Measuring almost 100,000 km2 (about twice the size of the Netherlands), the ice caps around Greenland's edges represent the largest glacierised area on earth, outside of the large ice sheets of Greenland and Antarctica. On a healthy ice cap, tens of metres of tightly packed snow are able to absorb meltwater in summer. In winter, that water refreezes, causing the total mass to remain more or less stable from year to year. However, increasing temperatures have knocked that yearly cycle out of balance. The amount of meltwater is so great that the tightly packed snow is now completely saturated with refrozen meltwater. That means that new meltwater cannot be absorbed by the snow anymore, causing it to run off into the sea.
Three times as much
To get a good idea of the status of Greenland's ice caps - the smaller masses of ice around the edges of the island - the researchers focused on 12 areas around the island. The data show that the ice caps in each of those 12 areas have been losing mass since 1997, with increases in meltwater run-off varying from 17% in the south to 74% in the northernmost area. Currently, Greenland's ice caps are losing about three times as much mass per year as they were before 1997.
Brice Noël, PhD candidate at Utrecht University and lead author of the publication, expects the meltwater run-off to only increase in coming years. "Higher altitudes are colder, so the highest ice caps are still relatively healthy at the moment. However, we see melting occur higher and higher. That's a big problem, because that 'melting line' is moving towards the altitude where most of the ice mass is." Noël's research is focused on the ice caps around the edge of the island. "The main ice sheet in the interior of Greenland is much more elevated and isn't doing too bad yet," says Noël. "But we can already see an increase in the altitude of the 'melting line' there as well."
More detailed model
Noël developed a more detailed model to study the ice caps, because the model used previously was too coarsely gridded to properly study the topographically complex ice caps. "The new model shows a striking agreement with observations", says Utrecht researcher Bert Wouters who computed changes in the volume of the ice caps using height measurements made by satellites, such as the European CryoSat-2 mission. "This gives us confidence that the model is reliably reproducing the ongoing changes." With the model, Noël analysed a number of variables that influence ice mass in enough detail to show not only the ice caps' mass loss, but also the underlying causalities. After increasing the resolution from 11 km to 1 km, the researchers saw that the mass loss correlates directly with meltwater run-off, which is in turn directly influenced by an increase in temperature.
One fifth to one quarter
About one third of today's sea level rise is contributed by the melting of Greenland's ice. In a scenario of continued global warming, the ice caps around Greenland's edges may lose one fifth to one quarter of their volume by the year 2100, adding an extra 4 cm of sea level rise.
A tipping point in refreezing accelerates mass loss of Greenland's glaciers and ice caps
Brice Noël*, W. J. van de Berg*, S. Lhermitte**, B. Wouters*, H. Machguth, I. Howat, M. Citterio, G. Moholdt, J. T. M. Lenaerts* & M. R. van den Broeke*
* researchers associated with Utrecht University
Published in Nature Communications, 31st March 2017, DOI 10.1038/NCOMMS14730.