Why Firn Sponginess Causes Ice Sheets to Melt Faster Than They Refreeze

Scientists have known from ice core research that it's easier to melt an ice sheet than to freeze it up again. Now, they know at least part of the reason why, and it has to do with ice's "sponginess," according to a new study published today in The Cryosphere.

The study uses a physics-based numerical model to assess the impacts of warming and cooling on firn, the porous layer between snow and glacial ice, over the entire Greenland Ice Sheet. Megan Thompson-Munson, a CIRES and ATOC PhD student, led the study alongside her advisors: CIRES Fellow Jen Kay and INSTAAR Fellow Brad Markle.

"The amount of change that occurs within the firn layer due to warming and cooling is not equal in magnitude,” Megan Thompson-Munson said. “If we look at thousands or millions of years, we see asymmetric ice sheet behavior overall: Ice sheets can melt away quickly, but take a long time to grow. This firn asymmetry we identify is a small piece of that puzzle."

Firn covers about 90 percent of the Greenland Ice Sheet, located at higher elevations where, along with snow, it covers hundreds of meters of ice and acts as a buffer against sea level rise — making it integral to preserving Arctic glaciers in a warming climate. Firn is porous and spongy, which allows water to pass through on its way to the solid ice layer below, where it can refreeze, adding to the existing ice sheet instead of flowing to the ocean.