At the end of the last ice age, parts of an enormous ice sheet covering Eurasia retreated up to a startling 2,000 feet per day — more than the length of the Empire State Building, according to a study released Wednesday. The rate is easily the fastest measured to date, upending what scientists previously thought were the upper speed limits for ice sheet retreat — a finding that may shed light on how quickly ice in Greenland and Antarctica could melt and raise global sea levels in today’s warming world.
Scientists monitor ice sheet retreat rates 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, boosting 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 on the Eurasian Ice Sheet far outpaces the fastest-moving glaciers studied in Antarctica, which have been measured to retreat as quickly as 160 feet per day. Once the ice retreats toward the land, it lifts from its grounding on the seafloor and begins to float, allowing it to flow faster and increase the contribution to sea level rise.
If air and ocean temperatures around Antarctica were to increase as projected and match those at the end of the last ice age, researchers say ice marching backward hundreds of feet in a day could trigger a collapse of modern-day glaciers sooner than previously thought. That could be devastating for global sea levels.
“If temperatures continue to rise, then we might have the ice being melted and thinned from above as well as from below,” said lead author Christine Batchelor, “so that could kind of end up with a scenario that looks more similar to what we had [off] Norway after the last glaciation.”
In the new study, Batchelor and her colleagues analyzed former beds of two major ice streams across the Norwegian continental ice shelf dating back to 15,000 to 19,000 years ago. Using ship-borne imagery, the team calculated the rates of retreat by studying patterns of wavelike ridges along the seafloor. They determined the orderly ridge patterns were probably created as the front of the glacier bounced on the seafloor from daily tides. The team mapped the spacing of the more than 7,000 ridges to calculate the rate of retreat.
“The ice sheet can move over several kilometers per day, touching down at the low tide and the high tide producing these ridges as it does so, kind of bobbing up and down on the tide,” said Batchelor, a physical geographer at Newcastle University.
The team found the 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 couldn’t be sustained for much longer. If an ice sheet retreated around 600 meters per day for a year, Batchelor said, you probably wouldn’t have any ice left.
“This is not a model. This is real observation. And it is frankly 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 retreat rates detected for a glacier was at Pope Glacier in West Antarctica, a smaller glacier that’s very close to the enormous Thwaites Glacier, nicknamed the “doomsday glacier” because of its relatively large melting contribution to sea level rise. During a period in 2017, based on satellite calculations, Pope Glacier retreated at a speed of about 105 feet (32 meters) per day. That’s quite fast — but still nothing like the rates of as much as 2,000 feet per day, the study found for the Eurasian ice sheet.
The upper 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 at Pope has now slowed down, and similarly, for the Eurasian ice sheet, the pulse of extremely rapid retreat would have been temporary. Still, it’s worrying, said Rignot, one of the scientists who published a 2022 paper that documented Pope Glacier’s retreat.
“Ice sheets are retreating fast today, [especially] in Antarctica,” said Rignot, a scientist at the University of California at Irvine. “But we see traces in the seafloor that the retreat could go faster, way faster, and this is a reminder that we have not seen everything yet.”
Not all ice beds are susceptible to these extremely rapid rates of retreat. The study found the fastest melting occurred on the flattest areas of the ice bed.
Batchelor and her colleagues theorize swift rates of retreat appeared across flatter areas because the ice slabs were more buoyant compared to a steeper glacier slope. She said the uniformly thick, flat ice bed was already close to floating, as it sat on top of a flat seafloor surface. Minor melting could push the flat ice bed up and move it toward the land, like a large ice cube that can float off the ocean bottom with enough thinning.
A sloping ice bed, she explained, would be harder to pop off the seafloor. The inclined ice surface at the top of the ice sheet would increase “driving stress,” which would push the ice more tightly into the ground. The steeper ice beds would be less likely to lift from the seafloor during tidal movements and become buoyant.
The findings are concerning for several current glaciers in Antarctica near flat ice beds, Batchelor said. For example, Thwaites Glacier is pinned on a ridge but has a flat area of its bed just four kilometers farther inland. Batchelor said if the ice were to retreat back to this flat area of its bed, then “we could well see one of these pulses of really rapid retreat taking place 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 away about 15,000 years ago and what is happening in Antarctica now — in part because sea level was rising much faster back then.
“This may be a critical driver of past rapid change that we won’t see repeated at the same scale now or in the near future,” he said. Norway is also located in a much more temperate place than Antarctica and would have had a much more vulnerable ice sheet.
In any case, he said “the finding that ice sheets are capable of extremely rapid change as they essentially lift away from their bed is an exciting one” and aligns with his own team’s research. Similarly analyzing ridges along the seafloor near Thwaites, Graham was “astonished” to document the glacier had pulses of retreat up to 32 feet (10 meters) per day at the ice front. He agrees that the glacier is likely to see “pulses” of quick retreat in the coming decades to centuries, but is uncertain if it will be on the order of hundreds of meters per day.
Yet Glaciologist Andrew Shepherd of Northumbria University, who was not involved in the study, said the retreat speeds of the Eurasian ice sheet may not actually be so different from what can happen today. That’s because the new research is based on seafloor markings that would have happened twice daily. In contrast, today’s satellites have generally been used to make annual measurements of change, Shepherd said, so they’re not good at detecting rapid, but relatively short-lived, periods of change.
“I don’t think that [the new research] changes what we think about … retreat on longer (e.g. annual) time scales, it just tells us that retreat happens in short intense bursts that are likely interspersed with long pauses,” Shepherd said in an email.