Researchers from The University of Texas at Austin, in collaboration with NASA’s Jet Propulsion Laboratory and the Geological Survey of Denmark and Greenland, have discovered a new mechanism for the flow and freezing of ice sheet meltwater, which could lead to more accurate estimates of sea level rise globally. The study by Mohammad Afzal Shadab, a graduate student at UT’s Oden Institute for Computational Engineering and Sciences, was published in Geophysical Research Letters, with supervision from Marc Hesse and Cyril Grima. The Greenland and Antarctica ice sheets, covered in porous firn, can see melted snow draining down and freezing again rather than running into the sea, reducing meltwater runoff by about half. However, impermeable ice layers can form that divert meltwater and accelerate the rate of runoff into the oceans.
The formation of impermeable ice layers in firn can act as barriers for meltwater, diverting it to the sea, and playing a crucial role in estimating sea level rise. Previous studies had found that ice layers are formed in mountain firn when rainwater accumulates and refreezes, but this mechanism did not seem to apply to ice sheets. The new research presents ice layer formation as a competition between warmer meltwater flowing down through the firn and cold ice freezing the water in place. The depth at which heat conduction dominates over heat advection determines where a new ice layer forms, allowing for better predictions of meltwater retention in firn.
To validate their new mechanism, the researchers compared their models to data collected in Greenland in 2016, involving thermometers and radar to measure meltwater movement. Previous hydrological models had deviated from the measurements, but the new mechanism successfully mirrored observations. An unexpected finding was that the location of ice layers may act as a record of the thermal conditions when they formed, with ice layers forming deeper in warmer conditions in a top-down fashion and closer to the surface in colder conditions in a bottom-up scenario. This understanding of freezing dynamics within firn is crucial for estimating sea level rise accurately.
Currently, Greenland contributes about 270 billion tons of water runoff into the sea per year, surpassing Antarctica’s 140 billion tons. Predictions for how much these ice sheets will contribute to sea level rise by 2100 vary widely, from 5 to 55 centimeters. Ice layers, with their poorly understood role until now, play a key role in these predictions. The complexity of the processes involved in meltwater flow and ice layer formation in firn highlight the need for advancements in predicting sea level rise accurately. By improving our understanding of these mechanisms, it is possible to enhance the accuracy of estimates of sea level rise and its potential impacts on coastal communities worldwide.