The Antarctic glaciers are melting faster than ever before. In fact, the rate of melting in some glaciers is twice as fast as it was just 15 years ago. Part of the reason for this is that Antarctic shelf ice, which is the part of Antarctic glacial ice which juts into the sea, melts more quickly where it interacts with ocean currents. This type of glacial melting is known as dynamic thinning.
Most measurements in this field involve the Pine Island Glacier Basin, which makes up 10% of the West Antarctic Ice Sheet. The 2007 study, conducted by the British Antarctic Survey, determined that ocean waters in the vicinity of Pine Island Glacier had warmed up by around 0.4 degrees F. Previous models of Antarctic glacial melting were based on this kind of ocean warming. However, the small rise in regional ocean temperature was not enough by itself to account for the drastic increase in melting.
An automated submarine which was sent under the ice shelf discovered that an undersea ridge had previously impeded the access of ocean currents to the ice shelf. When the glacier melted enough to detach from that ridge, warm ocean currents were able to reach parts of the shelf which had previously been inaccessible. In fact, the warmest water now flowed directly to the thickest parts of the ice.
At the same time, warm incoming seawater displaced the cold meltwater. This accelerated the speed of melting. As long as the cold meltwater had been trapped, it kept the surrounding seawater so cold that it was very difficult for the area to heat up enough to melt more of the glacier. However, once that cold meltwater was removed by the warm current, it no longer slowed down further melting.
The combination of incoming warm ocean currents and the removal of cold meltwater means that the coldest, most inaccessible, most protected parts of the Pine Island Glacier ocean shelf are now completely exposed to warm ocean currents, which melt it from underneath. As a result, the speed of melting has increased drastically.
This kind of melting creates cavities under the sea shelf. The cavities thin and weaken the ice shelf. When it gets thin enough, it will break under its own weight. This kind of glacial calving creates monster icebergs, which are a threat to ocean shipping.
In the Pine Island Glacier, the existence of the underwater ridge made the difference. Once the glacier melted enough to open up a new water channel between the ice and the ridge, warm ocean currents immediately flowed in and speeded up melting. Currently, the Pine Island Glacier is melting at a rate of nearly 20 cubic miles a year.
Other Antarctic glaciers which have thinned in recent years may have similar seabed topography. These kinds of undersea ridges often form at the edges of sea basins, even where glaciers are not involved, because of the interaction between the sea basin and the open ocean currents.
However, in the Antarctic, the interaction of the glacier with the ridge had created pockets of protected water. Once the glacier recedes from the ridge, those pockets are no longer protected from warmer ocean currents. Thus, accelerated melting can be expected in other ice shelves as well, when those glaciers retreat and expose previously unknown undersea ridges.
Updated measurements from a 2011 study confirm that the rate of melting under the Pine Island Glacier is still increasing. So is the local water temperature under the ice, which is now more than 7 Fahrenheit degrees above freezing. The current high rate of melting is due to stronger sub-ice-shelf circulation, as ocean currents continue to enlarge and warm the cavities underneath the glacial ice.