Icecaps send shivers through climatologists
Just how much is the sea set to rise over the coming century? Nobody can say for sure at present. However, worrying signs are multiplying at our planet’s two poles.
A new global warming threat has recently been added to an already long list. In the next few years, this threat could well grab the limelight, given its considerable destructive potential. Although everyone is now familiar with the global warming risks of lower agricultural yield, diminished biodiversity, natural disasters and epidemics, we are now forced to ponder the possibility of a rapid and dramatic rise in sea level.
This is nothing new in itself. For many years, scientists have known that locked up in the two enormous ice sheets of Greenland and the Antarctic continent, both several kilometres high, is a large portion of the world’s water. They also know that throughout geologic time, whenever there has been global warming, these icecaps have melted, raising the sea level – whilst glaciations have caused them to grow, lowering sea level by the same amount. At the peak of the last glacial period, around 20 000 years ago, sea level was 120 metres lower than it is today – attaching the British Isles to the European continent.
What is new, though, is that these melts could well happen at a faster than predicted rate. Early this century, these gigantic masses of ice were thought to be highly stable – especially in view of the fact that, while Greenland has recently warmed by 4°C, the Antarctic seems to have instead become colder. This led most researchers to speculate that, in terms of sea-level rises, humanity would be left in relative peace for at least one or two more centuries, even if significant warming were to take place. The current sea level rise has been measured at around 3 millimetres per year. According to the latest report by the Group of Inter-agency Experts on Climate Development (GIEC) in February 2007, the estimated total rise is therefore expected to be between 18 and 59 centimetres over the next 100 years.
However, this range is accompanied by a much less reassuring remark. According to the GIEC, these figures “do not include the full effects of changes in ice sheet flow, because a basis in published literature is lacking.” These flows are the result of hundreds of huge glaciers flowing from the icecaps to the ocean. As understanding of the effects of this major phenomenon is limited, the GIEC did not wish to include it in its projections.
Triple the speed
Satellite images reveal that the movement of the Greenland glaciers is accelerating. In just a few years, the largest glacier, Jakobshavn Isbrae, has speeded up by a factor of three to a staggering 15 kilometres per year. The same is happening with the other glaciers. Now, small blue patches can be seen dotted more or less all over the icecap, which are in fact meltwater lakes flowing towards the interior of the glacier. In Antarctica, the situation is more complex. The main body of the ‘White Continent’, which contains most of the world’s ice, is not evolving. Although a number of small glaciers are showing signs of acceleration, most scientists believe that the total mass of ice has in fact increased slightly owing to heavier snow cover. By contrast, the Antarctic Peninsula, which lies to the east of this region and is approximately the size of Greenland, is further away from the pole and is surrounded by water, so it is not so cold. And there, as in Greenland, the glaciers have been speeding up in the past few years.
It is still not possible to make forecasts using these alarming data, as our knowledge of these extreme environments is so recent and partial, and our models of icecap dynamics still rudimentary. For instance, we cannot rule out the possibility that heavier snowfall might have caused the glaciers to accelerate. Neither can we exclude the possibility that natural phases of acceleration and slowdown of these solid ‘rivers’ might be at work that are totally unconnected with global warming. However, most experts favour a different view, at least in the case of Greenland: the meltwater from the surface is filtering down through the ice to form a lubricating layer between the glacier and its bedrock, causing the acceleration. This is compounded by the shrinking sea ice (pack ice), which can no longer play its role of stemming the flow. Furthermore, the sea level rise already recorded could be aggravating the situation by lifting the glaciers at their mouth, which once again would reduce the friction.
Lastly, if the disintegration of the icecaps of Greenland and West Antarctica accelerates, we have no idea whether it will happen gradually or whether the icecaps will suddenly subside. In 2002, glaciologists were astounded when they saw the images by the United States National Aeronautics and Space Administration (NASA) of the disintegration – in just a few days – of the Larsen Ice Shelf in the Weddell Sea, measuring 3,200 km² and weighing 500 billion tons. This serves as a reminder that in nature subsidence can be drastic and unpredictable.
A rise of several metres?
In theory, the total volume of Greenland’s ice represents a sea level rise of seven metres, and the West Antarctic icecap is the same order of magnitude. Where might this lead? Climatologists are unable to agree on this point. “It is perfectly reasonable to envisage a one metre rise if the Greenland icecap continues to melt at its current rate,” says Jérôme Chappelaz, deputy director of France’s prestigious Laboratory of Glaciology and Environmental Geophysics (LGGE).
In January 2007, Professor Stefan Rahmstorf, from the renowned Potsdam Institute for Climate Impact Research (PIK) in Germany, published in the journal Science a correlation between temperature and sea levels in the past 120 years. He concluded that each degree of warming causes a 3.4 millimetre acceleration in sea level rise per year. Applied to the GIEC global warming projections, this would lead to a total rise of 1.4 metres by the year 2100 under a business-as-usual scenario. However, one of today’s leading experts, Eric Rignot, from the NASA Jet Propulsion Lab, considers a rise of two to three metres to be perfectly feasible.
James Hansen, the ‘climatology guru’ (who was already sounding warning bells about global warming as far back as the 1980s), points out that several recent articles have found rises of five metres per century to have occurred in the past quite naturally. So such high figures must therefore be envisaged.
What would the consequences be? “The problem with sea level rise is not what happens on days when the sea is calm”, says Stefan Rahmstorf. “It is what happens when the inevitable coastal storms arrive. It has been calculated that, in the case of New York, a rise of only one metre would cause the kind of damage now associated with a centennial storms to occur every three years (destruction, flooded metro system, economic disruption, and so on). The cost would be astronomical.” Now, more than half of the world’s human population lives on the coast, where resources and infrastructure are concentrated, and the figure is set to increase.
The effects of even larger rises of two, three or four metres are bewildering to contemplate. They would be rather comparable to the effects of a world war. And not only towns and cities would suffer. Many of the Earth’s major deltas (the Ganges, Mekong, Nile, etc.), whose fertile soil has led to large-scale agricultural production, would either be submerged or overrun with salt and made sterile. Stefan Rahmstorf drives home the following message: “Even if there is only one chance in a hundred of all this happening, we must do our utmost to avoid it. We would never agree to build a bridge with one chance in a thousand of collapsing – so why should we take such a risk with the climate?”