An uncertain future for the Greenland ice sheet

Xavier Fettweis, a researcher at the Laboratory of Climatology and Topoclimatology of the University of Liege, has been studying the Greenland ice sheet for more than 10 years in an attempt to predict fluctuations in its surface mass balance (see also Greenland: the surface of the ice sheet is melting quicker than predicted). His recent work (1) shows that melt of the ice sheet is accelerating. The year 2012 can henceforth be considered as a record year for melting. A great deal of uncertainty surrounds the future of this floating island and the consequences of the flow of meltwater into the oceans.

For 4 million years the permanence of the Greenland ice sheet has been guaranteed by snowfall that accumulates in the center of the island and which is then transformed into ice. Every year, the Greenland ice sheet gains mass in winter thanks to snowfall, and loses mass in summer due to surface melt and discharge from icebergs into the ocean. In spite of the increase of melt, the annual surface mass balance of the ice sheet, that is to say the difference between the gains (snowfall) and the losses (melt) of the surface, remains positive. The interior and the peaks of the ice sheet - the “accumulation zone”- continue to gain mass thanks to snowfall which is greater than the summer melt. On the contrary, the edges of the ice sheet which are lower in altitude tend to be worn away rapidly. In this “ablation zone”, the mass lost by melting is continuously increasing. In summer, once the winter snow has disappeared, bare ice appears, which increases surface melt even more because it is less white than the snow and thus absorbs more infrared radiation. The surface mass balance is therefore in deficit. In spite of the extension of the melt zone, the Greenland ice sheet remains balanced thanks to ice dynamics which redistributes the snow mass accumulated in the center to the edges of the ice sheet.

2012, a record year

Since the end of the 1990’s (and in particular since 2007), the Greenland ice sheet has been continuously losing mass. There are two main reasons for this: the increase of the surface melt and in discharge of icebergs into the ocean. In 2012, the absolute melt maximum in fifty years was reached (more than 100 % melt in respect to the average). Mid-July 2012, the whole surface of the ice sheet was melting. “The 2012 melt largely exceed all previous records. The ice sheet is really melting in a significant way”, explains Xavier Fettweis, FNRS postdoctoral researcher at the Laboratory of Climatology and Topoclimatology of the University of Liege.

This is explained not only by the global warming but a change in summertime atmospheric gneral circulation abover Greenland. For ten years, a occurence increase of anticyclones centred on the South of Greenland has been observed. Favoring winds from the South, these anticyclones bring warm air to the West coast of Greenland and accelerate the process of surface melt.

While the ice sheet had always managed to keep in balance between its gains and losses, the record melts of recent years have led to fear of a reversal of the situation. In the month of August 2012, Greenland’s surface mass balance of reached the absolute minimum recorded for at least 50 years. The ice sheet was therefore no longer balanced with the surface melt and detachment of icebergs exceeding snowfall. The atmospheric conditions of the next few months will be determining factors for the balance in 2012. If the direction wind does not change and melt continues to occur, the 2012 mass balance could finish in deficit and the gain/loss balance could be broken.

What are the consequences for the climate?

The island of ice is attracting researchers from all over the world due to its consequences for the environment. A melt of the Greenland ice sheet will have an impact on our climate.

Even partial melt of the ice causes an irreversible increase in sea levels, with the meltwater from the ice sheet flowing into the Atlantic Ocean. Currently the losses in mass of the Greenland ice sheet correspond to an increase in sea levels of nearly 1 to 2 mm by year. The meltwater flow from Greenland could cause an increase of the sea level of around 5 to 20 cm by 2100, representing a danger to certain coastal countries situated at low altitude such as the Netherlands. In addition to this, the possible accelartion of icebergs discharge into the ocean will contribute to further losses in mass of up to 20-30 %.

The flow of meltwater into the North Atlantic is likely to disrupt the thermohaline circulation, which controls ocean currents such as the Gulf Stream. A massive arrival of cold fresh water caused by the melt of the Greenland ice sheet would reduce the density of the Gulf Stream which is normally salty. This ocean current, whose salinity allows it to sink along the coasts of Greenland, would no longer be able to sink deeply enough to return to the South. A catastrophic scenario would predict collapse of the Gulf Stream; this means it would stop to supply heat to the coasts of Europe and cause a significant drop in temperature even though the planet is heating up at a global scale. It is more likely, however, that the addition of fresh water to the Gulf Stream would not stop it but rather slow it down, and the consequence of this would be to mitigate the global warming in our regions. 

