Extreme melt events can permanently alter structure of an ice sheet

During the summer of 2012, a high-pressure system stalled over Greenland and temperatures soared. Sheens of meltwater pooled and glistened across large expanses of the island's ice sheet.

According to a new study, published Tuesday in the journal Nature Communications, the short-term melting event had a lasting impact on the ice sheet's structure and its ability to capture meltwater runoff.

As ice-penetrating radar data revealed, the meltwater from the 2012 event refroze, forming a slick layer of ice beneath Greenland's snowpack.

Imaging showed the layer of refrozen ice even extended across the middle of the ice sheet where surface melting is typically minimal.

Importantly, scientists determined this new layer of ice has impeded the ice sheet's ability to store future meltwater.

"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," lead study author Riley Culberg said in a news release.

"This continental-scale picture helps us understand what kind of melt and snow conditions allowed this layer to form," said Culberg, a doctoral student in electrical engineering at Stanford University.

Because ice sheets are such giant slow-moving entities, they're typically able to shrug off annual anomalies.

But the latest research suggests that the combination of climate change and extreme weather can leave ice sheets and glaciers more vulnerable to short-term warming events.

For the study, scientists relied on measurements captured by NASA's Operation IceBridge from 2012 to 2017.

The radar instruments on the research plane are typically used to study the bottom of Greenland's ice sheet, but the authors of the latest paper deployed a unique modeling technique to capture meltwater data near the surface 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," said senior author Dustin Schroeder.

"It turns out we've been building records that, as a community, we didn't fully realize we were making," said Schroeder, an assistant professor of geophysics at Stanford.

What happened in 2012 in Greenland may be familiar to anyone who has experienced a day of warm weather after a snowstorm, followed by a frigid night.

Snow melts in the midday sun before refreezing overnight, leaving a dangerous layer of slick ice -- on roads, sidewalks and steps.

According to the new study, the layer of refrozen ice formed by the 2012 event has prevented the upper layers of the ice sheet from effectively storing meltwater.

As a result, more meltwater may flow to the bottom of the ice sheet, creating more slippery conditions and accelerating the descent of ocean-bound coastal glaciers.

Because climate change has had an outsized impact on the Arctic, researchers suggest Greenland can offer clues about how Antarctica's ice will behave during the decades ahead.

"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," Schroeder said.

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