Sea level rise uncertain

clipped from www.newscientist.com Ice sheet complexity leaves sea level rise uncertain Ice shed from the giant sheets covering Antarctica and Greenland is responsible for just 12% of the current rate of global sea level rise, according to a new review. The authors emphasise that it is now clear that the ice caps are losing ice faster than it is being replenished by snowfall. But exactly why this is happening remains unknown, making it difficult to predict the extent of future sea level rises. The remaining 88% of the current rise is due to the expansion of water as it warms, and melting from mountain glaciers and ice caps outside Greenland and Antarctica. Yet the shrinking of Greenland and Antarctica remains crucial because together they hold enough water to make sea levels rise by 70 metres, submerging vast swathes of land and displacing millions. The bases of the glaciers appear to be able to slip more easily at their basethan in the past, so they slide into the sea faster.

Ice shed from the giant sheets covering Antarctica and Greenland is responsible for just 12% of the current rate of global sea level rise, according to a new review. The authors emphasise that it is now clear that the ice caps are losing ice faster than it is being replenished by snowfall. But exactly why this is happening remains unknown, making it difficult to predict the extent of future sea level rises. The remaining 88% of the current rise is due to the expansion of water as it warms, and melting from mountain glaciers and ice caps outside Greenland and Antarctica. Yet the shrinking of Greenland and Antarctica remains crucial because together they hold enough water to make sea levels rise by 70 metres, submerging vast swathes of land and displacing millions. Over the past 10 years, satellite measurements have vastly improved the quality of data detailing changes in the ice sheets, say Duncan Wingham from University College London and Andrew Shepherd from the University of Edinburgh, both in the UK. Having reviewed the latest data, the pair conclude that losses from the ice sheets of Greenland and Antarctica contribute 0.35 millimetres per year to the total rate of sea level rise, estimated at 3 mm per year. This contribution is close to the Intergovernmental Panel on Climate Change's latest estimate of 0.41 mm from the Greenland and Antarctic ice sheets. According to the IPCC, measurements since 1993 show that the thermal expansion of water is responsible for 1.6 mm of the annual rise and other melting glaciers and ice caps for 0.77 mm. Ice flow The satellite data have revealed how the ice sheets are losing mass. "It has become very clear over the past five years that these sheets are not losing most of their mass through melting, they are losing it because the ice is flowing into the ocean faster than the snow is replacing it," Wingham told New Scientist (see Greenland's glaciers are speeding to the ocean). The bases of the glaciers appear to be able to slip more easily at their basethan in the past, so they slide into the sea faster. And this is where uncertainties arise. In Greenland, it is possible that water from melting ice at the surface of the glaciers is boring holes through the ice sheets and lubricating their base. "It is at least possible," says Wingham, that global warming is causing this to happen now more than before. But Antarctica is much colder than Greenland, so there is no such melt-water to travel down to the base of the glaciers. "Most people don't realise that Antarctica is so cold there isn't much melting going on," Wingham told New Scientist. Whatever is causing Antarctic glaciers to flow faster than before, he says, it is not melting of their surfaces. Increased snowfall Wingham and Shepherd's review of recent research on Antarctica did find that four Antarctic glaciers that are retreating in unison share a common feature: they are all in direct contact with the sea. "Our assessment confirms that just one type of glacier in Antarctica is retreating today – those that are seated in deep submarine basins and flow directly into the oceans," says Shepherd. "These glaciers are vulnerable to small changes in ocean temperature, such as those that have occurred over the 20th century and those predicted for the 21st century. A rise of less than 0.5 °C could have triggered the present imbalance." Climate modelling predicts that snowfall on the ice caps will increase over the 21st century. But the researchers warn that the processes that are causing ice sheets to shrink – with surface melting in Greenland and changes in sea temperatures in Antarctica as possible candidates – could in the 21st century rapidly counteract the ice gained from increased snowfall. They say continued observations by satellite and on the ground in both regions are essential to improve predictions of sea-level rises. In a separate review paper, also in Science on 15 March, David Vaughan and Robert Arthern of the British Antarctic Survey agree that there are two main obstacles to predicting the future of ice sheets: more needs to be known about what is going on underneath the ice sheets, and what happens at their edges where they meet ocean waters. Journal reference: Science (vol 315, p 1529, p 1503) Climate Change - Want to know more about global warming – the science, impacts and political debate? Visit our continually updated special report.