Between optimism and pessimism, an uncertain future

Quantifying and predicting the melt of the Greenland ice sheet is therefore not without interest, the destiny of the island of ice is intimately linked to our own. Xavier Fettweis is a modeler. From a regional climate model (the MAR model) and satellite observations, he is attempting to project the future of the ice sheet and the changes of its mass balance.

The GIEC, for its next assessment report in 2013, proposes several scenarios relative to our lifestyle and our future greenhouse gas emissions. The first, which is relatively optimistic, predicts a doubling of the current concentration of greenhouse gases. The contribution of the surface melt to sea levels in 2100 will be about 4 cm (4 ± 2 cm). Yet another more pessimistic scenario predicts a tripling of the current concentration of greenhouse gases. Sea level could rise by up to some ten centimeters in 2100 (10 ± 4 cm).

The future melt of the ice sheet is no longer in doubt. However, the extent of the damage this will cause remains to be determined. A great deal of uncertainty exists with regard to climatic scenarios and modeling which makes predictions precarious. The natural variability of the climate makes it impossible to predict atmospheric circulation in summer with any certainty knowing that it is this atmospheric circulation which is the main cause of the current records  of melt. In order to model the different possible scenarios, Xavier Fettweis uses a fixed topography of the ice sheet. This does not take into account either the changes in altitude due to the melt of ice, nor ice dynamics, nor discharge from icebergs into the ocean. Knowledge about the ice sheet itself still remains very incomplete and new discoveries constantly change predictions.

As proof of the uncertainty surrounding these results, a recent American publication in the journal Nature (Harper et al., 2012) adds another decade to most of the forecasts made by Xavier Fettweis. In their study, the American researchers show the importance of the “percolation zone”. According to them, the meltwater seeps into the empty pores of the snowpack and refreezes in winter without having a direct impact on ocean levels. “In my model, I consider that if the quantity of liquid water present into the snowpack is higher than 7 %, the meltwater flows into the ocean. However, the Americans show that certain zones of the ice sheet have a much higher storage capacity than my model consider. The water which, according to my model, flows into the ocean does not immediately do so”.

In the short-term, the contribution of Greenland to the sea level rise is therefore partly compensated by the storage of water in the percolation zone. However, this discovery merely postpones the forecasts of Xavier Fettweis by around ten years. Once the storage capacity of the snow is exceeded, the meltwater will have no other possibility than to flow into the ocean, the surface of Greenland will then form one layer of impermeable ice.

A lot of unknowns remain but it is sure that the ice sheet will significantly contribute to an increase of the sea level. “All the effects are contributory factors: by melting, the ice sheet loses altitude and reheats which in turn accelerates surface melt. The snow that melts becomes salty and absorbs then more solar energy which also contributes to accelerate melt. ”, explains Xavier Fettweis. The melt of Greenland has therefore become inevitable. However, to put things into perspective: total disappearance of the ice sheet should take a lot of time. At the current rate, several hundred or even thousands of years will pass before it melts completely. Enough time for us to change our behavior?

(1) Fettweis, X., Franco, B., Tedesco, M., van Angelen, J. H., Lenaerts, J. T. M., van den Broeke, M. R., and Gallée, H.: Estimating Greenland ice sheet surface mass balance contribution to future sea level rise using the regional atmospheric climate model MAR, The Cryosphere Discuss., 6, 3101-3147, doi:10.5194/tcd-6-3101-2012, 2012a.

Fettweis, X., Hanna, E., Lang, C., Belleflamme, A., Erpicum, M., and Gallée, H.: Brief communication "Important role of the mid-tropospheric atmospheric circulation in the recent surface melt increase over the Greenland ice sheet", The Cryosphere Discuss., 6, 4101-4122, doi:10.5194/tcd-6-4101-2012, 2012b.

Harper, J., Humphrey, N., Pfeffer, W.T., Brown1 J.  and X. Fettweis, Greenland Ice Sheet Contribution to Sea Level Rise Buffered by Meltwater Storage in Firn, Nature, doi: 10.1038/nature11566, 2012.