Last weekend it was stormy and on Saturday afternoon I stood in the Schiermonnikoog marina looking at the tidal flats at low tide. Put on a rain suit, lean into the lashing western storm. I noticed that everywhere small wading birds were foraging unperturbed over the sandbanks for bottom animals, as if the wind had no influence on their small bodies at all.
The next morning it was spring tide, and in combination with the persistent strong wind, the entire ferry port and the ferry dam were frozen underwater. On the North Sea side the water was up to the beach entrance. Nothing serious, that happens a few times a year. Yet, when I saw that scene, I couldn’t help but think of the distant future of our country. I am curious whether the current housing crisis is a piece of cake compared to that in the year 2125, in which millions of Westerners will have to give up their homes that have become worthless and have to start over in one of the ten new major cities in Gelderland and Brabant. All because a hundred years earlier people were too busy with their own short-term problems to think about the people in the future.
In glaciology, we understand that more ice will melt as the world continues to warm. The highest estimates of future sea level rise we find in the climate scenarios with the most warming. Yet one melting glacier is not the same as another. That’s due to gravity. The ocean is not a bathtub, but shaped after the Earth’s gravitational field. Due to variations in the thickness of the earth’s crust and the underlying mantle, this gravity field is folded over the earth’s surface with a difference in height of tens of meters. Anyone sailing from Europe to America will unknowingly slide down a slide with a height difference of a hundred metres. It could be even crazier. On the way from the Suez Canal to East Asia you sail through a hole more than a hundred meters deep, first steeply down towards Sri Lanka, and then steeply up again on the other side. Yet you don’t notice it, because everywhere on the ocean, gravity is exactly perpendicular to the ocean surface. The slope is definitely there, but you don’t really slide down.
Then the ice cream. Gravity has a pass there recently discovered effect on sea level. The enormous mass of the ice sheets of Greenland and Antarctica pulls the ocean towards itself. The ocean is, as it were, slightly leaning against the ice cap. Then, when the ice cap starts to melt, dizzyingly counterintuitive things happen. It is true that on average worldwide sea levels rise when ice melts. But in the immediate vicinity of the ice cap, the sea level is actually falling. This is because a melting ice sheet decreases in mass and therefore pulls the ocean less towards it. Sea levels do rise a little further from the ice sheet, but due to the loss of gravitational pull, they rise slightly less than the global average. Very far away, thousands of kilometers away, sea level rise is greater than the global average. The further away from the melting ice, the greater the impact.
For the Netherlands, all this means something very remarkable. If the melting of the relatively nearby Greenland ice cap causes global sea levels to rise by an average of one meter, then the sea level off the coast of the Netherlands will rise by only 25 centimeters. But if exactly the same amount of ice melts from the far-away Antarctic ice sheet, we will have a sea level rise in our country of about 125 centimeters. That is why everything that happens in Antarctica is five times as interesting to us Dutch as the fate of Greenland.
The future of Greenland is a lot less relevant to our country than to, for example, Argentina or Australia. But it is very interesting: it is far from certain how much the sea level will rise due to the melting of the Greenland ice cap, even if it were already known how much the earth would warm up. There is a layer of snow tens of meters thick on the ice sheet, which is slowly compressed into ice under its own weight. The snow layer acts like a sponge, into which the meltwater can seep through and refreeze. In fact, half of all the meltwater in Greenland is currently absorbed into that sponge. Only the other half reaches the ocean and leads to sea level rise. The snowpack is saving us at the moment half of Greenland’s sea level rise.
The big question is how long we will continue to be treated like this. If the melt in Greenland increases further in the future, it is not exactly clear what will happen to the snow layer. It is likely that more meltwater will seep into the snow layer and refreeze. This refreezing causes the snow layer to freeze and the sponge effect to decrease. But how exactly this happens and which physical processes are important is the subject of ongoing research. It creates significant uncertainty in Greenland’s future contribution to sea level.
For the future of the Dutch coastal provinces and the Wadden Sea, it will probably not matter much what happens in Greenland in the coming century. However, it is better not to let it come to a situation in which the Greenland ice sheet melts significantly. Because in that scenario, Antarctica, eight times larger, is also moving, and the consequences of the melting of Antarctica will be felt all the more in the Netherlands.
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