A newly identified mechanism for the movement and freezing of meltwater from ice sheets has the potential to enhance global sea level rise projections.
Scientists from The University of Texas at Austin, in partnership with NASA’s Jet Propulsion Laboratory (JPL) and the Geological Survey of Denmark and Greenland (GEUS), have discovered a new process that illustrates how impermeable horizontal ice layers develop beneath the surface. This process is essential for understanding the role of meltwater from ice sheets in rising sea levels.
The research, conducted by Mohammad Afzal Shadab, a graduate student at UT’s Oden Institute for Computational Engineering and Sciences, was published in Geophysical Research Letters. Shadab’s research was guided by co-authors Marc Hesse and Cyril Grima from UT’s Jackson School of Geosciences.
The Greenland and Antarctica ice sheets, which hold the world’s largest freshwater reserves, are blanketed in ancient snow called firn that has not yet solidified into ice. This firn is porous, allowing melted snow to seep down and refreeze instead of flowing directly into the ocean. This phenomenon is believed to reduce meltwater runoff by roughly 50%.
However, impermeable ice layers can also form, acting as blocks to divert meltwater toward the sea, according to Shadab.
“In some scenarios, these ice layers within firn can actually speed up the meltwater discharge into the oceans,” he explained.
Understanding how meltwater can either freeze within firns or escape over ice barriers is crucial for accurate sea level rise predictions, the researchers note. Prior studies on firn in mountainous regions, which also contain ice layers, indicated that these layers form when rainwater accumulates and freezes on older firn layers. Yet, Hesse noted that this process appears different for ice sheets.
“When we analyzed data from Greenland, we found that the meltwater generated during even significant melting events isn’t sufficient to create ponds,” Hesse stated. “This discovery led us to identify a new mechanism for the formation of ice layers.”
This innovative research describes the formation of ice layers as a balance between two processes: warmer meltwater flowing through the porous firn (advection) and the cold ice freezing the water through heat conduction. The depth at which heat conduction becomes more dominant than advection determines where new ice layers will form.
“Now that we’ve grasped the physics behind ice layer formation, we can improve predictions on how much meltwater can be retained by firn,” commented Surendra Adhikari, a geophysicist from JPL and a co-author of the study.
Anja Rutishauser, a previous postdoctoral researcher at UT and currently at GEUS, also contributed to the study.
To validate this new mechanism, the researchers compared their models against data from a 2016 study, during which scientists excavated a site in Greenland’s firn, equipping it with thermometers and radar systems to measure the behavior of meltwater. While older hydrological models struggled to align with the gathered data, the new mechanism accurately reflected the observations.
An intriguing outcome from this research revealed that the position of the ice layers could indicate the thermal conditions during their formation.
“In a warming environment, the ice layers were found to form deeper chronologically in a top-down manner,” Shadab noted. “In contrast, under colder conditions, they formed nearer to the surface in a bottom-up fashion.”
Currently, the volume of water entering the ocean from Greenland exceeds that from Antarctica—approximately 270 billion tons per year as compared to Antarctica’s 140 billion tons. Together, this amounts to over two and a half Lake Tahoes worth of water each year. Yet, estimates regarding future contributions from the two ice sheets to sea level rise range widely, from 5 to 55 centimeters by 2100. It’s evident that ice layers play a critical, yet previously poorly understood, role in this dynamic.
“Reality is far more complex than what existing models have captured,” remarked Adhikari. “To truly enhance our predictions, we’re making significant strides in advancing our understanding.”
