Climate researchers racing to calculate how fast and how high the sea level will rise found new clues on the seafloor around Antarctica. A study released today suggests that some of the continent's floating ice shelves can, during eras of rapid warming, melt back by six miles per year, far faster than any ice retreat observed by satellites.
As global warming speeds up the Antarctic meltdown, the findings "set a new upper limit for what the worst-case might be," said lead author Julian Dowdeswell, director of the Scott Polar Research Institute at the University of Cambridge.
The estimate of ice shelf retreat is based on a pattern of ridges discovered on the seafloor near the Larsen Ice Shelf. The spacing and size of the ridges suggest they were created as the floating ice shelves rose and fell with the tides while rapidly shrinking back from the ocean. In findings published today in Science, the researchers estimate that to corrugate the seafloor in this way, the ice would have retreated by more than 150 feet per day for at least 90 days.
Ice shelves float on the ocean but they are fastened to land and act as stoppers that prevent Antarctic ice sheets that are as big as the U.S. and Mexico combined from sliding into the sea. The shelves are frozen to outcrops on the seafloor, but when they melt away from those anchor points, the flow of ice into the ocean speeds up, accelerating sea level rise.
If the rate of retreat estimated by the new study extended across an 18-mile-wide and half-mile-thick ice shelf, as found in the closely watched Pine Island Glacier in West Antarctica, the researchers calculated it would release 138-gigatons of ice per year— three to five times more ice than is currently lost annually from that glacier system.
Discovery May Show Precedent for Rapid Melting
Sea level rise can't be directly extrapolated from the rate of ice shelf retreat, said University of Liège ice researcher Xavier Fettweis, but the results of the new study suggest it could accelerate in coming decades.
"With such retreat rates, the sea level rise contribution from Antarctica could be a lot higher and quicker than expected, as the models are tuned to represent the current observed retreat rates," he said.
Currently, the fastest retreat rates are more than half a mile per year for the ice shelves extending from the Pine Island and Thwaites glaciers, and 1.2 miles per year for the one below the Smith Glacier, which is also in West Antarctica.
There's still no way to know exactly how fast the meltdown will happen with increasing human-caused warming, Fettweis said. But between 12,000 and 15,000 years ago, global sea level rose about 1.5 to 2.3 inches per year for several centuries, raising sea level by 82 feet over a 500-year period.
There's enough ice left on Antarctica to raise sea level by about 200 feet if it all melted. That makes the new evidence of how quickly the previous melt occurred critically important for millions of people in coastal areas whose homes and fields are already at risk, said Twila Moon, an Arctic ice researcher with the National Snow and Ice Data Center, who was not involved in the study.
Rapid ice melt would increase risks of flooding, saltwater intrusions into drinking water supplies and coastal erosion, she said.
"We need to both decrease emissions to stave off the worst rates of sea level rise, and we have to start adapting for the amount that is already guaranteed," she said. "I do worry that most communities and societies are behind the ball on both fronts."
Ice retreating six miles per year is "unheard of," said glaciologist Eric Rignot, with the University of California, Irvine and Caltech's Jet Propulsion Laboratory. It's "a wow moment," he said of the findings.
"Glaciers in Greenland retreat at 500 meters (1,640 feet) per year," he said. "Glaciers in alpine landscapes retreat more like 100 meters (328 feet), and that's already something overwhelming for people visiting glacier parks," said Rignot, who was not involved in the new research.
Washboard Sea Floor Likely Carved by Ice and Tides
The new estimates came from a 2019 research project in the Weddell Sea, east of the Antarctic Peninsula. The scientists sent underwater drones diving 500 meters deep, where they captured clear images of a 12,000-year-old pattern of seafloor ridges spread across about 3.5 square miles around the Larsen Inlet, like a giant washboard.
The ridges are about three feet tall and spaced about 60 to 80 feet apart, suggesting the ice shelf was retreating as it was repeatedly lifted and lowered by the tides, Dowdeswell said. The shelf withdrew up to 160 feet every day, so each time the tide fell, the massive lower lip of the ice carved a new ridge in the sediments on the seabed.
The corrugated pattern they found doesn't show that all Antarctic ice shelves will melt at that rate for long periods of time, he added, but it may indicate the upper limits of what is possible.
The researchers have no other theories that adequately explain the formation of the ridges at such regular intervals. And there's evidence of comparable glacial activity elsewhere on the planet. In the Baltic Sea, similar patterns trace the retreat of the Scandinavian Ice Sheet at the end of the last ice age, said University of Stockholm marine geologist Martin Jakobsson, who wrote a Perspective piece in the same issue of Science to accompany Dowdeswell's paper.
"I agree with them that there is no other better explanation for the pattern," he said.
More research into the processes that caused the ridges is already under way, Jakobsson said, and that could also help show whether to expect such retreat again and on how wide a scale.
A Mysterious Past and an Uncertain, but Worrisome, Future
Ted Scambos, a senior scientist with the Cooperative Institute for Research in Environmental Sciences at the University of Colorado, Boulder, said he was surprised at the confidence with which the ridges are linked with tidal cycles.
He was not involved in the study, but recently reviewed another paper showing seabed scars from massive ice collapses, also suggesting that an Antarctic meltdown could happen faster than most climate models show.
"Even then I had some reservations about whether the scars in the seabed were incontrovertibly a result of really tall icebergs breaking off," he said. "This paper is interesting ... If it's really a tidal cycle, then yes, that's a really fast retreat."
Although the paper doesn't prove 100 percent that the ridges are linked with the tides, at face value, Scambos said, they do show that something was changing the behavior of the ice shelf in a significant way.
"What they are implying is that something starved the glacier to make it retreat at a really rapid rate," he said. "Was it starved of snowfall because of changing weather patterns? Or was it rapidly being melted at the base of the ice? It all implies big changes in climate."
The new study highlights the "deep uncertainty on how fast an ice sheet can retreat," said Rignot. "Along the Antarctic Peninsula, we've already seen glaciers speeding up by a factor of eight following the collapse of an ice shelf," he said. "Now we hear about glaciers retreating 10 kilometers per year."
The findings could mean ratcheting up projections for sea level rise "big time," he said. Model projections are conservative, he said, because they don't include mechanisms that can trigger rapid and extreme ice shelf retreat, such as serially calving icebergs, shifting ocean currents or the splitting action of surface water that seeps deep into the ice and refreezes, causing large-scale disintegrations.
"Let us take a step back and admit that no model would have predicted what we are seeing today," he said. "So we have to be humble about this and recognize that there are still a lot of elements we do not know in terms of how fast an ice sheet can fall apart."
While the new findings shouldn't affect current projections that sea levels will rise between 1 and 4 feet this century, beyond that they could have substantial implications, Rignot said.
"The most important message to take home is that the current projections are too conservative. We know it," he said. "The real drama in all of this is that the faster rates of retreat may turn out to be the most probable in some places, and as of now we do not know where and when."