Each winter the surface of the sea freezes around Antarctica, over a vast area, mostly to a depth of about one metre. But this is starting to change. Last year, the sea ice reached an unprecedentedly low maximum extent of only 17 million square kilometres.
Why aren’t we talking about sea ice? Perhaps it’s because most people haven’t even heard of it, which is a shame because it’s important.
Each winter the surface of the sea freezes around Antarctica, over a vast area, mostly to a depth of about one metre. The continent effectively doubles in size, with 18-20 million square kilometres being covered by floating ice. That’s an area 2.5 times that of Australia; 4% of Earth’s surface.
But this is starting to change. Last year, the sea ice reached an unprecedentedly low maximum extent of only 17 million square kilometres. Although this year looks like being a little less extreme, a clear and concerning trend appears to be under way. This is emphasised in the ice minimum values in late summer. By February each year the sea ice extent shrinks typically to about three million square kilometres (mostly in two large embayments, the Weddell Sea and the Ross Sea), but through most of the present decade it has dwindled to below two million.
Why does this matter? Well for a start, it is the underside of this huge area of sea ice where algae live and multiply, which feed the shrimp-like krill that in turn sustains an entire ecosystem: fish, seals, penguins, whales, the lot. The upper surface of sea ice is also crucially important. Its albedo, or reflectivity, means that 80-90% of the incoming summer sunshine is bounced back into space. Replace the ice with dark ocean and only about 9% is reflected, the rest going to warm the water. So the loss of sea ice is not only a symptom of climate change, it also contributes to it, in a feedback loop that might accelerate.
There’s more. When sea water freezes, the developing ice crystals comprise nearly pure water. Most of the salt is extruded as a heavy brine, and this cold, dense water sinks, becoming the Antarctic Bottom Current. This circulates around the Southern Ocean before spinning off into the other major ocean basins. As this deep cold flow moves north it displaces warmer water which then up-wells and forms the main surface currents. Without the annual ‘push’ of the Antarctic Bottom Current, these warmer currents might slow and cease.
The global ocean is so vast that it changes very slowly. We are only now beginning to see the results of the ocean’s absorbance of a century of industrial environmental heating, in the form of anomalously warm seas particularly this year. Any pronounced weakening of the ocean circulation due to sea ice loss will be slow – but inexorable.
The results, which are probably not going to happen in our own lifetimes but could well become part of our legacy to future generations, are likely to be dire. It could eventually mean goodbye to the Gulf Stream and the other currents which maintain benign climates on the European Atlantic coast, around Japan, and elsewhere in the northern hemisphere.
The possible consequences of such climate change for human societies are truly the stuff of nightmares.
Jeremy Smith
Jeremy Smith was born and educated in Britain. After obtaining an MA in Natural Sciences at Cambridge and a Diploma in Education in Singapore, he moved to Australia where he completed in 1974 a PhD on New Guinea high altitude flora at ANU, Canberra. Moving to Armidale NSW (where he still resides), he lectured in Geography at UNE before serving as Station Leader of Australian Antarctic Stations in 1996 and from 2000 to 2010.