The year 2012 was a disaster year in terms of climate. And we don’t mean the movie “2012”, but the reality in the far north. At that time, the Arctic had experienced the greatest decline in sea ice cover ever recorded. And Greenland was not spared either. Within a few weeks, almost 97 percent of the ice surface had melted. Scientists have now discovered that this process is still having an effect today and is probably massively reducing the stability of the ice sheet.
The results of the study show that after the melting a thick layer of ice had formed on the surface. Per se, this discovery would not be particularly exciting if it were not for the extent and location of this layer. This is because it can still be found even in the middle of the ice sheet, where the growth zone of the individual glaciers is also located and where normally no major melting processes take place. Here the snow is deposited, becomes firn and then turn into the ice that makes the glaciers grow. However, the ice layer formed in 2012 alters the structural properties of the shield and thus the response to subsequent melt events and the behavior of meltwater. Riley Culberg, a doctoral student at Standford University and lead author of the study, explains. “When you have these extreme, one-off melt years, it’s not just adding more to Greenland’s contribution to sea-level rise in that year – it’s also creating these persistent structural changes in the ice sheet itself,” he explains.
The data that the research team could draw on were actually all already there. Radar data obtained by reconnaissance flights as part of NASA’s Operation IceBridge only needed to be remodeled and analyzed. Instead of going all the way to the bottom of the ice sheet, Culbert and his supervisor, Dr. Dustin Schroeder, a geophysicist at Stanford University, examined only about the top 15 meters of the ice sheet. “Once those challenges were overcome, all of a sudden, we started seeing meltwater ice layers near the surface of the ice sheet,” explains Dustin Schroeder. “t turns out we’ve been building records that, as a community, we didn’t fully realize we were making.” This gave the team a glimpse of a previously unnoticed layer.
The researchers were then able to analyse the consequence of this layer formation and show that the ice sheet’s ability to store meltwater in the growth zone of glaciers is massively limited by this layer: instead of being stored in the first 50 metres of the surface, the water can only be stored in the first 5 metres, the rest runs off. “In the high-elevation accumulation zone, surface meltwater does not flow directly to the ocean or glacier bed, but instead percolates into the porous near-surface snow or firn. Here meltwater can be stored in perennial firn aquifers or refreeze locally,” the researchers write in the paper. However, with the ice cover due to the melting event, this storage is greatly reduced and the water flows under the glaciers, accelerating their flow velocity. The team was thus able to show that individual events have long-term effects on the ice sheet.
“This is really one of the first cases where you can say, shockingly, in some ways, these slow, calm ice sheets care a lot about a single extreme event in a particularly warm year,” Dustin Schroeder says. And since 2000, a total of five such events have been recorded in Greenland, the most recent in 2019, when the melt area was smaller but the mass of ice loss set new records. The likelihood that such extremes will become even more frequent and severe has already been shown by other scientists. “These structural changes mean the way the ice sheet responds to surface melting is going to be impacted longer term,” says Riley Culbert.
Dr Michael Wenger, PolarJournal
Link to the study: Culberg, R., Schroeder, D.M. & Chu, W. Extreme melt season ice layers reduce firn permeability across Greenland. Nat Commun 12, 2336 (2021). https://doi.org/10.1038/s41467-021-22656-5
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