Scientists track rates of ice sheet retreat to better estimate contributions to global sea level rise. Antarctica and Greenland have lost more than 6.4 trillion tons of ice since the 1990s, raising global sea levels by at least 0.7 inches (17.8 millimeters). Together, the two ice sheets are responsible for more than one-third of total sea level rise.
The rapid retreat found in the Eurasian Ice Sheet far outpaces the fastest-moving glaciers studied in Antarctica, which have been measured to retreat as fast as 160 feet per day. Once the ice retreated towards the landit rises from its anchorage on the seafloor and begins to float, allowing it to flow faster and increase its contribution to sea level rise.
If air and ocean temperatures around Antarctica rise as expected and match those at the end of the last ice age, researchers say the ice march back hundreds of feet a day could -triggered the collapse of modern glaciers earlier than thought. That could be devastating for global sea levels.
“If the temperature continues to rise, the ice can melt and thin from the top as well as from the bottom,” said lead author Christine Batchelor, “so there could be a scenario more like what we do. had [off] Norway after the last glaciation.”
In the new study, Batchelor and his colleagues examined the former beds of two major ice streams across the Norwegian continental ice shelf that formed between 15,000 and 19,000 years ago. Using ship-borne imagery, the team calculated retreat rates by studying wavelike ridge patterns along the seafloor. They determined that the smooth ridge patterns were likely created as the front of the glacier bounced off the seafloor from daily high tides. The team mapped the spacing of more than 7,000 ridges to calculate the rate of retreat.
“The ice sheet can move several kilometers per day, hitting the low tide and the high tide creating these ridges as it does, kind of rippling up and down the water,” said Batchelor, a physical geographer at Newcastle University .
The team found that retreat rates ranged from 180 to 2,000 feet per day. Extreme rates only last on a scale of days to months and are unlikely to be sustained much longer. If an ice sheet retreats about 600 meters per day for a year, Batchelor says, you probably won’t have any ice left.
“This is not a model. This is true observation. And honestly it’s scary. Even to me,” Eric Rignot, a glaciologist who was not involved in the study, said in an email.
In the past, one of the fastest rates of retreat seen for a glacier was at Pope Glacier in West Antarctica, a smaller glacier very close to the massive Thwaites Glacier, nicknamed the “doomsday glacier” because of its relatively large size. its contribution to sea level melting. rise At one time in 2017, based on satellite calculations, Pope Glacier retreated at a rate of about 105 feet (32 meters) per day. That’s pretty fast — but still not the same as rates of up to 2,000 feet per day, the study found for the Eurasian ice sheet.
The high rate found in the study released Wednesday is about 20 times higher than any retreat rate measured from satellites, and 12 times faster than any retreat rate inferred from similar landforms on the seafloor, Batchelor said.
The rate of retreat in the Pope has now slowed, and similarly, for the Eurasian ice sheet, the pulse of rapid retreat is only temporary. Still, it’s worrying, says Rignot, one of the scientists who published a 2022 paper that Pope Glacier’s retreat is documented.
“The ice is receding fast now, [especially] in Antarctica,” said Rignot, a scientist at the University of California at Irvine. “But we see traces on the seafloor that the retreat can go faster, faster, and it’s a reminder that we haven’t seeing everything.”
Not all ice beds are susceptible to them so quickly shrinkage rates. The study found that the fastest melting occurred in the flattest areas of the ice bed.
According to Batchelor and his colleagues, retreat rates appear faster in flat areas because the ice slabs are more buoyant compared to the glacier’s steeper slopes. He said that a uniformly thick and flat bed of ice would soon float, as it sat on top of a flat sea floor. A little melting can push the flat ice bed and move it towards the landsuch as a large ice cube that can float to the bottom of the ocean with enough thinning.
A sloping ice bed, he explained, would make it harder to sink to the bottom of the sea. The inclined surface of the ice at the top of the ice sheet will increase the “driving stress,” which will push the ice harder into the ground. Steeper ice beds are more likely to rise from the sea floor during tidal movement and become buoyant.
The findings concern some current glaciers in Antarctica near flat ice beds, Batchelor said. For example, Thwaites Glacier is pinned to a ridge but has a flat part of its bed only four kilometers away. Batchelor said if the ice retreats back to this flat part of its bed, then “we’re going to see one of those pulses of really fast retreat happening in that area.”
University of South Florida marine geophysicist Alastair Graham, who was not involved in the study, said he was “cautious about drawing direct lines” between how the last Norwegian ice melted about 15,000 years ago and what’s happening in Antarctica today – in part because sea levels were rising faster then.
“This may be a critical driver of past rapid change that we won’t see repeated on the same scale now or in the near future,” he said. Norway is also located in a more temperate area than Antarctica and will have less ice.
In any case, he said “the discovery that ice sheets are capable of very rapid change because they are actually lifting away from their bed is an exciting one” and in line with research of his own group. Similar to examining ridges along the seafloor near Thwaites, Graham was “amazed” to document that the glacier had pulses of retreat of up to 32 feet (10 meters) per day across the ice front. He agrees that the glacier is likely to see a “pulse” of rapid retreat over the coming decades to centuries, but is uncertain whether it will be on the order of hundreds of meters per day.
But Glaciologist Andrew Shepherd of Northumbria University, who was not involved in the study, said the rates of retreat of the Eurasian ice sheet may not have been that different from what might be happening today. That’s because the new research is based on seafloor markings that occur twice daily. In contrast, today’s satellites are typically used to make annual measurements of change, Shepherd said, so they are not good at detecting rapid, but relatively short-lived, periods of change.
“I don’t think about that [the new research] changes what we think about … retreat on longer (eg annual) time scales, it simply tells us that the retreat occurs in short intense bursts likely accompanied by long pauses,” Shepherd said in an email.
