|October 10, 2019|
How Antarctic ice sheets are melting from above and below
|The frozen mountains and icy plains of Antarctica hold enough water to raise global sea levels nearly 200 feet. Thankfully, over three-quarters of the continent is girded by ice shelves, the floating extensions of glaciers that protect the land-bound ice behind them like walls surrounding a vast, icy castle. But scientists are discovering new vulnerabilities that could weaken those walls from above and below.|
In the frigid realm of East Antarctica, tens of thousands of brilliant blue lakes are forming across ice shelves in the summertime---far more than scientists previously realized, according to a study published last month in Scientific Reports. Meanwhile, in rapidly-melting parts of West Antarctica, "upside down rivers" of warm water are gnawing away at the ice shelves' weak spots from below, according to a study published Wednesday in Science Advances.
As the Earth continues to warm, both processes could hasten the demise of Antarctica's icy armor and the giant glaciers it holds back.
Invasion of the blue lakes
For much of the year, the ice blanketing Earth's polar regions is locked in a deep freeze. But on mild summer days, the surface of the ice begins to melt, draining into depressions and forming topaz blue lakes.
Beautiful as they are, these lakes are bad news for ice. Because of their dark color, they absorb more of the sun's energy, triggering further warming. And under certain conditions, clusters of lakes can drain rapidly into the ice below them, causing it to break apart in a process known as "lake-induced hydrofracturing."
They found a lot more lakes than they were expecting.
In fact, in satellite data from January 2017, the researchers identified a whopping 65,000 lakes and ponds spread across the East Antarctic coastline. "Pretty much everywhere we looked around the margin, you can see lakes," says lead study author Chris Stokes, a glaciologist at Durham University in the UK. "That surprised us."
More troubling than the number of lakes, though, was the fact that many of them seemed to be clustered in regions of ice shelves that could be vulnerable to collapse via hydrofracturing. "We're much closer to those kinds of densities that can cause hydrofracturing than we thought we were," Stokes says.
The study only looked at one melt season, and a weird one at that: In late 2016 and early 2017, mild weather and unusual atmospheric circulation patterns gripped Antarctica's coastlines, causing its sea ice to crash. The scientists are planning to repeat their analysis with additional years of satellite data, Stokes said. But even if 2017 was an outlier, the study suggests some of East Antarctica's ice shelves could be more vulnerable to warm years than we thought.
And as Alison Banwell, a glaciologist at the University of Colorado Boulder who wasn't involved with the paper, notes, "Atmospheric temperatures are warming, so we should expect to see more warm years."
As lakes raise concerns about the future of East Antarctica's ice shelves, to the west an unseen force is attacking ice from below. Blobs of warm water are rising up from the deep, forming river-like channels that eat away at the bottoms of those ice shelves.
Several years back, Karen Alley, a glaciologist at the College of Wooster, began studying these so-called 'upside down rivers' via satellite imagery. By examining depressions at the surface of the ice, she could tell that some of them were enormous---up to three miles wide, tens of miles long, and hundreds of feet deep. She also noticed that the rivers frequently formed below what glaciologists call "shear margins," weak points at the edges of ice shelves.
In their latest research, Alley and her co-authors used satellite imagery to try and understand why that is. As they report in their new paper, the formation of rivers beneath shear margins seems to start on the land, when flowing ice streams are stretched and thinned along their edges. When those stretch zones reach the ocean, they rise up relative to the thicker ice surrounding them, creating what looks like an inverted river bed running across the bottom of the shelf. Rising warm water gets funneled into that bed, forming an upside down river.
The researchers found that the rivers are most likely to form beneath fast-flowing ice, including the ice shelves protecting West Antarctica's imperiled Pine Island and Thwaites glaciers. And in general, "there were a lot more of these ice shelf channels on shear margins than we thought," Alley says.
Nobody's sure just how important upside down rivers are in hastening ice shelf breakup. But in earlier work on the Getz ice shelf, Alley discovered that the rivers can deepen rapidly, carving as much as 30 feet of ice out of the base of the shelf in a single year. If the water within these rivers becomes even a little warmer due to climate change, that thinning process could speed up dramatically.
"This could matter a little, or it could matter a lot," Alley said. "But we know this makes it more likely that we lose ice shelves. These channels make the weak points weaker."
A two-pronged attack
Taken together, ephemeral lakes and hidden rivers point to poorly-understood weaknesses in Antarctica's icy fortress. And the two phenomena could work in tandem to weaken ice.
Banwell said that the impact of surface lakes on ice shelves often depends on the strength of the shelf's shear margin. A shear margin that's been attacked from below by warm water, she said, might be more vulnerable to breakup due to lakes. And with both the atmosphere and the oceans warming due to climate change, the assault could be heating up on both fronts.
"I think in the future you're going to get more and more of these lakes due to atmospheric warming," Banwell says. "And a greater intensity of melting and ponding combined with these subsurface processes simply means these ice shelves are going to become more unstable."