Scientists have added a new item to the long list of Greenland Ice Sheet woes. Along with snow-darkening algae and increasing rainfall, giant slabs of ice have been thickening and spreading under the Greenland snow at an average rate of two football fields per minute since 2001, new research shows.

The slabs prevent surface meltwater from trickling down and being absorbed by the snow. Instead, more water pours off the surface of the ice sheet and into the ocean.

That's speeding Greenland's contribution to sea level rise, said University of Liege climate researcher Xavier Fettweis, a co-author of a study published Wednesday in the journal Nature. "It is very likely that the current climate models overestimate the meltwater retention capacity of the ice sheet and underestimate the projected sea level rise coming from Greenland ... by a factor of two or three," he said.

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This past summer was either Greenland's worst melt season on record or close to it, scientists said. Melting spiked in June. And in July and August, a heat wave led to melting over more than 60 percent of the ice sheet, including sending temperatures above freezing at the ice sheet's summit. All summer long, the average melt extent was much higher than the 1981-2010 average. There was also open sea ice north of Greenland at a record-early date, and Arctic sea ice hovered near record-low extent all summer.

The formation of the ice slabs is connected to extreme melt seasons like the one Greenland just experienced, said Mike MacFerrin, the study's lead author and a University of Colorado climate researcher who has traversed the Greenland Ice Sheet each spring for several years to measure the phenomenon.

"Ice slabs are forming where they never used to exist because big melt summers are happening ever-more-frequently, and 2019 was one of those years," he said.

If greenhouse gas emissions continue at a high rate, fueling more global warming, the expansion of the ice slabs could increase Greenland's surface runoff contribution to global sea level rise by half an inch to nearly 3 inches by 2100, MacFerrin said.

It is a feedback cycle, he said, but not necessarily "runaway" feedback. "If you cooled the atmosphere, the melting would slow down," he said. "But the Arctic keeps warming, so we see these slabs continuously getting thicker and more extensive, reaching higher-and-higher portions of the ice sheet. Already, we're set to roughly double the size of Greenland's runoff zone by the end of the century, perhaps much more, if warming continues."

That carries important warnings for coastal communities about sea level rise.

"There is no question now that we're seeing melt from both Greenland and West Antarctica that is far ahead of schedule relative to what most models previously predicted," said Pennsylvania State climate scientist Michael Mann. "Only a decade and a half ago, the IPCC was predicting a foot or so of sea level rise by the end of the century. Now, we can no longer rule out 6-8 feet by the end of the century."

The Melt Zone Is Expanding

To understand how the growing ice slabs affect runoff, it's helpful to know that the Greenland Ice Sheet has three zones.

In the coastal melting zone, snow that falls in winter melts away completely, exposing darker ice that melts quickly during warm summer weather. High on the ice sheet is the permanent dry snow and ice zone, where there has been little melting, at least up to now.

Sandwiched in between, where the ice sheet curves upward, is the percolation zone where surface meltwater trickles down into the snow and refreezes. This is the zone where MacFerrin and other researchers first discovered the growing ice slabs in 2012. Using observations and modeling, scientists measured the growth of the ice slabs since 1990, when the ice sheet was stable. In 2002, they determined, the slabs started growing fast.

Some of the ice slabs have thickened from 5 meters to 7 meters in just five years, and they are expanding inland.

The thick layers prevented water from percolating down, and so the scientists found slush fields and runoff channels on the surface that sent torrents of water rushing toward the ocean.

(Animation by NASA, with audio by Laura Krantz of CIRES.)

"Up until recently, nobody was paying attention to the cold slow moving ice in the interior, but now it's waking up, turning into warm wet ice very quickly," MacFerrin said. "These interior regions of Greenland didn't use to run off, so you didn't have to worry about them in terms of sea level rise."

The entire Arctic system, from its thawing permafrost to its diminished sea ice, is in a state of transition as global temperatures rise, said Mark Serreze, director of the National Snow and Ice Data Center.

"These ice slabs are not the sort of thing we were thinking about 20-30 years ago. Basically, the ice slabs are making the runoff process more efficient. Every time we look, we see changes that suggest more rapid melting of the Greenland ice sheet than we thought 20 years ago."

The Kind of Melting Expected in 2050

The growing ice slabs contributed to what will likely be a record melt year on Greenland, once the final data is compiled, Fettweis said. In all, the ice sheet shed about 500 gigatons tons of ice in 2019, "very close to the previous record of mass loss in 2012, mainly as a result of a surface melt that was twice the average," he said.

The extreme melting this year and in 2012 is linked with changes in atmosphere above Greenland. During both summers, winds that usually transport weather systems from west to east stalled, keeping a big area of warm, sunny weather right over the ice sheet.

This year, one of those stalls happened in late June, said Ted Scambos, a polar ice expert with the National Snow and Ice Data Center. The second big melt event, in July and August, was when a European heat wave migrated over to Greenland, he said.

Fettweis said the extreme melting this year and in 2012 "represent what the Intergovernmental Panel on Climate Change projects for a normal summer in the 2050s." But if weather patterns over the Atlantic continue to stall, the tipping point, beyond which the Greenland Ice Sheet would continue disintegrating, could be reached twice as fast as estimated, and "could even be reached at the end of this century if we do not drastically cut our carbon emissions," he said.

MacFerrin said the implications of the rapid growth of the ice slabs are alarming.

"These features are going to play a bigger role as they get larger. The fact that these layers are very near the surface tells you this is recent in these areas," MacFerrin said. "We have a choice, our hand is on the controls. It's going to get worse, but how much is up to us."