Modeling the Slips and Stresses of Polar Ice on the Move
If melted, all of the ice on the Earth’s surface could raise global sea level by more than 200 feet. Because most of this ice resides in the ice sheets of Greenland and Antarctica, interest in understanding ice sheet dynamics is growing, particularly among scientists responsible for designing the Community Climate System Model (CCSM).
Recently, scientists modified the Community Land Model (CLM), a component of CCSM, to compute ice sheet surface mass balance—the net accumulation of snow minus melting or evaporation. But to obtain a full picture on ice sheet behavior, they must pair information on surface mass balance with estimates of dynamic changes in ice flow. If melted, all of the ice on the Earth’s surface could raise global sea level by more than 200 feet. To achieve this end, William Lipscomb and Steve Price, scientists at Los Alamos National Laboratory, are in the process of adapting GLIMMER, an ice sheet model created at the University of Bristol, for inclusion in CCSM4, the latest version of the community model.
“In retooling GLIMMER, it is important to be able to represent both ice sheet dynamics—vertical shear stress, lateral shear stress, and longitudinal normal stress—and surface mass balance, at scales of 10 kilometers or less,” says Lipscomb.
Surface mass balance is hard to model with precision because, over small distances, ice sheets tend to have steep edges and undulating topography. Also, the rate of ice sheet melt is sensitive to temperature and elevation changes, with temperatures decreasing as elevation increases. Accounting for surface processes, such as percolation and refreezing of water within the ice and snow, can further complicate mass balance modeling. Despite this, Lipscomb says, CLM’s excellent surface energy and snow scheme is capable of generating accurate surface mass balance estimates.
GLIMMER is expected to become part of CCSM4 by the end of 2008. It will not have all of the desired ice sheet dynamics capabilities, but the team is working on additional improvements that will be included in future CCSM versions. Currently, Lipscomb, Price, and other CCSM contributors are testing the model to make sure all of the pieces work together as anticipated. These efforts will mean that, for the first time, CCSM will include a dynamic Greenland ice sheet component. This represents a significant improvement; to date, the thickness and extent of the Greenland and Antarctic ice sheets in CCSM have remained static.
Further out, the team plans to create a Community Ice Sheet Model (CISM) that will allow dynamic simulations of the Antarctic and paleo ice sheets, in addition to Greenlandic ice sheets. CISM will help scientists predict and hindcast regional and global ice sheet retreat and the resulting sea level rise. Also, researchers expect that CCSM’s ocean and ice sheet models will be fully coupled. This will let the model replicate intrusions of warm water beneath ice shelves, shedding insights on how this phenomenon may feed back on itself and cause ice margins to destabilize. Scientists think that this mechanism might trigger the rapid retreat of the West Antarctic Ice Sheet—which could raise sea level by a meter or more within a century or less.
