Ten years ago or so, when IPCC reports were still rather hesitant, some climatologists were already warning us to prepare for the inevitable consequences of climate change. Today, scientists have a more precise view (1): during this century, sea level will increase by several tens of centimetres according to the different scenarios envisaged. If we don't do anything, tens of millions of people will be flooded every year after 2100 in coastal areas, concludes a study published in PNAS magazine. Xavier Fettweis, from the University of Liège’s Laboratory of Climatology and Topoclimatology, took part in this study. The cost for society will be exorbitant. It's time to think of building dykes.
According to the majority of climatologists, the question today is no longer whether or not the waters will rise, but by how many centimetres. Between 25 and 123 cm, according to the IPCC’s latest estimations, included in this study, published in February 2014 in PNAS (2). "It will cost less to prepare ourselves for the floods than waiting for them to happen and then repairing the damage", believes Xavier Fettweis, FNRS research associate at the University of Liège’s Laboratory of Climatology and Topoclimatology, and co-author of the study. "If we don’t do anything by 2100, between 0.2 and 4.6 % of the world’s population will be flooded every year. These climate disasters will lead to a loss of 1 to 10 % of the world’s riches. Of course, it will cost a lot of money to build dykes along all the threatened coastlines: between 12 and 70 billion dollars in 2100. But despite the high level of uncertainty regarding the exact increase in sea level in 2100, dyking up the coastline will cost far less than doing nothing.”
The study recalls the three factors that contribute to the sea level rise. First of all, there is what the specialists refer to as ‘thermal expansion’. Global warming raises the temperature of the water and, just like in a saucepan, the heated water takes up more room. Then there is the melting of the continental glaciers (Alps, Himalayas, etc.). Finally, there is the melting of both Antarctica and Greenland ice sheets.
To conclude, it should be mentioned that the rise in sea level worldwide won’t be uniform, owing to sea currents that will prevent the oceans from heating up in the same way everywhere, and changes in the Earth’s gravitational fields (isostasy) following the melting of the continental ice.
![Sea level rise]( "Sea level rise.")
The role of the Greenland ice sheet
Concerning the Greenland ice sheet, the PNAS study used projections provided by Xavier Fettweis. Thanks to satellite images and climate models, the researcher from Liège has been studying this immense ice sheet (2 million km2) for the past ten years or so. "Since the end of the 1990s, Greenland has been losing ice and this is clearly contributing to 10 to 15 % of the observed sea level rise (~ +20cm since 1900)."
Greenland’s ice sheet is a legacy from a distant past, some several million years ago, a time when it was far colder than now. It is gigantic (3 x the surface area of France), with permanent ice covering 80 % of the island and reaching a peak of more than 3000 metres in altitude! We can summarise the yearly dynamics of this frozen island by the ratio between the mass of snowfall and the ice/snow that disappears into the ocean, either in the form of meltwater or in the form of icebergs that are discharged by the glaciers. If this ratio is higher than one, the ice sheet is gaining mass; if the ratio is less than one, it is losing mass.
Until the end of the 1990's, the ice sheet has been maintained through a balance of 'entries’ and ‘exits’. The annual theoretical balance is set at approximately 600 gigatons/yr of snowfall (representing about 2 mm in sea level) and 600 gigatons/yr of lost ice. But for the past fifteen years or so, the mass balance is negative nearly every year. The Greenland ice sheet is shrinking before our very eyes, at least in the ‘eyes’ of the satellite images.
“2012 was particularly spectacular from this point of view”, Xavier Fettweis insists. “A great deal of ice melted on the surface of the ice sheet (approximately 700 gigatons) as well as icebergs breaking off from it (approximately 500 gigatons). Snowfall (still near 600 gigatons) definitely didn’t compensate for these losses.” No need to be a mathematician like Xavier Fettweis to work out that the Greenland ice sheet lost around 600 gigatons of ice in 2012 (1200 – 600 = 600). A lay person might, however, find it rather more difficult to translate that into the level at which the sea is rising. Xavier Fettweis has done the calculation for us: “The oceans have risen by two millimetres as a result of the melt in Greenland in 2012 alone.”
NAO is a guilty party
Why does this sudden imbalance occur at the turn of the 1990s in Greenland? After all, according to temperature records, climate change goes back to the beginning of the 20th century, with a temperature increase of about 0.8°C since 1900, while the temperature in Greenland has increased by 3°C in the past 30 years.
