Climate: Extreme Greenland Ice Sheet melting episodes change runoff regime

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Extreme melting on the surface of the Greenland ice sheet could change the sea level rise equation.

Study shows that 2012 melting created a dense ice cap

Staff Report

When warm temperatures in 2012 caused an extreme melting episode across much of the Greenland Ice Sheet, it may have fundamentally altered the way the near-surface snow layers absorb water, according to a new study published in Nature Climate Change.

The melting resulted in the formation of a thick layer of ice atop the previously porous surface. Subsequently, meltwater ran off the surface and to the ocean, with potential impacts on sea level, according to York University Professor William Colgan.

Together with Horst Machguth of the Geological Survey of Denmark and Greenland, Mike MacFerrin of the University of Colorado at Boulder and Dirk van As of the Geological Survey of Denmark and Greenland Copenhagen, Denmark, Colgan carefully studied data from three expeditions to Greenland in 2012, 2013 and 2015.

The field research included drilling into the surface of the ice sheet.

“We were interested in the thin porous near-surface firn layer, and how its physical structure is changing rapidly with climate change,” Colgan explained in a press release. “The study looked at very recent climate change on the ice sheet, how the last couple of years of melt have really altered the structure of the ice sheet firn and made it behave differently to future melt.”

The researchers also towed a radar unit behind their skidoos to gather profiles between core sites along a 100-kilometre path from the low elevation ice sheet margin into the high elevation ice sheet interior. They analysed the firn cores on the spot by cutting them into small sections to quantify their properties, such as their density, so they could compare them with samples collected the following year.

“The year-on-year firn changes were quite dramatic,” said Colgan. The findings overturned the idea that firn can behave as a nearly bottomless sponge to absorb meltwater. Instead, we found that the meltwater storage capacity of the firn could be capped off relatively quickly,” he said.

“Basically our research shows that the firn reacts fast to a changing climate. Its ability to limit mass loss of the ice sheet by retaining meltwater could be smaller than previously assumed,” said Machguth.

Because the models scientists use to project Greenland’s sea level rise contribution do not presently take firn cap-off into consideration, it means that Greenland’s projected sea level rise due to meltwater runoff is likely higher than previously predicted. Getting this newly observed physical process into these models is an important next step for the team.

Using unmanned aerial vehicles, Colgan also plans to begin surveying the changes in ice sheet surface reflectance caused by the development of massive ice layers associated with firn cap-off. There are preliminary indications that firn cap-off is also occurring in the ice caps of the Canadian High Arctic.

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