Understanding Ice Loss in Earth’s Coldest Regions: Melting Under the Antarctic Ice Sheet’s Skin
A puzzle for Antarctic ice sheet change has been to understand how ice melts in places where surface conditions are too cold for ice to melt. A team of scientists, including a Department of Energy-supported researcher at Los Alamos National Laboratory, has studied this problem using ice models. Their field location is the McMurdo Dry Valley glaciers of Antarctica, where summer temperatures never rise far above the melting threshold. Active melting is rarely observed, yet runoff from these glaciers is the primary source of water to streams, lakes and associated ecosystems in the valleys, which are among the coldest and driest ecosystems on Earth. The processes generating melt under these marginal conditions are not well understood, and traditional melt modeling techniques are inadequate to explain the observed runoff from these glaciers.
The team investigated two processes: 1) penetration of solar radiation into the ice, and 2) drainage of subsurface melt from the ice, as well as their roles in generating runoff from Dry Valley glaciers. The researchers successively added these processes to an energy balance model and applied the model to three glacier sites using 13 years of hourly meteorological data. Model results show that inclusion of both processes is necessary to accurately model ice loss, ice density, and ice temperature on these glaciers. Melt on the glacier surface is rare, but internal melting 5-15 cm below the ice surface is extensive, and its drainage accounts for ~50% of all summer ice loss. This finding is consistent with field observations of subsurface streams and formation of a weathering crust. The team identified an annual cycle of weathering crust formation in summer and its removal during the 10 months of winter sublimation. Due to the complexities of ice melt at air temperatures close to the melting temperature, these glaciers will respond differently to changes in climate than glaciers in warmer climates. This behavior also will apply to other glaciers at very high latitudes and elevations and many extraterrestrial glaciers such as those on Mars.
Hoffman, M. J., A. G. Fountain, and G. E. Liston. 2014. “Near-Surface Internal Melting: A Substantial Mass Loss on Antarctic Dry Valley Glaciers,” Journal of Glaciology 60(220), 361-74. DOI:10.3189/2014JoG13J095.