Scientists monitor 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 a third of the total rise in sea level.
The rapid retreat found in the Eurasian ice sheet far exceeds the fastest moving glaciers studied in Antarctica, which have been measured to retreat as fast as 160 feet per day. Once the ice recedes down to earthit rises from its base on the seafloor and begins to float, allowing it to flow faster and increasing its contribution to sea level rise.
If air and ocean temperatures around Antarctica were to rise as projected and coincide with those at the end of the last ice age, researchers say ice retreating hundreds of feet in a day could trigger the collapse of modern glaciers. sooner than thought. That could be devastating for global sea levels.
“If temperatures continue to rise, then we could have the ice melting and thinning from both the top and bottom,” said lead author Christine Batchelor, “so you could end up with a scenario that looks more like what we thought.” had (outside) Norway after the last ice age.”
In the new study, Batchelor and his colleagues analyzed ancient beds of two large ice streams along the Norwegian continental ice shelf dating to 15,000 to 19,000 years ago. Using shipboard imagery, the team calculated retreat rates by studying patterns of wavy ridges along the seafloor. They determined that the ordered patterns of the ridges were likely created when the glacier front bounced off the seafloor by daily tides. The team mapped the space of the more than 7,000 ridges to calculate the rate of retreat.
“The ice sheet can move several kilometers a day, landing at low tide and high tide producing these ridges as it does so, sort of bobbing up and down with the tide,” said Batchelor, a physical geographer at the University of of Newcastle. .
The team found that retreat rates ranged from 180 to 2,000 feet per day. The extreme rates only lasted on a scale of days to months and probably could not be sustained for much longer. If an ice sheet receded about 2,000 feet a day for a year, Batchelor said, there probably wouldn’t be any ice left.
“This is not a model. This is a real observation. And it’s downright terrifying. Even for 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 detected for a glacier was at Pope Glacier in West Antarctica, a smaller glacier that is very close to the massive Thwaites Glacier, nicknamed the “Doomsday Glacier” because of its relatively large contribution to sea level melt. raise. For a period in 2017, based on satellite calculations, Pope Glacier retreated at a rate of about 32 meters (105 ft) per day. That’s pretty fast, but still nothing like rates of up to 2,000 feet per day, the study for the Eurasian ice sheet found.
The higher rate found in the study published 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 at Pope has now slowed, and similarly for the Eurasian ice sheet, the extremely rapid retreat pulse would have been temporary. Still, it’s worrisome, said Rignot, one of the scientists who published a 2022 paper that documented retreat of the Papa Glacier.
“Ice sheets are retreating rapidly today, (especially) in Antarctica,” said Rignot, a scientist at the University of California, Irvine. “But we see traces on the seafloor that the withdrawal could be faster, much faster, and this is a reminder that we haven’t seen everything yet.”
Not all ice beds are susceptible to these extremely rapid changes. withdrawal fees. The study found that the fastest melting occurred in the flattest areas of the ice bed.
Batchelor and his colleagues theorize that the rapid retreat rates appeared in flatter areas because the ice sheets were more buoyant compared to a steeper glacial slope. She said the flat, uniformly thick bed of ice was already close to floating, since it lay on a flat surface of the seabed. Minor melting could push the flat ice bed up and move it down to earthlike a large ice cube that can float to the bottom of the ocean with enough thinning.
A tilted ice bed, he explained, would be more difficult to dig up from the seafloor. The sloped ice surface on top of the ice sheet would increase the “driving stress”, which would push the ice more forcefully towards the ground. Steeper ice beds would be less likely to lift off the seafloor during tidal movements and become buoyant.
The findings are worrisome for several existing glaciers in Antarctica near flat ice beds, Batchelor said. For example, Thwaites Glacier is anchored on a ridge but has a flat area of its bed just four kilometers inland. Batchelor said that if the ice were to recede into this flat area of its bed, then “we might well see one of these really fast receding pulses in that area.”
University of South Florida marine geophysicist Alastair Graham, who was not involved in the study, said he is “cautious about drawing direct lines” between how the last ice sheet over Norway melted about 15,000 years ago and what is happening in Antarctica now, partly because the sea level was rising much faster back then.
“This may be a critical driver of rapid changes in the past that we will not see repeated on the same scale now or in the near future,” he said. Norway is also in a much warmer place than Antarctica and would have had a much more vulnerable ice sheet.
In any case, he said that “the finding that ice sheets are capable of changing extremely rapidly, as they essentially break away from their bed, is exciting” and aligns with research your own team. Similarly analyzing ridges along the seafloor near Thwaites, Graham was “shocked” to document that the glacier had retreat pulses of up to 32 feet (10 meters) per day on the ice front. He agrees that the glacier is likely to experience rapid retreat “pulses” in the coming decades or centuries, but he is not sure if it will be on the order of hundreds of meters per day.
However, glaciologist Andrew Shepherd of Northumbria University, who was not involved in the study, said the Eurasian ice sheet retreat rates may actually not be that different from what may happen today. That’s because the new research is based on marks on the seafloor that would have occurred twice a day. By contrast, today’s satellites have generally been used to make annual measurements of change, Shepherd said, so they are not good at detecting rapid but relatively short periods of change.
“I don’t think (the new research) changes what we think about … withdrawal on longer time scales (for example, yearly), it just tells us that withdrawal occurs in short, intense bursts that are likely interspersed with long breaks,” Shepherd said. in an email.
