A new tipping point has been identified for the climate. A relatively small ice plug is holding back an enormous volume of ice in East Antarctica. Although it would take millennia for the full effects to flow through, the melting of this obstacle would lead to the global ocean rising 3-4m on its own.
"East Antarctica's Wilkes Basin is like a bottle on a slant," says lead-author Matthias Mengel, a PhD student at the Potsdam Institute for Climate Impact Research. "Once uncorked, it empties out." However, the analogy may create a slightly misleading impression. The rock in most of the basin sits well below sea level, a fact disguised by the thickness of the ice sheet above. Recent efforts to more accurately map Antarctica's topography have presented new opportunities to see how climate change will affect basins like this.
Most of the world's frozen water lies in East Antarctica. While Greenland contains enough ice to raise global sea levels more than 7m and West Antarctica slightly less, for East Antarctica the figure is 55m.
The Wilkes Subglacial Basin sits south of Tasmania, and is bounded on the west by the Prince Albert Mountains. While it contains less than 10% of East Antarctica's ice, Mengel reveals in Nature Climate Change it is unusually vulnerable.
"Until recently, only West Antarctica was considered unstable, but now we know that its ten times bigger counterpart in the East might also be at risk," says Professor Anders Levermann, Mengel's supervisor. The latest IPCC report projects that just 16cm of the sea level rise to be experienced this century will come from Antarctica, but Levermann says, “If half of that ice loss occurred in the ice-cork region, then the discharge would begin. We have probably overestimated the stability of East Antarctica so far.”
Air temperatures in this part of Antarctica rarely get above freezing. However, when waters off the coast warm up they can melt the ice with which they are in contact. Currently the ice in contact with the ocean sits on a ridge. If the line where the ice and water meet retreats more ice will be exposed to the sea, speeding the melt rate in what could quickly become a vicious circle.
In one sense this is a very long term threat. Mengel and Livermann's simulations suggest it would take 5-10,000 years for the entire region to melt, and the effect on sea levels over the two next centuries would be minor compared to thermal expansion and other melting. However, if the process starts it would be almost impossible to reverse even if we get the global climate under control.
"This is the underlying issue here," says Mengel. "By emitting more and more greenhouse gases we might trigger responses now that we may not be able to stop in the future." So even if the population of the 22nd Century ceases all carbon emissions and removes what we have put into the atmosphere they may still need to abandon low lying cities such as New York, Tokyo and Shanghai in the face of rising waters.
If that is not nightmare enough, there are several more basins in East Antarctica with similar topography that have yet to be studied in this depth, although none holds as much ice as Wilkes. Most of West Antarctica's bedrock is similarly below sea level, including the Pine Island Glacier from which a giant iceberg broke off last month.