A heat wave in 2012 continues to have a measurable effect on Greenland’s ice sheets, causing meltwater that would otherwise be soaked up by a porous layer of ice to instead flow freely out to sea.
(CN) — Greenland’s massive ice sheets have been melting at an alarming rate for more than a decade. Scientists have now detected structural changes causing the ice itself to store meltwater less efficiently, further contributing to sea-level rise.
A similar effect has been seen in other icy regions of the globe, such as the Svalbard archipelago off the coast of Norway. Porous firn is a layer of ice occurring in the outer portion of an icesheet, measuring up to 120 feet deep from the surface, it soaks up excess meltwater before it reaches the sea — unless the porous firn itself melts and refreezes, in which case it’s like trying to soak up a spilled drink with a wet sponge.
According to scientists, that’s exactly what happens during extreme weather events, such as the one Greenland experienced in 2012. Researchers from Stanford University describe the problem in a study published Tuesday in the journal Nature Communications.
“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,” said lead author Riley Culberg, a doctoral student in electrical engineering, in a related statement. “This continental-scale picture helps us understand what kind of melt and snow conditions allowed this layer to form.”
The authors used a new form of ice-penetrating radar to reveal Greenland’s compromised porous firn layer. Rather than having a spongy consistency through which water can freely percolate, the layer refroze into something closer to that of an ice cube, forcing meltwater around it and eventually into the sea.
Unusually warm temperatures coupled with a high-pressure zone over Greenland led to 2012’s extreme melt season, both of which were likely exacerbated by climate change. Five record-breaking melting events have occurred in Greenland over the past two decades, most recently in 2019.
“Normally we’d say the ice sheet would just shrug off weather — ice sheets tend to be big, calm, slow things,” said senior author Dustin Schroeder, an assistant professor of geophysics at Stanford’s School of Earth, Energy & Environmental Sciences, in a related statement. “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.”
For their analysis researchers retooled existing airborne radar data, which is usually used to study the bottom of an ice sheet. They overcame previous limitations to reanalyze radar data collected from NASA’s Operation IceBridge flights over Greenland between 2012 to 2017 and used this data to understand how the ice is melting from the surface down to a depth of nearly 50 feet.
“Once those challenges were overcome, all of a sudden, we started seeing meltwater ice layers near the surface of the ice sheet,” Schroeder said in a related statement. “It turns out we’ve been building records that, as a community, we didn’t fully realize we were making.”
Meltwater runoff from the surface comprises the majority of mass lost from Greenland’s ice sheet in recent years. The authors said the 2012 melting event is influencing how the ice sheet responds to surface melt even to this day, particularly in the northwest near Camp Century, an old nuclear-powered cold war outpost, in the southern saddle and at the southern dome. It’s basically a race: Pore space and cold content in the firn need to regenerate above the melt layer faster than extreme heat events can liquify it or the result is ever larger quantities of water flowing out to sea.
Ice sheets that haven’t gone through an extreme heat event like the one in 2012 can typically store water down to a depth of about 150 feet. Afterward, that drops dramatically to about 15 feet in some areas, significantly reducing the volume of water that the ice can sponge up. According to the authors, this lesson can also be applied to Antarctic ice sheets.
“I think now there’s no question that when you’re trying to project into the future, a warming Antarctic will have all these processes,” Schroeder said. “If we don’t use Greenland now to better understand this stuff, our capacity to understand how a warmer world will be is not a hopeful proposition.”