It’s just two weeks since a paper in Nature flagged an ominous thinning of ice shelves along parts of the Antarctic coast lying due south of the Pacific Ocean. The ice appears to be melting from below, as changing ocean currents are bringing relatively warm water to bathe the shelves’ undersides — and as the ice shelves lose mass, they also lose their ability to slow land-based ice in its slide toward the sea.
Now there’s something new to worry about. A pair of brand-new studies published today, one in Nature and one in its sister publication Nature Geoscience, are pointing to yet another danger zone, this one on Antarctica’s Weddell Sea coast, nestled in the armpit of the Antarctic Peninsula. The first study asserts that warm ocean currents are likely to eat significantly into the huge Filchner-Ronne Ice Shelf by 2100; the second argues that the lay of the land underneath the shelf makes the ice even more unstable than it would otherwise be. “We don’t necessarily have any evidence for a dramatic change right now,” said Martin Seagirt of the University of Edinburgh, a co-author on the second paper, in a press conference, “but it’s on the threshold.”
The reason, say Seagirt and his colleagues, is that airborne radar shows that the ice shelf sits atop a depressed basin of bedrock about 60 miles wide by 160 miles long by up to a mile and half below sea level at its deepest. Right now, the so-called grounding line — the place where a shelf makes the transition from grinding along the rock to floating freely in the sea — lies at the outer rim of that basin. As warmer water melts the ice back, it can flow into the basin and cause the ice within to detach from the bedrock relatively quickly. “Its very nearly afloat already,” Seagirt said. “It needs some push and we don’t believe the push needs to be very hard.”
It’s exactly that sort of push that emerges from the work of Hartmut Hellmer, of Germany’s Alfred Wegener Institute for Polar and Marine Research and his colleagues. The scientists used a state-of-the-art climate model known as HadCM3, which simulates the responses of both atmosphere and oceans in a warming world, to test what might happen to the frigid waters off Antarctica as temperatures rise. They found that a 7°F warming of the atmosphere — on the high end of what scientists expect by 2100, but still well within the plausible range — could warm the Weddell Sea by 3.5°F. Since the floating sea ice in the Weddell has already begun to disintegrate, that water would have easy access to the ice sheet.
If the Filchner-Ronne lifts off the bedrock, land-based glaciers that feed it will be able to move more quickly to the sea, especially, Seagirt said, because the inland part of the basin, where it slopes back upward toward the center of Antarctica, has a very smooth floor, which is easy for ice to slide along. This suggests it used to sit at the bottom of an ocean where sediment would have plastered over outcroppings of rock— a glimpse, perhaps, of things to come.
Since the Filchner-Ronne is mostly fed by ice from the West Antarctic Ice Sheet, it’s natural to think that the much larger East Antarctic Ice Sheet, which holds enough ice to raise sea level by 160 feet or more, is safe (although with 20 feet of sea level rise potential itself the West Antarctic sheet isn’t anything to sneeze at). But the safety is by no means guaranteed: the two sheets, which are separated by the Transantarctic Mountains, aren’t completely isolated from each other. If you lose the Filchner-Ronne, Seagirt said, “there will be knock-on effects. There will be consequences for East Antarctica.”
None of this means the world is necessarily headed for apocalyptic sea level rise by the end of this century. Current projections still put the most likely increase by 2100 at about 3 feet, which is bad enough. But scientists still barely understand the dynamics of the world’s great ice sheets. They could turn out to be more stable than glaciologists expect.
Or, as these new results seem to imply, they could be much less.