The most complete assessment to date of Antarctica's ice sheets confirms that the meltdown accelerated sharply in the past five years, and there is no sign of a slowdown.
That means sea level is expected to rise at a rate that will catch some coastal communities unprepared despite persistent warnings, according to the international team of scientists publishing a series of related studies this week in the journal Nature.
The scientists found that the rate of ice loss over the past five years had tripled compared to the previous two decades, suggesting an additional 6 inches of sea level rise from Antarctica alone by 2100, on top of the 2 feet already projected from all sources, including Greenland.
"That may not sound like a lot, but it's a big deal for people living along coasts," said University of Leeds climate researcher Andrew Shepherd, who led the assessment, supported by NASA and the European Space Agency. "And the signals are not abating. The ice shelves continue to retreat, warmer ocean water continues to melt them from below, which all means we are progressively going to lose more and more ice from the interior."
Coastal flooding is becoming an increasing problem for some U.S. cities as sea level rises. A study released last week by the National Oceanic and Atmospheric Administration found that the frequency of high-tide flooding had doubled in the past 30 years, with some cities experiencing more than 20 days of it over the past year.
Pine Island and Thwaites Glaciers
University of California, Irvine climate scientist Eric Rignot said the compilation of data on Antarctica's ice loss gives a detailed and thorough understanding of how the rapid changes in ice flow raise sea level worldwide.
"The large signal we see now will only grow bigger with time," he said. As the Southern Ocean warms, there will be additional ice shelf disintegration. That will speed up the flow of land-based ice from the interior to the sea, where it raises sea level.
Between 1992, when detailed satellite measurements started, and 2012, Antarctica lost about 76 billion tons of ice per year. But since 2012, that rate has tripled to about 219 billion tons of ice loss per year, the scientists found.
The recent acceleration is partly driven by natural climate cycles, including El Niño and the Pacific Decadal Oscillation, as well as rising temperatures, Rignot said. In West Antarctica, most of the ice loss has been from the massive Pine Island and Thwaites Glaciers, which have been retreating rapidly.
In the Amundsen and Bellingshausen seas, ice shelves that slow the flow of the glaciers behind them have thinned by up to 18 percent since the early 1990s as warming water has reached underneath them. In some areas, the ice is thinning by as much as 19 to 20 feet per year. A November 2017 study suggested that a rapid collapse of both glaciers is possible, which would raise sea level more than 3 feet by 2100.
Around the Antarctic Peninsula, where air temperatures have risen sharply, more than 13,000 square miles of ice shelf area has been lost since the 1950s, according to the study.
The new assessment used data from satellite altimeter instruments that can measure tiny changes in ice sheet elevations, as well as from gravity-sensing instruments that can track subtle shifts in the distribution of Earth's gravitational field to show where ice loss is happening.
All the data together show that the problem of sea level rise is "growing in severity each year," said UC Irvine researcher and co-author Isabella Velicogna.
Ice Regrew in Past, But Took Thousand of Years
One of the related studies in the series found a surprising pattern of ice sheet retreat and advance in West Antarctica during the past 10,000 years.
As the heavy ice melted rapidly at the end of the last ice age, the land beneath rebounded. The mountains grew higher, colder and snowier, and underwater mountains offshore also rose up, creating new anchor points for floating shelves that blocked the glaciers flowing from land and enabled them to build up temporarily again, even during the post-ice age warm-up.
That could happen again, but not fast enough to help coastal communities facing the threat of sea level rise in the coming decades said Torsten Albrecht, from the Potsdam Institute for Climate Impact Research.
Albrecht's team used data from ice radar, chemical analysis of sediment deposits at the base of the ice and climate simulations to show that the ice retreated inland by more than 600 miles in a period of 1,000 years, then regrew by 250 mile—but that regrowth took 10,000 years, he said.
Some of the radar data came from a 2014 field survey by co-author Jonny Kingslake, with Columbia University's Lamont-Doherty Earth Observatory. Unusual cracks in the base of the Weddell Sea Ice Shelf suggested the ancient ice had, at one point, been stretched or squished rapidly in an area where scientists assumed only slow ice movement. Sediment data showed a similar record, and together, the information helped the Potsdam climate modelers create an accurate picture of the ice sheet cycle.
"People are using the history of the ice flow to tune their models. Knowing more about that past can help us understand what's going to happen in the future," Kingslake said.
Warm Water Intrusions Lead to More Fractures
Another study, published June 13 in the journal Science Advances, raises even more concerns about the vulnerability of Antarctic ice to global warming.
Data from radar and laser readings of the ice enabled scientists with the University of Texas at Austin and the University of Waterloo to map a vast network of channels in the base of many ice sheets formed by intrusions of warm water. Some are several kilometers wide, said University of Texas at Austin researcher Jamin Greenbaum.
They found the channels everywhere they looked, including beneath the ice shelf of the Totten Glacier, in East Antarctica, as well as in Greenland. While scientists have known about the channels for quite some time, the new study found a relationship between what's happening below the ice and on the surface.
Warm water intrusions are melting the ice from below so much that the ice in those channels is cracking. That allows surface meltwater to flow into the fractures, which can destabilize the ice shelf and increase the chances that big chunks will break off, Greenbaum said.
"These things are conspiring to increase loss of ice," Greenbaum said. "It's all a positive feedback system, with the ice getting thinner, more strained, and more susceptible to all these processes."
"In a warming climate, we expect to see more and more melting rivers of surface meltwater, and if they interact with these fractures, you could see more rapid melting," he said. "It could mean we are underestimating the magnitude and the speed of the meltdown."