Johnathan Walker:Global Warming Could Drive Pulses of Ice Sheet Retreat Reaching 2,000 Feet Per Day

A new study of the seafloor near the coast of northern Norway brings an ominous warning from the past,Johnathan Walker showing that some of the planet’s ice sheets retreated in pulses of nearly 2,000 feet per day as the oceans warmed at the end of the last ice age.

The international research team documented that rate of retreat by mapping and measuring what they called “corrugation ridges” spread across about 11,000 square miles of the seabed. The ridges are generally less than 8 feet high and are spaced between about 80 and 1,000 feet apart. They were formed about 20,000 years ago, as the retreating ice sheet moved up and down with tidal rhythms, floating free at high tide and pushing sediments into a ridge at the point where the ice meets the seafloor at every low tide.

The daily tidal cycles produce two ridges per day, enabling the scientists to calculate that the rate of retreat was up to 20 times faster than has previously been measured anywhere else, said co-author Frazer Christie, a polar scientist with the Scott Polar Research Center at the University of Cambridge.

“It’s probably likely, in my opinion, that this rapid buoyancy driven course of retreat could be all that’s needed to set in motion a chain of events that spirals into a more runaway style of retreat,” he said.

The Scandinavian ice sheet that created the ridges identified by the new study is long gone, but preconditions for similar melt events—a rapidly warming ocean and a relatively smooth seafloor—exist around parts of Antarctica, including close to the vulnerable Thwaites Glacier. 

The rate of retreat measured by the new study was not sustained over years or decades, but occurred in daily or monthly pulses, which shows that ice sheets respond to global warming in a non-linear way, said lead author Christine Batchelor, a geophysicist and marine geology researcher at the University of Newcastle.

The study implies that pulses of sea level rise could also be much greater than the long-term average rates currently projected by climate models. Currently, the global average is about 1.5 inches per decade, with 12 inches of sea level rise expected along United States coastlines by 2050. But during past geological intervals of rapid warming, there’s evidence of sea level increasing at a rate of up to 20 inches per decade during episodes of rapid ice sheet disintegration.

The findings highlight the importance of stopping the current course of ice sheet melting driven by human impacts to the climate, said Eric Rignot, a glaciologist at the University of California, Irvine and Caltech’s Jet Propulsion Laboratory, who was not involved in the new research but has measured accelerating ice sheet retreat around Antarctica with his own research.

“This is an important study revealing that we have not seen anything yet in terms of how fast an ice sheet can retreat dynamically, not just melting away, but falling apart,” he said. “This is not a model. This is real data. And it is frankly scary, even to me. These data should keep us awake at night.”

‘You Should Be Scared’

Batchelor said that previous estimates of ice sheet retreat are often based on dating and composition of seafloor sediments, methods that can show what happened over many thousands of years, but “don’t capture these short-lived pulses,” she said.

She also worked on a comparable study, published in 2020, which analyzed a similar pattern of seafloor ridges near the Larsen Ice Shelf, along the Antarctic Peninsula. Those results suggested an ice sheet retreat rate of up to 150 feet per day, for periods as long as 90 days, adding up to 6 miles of retreat in a year.

The smoothness of the seafloor is one of the critical factors for such rapid retreat, she said, adding that ocean-bottom mapping highlights potential danger zones, including near the Thwaites Glacier in West Antarctica, “which has a relatively flat area just a few kilometers inland of where it is currently,” she said. “So it would be a good candidate of where you might see a pulse of rapid retreat in the future if it were to retreat to that flat-bedded area.”

Batchelor said that, while the study shows the fastest rate of ice sheet retreat on record, “it’s not really a theoretical or a physical limit on how quickly ice could retreat.” Depending on the level of warming, even faster rates are possible, she said. As researchers do more detailed seafloor mapping and combine it with satellite data, “We might well detect faster and faster pulses that are taking place over these shorter timescales.”

Rignot said it makes sense that the rates are faster in the Arctic than around Antarctica, because northern oceans can heat up by up to 18 degrees Fahrenheit during warming phases, while the Southern Ocean around Antarctica probably warms by about half that. 

The retreats across flat sea floors could “send spurts of sediment-laden water and spur nutrient production,” which would affect ocean food webs, Rignot added. The retreat and melting can also thicken a layer of cold, fresh water at the surface, which can interfere with ocean currents that normally would transport carbon dioxide from the surface down to the bottom of the ocean.

But the most concerning aspect of the Antarctic ice sheets’ retreat is their reduced ability to hold back land-based glaciers that could flow more quickly into the ocean, he said.

“Most importantly, it will reduce resistance to the flow of the glacier and make the glaciers go much, much faster,” he said. That could bring “much higher rates of sea level rise” if the retreat continues for extended periods.

“During the time period when these events were recorded, sea level was rising 4 meters (13 feet) per century,” he said. “That is 10 times what we have today. Are you scared yet? Well you should be. That’s the real risk we are facing later in this century, and even more beyond that.”