A recent study reveals significant differences in how temperature changes affect a crucial layer of ice.
Researchers have long understood from ice core studies that melting an ice sheet is easier than refreezing it. A new study, published on July 24 in The Cryosphere, sheds light on part of the reason for this, focusing on the “spongy” characteristics of ice.
The research employs a physics-based numerical model to examine the effects of both warming and cooling on firn, the porous layer located between snow and glacial ice, across the entire Greenland Ice Sheet. Megan Thompson-Munson, a PhD student at CIRES and ATOC, led the study with guidance from her advisors: CIRES Fellow Jen Kay and INSTAAR Fellow Brad Markle.
“The changes seen in the firn layer due to warming and cooling are not equal,” said Megan Thompson-Munson. “Over thousands or millions of years, we can observe that ice sheets behave asymmetrically: They can melt rapidly but take much longer to reform. The firn asymmetry we uncovered is a small part of this larger dynamic.”
Firn constitutes about 90 percent of the Greenland Ice Sheet, primarily at higher elevations, where it, along with snow, blankets hundreds of meters of ice and helps mitigate sea level rise—playing a critical role in safeguarding Arctic glaciers in a warming world. Its porous and spongy nature allows water to trickle down to the solid ice beneath, where it can freeze again, thereby contributing to the ice sheet instead of draining into the ocean.
The study reveals that rising temperatures are affecting how well firn can store meltwater, while cooling temperatures may not restore the firn’s capabilities as effectively as previously thought.
“Warming diminishes what we call the ‘firn air content’ or the ‘sponginess,'” Thompson-Munson explained. “More sponginess is lost through warming than can be regained during cooling. This is crucial because this porous firn layer helps to regulate the ice sheet’s contribution to rising sea levels.”
To investigate firn’s response to varying temperatures, the team utilized a physics-based computer model known as SNOWPACK, focusing specifically on temperature as a key variable. This study marks a groundbreaking effort in two ways: it examines both warming and cooling influences on Greenland firn, and it encompasses the entire ice sheet, a broader perspective than previous studies that looked at smaller regions.
“The Greenland ice sheet is losing mass more quickly due to warming than it is gaining due to cooling,” explained Kay. “This study’s key finding is that the firn in Greenland significantly contributes to this asymmetric response, where warming has a greater effect than cooling.”
Thompson-Munson raises an important issue regarding geoengineering and our potential to reverse global warming. Any geoengineering initiatives aimed at reducing Arctic temperatures may not effectively preserve ice and snow as intended; the extent of cooling would need to exceed the degree of warming to enable firn and glaciers to return to their former state.
“To revert to initial conditions, we would need significantly greater cooling or begin to alter other factors as well,” Thompson-Munson noted. “Reversing our current situation is quite challenging.”
