Ice caps and glaciers along the coast of Greenland passed a tipping point in 1997, when a layer of snow that once absorbed summer meltwater became fully saturated. Since then, the coastal ice fields—separate from the main Greenland Ice Sheet—have been melting three times faster than they had been, according to a new study published Friday in the journal Nature Communications.
"The melting ice caps are an alarm signal for the ice sheet. It means long-term ice mass loss is inevitable. It will increase and accelerate if nothing changes," said lead author Brice Noël, a scientist at the University of Utrecht Institute for Marine and Atmospheric Research. "It's very unlikely the ice caps will recover. It's a climate tipping point—the time at which a change or an effect cannot be stopped."
Climate scientists are wary of tipping points, when a series of small changes make a much larger change inevitable. The fear is a total meltdown of the Greenland Ice Sheet, which would raise global sea level by 24 feet, Noël said. Overall, the rate of ice sheet melting is accelerating, according to peer-reviewed studies cited in the most recent Arctic report from NOAA.
"On a warming planet, there will be less snow and more rain. That will limit the formation of healthy snow that could absorb the runoff in summer. Additional melt will just run off toward the ocean, raising sea level," he said. "What we saw there in normal conditions, before 1997, is that the snow was able to absorb most of the melt and then refreeze. So the melting was not contributing to sea level rise before 1997, even though warming was already ongoing."
The new study, which included scientists from the Netherlands, Switzerland, Norway, Denmark and the United States, focused on coastal ice caps and glaciers at 12 locations around Greenland, tracking the melt-freeze cycle from 1958 to 2015. The sites represent a total of 38,000 square miles of ice, a little larger than Indiana.
The ice fields are Earth's largest glaciers apart from the ice sheets on Greenland and Antarctica. The study found most of the fields are likely to melt by 2100, raising global sea level by about 1.5 inches. All the ice caps at the 12 study sites have been losing mass since 1997, with increases in meltwater runoff varying from 17 percent in the south to 74 percent in the northernmost area.
Ohio State University glaciologist and co-author Ian Howat said the study provides new evidence of how rapid climate change happens. Since the tipping point was reached in the late 1990s, before the recent spike in global warming, it suggests the ice along the edge of Greenland is particularly sensitive to temperatures, he said.
Howat provided an ice mapping model that detailed the surface and exact boundaries of the glaciers, showing exactly where and how the ice caps and glaciers melted during the study period. He said the main Greenland Ice Sheet faces a similar danger, but not as soon because of its vast mass.
Utrecht University researcher Bert Wouters found that data closely matched observations from the European Space Agency's CryoSat-2 satellite. The measurements showed that, for the last 20 years, mass loss has been exactly equal to the amount of meltwater runoff lost to the sea, crucial information for scientists trying to project future sea level rise.
University of Colorado at Boulder scientist Mike MacFerrin, who has spent several field seasons studying the melting ice, said the trend is headed in the same direction for the main Greenland Ice Sheet. "I would anticipate that if we continue warming, we would reach this same sort of tipping point," he said. In a paper published in January, MacFerrin documented how surface water runoff increased as layers of firn atop the ice sheet became more saturated.
Marco Tedesco, an Arctic researcher at Columbia University's Lamont-Doherty Earth Observatory, said the new study's conclusions were not surprising. The mid-90s saw significant shifts in many Arctic systems that affect Greenland's ice, he said, including changing ocean currents and decreasing reflectivity of the surface of the ice sheet.
"Things really changed in the way the snow captured water, and the runoff increased. The new study is a huge help to project what's going to happen on the Greenland ice sheet in the future," he said.