Greenland's "Sponge" Affected By Atmospheric Warming
Changing climate limits the ability of the ice sheet to retain meltwater
A new study of snow and firn layers high on the Greenland ice sheet, published in Nature Climate Change, shows that recent atmospheric warming is changing the ability of near-surface firn layers to store meltwater, which can result in a faster release of runoff from the ice sheet.
“The near-surface of the large ice sheet interior is comprised of snow that is slowly being converted into glacier ice,” explains Horst Machguth from the Geological Survey of Denmark and Greenland, who is the lead author of the study. "This prous firn layer can be up to 80 m thick. Earlier research has shown that firn acts like a sponge, and stores meltwater percolating down from the ice sheet surface by refreezing it to form ice layers. But after the Greenland Ice Sheet was hit by a series of warm summers, it was unknown how the firn reacted to exceptional amounts of meltwater. Our research aimed to clarify whether the firn was indeed capable of retaining the meltwater, or whether the sponge had been overwhelmed.”
The scientists drilled several 20 m deep cores to sample the firn, also targeting sites where similar cores had been drilled 15 to 20 years ago. At many locations, a comparison of the new and old cores revealed that the amount of refrozen ice layers in the firn had indeed increased substantially over the past two decades, consistent with the sponge theory. This sponge-like behaviour, however, was not found everywhere. Cores drilled at lower elevations indicated that the recent exceptional meltwater production had only percolated to shallow depths within the firn, aggregating into massive ice layers directly below the ice sheet surface.
“It appears that the firn was overwhelmed by the melt to a degree where so many ice lenses had formed that they started to hinder percolation of further meltwater. Initially small ice lenses grew to form ice layers of several meters in thickness that act as a lid on top of otherwise sponge-like firn. Radar measurements identified that these massive ice lenses were continuous over tens of kilometres,” says Dirk van As, a co-author of the study from the Geological Survey. “Surface meltwater wants to refreeze in firn locally, which it does at higher elevation, but at lower elevations it hits that lid of ice and is forced to stay at the surface where it cumulates.”
Satellite imagery from recent summers shows that the meltwater then formed rivers on the surface that flow towards the margin of the ice sheet. “In contrast to storing meltwater in porous firn, this mechanism increases runoff from the ice sheet,” says CIRES researcher Mike MacFerrin, a second author on the study. “This process has not previously been observed in Greenland. The extent of this ice lid capping the ice sheet firn remains unknown. For this reason, the total amount of additional ice sheet runoff associated with this newly observed process cannot yet be quantified.”
Similar changes in firn structure have been observed on various ice caps in the neighbouring Canadian Arctic, which leads to the conclusion that this phenomenon could be widespread in Greenland. “To investigate this question, we are now combining our core data with remotely sensed radar measurements from NASA, which cover the entire ice sheet and will give us a clearer picture of just how widespread this is,” explains MacFerrin.