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For a change, once no football fields. Imagine a big cube instead. A really, really big one. A cube whose edges are each one kilometer long. If you were to fill such a cube with water, then the contents of about 400,000 Olympic swimming pools fit into it. These gigantic dimensions can help, if one wants to understand the answer to a simple question: How is the ice of Greenland?
A giant cube full of water equals one gigatonne, or one billion tons. Now imagine, someone has placed many such gigantic cubes in a long row behind each other, almost 400 of them.
According to Danish researchers, Greenland has lost no less than 392 gigatons of ice this summer: melting the surface and draining the water, calving the glaciers and allowing warmer ocean water to nibble the huge ice tongues from below.
Often, when talking of ice loss in # Greenland, gigatons come into play. This visualization nicely shows what that actually means. # AGU18 pic.twitter.com/W5KLq2dhkn
- Christoph Seidler (@ chs42) December 11, 2018392 gigatons - this value is on the one hand below the negative record of the horror year 2012, when the minus on Greenland ice was even 458 gigatons. On the other hand, the long-term average of the years 2002 to 2016 is only 260 gigatons.
This week, the UN IPCC will present its special report "Oceans and the Cryosphere in Climate Change". But it is certainly no secret to tell in advance that the melted ice of Greenland does not simply disappear. It rather contributes to the rise of global sea level. That's a difference to the Arctic sea ice. That floats on the ocean and at the time of its disappearance at least does not let the water levels climb any further, but Greenland's melt already.
A rule of thumb is that around 320 gigatons of vanished ice cause one more millimeter of sea-level rise. Depending on which scenarios are used, the melting of the Greenland ice should provide by the end of the century for an increase of 5 to 33 centimeters, scientists have recently calculated.
A new research now shows that there may be more to the previously forecast amounts of meltwater. This has to do with a change that is taking place in parts of Greenland's subsurface: the top layer of the ice sheet has absorbed meltwater that has formed in the summer, like a huge sponge. And this ability is lost to him in many places - which provides additional outflow into the oceans.
"We have mass loss in areas where this did not exist before," warns glaciologist Achim Heilig of the University of Munich in an interview with SPIEGEL. He is co-author of a recent article in the journal "Nature" published article. In it he describes, together with other researchers, that in Greenland's subsoil an ice layer continues to spread, which prevents the infiltration of melt water into the ground like a kind of cover.
The accumulated surface mass balance (SMB) for the "SMB year" 1 September 2018 to 31 August 2019 ended at 169 Gt (= km³), which puts 2018/19 on rank 7 of the last 40 years. Note this is SMB only, not the total mass balance, which includes calving and melting in warm seawater! pic.twitter.com/G8ocJwzo5c
- Martin Stendel (@ Martin Stendel) 1 September 2019An ice cap in the underground of Greenland - that sounds a bit unnerving. If you want to understand why the team's observations are still a problem anyway, you have to work your way into the depths of the ice sheet: At the top is the last fallen snow, maybe a meter thick, maybe a bit more. This snow weighs about 350 kilograms per cubic meter, so it still contains a lot of air.
Below this is the so-called firn. This is snow that has survived a summer or more. He reaches tens of meters deep. At a depth of 20 meters it has a weight of 500 kilograms per cubic meter - that is, it still consists of half of the air. Only in about 100 meters depth then begins the "real" ice, in which there are only small bubbles.
The pores in the firn usually have a practical feature: when the snow melts on the surface on a summer's day, the water runs down and is caught there. It cools down, freezes - and does not flow away. Using radar data and drill cores, Heilig and the other scientists have now shown that this process is disrupted in more and more places in Greenland. Instead, directly under the snow forms a partly meter thick layer of ice. The meltwater can then no longer be kept and makes its way towards the ocean.
Sean Gallup / Getty Images
Deep blue, this is what meltwater lakes on Greenland ice cream look like in summer
The reason for the spread of the impermeable ice layer in the ground are the excessive amounts of accumulating meltwater. In the record summer of 2012, an incredible 97 percent of the surface of Greenland had melted on a July day. In 2019, the peak on 1 August was still 58 percent. And yet the summer was extraordinary: For several days straight, there were plus degrees at the "Summit" research station at the highest point of the ice sheet. That's ice cores prove that has not happened ten times in the past 2000 years.
More than seven inches of additional sea-level rise possible
Greenland has been struggling with several warm summers in recent years. Their consequences can be detected in the underground even years later. "The problem is the frequency of record smelters," says Heilig.
A team around the Swiss glaciologist Horst Machguth had already shown in 2016 that the ice cover effect exists. The researcher from the University of Friborg in Switzerland is also part of the current writing team. "The new article shows that the phenomenon is occurring in large areas of Greenland," he explains. Another new feature is that the group has now calculated for the first time the possible contribution to further sea-level rise: Depending on which scenario is assumed for global CO2 emissions in this century, the plus is 0.7 to 7.4 centimeters until the year 2100. "That's in addition to what's already coming," says Heilig.
Milestone for Arctic research
According to the researchers, the area of Greenland, where meltwater can no longer seep underground, grew by about 65,000 square kilometers between 2001 and 2013 - simply because the ice cover in the subsurface got bigger and bigger. You can see that for example with the help of soccer fields - now, after all. By two of them, the impermeable surface in the ground is statistically larger. And per minute.