![Anomalies temperatures]( "Anomalies temperatures.")
The explanation can be found in a meteorological phenomenon known as NAO (North Atlantic Oscillation). For the past fifteen years or so, the Azores High has tended to go northwards in summer, thus increasingly exposing Greenland to tropical air currents. Under the influence of these abnormal winds from the south, the average summer temperatures have risen mainly in Greenland and Baffin Island. “2012 was particularly warm”, explains Xavier Fettweis. “That summer, we observed urface melt over the whole ice sheet, even at its summit at an altitude of more than 3000 metres. That’s exceptional.” (see also: Greenland: the surface of the ice sheet is melting faster than predicted and An uncrtain future for the Greenland ice sheet).
But the NAO remains a relatively unknown phenomenon and we still don’t know why it varies in time and if the changes in circulation (frequency increase of southern winds) observed in the North Atlantic since the end of the 1990s are a long term trend or just due to the natural variability of climate. Indeed, contrary to what has happened over the past 15 years, the Azores High returned to its normal position in the summer of 2013. “In 2013, we returned to temperatures comparable with those observed between 1960 and 1990. And Greenland didn't lose any mass last year. But it would be very difficult for us to forecast exactly what will happen in 2014 and in following summers.”
![Evolution melt Greenland]( "Evolution melt Greenland.")
Added to these meteorological vagaries, at longer time scale, they are uncertainties regarding the complex dynamics of glacier such as the one in Greenland. These dynamics haven’t yet been sufficiently integrated into the models.
The accelerated surface melt of the whole of the ice sheet over the past few years has uncovered deeper, older layers of ice, which are therefore dirtier and darker than fresh snow. And these less white surfaces have a lower albedo than snow, which will further accelerate the melt effect, for a same air temperature. “This albedo reduction (partly resulting from human activity which dirties the snow in Greenland owing to the black soot emitted when coal is burnt, for instance) is currently underestimated”, Xavier Fettweis believes.
Another unknown: to what extent does the meltwater from the surface of the ice sheet run into the ocean? The water that runs closest to the edge of the glacier, in the so-called ‘ablation’ zone (the sides of the island where the mass lost through the melt in the summer is higher than the mass gained by snowfall in winter), runs off and ends up in the surrounding seas. But the water that melts at the centre of the ice sheet, hundreds of kilometres from the coast, percolates, slips between the interstices of the snow cover (which is like a sieve) and ends up being frozen again the following winter. This water doesn’t contribute to the sea level rise. “Until the snow is completely saturated”, Xavier Fettweis points out. “When this happens, all the additional meltwater will flow into the sea.”
The shape of the glacier is evolving
Just to make things more complicated, the ablation zone is expanding. With the acceleration of the melt observed over the past few years, this zone has a tendency to rise to the summit of the ice sheet (which more or less has the shape of a hill). This contributes to an increase in the meltwater run-off towards the ocean. To what extent? This has yet to be determined.
Another phenomenon to keep an eye on: the shape of the ice sheet. Over the years, the ice sheet has tended to get thinner in the ablation zone; the sides of the ice sheet are losing mass. Each time a height of 100 metres of ice is lost, the temperature increases by approximately one degree, which contributes to acceleratethe melt in this ablation zone.
The role of icebergs will also evolve. There has been a considerable increase in icebergs calving into the sea in the 2000s. “This is probably partly linked to the meltwater, which causes a lubricating effect between the ice and the island’s bedrock surface”, Xavier Fettweis explains. But this isn’t the only factor because for the past few years, the number of icebergs discharge has decreased while the meltwater run-off is still increasing. How will this evolve in the coming years? Difficult to predict exactly because the shape of the fjords through which the glaciers flo down would also appear to play a major role.
However, at a longer time scale, the icebergs may well contribute less and less to the sea level rise because if the ice sheet continues to shrink as it is currently doing, the ice will ultimately desert the edges of the island and will no longer be in contact with the surrounding seas as is currently the case.
When all these feedbacks has been clarified and integrated into the climate models, it is likely that the researchers will suggest other estimations concerning the evolution of Greenland and its impact on the sea level rise. “In this area of climatology, the more knowledge progresses, the more the uncertainties increase”, Xavier Fettweis states. “In a few years time, the range concerning rising sea levels may well be more extensive than that currently predicted. But nobody seriously thinks that the trend can be reversed. The rise in the water level in the 21st century is inevitable even if its exact evolution remains unsure.”
