Tuesday, April 5, 2022

Antarctic ice shelves are shattering. How fast will seas rise?

The Thwaites ice shelf in West Antarctica is the floating end of the Thwaites Glacier, where the glacier flows into the sea. Already, about two-thirds of the ice shelf has fallen apart. 
n December 2021, scientists saw worrisome signs of disintegration in the remaining bit.
Ice shelves slow the flow of ice from land to sea, keeping sea level rise in check. 
Photo by Jim Yungel, NASA Earth Observatory

From National Geographic by Alejandra Borunda

An unexpected ice shelf collapse in East Antarctica, after temperatures spiked 70°F above normal, highlights bigger problems in the West, where one glacier could singlehandedly raise global sea levels several feet.

All scientist Erin Pettit could see when she looked at the satellite photos of the ice shelf in front of the Thwaites Glacier in West Antarctica was the giant crack that stretched across most of the image.

Two years before, when she and her colleagues were deciding where to put their research camp, the entire floating ice shelf—a tongue of ice poking out from the enormous glacier behind it—was solid.
It was plenty safe to plan a camp there, they thought.

But last December, when they were preparing to go to the camp, the images revealed enormous cracks in the ice pointing straight at it.

It was unlikely the cracks would grow fast enough to endanger them.
But to Pettit, it signified something even scarier: the start of ice shelf's disintegration, which is a step toward a larger disintegration of the glacier itself.

In March, East Antarctica—the other, colder side of the continent—saw its first-ever ice-shelf collapse.
As a late Austral summer heat wave brought extraordinary temperatures and high winds to the region, the Conger ice shelf disintegrated within days.
The unexpected collapse highlighted the importance of—and uncertainty about—the continent’s ice shelves, which act like bottle stoppers controlling the flow of ice from land to sea.
Their incipient demise, scientists fear, could be the beginning of more ice loss—and much more sea level rise that would affect countries all over the world.

Despite Conger’s collapse, the most pressing concern is still the ice shelves fringing West Antarctica, where Pettit works.
Their December 2021 discovery suggested the Thwaites ice shelf could disintegrate within the decade, leaving the enormous and unusually precarious glacier unprotected.
The size of Florida, the Thwaites Glacier holds enough ice to raise global sea levels two feet.
It’s also a bottleneck protecting the larger West Antarctic ice sheet, which would raise sea level 10 feet if it were to melt completely.
And because of some crucial, frightening quirks of geology and geography, Thwaites could one day become one of the most significant drivers of global sea level rise.
“It is the most important glacier in the world,” says Julia Wellner, a marine geologist at the University of Houston.

And the trajectory it seems to be on is “alarming,” says University of Colorado glaciologist Ted Scambos, who co-leads a major multi-year research program at Thwaites.
“All by itself, it could change the story. It could change the game of what we need to do by the end of the century” and beyond to adapt to sea level rise, he says—from building “hard” protection like seawalls or levees, to retreating from the coast.

Seas are rising now

Though a precise forecast is impossible, it's clear where sea level is headed: Up, possibly a lot, possibly soon.
Most coastal communities are struggling even to acknowledge the reality, says A.R. Siders, a sociologist at the University of Delaware.
“It’s not a question of if seas will rise two feet, it’s when. We just have to make the decision [to adapt], even with some uncertainty.”

Globally, seas have risen a little over 8 inches since 1900, but the rise is accelerating: A quarter of it has happened since 2006.
In the latest reportfrom the Intergovernmental Panel on Climate Change (IPCC), released in 2021, scientists determined global average sea level is now rising around 0.15 inch (3.7 millimeters) each year.
The IPCC projected with “medium confidence” that it would rise another 15 to 30 inches by 2100, and will keep rising for centuries.

Some regions are seeing a faster rise than the global average.
The U.S. East Coast, for example, is hit harder in part because the Gulf Stream is slowing and funneling less water away from the coast. U.S. coasts on average will likely see a foot of sea level rise by 2050 and two feet by 2100, the National Oceanic and Atmospheric Administration projected earlier this year.

Already, some 110 million people worldwide live in zones vulnerable to flooding by high tides.
With seas a foot higher, hundreds of thousands of U.S. homes on all its coasts, but particularly in the East and Gulf regions, could find themselves flooding as often as weekly.
Two feet would submerge much of the Maldives and other small island nations.

But even by 2100 the sea level rise could be greater than two feet. If we don’t control emissions and the planet warms 9ºF, the NOAA report says, there’s a 50 percent chance of seas rising more than three feet by 2100—and a 10 percent chance of them rising more than six feet.

The biggest source of uncertainty, aside from how fast we’ll choose to curtail emissions, is how fast an overheated ice sheet can crumble and melt—especially the Antarctic ice sheets, which hold enough water to raise sea level by 190 feet.

Billions of tons of Antarctic ice are already falling into the sea each year, but they contribute only a small fraction, about 10 percent, of total sea level rise.
The bulk of the rise comes from seawater expanding as it gets warmer, from mountain glaciers, and from melting Greenland ice, which will likely accelerate toward the end of the century.

Sometime in the future, Antarctica will also start discharging a lot more melt into the oceans.
The question is whether that change will take centuries to play out, reshaping coastlines slowly enough that communities might adapt, or whether it will happen faster.

But the dynamics of the ice shelves and glaciers are fiendishly difficult to predict, especially because warming of this speed and magnitude is unprecedented during the era of human observation.
"We are probably not going to definitively figure this out in the next few decades," says Bob Kopp, a sea level rise expert at Rutgers University.

In the meantime, he and other scientists fear, West Antarctica could cross a tipping point beyond which massive, accelerating ice loss becomes inevitable.

