Under the Microscope:If global warming melts the land-based polar ice-sheets the world will experience a disastrous rise in sea levels. However, I regret to bring you the news that the situation is even worse than that.
Much liquid water has been discovered under these great ice sheets which could exacerbate the destabilising effects of global warming, allowing them to slide into the ocean before melting, thereby causing the disastrous sea-level rise even sooner than would otherwise be the case. Robin E Bell describes this unsettling scenario in the current issue of Scientific American.
Melting of ice already in the sea will not raise sea levels. You can demonstrate this as follows. Take a glass and half-fill it with water. Add a fistful of ice cubes and note that the water level rises. Mark the new level. Wait until the ice fully melts and note that the new level remains unchanged. Ninety nine percent of the ice on earth that would raise sea levels if global warming melts the ice is located in three great ice sheets, each resting on bedrock - one covering Greenland and two covering east and west Antarctica respectively.
If the west Antarctic ice sheet melts, sea levels will rise by 19 feet (5.8m). If the Greenland ice sheet melts also, levels will rise by a further 24 feet (7.3m), and if the east Antarctic ice sheet melts, levels will rise by a further 170 feet (58m) - a total rise of 213 feet (65m) if all the ice melts. The seriousness of this can be quickly appreciated by the fact that one third of the world's population lives within 300 feet (91.4m) above sea level and almost all the great cities are built near the oceans. For example, a sea rise of 170 feet (58m) would inundate the entire state of Florida!
The ice sheets don't just sit there, they exist in a state of dynamic equilibrium. The ice slowly moves from the ice sheet to the ocean, ocean water evaporates and falls back on the ice sheet as snow which is compacted into ice. For a long time, until recently, the two processes - loss of ice to ocean and formation of new ice - balanced each other out and the ice sheets remained fairly constant in mass.
However, in recent times, loss of ice to sea exceeds the formation of new ice in the Greenland and west Antarctic ice sheets. And now, the discovery of much liquid water beneath the ice sheets and the realisation of its possible effects raises the spectre of a relatively sudden and disastrous sliding of ice into the sea.
IN ANTARCTICA, 90 PER CENT of the ice that reaches the sea from the ice sheet is carried in enormous "ice streams" about 60 miles (97km) wide and as thick as the ice sheet itself (3,500-6,500 feet or about 1km-2km). These streams run more than 500 miles (about 800km) inland from the sea and their edges are marked by crevasses as they move forward. They move between 650 and 3,500 feet (200m and 1km) a year while the adjacent main ice sheet moves very slowly. The major ice streams in Greenland are bounded by mountains.
The recent loss of ice from the Greenland and west Antarctic ice sheets is caused by a speeding up of the ice stream flows. The base of the ice stream must scrape along the underlying bedrock and anything that serves to grease its passage will speed it up. Also, where the ice sheet meets the sea it grows on out to sea as an ice shelf that floats in the water. If anything causes this ice shelf to break up this will remove an impediment to the flow of the ice streams allowing them to speed up.
Researchers now know that there is a lot of water under the ice sheets and this can lubricate the contact between the ice and the bedrock. In Greenland, surface melt water gushes into crevasses and tunnels its way to the base of the ice-sheet. The recent warming Arctic climate has greatly increased surface melt water and this is linked to acceleration of ice movement towards the sea.
In Antarctica, friction between the base of the ice sheets and bedrock melts ice into liquid water and geothermal energy from the earth beneath has the same effect, thus lubricating the passage of the ice streams over the bedrock. The warming sea and air is causing accelerated disintegration of the ice shelves at the periphery of the ice sheets, thus removing a "cork" that previously slowed down the rate of flow of the ice streams towards the sea.
In summary, and to quote Robin C Bell: "Abundant liquid water newly discovered underneath the world's great ice sheets would intensify the destabilising effects of global warming on the sheets. Then, even without melting, the sheets may slide into the sea and raise sea level catastrophically."
Bell cites convincing evidence that both the Greenland and the west Antarctic ice sheets disappeared in the recent geological past. This could happen again. Marine microfossils found at the bottom of a borehole in the west Antarctic ice sheet are of a kind that only form in open sea conditions. These open-water life forms lived there as recently as 400,000 years ago.
The behaviour of the polar ice sheets is the biggest uncertainty in predicting the effects of global warming. Worryingly, none of the climate models used to-date takes major features such as ice streams or accurate representations of the bottom of the ice sheets. Intensive research is needed to update these climate models. William Reville is associate professor of biochemistry and public awareness of science officer at UCC - understandingscience.ucc.ie