23/08/2019

West Antarctica Is Melting—And It’s Our Fault

National Geographic - Alejandra Borunda

 The fingerprints of human-caused climate change have made it to Antarctica, a new study shows
Pine Island Glacier, in West Antarctica, is retreating quickly. In 2014, this iceberg, 20 miles wide, broke off the tongue of the glacier and floated away. Other chunks of ice continue to shear off the glacier. Photograph by Jeff Schmaltz, NASA/GSFC/MODIS Land Rapid Response Team
The towering glaciers of West Antarctica hold the fate of the world’s coasts in their flanks. Their collapse could send sea levels up by at least a foot by 2100—and potentially much more.
For years, scientists have watched and learned that those glaciers are crumbling and melting, the rate speeding up over the decades and imperiling the stability of the entire ice sheet. But while the science was clear that human influences on climate would affect the ice down the line, it has been hard to tell whether human-driven global warming has affected the melting already underway.
Now, a team has unraveled evidence of that human influence. In a study published Monday in Nature Geoscience, a team of scientists showed that over the past century, human-driven global warming has changed the character of the winds that blow over the ocean near some of the most fragile glaciers in West Antarctica. Sometimes, those winds have weakened or reversed, which in turn causes changes in the ocean water that laps up against the ice in a way that caused the glaciers to melt.
“We now have evidence to support that human activities have influenced the sea level rise we’ve seen from West Antarctica,” says lead author Paul Holland, a polar scientist at the British Antarctic Survey.

The ocean eats the ice
The massive West Antarctic ice sheet holds something like 6 percent of the world’s fresh water frozen in its guts. If it all melted away, global sea levels would rise by about 10 feet or more. That’s not likely to happen anytime particularly soon, scientists think, but some parts of the ice sheet are particularly vulnerable, in danger of crossing a crucial “tipping point” if they retreat too far. (Read about the "tipping point" here).
In the past decades, some glaciers in the region have been retreating shockingly quickly. Pine Island Glacier and Thwaites Glaciers, for example, are losing about 100 billion tons of ice each year, and more in bad years. (See what a 10 billion ton chunk of ice looks like in this video).
The glaciers have been receding because their snouts spill over the edge of the continent into the surrounding ocean, which is warmer than the ice. The warm water melts away the ice.
Just how warm the ocean is, though, matters a lot. Over decades, the temperature of the water has waxed and waned, driven in part by natural climate cycles that send different water masses close to the edge of the ice sheet at different times, cycling through from cold to a little less cold every five years or so.

See the crack splitting an Antarctic ice shelf in two
Stunning drone footage shows how an iceberg the size of Houston, Texas is holding on by a thread.
Jaskiran Nagi

The main thing that controls whether warm water makes it to the edge of the ice sheet, it turns out, is the strength of the winds a little bit farther offshore, in the heart of the icy, bitter Amundsen Sea. Sometimes, those winds—cousins of the famous raging band of Southern Ocean winds known as the Roaring 40s—slacken or even reverse. When they do, more warm water ends up near the edge of the ice sheet, which means more ice melts away. (See what the world would look like if all the ice melted away).
“In the 1920s, the winds were pretty much consistently blowing toward the west,” says Holland. “So in the old days, it was cold all the time—it flopped between cold and very cold.”
But now, because of the slow warming of the planet, the whole baseline has moved up. Instead of the cycle flipping between cold and very cold, the flip is between warm and cold.
Scientists knew that the strength of the winds in this region of the Amundsen Sea affected the water temperature. Records of wind strength and direction only went back until 1979. But the patterns in this region match up nearly perfectly with conditions far away, in the tropical Pacific Ocean, where much better, longer-term records exist—so the team could extrapolate how the polar-region winds have changed over the last century.
They used a suite of climate models to look at how the wind patterns would have evolved over the last 100 years if human-caused global warming weren’t in play, and compared that with what the winds actually did. Today’s pattern—with about equal west-flowing and east-flowing winds—means the whole region ends up quite a bit warmer than it was 100 years ago, when the wind flowed toward the west most of the time.

Ice out of Balance
In the past, and even up to the early part of the record the scientists looked at in the 1920s, ice melted during warm phases and grew back during cold phases. But over the last century, that balance has come undone. The shifting winds and warm ocean phases have eaten away at the ice more quickly than it’s being replaced.
Several particularly notable moments of wind-flipping, like in the 1970s, matched up closely with major retreats of the Pine Island and Thwaites glaciers.
Those glaciers are particularly sensitive to melting at their snouts. The ground underneath them, it turns out, is concave, like a bowl. The glacier ice is attached the “rim” of the bowl, but if it melts back past that edge, warm ocean water can spill underneath it and melt it even more quickly from the bottom.
In 1974, one of these strong moments of melting pushed the glaciers past one of these “rims,” and since then the glacier has melted much more quickly than it did before—at least 50 percent more melt after that un-groundig than before, said Eric Steig, an atmospheic and ice core scientist at the University of Washington and an author of the paper.

The suspect has been identified—and it’s us
The ultimate cause of the wind patterns, they found, is human-caused climate change. The extra greenhouse gases humans have pumped into the atmosphere over the past few hundred years have changed the way heat moves around the planet so thoroughly that they’ve changed the shape of the basic wind patterns at the poles.
The Antarctic ice sheet sat more or less stable in shape and size for many thousands of years. But about a century ago, pieces of it started to retreat in measurable ways. That’s well within the time frame when carbon dioxide and other greenhouse gases had started to accumulate thickly in the atmosphere, so it seemed logical to think that human influence was affecting the ice. But Antarctica is a complicated place that changes a lot because of natural variability, so it has been challenging to pinpoint the extent of human influence on the changes.
“It was very hard to imagine that the ice sat around happily for millennia and then decided to retreat naturally just as humans started perturbing the system, but the evidence for forcing by natural variability was strong,” writes Richard Alley, a climate scientist at Pennsylvania State University, in an email.
But a warming planet has very clearly changed the way winds move around Antarctica—and that change is likely to continue, unless something drastic happens to slow or reverse the warming process.
“If we carry this pattern forward, we may move to a situation where we’re flipping between warm and very warm,” says Holland. And that could be devastating for the ice.
But the future isn’t yet written, Steig stressed. Keeping future greenhouse gas emissions in check would go a long way toward keeping those crucial winds from weakening further, the water under the edge of the ice chilly, and the ice frozen.
“[West Antarctic Ice Sheet] melting will affect everyone,” says Steig. “The effects will be global, because sea level will rise globally.”

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