The Heat Is Online

Rate of Antarctic meltdown tripled since 2012

After Decades of Losing Ice, Antarctica Is Now Hemorrhaging It

Global warming has already cost the continent 2.7 trillion tons of mass.

TheAtlantic.com, June 13, 2018
 
Climate change has not been kind to Antarctica. According to a comprehensive new study, global warming has already bled the frigid continent, which is larger than Europe, of about 2.7 trillion tons of ice. This enormous amount of ice has already raised global sea levels by as much as a centimeter.

This outflow seems to be increasing: Almost half of all losses have occurred in just the last five years. And the continent is hemorrhaging that mass in a way that will lead to especially high sea levels on the East Coast of the United States.

“The continent is causing sea levels to rise faster today than at any time in the past 25 years,” said Andrew Shepherd, a leader of the new study and a professor of Earth observation at the University of Leeds, in a statement.

“If you take the big picture, when we compared before and after 2012, there was a three-times increase in the amount of melting,” said Beata Csatho, an author of the paper and a professor of geophysics at the University at Buffalo.

The results, published Wednesday in the journal Nature, present what is generally considered to be highest-quality census of the world’s glaciers and ice sheets. Written by 84 scientists across 44 different institutions, including NASA and the National Snow and Ice Data Center, the paper combines several different modes of measurement to produce one consensus result.

“This is the gold standard in terms of demonstrating that ice sheets are changing,” said Robin Bell, a professor of geophysics at Columbia University who was not involved in the paper. “You have three measurements, three approaches, from three different instruments, and they all show pretty much the same thing.”

When something is the size of a continent, how can you tell if it’s shrinking? Scientists have a few different tools at their disposal. First, Antarctica is so massive that it exerts its own gravity field, which can be sensed from orbit by satellites like NASA’s GRACE. Second, researchers can shoot radar or lasers at the surface of Antarctica to detect its surface altitude, which they can then combine with knowledge of ice physics and topography to compute its balance.

Finally, they use the “input-output method.” You may have heard of the 16th-century medical pioneer Santorio Santorio, who discovered human metabolism by weighing his food, his urine, his feces, and himself every day for 30 years. The input-output method applies the same idea to a continent: By measuring the velocity of moving glaciers (often with GPS), researchers can calculate how much snow is being added to a glacier and how much is disappearing into the sea.

The new study combines two dozen previous estimates of Antarctica’s changing mass, arrived at through all three techniques. “These are completely independent data sets,” said Csatho, the University at Buffalo professor. Her team worked on the altimetry data. She told me that she was pleased by how the different methods arrived at roughly the same conclusions. “We didn’t know until a few weeks ago where our results would sit relative to each other. It was a very nice surprise to see our results sitting right where they should be,” she said

The study contains two particular pieces of ominous news—especially for Americans. First, it finds that two glaciers in western Antarctica, named Thwaites and Pine Island, are losing mass at a particularly fast clip.
Recent research has suggested that these glaciers may be subject to a feedback loop called “marine ice-cliff instability,” in which huge walls of ocean-facing ice buckle under their own weight and tumble into the sea. It’s not yet clear whether marine ice-cliff instability will happen at these two glaciers, but if it does kick in, then Thwaites and Pine Island would begin rapidly disintegrating, catastrophically raising global sea levels. Under that scenario, the two glaciers could increase global sea level by more than four and a half feet by 2100, inundating the homes of more than 150 million Americans.

Rob DeConto, a professor of climatology at the University of Massachusetts, Amherst, published research that first warned of that scenario a few years ago. He said it was too early to know whether marine ice-cliff instability had taken hold. “It’s pretty amazing that there’s been such a big uptick in the pace of mass loss down there, but we still can’t say that it’s because this cliff mechanism is kicking in now,” he told me.

But the good news ended there. “This should get people’s attention, especially here in North America,” he said. “This little hook where the ice is going into the ocean, it’s at the worst possible place in terms of its impacts on North America.”

Why? It has to do with one of the stranger mechanisms in ice physics. Glaciers, it turns out, don’t just alleviate sea-level rise by freezing water and keeping it out of the ocean. Their gravity fields are strong enough that they actually attract ocean water from elsewhere on the planet. The farther you go from a certain patch of glacier, the greater the gravitational effects—and West Antartica is very far from the United States. So Thwaites and Pine Island glaciers essentially swaddle themselves with water that would otherwise slosh against the beaches of the East Coast.

But if West Antarctica’s ice melts, and it loses mass, then its gravitational field will also lose its protective power. And North America will suffer the consequences. For example, for every bit of West Antarctic ice that tumbles into the sea, sea levels in Boston will bear an additional sort of gravity tax of 25 percent.   

“For every centimeter [of sea-level rise] from West Antarctica, Boston feels one and a quarter centimeters. And that extends down the East Coast,” said DeConto.

And all of that is only the first bit of ominous news.

The second is more straightforward. For years, researchers have generally believed that the eastern two-thirds of Antarctica—dubbed “East Antarctica”—have been growing in mass. As the climate changed, snowfall had seemed to increase there, and the region seemed to be absorbing more sea level rise than it was contributing.

This no longer seems to be the case. “East Antarctica has begun to contribute to sea-level rise,” said DeConto. “It’s actually become a source now. That’s where most of the ice is—it’s vastly bigger than West Antarctica.”

This offers further evidence of the team’s largest contention that Antarctica’s 3-trillion-ton problem is rapidly getting worse. DeConto referred to a chart that showed the continent’s mass loss over time.

“If you look at the figure—it’s not a straight line going down, it’s like a downward-bending banana,” he told me.

“That’s acceleration. You don’t have to be a statistician to see the pace of mass loss is increasing.”

https://www.theatlantic.com/science/archive/2018/06/after-decades-of-ice-loss-antarctica-is-now-hemorrhaging-mass/562748/