmer water then makes its way into a cavity beneath the ice shelf melting it from below.”
The ice shelf in the Pine Island region has experienced nearly continuous thinning since scientists started observing it in the 1970s, the researchers explained. While previous studies have demonstrated that warm deep-ocean water was melting the ice from underneath, the investigators found that the issue is more complicated than it appeared.
While the deep ocean has been warming, the more important factor is that warm water has been finding its way to the ice shelf, the researchers said. When conditions are optimal, the continental shelf can become flooded by warm deep water, and that water can make its way to the glacier margin. Over the past 20 years, measurements indicate that a thick layer of warm water has persisted on the continental shelf that has come in contact with the ice shelf.
When Dr. Dutrieux and his associates returned to the region in order to collect additional data last January, they discovered that the warm water layer was actually far thinner than before, and the ice shelf was being surrounded and protected by a thicker than normal layer of cold water.
“They estimated half as much meltwater was being produced from the glacier compared to 2010, making 2012 the year with the lowest summer melting of the Pine Island Glacier on record,” explained the University of Washington, which provided atmospheric modeling expertise to the researchers.
Based on water temperature measurements and ocean circulation models, they found that the reduced melting observed in 2012 was due to the fact that less warm, deep water was able to make it across the underwater ridge.
The reduced flow was due to a change in winds, which are typically westerly but had been coming out of the east for the majority of the preceding year, the study authors noted.
“We found ocean melting of the glacier was the lowest ever recorded, and less than half of that observed in 2010,” Dr. Dutrieux said. “This enormous, and unexpected, variability contradicts the widespread view that a simple and steady ocean warming in the region is eroding the West Antarctic Ice Sheet. These results demonstrate that the sea-level contribution of the ice sheet is influenced by climatic variability over a wide range of time scales.”
“It is not so much the ocean variability, which is modest by comparison with many parts of the ocean, but the extreme sensitivity of the ice shelf to such modest changes in ocean properties that took us by surprise,” added co-author Professor Adrian Jenkins, also from BAS. “These new insights suggest that the recent history of ice shelf melting and thinning has been much more variable than hitherto suspected and susceptible to climate variability driven from the tropics.”