Old theories of Antarctic ice stability, proven wrong today by a report in Science, incorrectly disconnected ice flow in the deep interior from ice flow near the coast. Therefore, they probably significantly overestimated the stability of Antarctic ice.
Today's report in Science ties in with a NASA report in July in the journal of Glaciology which found rapid acceleration of ice loss in the coastal zone took place after the break up of the Larsen B ice shelf on the Antarctic peninsula. Warm sea water, likely caused by effects of global warming, melted the ice shelf melted from below then calved the huge Larsen B iceberg and set the large coastal glacier behind it in motion. When the ice shelf broke lose it was like a dam burst that rapidly accelerated glacial flow in the coastal region. Taken together, the results of these two reports are profound. Once a dam of ice grounded at the coastal outlet is broken by warm ocean water, glacial flow may accelerate into the continental interior through a network of glaciers. Therefore, the Antarctic ice cap is likely much less stable than existing models predict. New models need to be developed incorporating these two discoveries of enhanced ice flow to predict the stability of Antarctic ice.
The break up of the Larsen B ice shelf calved an enormous iceberg, then began a domino effect of glacial flow and mass loss over a large area of the Antarctic Peninsula.
The Larsen B ice shelf began disintegrating around Jan. 31, 2002. Its eventual collapse into the Weddell Sea remains the largest in a series of Larsen ice shelf losses in recent decades, and a team of international scientists has now documented the continued glacier ice loss in the years following the dramatic event. NASA’s MODerate Imaging Spectroradiometer (MODIS) captured this image on Feb. 17, 2002. (Credit: MODIS, NASA's Earth Observatory)
NASA space based view of the Antarctic Peninsula and the Larsen Ice Shelf
NASA time lapse imagery of the Larsen B ice shelf collapse & its effect on coastal glaciers
"Not only do you get an initial loss of glacial ice when adjacent ice shelves collapse, but you get continued ice losses for many years -- even decades -- to come," says Christopher Shuman, a researcher at UMBC's Joint Center for Earth Systems Technology (JCET) at NASA's Goddard Space Flight Center, Greenbelt, Md. Shuman is lead author of the study published online July 25 in the Journal of Glaciology. "This further demonstrates how important ice shelves are to Antarctic glaciers."
An ice shelf is a thick floating tongue of ice, fed by a tributary glacier, extending into the sea off a land mass. Previous research showed that the recent collapse of several ice shelves in Antarctica led to acceleration of the glaciers that feed into them. Combining satellite data from NASA and the French space agency CNES, along with measurements collected during aircraft missions similar to ongoing NASA IceBridge flights, Shuman, Etienne Berthier, of the University of Toulouse, and Ted Scambos, of the University of Colorado, produced detailed ice loss maps from 2001 to 2009 for the main tributary glaciers of the Larsen A and B ice shelves, which collapsed in 1995 and 2002, respectively.
"The approach we took drew on the strengths of each data source to produce the most complete picture yet of how these glaciers are changing," Berthier said, noting that the study relied on easy access to remote sensing information provided by NASA and CNES. The team used data from NASA sources including the MODerate Imaging Spectroradiometer (MODIS) instruments and the Ice, Cloud and land Elevation Satellite (ICESat).
The analysis reveals rapid elevation decreases of more than 500 feet for some glaciers, and it puts the total ice loss from 2001 to 2006 squarely between the widely varying and less certain estimates produced using an approach that relies on assumptions about a glacier's mass budget.
The authors' analysis shows ice loss in the study area of at least 11.2 gigatons (11.2 billion tons) per year from 2001 to 2006. Their ongoing work shows ice loss from 2006 to 2010 was almost as large, averaging 10.2 gigatons (10.2 billion tons) per year.
The same ice shelf break up processes that NASA observed on the Antarctic Peninsula are expected to affect other ice shelves around Antarctica, but when they break up, glacial flow may increase deep into the interior of the continent according to today's report in Science.
UC Irvine and JPL scientists used a high-resolution, digital mosaic of ice motion in Antarctica assembled from multiple satellite interferometric synthetic-aperture radar
data acquired during the International Polar Year 2007-2009 to determine ice motion.
They observed a pattern of ice flow from the continental interior into large Antarctic coastal glaciers similar to the pattern of water flow from the Rocky mountains into the Mississippi river basin. Dendritic drainage patterns, like the veins of a leaf, enhance the flow of ice from the deep interior of Antarctica like dendritic tributaries of the Mississippi enhance the flow of water from the Rocky mountains to the Gulf of Mexico.
Actual observations of continental scale ice motion reveal a new flow regime that initiates near topographic divides and involves a significant amount of
basal-slip motion. Much remains to be understood about the mechanisms of basal motion and patterned enhanced flow, but our observations already imply a tighter connection between coastal sectors and interior regions than in the hypothetical case of a uniform ice sheet flow because the concentration of ice fluxes along preferred channels enhances the diffusivity of perturbations. It is likely that this patterned enhanced flow is not unique to Antarctica but a common feature of ice sheets.
The data reveal widespread, patterned, enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland, over the entire continent. This view of ice sheet motion emphasizes the importance of basal-slip dominated tributary flow over deformation dominated ice sheet flow, redefines our understanding of ice sheet dynamics, and has far-reaching implications for the reconstruction and prediction of ice sheet evolution.
