A study reveals that warming more significantly impacts the firn layer on the Greenland Ice Sheet than cooling, complicating efforts to reverse melting through geoengineering.
Scientists have long understood from ice core research that melting an ice sheet is easier than refreezing it. A recent study published in The Cryosphere reveals part of the reason: the “sponginess” of ice.
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’s Role in Ice Sheet Stability
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.
In this study, researchers found warming temperatures are rapidly changing how efficiently firn can store meltwater, and cooling temperatures may not help the firn fully recover as much as scientists might have hoped.
“The warming depletes what we call the ‘firn air content’ or the ‘sponginess,” Thompson-Munson said. “So you lose more of the sponginess due to warming than can be regained due to cooling. And it’s important because this porous firn can buffer the ice sheet’s sea level rise contribution.”
Groundbreaking Firn Research
To understand how firn responds to both warming and cooling temperatures, the team used a physics-based computer model called SNOWPACK, and honed in on one variable: temperature. The study is the first of its kind in two ways. First, researchers looked at the impacts of both warming and cooling temperatures on Greenland firn. Second, the scope of the research covered the entire ice sheet, while previous studies focused on smaller geographical areas.
“The Greenland ice sheet loses mass faster under warming than it gains mass under cooling,” said Kay. “The key advance of this study is that Greenland’s firn contributes to this greater warming-than-cooling asymmetric response.”
Thompson-Munson said the study brings up an important question regarding geoengineering and the ability to reverse our Earth’s warming. Any geoengineering concepts designed to decrease temperatures in the Arctic might not preserve ice and snow as efficiently as imagined; the degree of cooling will have to exceed the degree of warming to help firn and glaciers return to normal.
“To get back to initial conditions, we’d have to cool a lot more or start changing other variables as well,” Thompson-Munson said. “It’s hard to reverse what we’ve already done.”
Reference: “Greenland’s firn responds more to warming than to cooling” by Megan Thompson-Munson, Jennifer E. Kay and Bradley R. Markle, 24 July 2024, The Cryosphere.
DOI: 10.5194/tc-18-3333-2024
4 Comments
“…, cooling temperatures may not help the firn fully recover as much as scientists might have hoped.”
The epitome of a scientist is a disinterested observer. That is, someone who is totally objective and doesn’t have their conjectures, hypotheses, or interpretation of data subjectively influenced by their hopes or beliefs.
Now, an engineer, on the other hand, is typically tasked with making something tangible, like a bridge; they have formulas (recipes, if you will) and regulations based on analysis of past failures to help guide them. When the budget or other constraints allow it, they will often increase the strength of structural materials by a 2 or 3X factor, and cross their fingers. They “hope” that their ‘fudge factor’ will be more than adequate. In the case of a geoengineer, attempting to do something that hasn’t been done before (like modifying the climate), they don’t have a lot of experience to guide them and are typically suggesting approaches that are theoretical. Now, they “hope” that they are right.
Perhaps the reason that climatology produces such sloppy research and such poor predictions is that so few climatologists realize just how a scientist is supposed to operate. Unlike an engineer building a rocket, they (and the public) might not discover that their predictions are wrong until long after they have retired. Unlike in Materials Science, they don’t get immediate feedback. That is, if a researcher is trying to make a new superconductor or photovoltaic cell, they find out immediately if the material works as predicted. Climatologists depend on supercomputers and computer code that can’t be verified for decades.
“The warming depletes what we call the ‘firn air content’ or the ‘sponginess,”
And, warm water holds less CO2 than cold water and, as it flows downward, fills up the voids containing air. In the light of those mechanisms, how reliable are ice core measurements of ancient CO2?
“The Greenland ice sheet loses mass faster under warming than it gains mass under cooling,”
That is really a meaningless statement without putting it in the context of the RATES of warming and cooling, the temperature of the firn before the warming/cooling episode starts, and the rates of snowfall. What they are observing is the current situation. The GIS exists in what is essentially a very cold, high desert with low rates of precipitation. The rate of growth is determined by the annual rate of snow accumulation (reduced by Winter sublimation and Summer evaporation/melting). However, the rate of melting/evaporation is primarily driven by how much and how fast the temperature exceeds the melting point of ice. Cloud cover is also important because much of the melting is driven by solar heating rather than ambient air-mass temperature.
“…, and honed in on one variable: temperature.”
How does temperature affect the other variables such as precipitation, windiness, and cloudiness? I think that they are only getting part of the picture.