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The glaciers of the Greenland Ice Sheet are running away

Even if snowfall rates returned to those of the 1980s, the ice sheet would continue to shrink. Image credit: Michalea King.

Even if snowfall rates returned to those of the 1980s, the ice sheet would continue to shrink. Image credit: Michalea King.

  • Greenland’s massive ice sheet will continue shrinking even if snowfall rates return to the higher levels of decades ago, when the ice sheet was stable, a new study shows. 
  • Rates of ice loss climbed dramatically in the early 2000s before settling at a higher, sustained state of decline.
  • For each kilometer that Greenland’s glaciers retreat, their rate of ice loss speeds up by 4 to 5 percent—a bleak trend that will accelerate sea-level rise.

The Greenland Ice Sheet is losing mass, and there may be no way to stop it, scientists have concluded. Even if large amounts of snowfall return to the ice sheet, it will continue to shrink, according to new research published in Communications Earth and Environment.

The problem appears to lie with Greenland’s glaciers, which deliver ice from inland areas to the sea. “They’re conveyor belts of ice,” said glaciologist Michalea King of The Ohio State University, first author of the study. “If you look at an aerial image of a glacier, it looks like a long finger of ice that extends off of an ice sheet.”

These glaciers act like dams that control how much ice flows into the ocean, said Michael Wood, a postdoctoral researcher at NASA’s Jet Propulsion Laboratory in Pasadena, California, who was not involved in the study. “What’s happened is that a lot of those dams have burst,” Wood told Mongabay. The new study, he said, shows this process “is fully or mostly responsible for the extra ice flooding into the ocean.”

Greenland’s glaciers are discharging more ice than snowfall can make up for. Image credit: Michalea King.

As Greenland’s thick shield of ice melts, it is the single largest contributor to global sea level rise. Between 1992 and 2018 it shed about 3.9 trillion metric tons, raising sea level about 11 millimeters (0.4 inches), a recent study in Nature reported. If the entire ice sheet melted, sea level would soar more than 7 meters (23 feet).

To gauge how the glaciers have changed, King and her colleagues analyzed more than three decades of satellite imagery and other data on the sizes and speeds of the island’s ice streams. They examined how the margins of the glaciers have moved and compared these shifts to estimated rates of ice loss, or discharge.

They observed that before the turn of the century, the ice sheet was in balance: accumulated snowfall kept pace with surface melting and glacier discharge. That stable rate was about 450 gigatons per year. (For scale, one gigaton is roughly twice the estimated weight of all humans on the planet.)

Even if snowfall rates returned to those of the 1980s, the ice sheet would continue to shrink Image credit: Michalea King.

But around 2000, the dam burst, and discharge rates began to rapidly climb. By 2006, those rates had settled at around 500 gigatons per year, outpacing rates of snowfall. Today, discharge remains approximately 14 percent higher than it was between 1985 and 1999.

As the glaciers shrink, they flow faster and remove more ice from the ice sheet—an effect that feeds on itself. For each kilometer a glacier retreats, the team found, ice discharge speeds up 4 to 5 percent. King used this analogy: “If you have a really long traffic jam, it can take a while for the cars to move. As you get a shorter traffic jam—or a shorter glacier—everything can move a little faster.”

The glaciers are much shorter now than they were before 2000, King told Mongabay. As a result, “we’re losing ice faster than before,” she said. It wouldn’t be enough to bring back the “healthy amounts” of snowfall present in the 1980s, the team concluded. Greenland’s glaciers are now retreating so fast that they would still lose mass.

All around the Greenland Ice Sheet, glaciers retreat is accelerating. Image credit: Michalea King.

Understanding these trends helps climate modelers anticipate how quickly sea levels will rise, said Wood. According to a study published last year in Nature Communications, China and some tropical countries in South and Southeast Asia will experience the impacts of rising seas more severely. Projections based on melting rates in Greenland could help such nations plan for changing coastlines, noted Wood.

Coverage of King’s study—and a statement from her university—depicted a “point of no return” for Greenland’s ice. But King has pushed back on this notion. “I don’t want to send the impression that we should do nothing about climate change,” she said. “There are different trajectories that our planet can follow, depending on how we act. We shouldn’t do nothing.”

Citations:
King, M.D., Howat, I.M., Candela, S.G. et al. Dynamic ice loss from the Greenland Ice Sheet driven by sustained glacier retreat. Communications Earth and Environment 1, 1 (2020). https://doi.org/10.1038/s43247-020-0001-2

Kulp, S.A., Strauss, B.H. New elevation data triple estimates of global vulnerability to sea-level rise and coastal flooding. Nature Communications 10, 4844 (2019). https://doi.org/10.1038/s41467-019-12808-z

Shepherd, A., Ivins, E., Rignot, E. et al. Mass balance of the Greenland Ice Sheet from 1992 to 2018. Nature 579, 233–239 (2020). https://doi.org/10.1038/s41586-019-1855-2

Nikk Ogasa (@nikkogasa) is a graduate student in the Science Communication Program at the University of California, Santa Cruz. Other Mongabay stories produced by UCSC students can be found here.