Sea ice area fraction in the Antarctic region, displayed on the Mercator Ocean MyOcean Viewer from September 2021 to February 2022.
Credit: Copernicus Marine Service
The vicious cycle threatening Thwaites

One place they are watching very closely for signs of catastrophe is the Thwaites Glacier, which is already responsible for 4 percent of global sea level rise.

Unlike most of the ice sheets in Greenland and East Antarctica, most of the West Antarctic one sits on bedrock that lies below sea level.
The ice, well over 6000 feet thick in some places, overflows a deep basin, only the rim of which pokes up above current sea level.
Beyond the rim the ice meets the ocean at the “grounding zone”—a giant underwater wall that rises from the seafloor.
At the surface the ice continues out to sea as a floating shelf, a bit like a mushroom cap.

As warm air and seawater melt the ice, the grounding line retreats.
A scary moment will come when it retreats past the rim of the rock basin; beyond that point, the bedrock under the ice slopes downward toward the Antarctic interior.
Any further retreat will just make the underwater ice-wall taller, exposing more ice to water, which can then melt it faster, which pushes the grounding even farther back—a vicious cycle of retreat.
The technical term for this effect is “marine ice sheet instability,” known as MISI.

At the Thwaites Glacier, the grounding line is already right near the rim of the bowl.

But there’s another risk that could accelerate the ice’s demise, one that scientists first recognized only a few years ago. It’s called the “marine ice cliff instability,” or MICI.

As a glacier loses its fringing ice shelf, its front becomes a tall, vertical ice cliff extending from the seafloor up above the sea surface.
Such a bare cliff is likely to be fundamentally unstable, “like a sandcastle,” says Jeremy Bassis of the University of Michigan.
That's because there’s a physical limit beyond which the material—sand or ice—can’t hold itself up anymore.

In 2012, Bassis and Catherine Walker suggested that if ice cliffs got taller than about 1,000 meters (3,200 feet)—a very real possibility in the Thwaites Basin—they might start to catastrophically collapse, accelerating the retreat and exposing ever taller ice cliffs, and so on.
Like the marine ice shelf instability, but on steroids.

When other scientists added this process into their models of the ice sheet, they found something shocking.
In a 2016 study, a team showed that under the worst-case emissions scenario, nearly all the West Antarctic ice sheet could be lost within 500 years.
By 2100 the region’s melt could add an extra 2.5 feet to the world’s oceans.

The Conger ice shelf, in East Antarctica, recently broke apart during an unusually warm spell at the end of Austral summer. It had been slowly disintegrating for years but then collapsed quickly, as winds rose and temperatures spiked as much as 70ºF above normal (to a still-chilly 10ºF).
Photo by NASA via AP

Since then, more detailed studies have quelled that fear somewhat, primarily by finding that when ice cliffs get too high, they may slump rather than collapse.
That slows ice loss considerably, cutting West Antarctica’s contribution to sea level rise to an extra 13 inches by 2100.
It could even be substantially less—just a few inches—if emissions get slashed immediately.

The first sign: a collapsing ice shelf

The first step in the disintegration of the Thwaites glacier is the disintegration of its protective ice shelves, which buttress it and slow its inexorable slide into the sea.
They have already disappeared along two-thirds of the glacier’s 75-mile-long coastline. In those places, ice flows away three times as fast.

That’s why Erin Pettit was so stunned last year when she saw the cracks cutting through the ice shelf near her camp, on the last 25-mile stretch.

The shelf there has been held in place by a tenuous connection to a ridge on the ocean floor that reached high enough to snag the bottom of the ice.
But this season, Pettit and her colleagues found that the ice shelf was no longer touching the ridge—and was beginning to come apart faster than they could have imagined.

This part of the ice shelf, Pettit explains, is shot through with thin breaks that are barely holding together.
It is “likely to shatter into hundreds of icebergs, just like your car window,” she says. That disintegration is likely within the decade and possible as soon as three years from now.

It won’t add to sea level because the ice shelf is already floating; it’s already in the ocean.
But the faster the glacier behind it spills more ice into the sea, the faster sea levels will rise.

How much, how fast?

The geometry of the West Antarctic is such that if the Thwaites Glacier were to collapse, a lot more ice would follow it.
It’s fairly certain, though, that Thwaites won’t have much effect on sea level before 2050, says Ben Hamlington, a sea level rise expert at Cal Tech’s Jet Propulsion Laboratory and one of the authors of the recent NOAA report.
Beyond that, he says, things get much less certain—because of the complex interplay between ice shelves, ice sheets, the shape of the bedrock, and uncertainty about how more carbon humans will emit,

Here we go again -- another Antarctic heatwave may be coming soon.
In the 10-day forecast, areas near the South Pole may reach more than 30C (54F) above average.
The heat will also be impacting West Antarctica and the Thwaites glacier.
While some the processes that could cause rapid, dramatic losses of ice could begin within the next few decades, their full effects are unlikely to spin up until well into the 2100s.

It’s not yet clear whether Thwaites has crossed the threshold of irreversible change, and a recent study suggests there’s still time to stave it off.
Keeping global warming to less than 2ºC, or 3.6ºF—the goal of the Paris Agreement, and still technically possible—should be enough to stave off, or at least dramatically slow, the decline of Thwaites and many other Antarctic glaciers, the study says.
The planet has already warmed 1.1C.

The instability of the West Antarctic Ice Sheet is, in essence, a tipping point: Cross it and there’s little hope of return.
Sustained warming of more than 3ºC, for example, could lock Earth into an eventual sea level rise of 20 to 40 feet over the next several hundred or 1000 years.
Even if in the future it became possible to remove enough carbon dioxide from the atmosphere to bring temperatures back down, the ice sheets would probably be unrecoverable: They are much harder to grow than to break.
“Already, we’re seeing much more retreat than can regrow in a human lifetime,” says Wellner.
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