For nearly 40 years we have known that the West Antarctic Ice Sheet is susceptible to the sudden loss of ice into the ocean, a process termed marine instability. The reason is that the 2,000 metre-high centre of the ice dome overlies a marine basin with a depth of 1,500 metres below present sea level.
The risk is that warm ocean water will melt the floating margins, thin the ice, and lead to a speed up in ice velocities as the ice retreats into deeper and deeper water. This loss of ice into the ocean would cause a global sea level rise of up to five metres if the ice sheet disappeared.
The issue is important because the latest IPCC predictions of sea level rise by 2100 under warm scenarios range from 53 to 98 centimetres and are due to a combination of thermal expansion and the melting of glaciers. Marine instability was excluded from the predictions because there was too much uncertainty about the process.
But the record of marine instability at the end of the last ice age shows that a rise in sea level exceeding one metre per century is possible. The real concern is that glaciologists are alarmed at the loss of ice from the Pacific margins of the West Antarctic Ice Sheet as revealed by satellite imagery. Some think it may have passed the point of no return.
One approach to this problem is to ask what happened to the ice sheet in the past during interglacial periods such as 130,000 and 205,000 years ago when the world was warmer than at present. Such conditions are a pointer to what we might experience by the end of the present century.
Scientists working on raised shorelines believe that sea levels 130,000 years ago were four to six metres higher than at present and look to the loss of the West Antarctic Ice Sheet as a source of water. Marine biological evidence agrees. Bryozoa and octopuses on the Pacific and Atlantic coasts of West Antarctica show similarities that imply a marine seaway existed across West Antarctica within the last million years or so. A collapse of the ice sheet during an interglacial period would expose such a seaway.
“We risk losing the marine portions of the West Antarctic Ice Sheet. If so, we have to plan for an additional rise in the global sea level of three metres”
Geographers from the universities of Edinburgh and Northumbria have recently published a paper in Nature Communications (doi: 10.1038/ncomms10325) in which they provide a new insight into West Antarctica during interglacial periods. They outline field evidence to show that the minimal configuration of the West Antarctic Ice Sheet in a warmer world involved regional ice sheets ~500 kilometres across that survived on three mountain blocks in West Antarctica, with marine seaways in between. The loss of ice raised global sea levels by about three metres. The research studied the geomorphology of three massifs in the southern Ellsworth Mountains in the heart of the Weddell Sea embayment.
The team studied boulders dropped by the ice sheet on the mountains and dated them using cosmogenic isotopes. Cosmic rays bombard the Earth’s surface and build up cosmogenic isotopes in surface rocks. Measure these and you can show how long a boulder has been exposed and sometimes how long it has subsequently been buried by ice.
The team found that while the ice sheet elevation fluctuated, there was evidence of persistent ice sheet conditions for 1.4 million years, a discovery demonstrating that at least a sizeable ice sheet survived several warm interglacial periods.
No evidence was found of complete deglaciation. There are two possible explanations. First, the main ice dome survived intact for 1.4 million years in which case the West Antarctic Ice Sheet has played little part in sea level change. Second, a regional ice sheet survived on the Ellsworth-Whitworth mountain block during warm interglacial periods, in which case the contribution to sea level rise would have been three metres.
This latter scenario is supported by the evidence of higher than present sea levels during interglacial periods and the biological evidence of a seaway across West Antarctica in the last million years.
The implications of this work are troubling. If the world continues on its warming trajectory, then we risk losing the marine portions of the West Antarctic Ice Sheet. If so, we have to plan for an additional rise in the global sea level of three metres, a future with frightening implications for the coastal populations of the world.
This was published in the June 2016 edition of Geographical magazine.