Melting of Greenland ice sheet makes sea rise 27.5 cm anyway

Even if humans immediately stop emitting greenhouse gases, the sea level will still rise on average by 27.5 centimeters due to the guaranteed melting of the Greenland ice sheet. A group of mainly European glaciologists has calculated how much rise is still ‘in the barrel’. They are higher than climate models predicted so far. Their results came out on monday in Nature Climate Change.

The fact that the melting of the Greenland ice sheet will continue for centuries, even after further global warming has stopped, has to do with its delayed response. “The ice sheet needs time to adjust to the new, higher temperature and to reach a new equilibrium,” says Bert Wouters, affiliated with Delft University of Technology and Utrecht University, and one of the authors of the article. When warming, the ice sheet loses mass on balance, because more ice melts in summer than snow falls in winter. “The ice sheet is retreating,” says Wouters. “At first, mainly on the edges.” If warming stops, the ice sheet will continue to shrink until summer melt and winter snowfall are balanced again.

Mass of snow and ice

For their calculation, the researchers determined the annual surface ratio between the so-called accumulation zone, the area of ​​the ice sheet where the mass of snow and ice increases, and the total ice sheet. That will be the accumulation area ratio called the AAR. They used data from 2000-2019. Each year they related this ratio to the total mass of the glacier. “You can then make a graph in which you plot the amount of mass loss against the AAR,” explains Wouters. From this it can be deduced how large the AAR should be if the ice sheet is in equilibrium.

They conclude, for example, that the Greenland ice sheet still needs to shrink by 3.3 percent to return to equilibrium in the current climate. The amount of ice that has to melt (a volume of 110,000 km³) will raise the sea level on average by 27.5 cm. Wouters emphasizes that it is the minimal increase. “Because global warming doesn’t stop today.”

The disadvantage of their method, says Wouters, is that they cannot say over which period the increase will spread. The researchers write that it could be 2,500 years, but also 200 years.

The melting of all the world’s glaciers, including the expansion of ocean water due to warming, will cause a sea level rise of between 30 cm and 1 meter over the next 80 years, according to the IPCC, the United Nations climate agency. The increase depends on how greenhouse gas emissions develop.

But the climate models on which these calculations are based do not include all processes that affect the melt. Alun Hubbard of the Arctic University of Norway in Tromsø, one of the other authors of the paper, describes some of those processes in an article he wrote about the now published research in front of The Conversation. Hubbard, for example, mentions the increasing growth of bacteria, which darkens the ice surface, so that it absorbs more heat and melts faster. Or the fact that warm ocean currents penetrate deeper into Greenland’s fjords and ‘eat down’ glacier tongues there.

The guaranteed sea level rise of 27.5 cm is a global average. It remains to be seen whether the sea level in the Netherlands will rise as a result. It has to do with gravity effects. Because of their enormous mass, ice caps attract other masses (water, for example). The water around an ice sheet is therefore higher than average. But when the ice sheet melts, its mass decreases, the gravitational pull decreases, water flows away from the ice sheet, and the sea level drops nearby. This effect can be seen up to 2,200 km from an ice sheet. Between 2,200 and 6,700 kilometers the effect is still there, but weaker. Even further away, the opposite effect occurs: the sea level rise is above average. The Netherlands is more than 3,000 km from Greenland, and therefore falls in the zone where a weak decline is expected. But the height of the sea level off the Dutch coast does not only depend on Greenland. Antarctica is also losing mass, and that has a major effect for the Netherlands.