Is the Greenland ice sheet lost forever – or can we still turn the tide?
A tale of three ice caps
Klassekampen, 28 August 2020
While a temperature of 54.4 °C was recorded in Death Valley – possibly a new world record – news surfaced that Greenland's ice sheet has melted to a point of no return, even if we manage to halt climate change. The study appeared on CNN on August 14th, among others. The Greenland ice sheet contains enough ice to raise global sea level by 7 meters. Should low-lying countries such as the Netherlands, Bangladesh and the Pacific island nations be considered as lost? And will the water also flood the center of Oslo?
If we look closely at the study, it appears that this is far from certain, although things are trending in the wrong direction. Greenland is by far the largest source of meltwater to the oceans: one fifth of the current sea level rise of more than 3 mm per year is due to the melting of the glaciers and the ice sheet on Greenland. That doesn't seem like much, but over the decades it adds up to quite a lot. Besides, the melt is increasing rapidly: last year enough Greenland ice melted over the course of just two months to raise the global sea level by more than 2 mm. At the beginning of this century, the loss of Greenland's ice also accelerated, and that's what the new study investigated.
If you think we've had a lucky escape, then I have some bad news for you
If you want to know how fast the ice in Greenland is disappearing, you need to figure out three things: how much snow falls on top of the ice sheet, how much meltwater is running off the sides of the ice sheet, and how fast the glaciers on the coast are calving. The researchers in this study mostly considered the latter and saw that the ice flow from glaciers had shown a step-wise increase at the beginning of this century. They concluded that ice discharge had reached a new, accelerated level. Even if the other two factors, snowfall and melting of the ice sheet, were to return to levels seen in the 1980s, Greenland would still lose ice due to the faster pace of the glaciers.
But it is not that simple. When Greenland's glaciers retreat onto land, their base will no longer be in contact with relatively warm water in the fjords and the calving of icebergs will cease. In that case, the arithmetic only consists of determining the amount of snow fall minus the amount of melt water. When the glaciers stop calving, it will take centuries, if not a millennium, for the two-kilometer thick ice sheet to completely melt away.
Incidentally, the new study did not investigate that future development at all. The researchers only determined the loss of ice over the past few decades. The claim that the Greenland ice sheet had reached a "point of no return" was only mentioned in the title of the overly enthusiastic press release – not in the scientific paper.
In other words, it is too soon to establish that Greenland's ice will be irreversibly lost. With a successful climate policy, it is not inconceivable that the ice sheet will end up in a new equilibrium in the long term. But if this is a process that takes place over many centuries, it may be less relevant for us to know if the entire ice sheet will melt away some time in the future rather than the amount of sea-level rise we can expect in the coming decades. Strangely enough, in this case Norway is benefiting from the disappearance of another ice sheet.
In the last ice age, more than ten thousand years ago, there was a thick ice sheet covering Scandinavia, just like the one in Greenland. The weight of a few kilometers of ice was enough to press the surface of the Earth deep down, and after the ice disappeared, it slowly rebounded. This process, called post-glacial lift, is extremely slow and lasts many millennia. It is still going on at a rate of a few millimeters per year. As it happens, the Earth's surface in Oslo is rising – for the time being – faster than the sea level.
In places on the coast, such as Bergen and Kristiansand, this increase is not so rapid, but there too, there is little need to worry about Greenland. Sea levels are not rising equally fast across the planet. Each year, several hundred gigatons of ice disappear from the Greenland ice sheet. This is such a large amount of mass that it slightly lowers gravity in the region. As a result, less seawater is attracted around Greenland and the strongest rise in sea levels, due to all this meltwater, happens on the other side of the planet: in the Pacific and Southern Oceans.
If you think we've had a lucky escape because of this, then I have some bad news for you. On the other side of the planet is a third ice sheet, the one on Antarctica. It contains enough ice to raise sea levels by nearly 60 meters – and in this case the change in gravity plays to our disadvantage. Furthermore, it is highly uncertain how stable the West Antarctic ice sheet will remain this century, which makes it possible that current predictions of sea level rise have been severely underestimated.
One thing is certain: as long as global warming continues, more ice will continue to melt. Are we going to stop this in time before we're up to our necks in meltwater?
This text originally appeared in Klassekampen on 28 August 2020