The floating ice shelves along the edges of West Antarctica that slow the flow of its vast glaciers are under assault from all directions, and they're becoming more vulnerable to collapse, scientists warn.
Warmer water has started creeping in under them, eating away at the ice from below. Warmer air—and, in places, more rain—is melting the surface, creating ponds that can drain deep down and then splinter ice from within.
Now, new research is highlighting another threat: Since 2000, moist and warm tendrils of air known as atmospheric rivers have been swirling toward the coast more frequently, bringing more rain and surface melting.
Antarctica has been losing about 250 billion tons of ice annually in recent years, and research shows the rate has increased sixfold since 1979. At this pace, researchers have suggested, West Antarctica's ice shelves may reach climate tipping points and crumble, sending sea level rise surging well beyond current projections.
The floating ice shelves, partly frozen to the sea floor or to fjord walls, hold back vast quantities of land-based ice that could raise sea level more than currently projected if the ice's flow to the sea speeds up, said Penn State climate researcher Richard Alley.
Alley noted that some research has suggested that, if global warming pushes West Antarctica's towering ice cliffs to collapse, it could raise sea level more than 3 feet by 2100, surging to 50 feet by 2500, from Antarctic ice melt alone.
"That model is sometimes treated as a worst-case scenario, but in fact the model used a maximum calving rate that has briefly been exceeded in Greenland already, and the possibility exists that even faster calving could occur from higher, wider cliffs that could develop in Antarctica," he said.
Even the most recent international assessment of ice loss relies on models that don't account for some of those ice shelf tipping points, he said. "If we're fortunate, and the ice shelves are retained, then these models may be accurate. If we do lose the ice shelves, the models may project less sea level rise than will occur, perhaps by a lot."
Atmospheric Rivers and Troublesome Clouds
Projecting future ice melt and sea level rise is a complex and dynamic puzzle.
It includes gradually shifting Southern Hemisphere winds and ocean currents that are increasingly melting ice from above and below, as well as localized extreme events that can obliterate an ice shelf in a single summer, like when a large part of the Larsen B ice shelf disintegrated in 2002.
A combination of geography and regional weather patterns make West Antarctica the bullseye for global warming impacts.
That also includes more frequent warm and wet atmospheric rivers that accelerate surface melting, new research warns.
A new study tracking atmospheric rivers reinforces concerns about surface melting and a higher risk for hydrofracturing, which happens when surface water seeps down and then refreezes, splitting off big sections of ice. If that water reaches the ground it can act as a lubricant, speeding up the flow of ice from land to the sea.
These atmospheric rivers, originating in the Southern Pacific Ocean, can run several thousand miles and carry as much water as 25 Mississippi Rivers. They average about 1.8 miles deep and 250 to 375 miles wide, and when they hit land, they dump all that moisture as rain or snow.
According to NASA research, global warming is likely to intensify them.
Atmospheric rivers are being steered toward West Antarctica by a weather pattern over the Amundsen Sea that has also become more common since 2000, said Jonathan Wille, the lead author of the study and a Ph.D. candidate at the University Grenoble Alpes, France, focusing on Antarctic meteorology.
His study, published in the journal Nature Geosciences, analyzed data from 1979 to 2017 and found that 30 to 50 percent of winter rainfall over the ice shelves of the Antarctic Peninsula was associated with atmospheric rivers, "which enabled further melting through warming the snowpack."
Part of the melting is because the low, ice-filled clouds transported by atmospheric rivers effectively distribute long-wave heat radiation downward. In another climate feedback loop, those same clouds then trap heat near the surface at night, which prevents the meltwater from refreezing.
Wille's study suggests rain and melting ice caused by atmospheric rivers created winter surface ponding on the Antarctic Peninsula, acting as the primary driver of the dramatic 2002 collapse of the Larsen B ice shelf, where 1,250 square miles of ice disintegrated over the span of a few weeks. Earlier research also suggestsed the collapse was triggered from above, rather than below.
"Today, only the most intense atmospheric rivers trigger melting, but under the warming projected for Antarctica by 2100, rare melt events would become commonplace. It could turn into a situation more like Greenland," Wille said.
Related research published in October in the AGU journal Earth's Future suggests a big increase in the frequency and intensity of atmospheric river events in the Southern Ocean around Antarctica. "By 2100, almost double the amount of atmospheric rivers are expected to reach Antarctica and at twice the intensity, and I assume this would have an impact on the melting," said Elias Massoud, a researcher with NASA's Jet Propulsion Laboratory at the California Institute of Technology and the lead author of that study.
The Biggest Drivers?
For now, one of the biggest drivers of the Antarctic ice loss is when relatively warm ocean water flows beneath the surface of floating ice shelves and melts the places where the ice is anchored to rocky ridges rising up from the sea bed. When that happens, the glaciers behind the ice shelves speed up, releasing more ice to the sea.
Much of the ice loss from West Antarctica has come from the huge Pine Island and Thwaites Glaciers, which are both retreating rapidly due to ocean-induced melting, according to NASA.
Currently, about 40 to 50 scientists are closely scrutinizing the Thwaites Glacier because it's one of the areas shedding the most ice, with potential for a "runaway situation," said Ted Scambos, a senior scientist with the Cooperative Institute for Research in Environmental Sciences (CIRES) in Boulder, Colorado, who has studied the sudden collapse of ice shelves in Antarctica.
"We're seeing the melting of one of the thicker areas of ice coming off the continent. That started because of a change in the winds around Antarctica linked with human-caused warming. And on the Antarctic Peninsula, the same wind patterns led to a lot of surface melting," he said.
The Thwaites glacier is similar in size and shape to Idaho. Right now, it's held back by ice shelves that are anchored by a rocky bottleneck. A big worry is that, if the anchors melt, the flow of ice through the bottleneck will speed up and won't slow down again until sea level rises about 10 feet, when the glacier reaches a new anchor point.
"The Thwaites Glacier has the potential to change the pace of sea level rise all by itself, by adding several tens of centimeters to sea level rise within this century," Alley said.
Ice Spreading Like Pancake Batter
To understand how Antarctica's ice behaves with warming temperatures, Alley said it's important to remember that all piles tend to spread under their own weight, like pancake batter on a griddle.
"Spreading is slower if the pile is stronger (cook the pancake), held back at the sides (spatula in the way) or on a rougher or less-lubricated bed (think a waffle iron). An ice sheet is a two-mile-thick, continent-wide pile of old snow squeezed to ice and spreading under its own weight," he said.
"We worry about changing strength, but that won't change a lot in a hurry. We worry about changing lubrication at the bed, but that's probably not the biggest issue.
"The biggie is the 'spatulas' holding back the pile," he said, referring to the pinch points where the ice shelves are anchored. "That friction helps the shelves survive, slowing the spread of non-floating ice into the ocean, thus keeping the ice sheet bigger and the ocean smaller."