How Scientists Tracked Antarctica's Stunning Ice Loss
When the Antarctic wants to rid itself of ice, it has to get creative. The cold is too stubborn to allow surface ice to gently melt into oblivion. Instead, crushed by the immense buildup, ice gets shoved slowly along valleys and gorges until it finally reaches the edge of the continent, walking the plank into its watery grave.
Back in the 1980s, scientists would plant stakes on these so-called “ice streams” to see how fast (or how slowly) they moved. The difficulty, cost, and danger limited the number of measurements. These days, however, scientists studying ice movement on the Antarctic are inundated by measurements—so much so that today, a group of scientists has published the combined results of 24 different surveys of Antarctic ice loss in an edition of Nature. Together, they suggest that the rate of melting has tripled in the past five years. Led by investigators Andrew Shepherd of the University of Leeds and Erik Ivins of NASA, they believe that the loss, now up to 219 billion metric tons per year, is driven both by faster-flowing ice streams and by warmer ocean waves lapping at the bottom of ice shelves.
The researchers studied the whole of Antarctica. They studied the Antarctic Peninsula (“the spiral arm that sticks up towards South America,” as Ivins describes it), pock-marked with more than 100 ice streams. They studied the inhospitable isolation of East Antarctica. They even studied the deep cracks that form where the ice streams hit the sea. “If you fall in there, you will not survive,” says Johan Nilsson, a coauthor and scientist at NASA.
So are scientists today just braver and more willing to put up with danger and discomfort than in the 1980s? Perhaps yes—but they likely won’t have the chance to prove it. Most of the data in these comprehensive studies was captured by satellites.
“It’s a fantastic way of doing science,” Nilsson says. American, Canadian, European, and Japanese governments and space agencies have placed millions of satellite images and data online, much of it freely available. The images cover vast sections of the earth all at once, and the satellites stay in orbit and keep sending new data every time they flit over Antarctica.
One category of satellites captures the height of the ice. These “altimeters” send out a laser or radar signal, racing from the ebony of space to the ivory of the ice at the speed of light. Then the signal bounces back to hit the satellite again. By measuring how long it takes for the signal to return, scientists can compute the height of the ice—and from that, its mass.
Other satellites make a cunning use of gravity to estimate the snow and ice mass. Still others capture radar images of the ice streams that help determine their speed. The earliest of these satellite missions date back to 1991 and 1992, so researchers can reliably track ice mass increase or decrease over 25 years. And because each technique is different, researchers can compare and merge independent sets of results.
The new paper estimates that between 1992 and 2011, Antarctica lost ice at a rate of 76 billion metric tons per year, but since then, the rate has increased to 219 billion metric tons per year. If these rates were not increasing, Ivins explains, then we wouldn’t have to worry so much about it. But instead, “we’re seeing a tripling of the rate in the amount of ice being lost from the Antarctic to the ocean.”
This loss of ice translates into an estimated sea level rise of 7.6 millimeters since 1992. “It sounds like the tiniest amount that you wouldn’t even notice,” says Gwenn Flowers, a professor of earth science at Simon Fraser University unaffiliated with the study. But she points out other glaciers are melting in Greenland and on snow-gemmed mountains, which also contribute to sea level rise. It’s an open invitation for more severe floods or storms along coastlines. “If you raise the floor of the basketball court by just a few inches, you’re going to get a lot more slam dunks,” says Twila Moon, a research scientist with the National Snow and Ice Data Center also unaffiliated with the study. “Unfortunately, this is the same.”
The results on ice loss weren’t always so unified. Back in 2011, at least a dozen research groups were using the satellite data and “the field was itching for a showdown,” Ivins recalls in an email, because of the discrepancies between each method.
In August of that year, Shepherd from the University of Leeds took a vacation with his family to Laguna Beach. He managed to squeeze in a couple of hours for coffee with Ivins in his backyard in Pasadena, and that was when they hatched a plan for collaboration. Eventually they set up an online portal through which individual researchers could submit their estimates of Antarctic ice loss. Shepherd and Ivins now lead a steering group that sets the guidelines and standards the results must adhere to. Then, they combine the data—resulting in comprehensive papers like the one published today.
Moon lauds their approach. “One of the wonderful strengths of this paper is that often in science, each individual study produces a paper, and you have to wait for those papers to pile up and you collect them together. But in this case, this team has organized to do that hard work for us”—work that took “thousands of science-hours” from 84 researchers across the globe.
There were complications; scientists have the same troubles with collaborative tech as any remote worker. “Sometimes we had Skype meetings where one person got the time zone wrong, so as the meeting was ending, the person popped up and said, ‘Hi everyone’,” says Pippa Whitehouse, a co-author from Durham University who sits on the committee. But otherwise, no serious disagreements or issues derailed them.
The group published their first combined results in 2012, covering both Greenland and the Antarctic. Today’s paper provides updated results for Antarctica spanning the last five years, and the next step will be to provide an updated report for Greenland as well. Satellites become worn and faded like anything else. But new missions are being launched with fresh instruments, so the space observations will continue.