Scientists have long understood from studies of ice cores that it is easier to melt a sheet of ice than to freeze it. A recent study published in the journal The Cryosphere reveals part of the reason: the “porosity” of the ice.
The study uses a physical numerical model to predict the effects of warming and cooling on firn, the porous layer between snow and glacier across the Greenland ice sheet. CIRES and ATOC graduate student Megan Thompson-Munson led the research with her advisors Jen Kay (CIRES) and Brad Markle (INSTAAR).
“The amount of change that’s happening in the firn sheet due to warming and cooling is not of the same magnitude,” said Megan Thompson-Munson. “When we look over thousands or millions of years, we see asymmetric behavior of the ice sheet as a whole: the ice sheet can melt quickly, but it takes a long time to grow. Discovering the asymmetry of the firn sheet is a small piece of the puzzle.”
The role of fire in ice sheet stability
Firn covers about 90 percent of the Greenland ice sheet at high altitudes, where it forms hundreds of meters of ice along with snow and acts as a buffer against sea level rise, making it an integral part of maintaining Arctic glaciers in a warming climate. Firn is porous and spongy, allowing water to pass through it into a solid sheet of ice, where it can freeze and contribute to the existing ice sheet rather than flowing into the ocean.
In this study, scientists found that warming rapidly alters the furnace’s ability to store meltwater, and that lower temperatures may not help the furnace recover as fully as scientists had hoped.
“Warming depletes what we call the ‘air content’ or ‘porosity’ of the kiln,” Thompson-Munson says. “So you lose more porosity through warming than you can regain through cooling. And that’s important because that porous firn can buffer the contribution of the ice sheet to sea level rise.”
Revolutionary work of fir trees
To understand how fire responds to warming and cooling, the team used a physical computer model called SNOWPACK, focusing on a single variable: temperature. This study is the first of its kind in two respects. First, the researchers examined the effects of both warming and cooling on Greenland firs. Second, the study covered the entire ice sheet, whereas previous work focused on smaller geographic areas.
“The Greenland ice sheet is losing mass faster with warming than it is gaining mass with cooling,” Kay said. “The key finding of this study is that Greenland fir contributes more to this asymmetric response to warming than to cooling.”
Thompson-Munson said the study raises an important question about geoengineering and the possibility of stopping global warming. Any geoengineering concept aimed at lowering Arctic temperatures may not be able to maintain ice and snow as effectively as imagined; the degree of cooling must exceed the degree of warming to help glaciers and ice caps return to normal.
“To get back to the original conditions, we would have to cool down more or start changing other variables,” Thompson-Munson says. “It’s hard to change what we’ve done.”
Source: Port Altele
