Antarctica holds an absurdly large amount of ice — 3.5 million cubic miles’ worth — so it’s reassuring to know that it isn’t all going to melt anytime soon. If it did, sea level would go up about 180 ft., causing unimaginable devastation. But even with any reasonable projection for global warming, the air over Antarctica simply won’t get warm enough to turn much of that ice into water.
That’s the good news.
The bad news is that ice doesn’t have to melt to raise sea level. It can simply plop into the sea, like an ice cube dumped into a nearly full glass of water. That’s been going on for untold thousands of years in both Antarctica and Greenland, but it’s been nicely balanced by snowfall in the landmasses’ interiors. Lately, though, faster-flowing glaciers in both places have upset the balance, with ice entering the ocean faster than it can be replaced.
And a new paper in Nature suggests an ominous reason: it’s the warming sea, not the air, that’s speeding the glaciers’ progress. “The oceans can do all the work from below,” lead author Hamish Pritchard, of the British Antarctic Survey, said in a press release.
The evidence comes from the ICESat satellite, which orbited the Earth from 2003 to 2009, measuring the thickness of ice in both the Arctic and the Antarctic by bouncing a laser beam off the ice surface and measuring how long it took to return. If the round trip got shorter over time, it meant the ice was thinner. And between 2003 and 2008, the period covered in the study, the satellite instruments found that many of the ice shelves surrounding the frozen continent have lost thickness. In other words, they’re melting.
They’re not melting from above, where they touch the air, however: the ice sheets are melting from below, as (relatively) warm ocean water bathes their undersides. Changing currents, driven by changing winds, driven ultimately, say the researchers, by global warming, are transporting warmer waters to Antarctica’s edges.
“[Previous studies combined with] our new results suggest Antarctica's glaciers are responding rapidly to a changing climate,” Pritchard said.
The ice shelves are already floating in the ocean, so the melting doesn’t add to sea level rise. But their enormous mass acts as a brake on glaciers flowing seaward behind them, and as the shelves thin out, the glaciers can move faster. Earlier studies have shown that Antarctica’s glaciers are indeed moving faster and that the continent is losing ice overall. The same thing is happening in Greenland, but it’s even more pronounced there — one reason that estimates of sea-level rise over the next century have increased since the last IPCC report in 2007.
It’s not clear whether Antarctica’s current rate of ice loss will continue as the planet warms even further, since ocean currents may continue to change. Ice loss could slow down. It might, on the other hand, speed up.
Understanding what’s going on will, unfortunately, be more difficult for the next couple of years. While satellites including GRACE will continue to monitor changes in ice from orbit, ICESat’s successor, ICESat2, won’t be launched until 2016.
Source : climatecentral.org