There isn’t a doubt in the world (among serious scientists, anyway) that the sea has been rising for the past century, by about eight inches in total since 1900. There’s little doubt, either, that the rise has been speeding up over the past couple of decades – the water has been inching up about as twice as fast lately as it was for most of the 20th century.
All of that is a powerful confirmation of what thermometers tell us: that the earth is warming – the result, say those same serious scientists, of human-generated, heat-trapping greenhouse gases. That heat makes seawater expand, and it also transforms land-based ice into even more water that swells the oceans further.
What nobody has firmly pinned down so far, though, is just how big a contribution all that new water makes to the rising seas. They’ve come up with an estimate, by calculating how much should come from heat expansion then blaming the rest on melting ice: about 1.8 millimeters per year, says University of Colorado physicist John Wahr. But that’s not as convincing as a direct measurement, and it doesn’t solve the mystery of where all the ice is disappearing from.
The mountain ice in the Himalayas (pictured above), Karakorams and other high ranges to the south of the Tibetan plateau, a region so icy it’s sometimes called the “third pole", has lost only about 1 per cent of the worldwide total. Credit: TopTenList.org
Now, however, thanks to Wahr and three other scientists, the measurement question and the mystery have both been answered. Using a high-flying pair of satellites known collectively as GRACE, the Gravity Recovery and Climate Experiment mission, they’ve been watching carefully since 2002 to see, among other things, which of the planet’s glaciers and ice sheets are shrinking and by how much. The answer, just reported in Nature: between 2003 and 2010, about 385 billion tons of ice have vanished into the sea each year – enough, says Wahr, “to fill Lake Erie with water eight times over, or cover the entire US with water to a depth of a foot and a half.”
As it turns out, that comes pretty close to what the estimators were saying before GRACE weighed in. That’s a good thing, since it means ice and sea-level experts were on the right track. They were also right that the greatest contribution by far is coming from the huge masses of ice sitting atop Greenland and Antarctica.
But there was one big surprise: the mountain ice in the Himalayas, Karakorams and other high ranges to the south of the Tibetan plateau – a region so icy it’s sometimes called the “third pole" – has lost only a paltry 4 billion tons of ice annually over the study period, or about 1 per cent of the worldwide total. It was, said Wahr, “a big surprise” that it was so little, and that the bulk of the loss outside Antarctica and Greenland is coming from other mountain ranges. But then, previous estimates of ice loss in that region have been based on very limited measurements on the ground, and weren’t trusted all that much in the first place.
Aside from that, the new, more reliable numbers on ice loss won’t give much comfort to those who have something to fear from the rising sea, which means hundreds of millions of people living in coastal cities around the world, from New York to Shanghai to Amsterdam to Calcutta. For the people who live on the Indian subcontinent and Southeast Asia, though, the news may be a little rosier: the great rivers that water the region have their source in those same high mountains. If the ice that feeds those rivers is mostly staying intact, there’s less chance that the rivers will start to run dry.
Average changes in ice thickness in centimeters per year from 2003 to 2010, as measured by NASA’s Grace satellites, in each of the world’s ice caps and glacier systems outside of Greenland and Antarctica. Blue represents ice mass loss, while red represents ice mass gain. Credit: NASA/JPL-Caltech/University of Colorado
But while the new research finally nails down what’s been happening during the past decade or so, Wahr warns that it doesn’t necessarily tell us what will happen in the future. “In 2004,” he says, “glaciers in southeast Greenland suddenly started moving faster toward the sea. Then they stopped, and glaciers in the north and west started to speed up.” The GRACE measurements will help scientists monitor future changes more accurately, but for now they’re just the beginning of the biggest wild card in figuring out how high, and how fast, the sea will rise.