Assessing Antarctica’s Ice Shelves for Vulnerability to Surface-Melt-Induced Collapse Using Scatterometry

Monday, 15 December 2014
Karen E Alley, University of Colorado at Boulder, Boulder, CO, United States, Theodore A Scambos, Univ Colorado, Boulder, CO, United States and David G Long, Brigham Young Univ, Provo, UT, United States
The disintegration of several ice shelves on the Antarctic Peninsula since 1995 initiated a rapid increase in ice flow, altering the regional mass balance. A key element of disintegration appears to be the formation of surface melt ponds, which can trigger a run-away hydrofracturing process. A shelf’s firn layer must be saturated with ice for ponds to form at the surface. This study presents a comparison of wintertime satellite-derived active microwave backscatter and surface melt-day data, revealing a distinctive pattern that can be used to assess the state of ice shelf firn. Low melt areas (1 to 10 days of melting per year) have few refrozen meltwater lenses within the firn to serve as scatterers, so they have low backscatter values (-10 to -5 dB σo). As the mean number of melt days and therefore the abundance of ice lenses in the firn increases, backscatter values rise significantly to a peak at approximately 30 to 50 days of melt and approximately -1 dB σo. With increasing melt beyond this threshold, mean wintertime backscatter declines, reaching -5 dB σo at 80 to 100 melt days/year. This drop in backscatter reflects an increase in specular reflections from an ice-saturated firn layer. All ice shelves that have previously collapsed plot above this threshold. This pattern mirrors the characteristics of snow facies as observed in these same data types for transects across the Greenland Ice Sheet. Backscatter values on Antarctic ice shelves are also sensitive to accumulation rate, with higher accumulation requiring more melt days to produce the same level of backscatter increase. We hypothesize that, as the atmosphere warms in the future, ice shelves will evolve upwards along this pattern until they reach the firn saturation threshold where collapse is imminent. Therefore, a comprehensive survey of Antarctic ice shelves using scatterometry indicates which shelves are presently most vulnerable to surface-melt-induced collapse, and which shelves are likely to be vulnerable in the relatively near future.