Greenland is the second largest ice reservoir on the planet, but is considered to be particularly sensitive to climate change. Researchers without climate models have now determined what this means for future sea levels based solely on measurement data. Accordingly, the current discrepancy between snow supply and ice melt ensures that the Greenland ice sheet will lose another 3.3 percent of its ice even if climate change ends immediately - this corresponds to a 27 centimeter rise in global sea levels. On the other hand, if climate change continues, sea levels could rise by up to 78 centimeters – just due to the Greenland ice melt. According to the scientists, a large part of this increase will take place before the end of this century.
The Greenland ice plays a key role in global climate and sea levels. Because when this second largest ice reservoir on the planet thaws, enormous amounts of meltwater flow into the sea. The giant island is already one of the regions most affected by climate change in the world. Because the Arctic is warming disproportionately and changing air currents are increasingly bringing warm air to Greenland, its ice sheet is already thawing exponentially in places. The ice loss is already overtaking all previous forecasts, and in the summer of 2021, rain instead of snow fell on the coldest, highest point of the Greenland ice sheet for the first time since observations began. In view of the rapid melting of glaciers, especially in southwest Greenland, some climate researchers are even seeing the first signs that the glaciers there are approaching a tipping point - a threshold at which there is a risk of complete thawing.
Accumulation zone of the glaciers in view
Jason Box from the Geological Survey of Denmark and Greenland (GEUS) and his colleagues have now determined what this means in concrete terms for the ice masses in Greenland and for future sea levels. Unlike many previous studies, they did not use climate models for this, but based their calculations solely on observational data. "This is a complementary approach to determine mass loss," explains Box. Because models have so far been unable to fully capture the complex interactions between ice, atmosphere and ocean, forecasts for some regions of Greenland in particular are subject to great uncertainty. Instead, the researchers used the observation and measurement data on the condition of the Greenland ice sheet and in particular on the mass balance of the glaciers that has been collected in the last two decades from 2000 to 2019.
To do this, the scientists analyzed the surface area of the individual glaciers occupied by their accumulation zone. This part of a glacier lies above the so-called snow line, the area that is cold enough not to lose ice even in summer. Therefore, the ice gradually increases in this area due to snowfall - it is in a sense the replenishment zone of a glacier. The proportion of this zone compared to the total glacier size gives an indication of whether the glacier is gaining enough ice to be sustainable. For their study, Box and his colleagues determined whether the current accumulation areas are still sufficient to maintain the balance between losses and gains - and how large the discrepancies are.
A rise in sea level of 27 centimeters is unavoidable
The analyzes showed: In relation to the entire currently glaciated area of Greenland of around 1.78 million square kilometers, the Greenland ice sheet is already clearly in the red. The accumulation areas are already 3.3 percent smaller than they would have to be to maintain the current ice mass. This imbalance results in an already inevitable loss of ice of 110,000 cubic kilometers in volume and 59,000 square kilometers in area,” report Box and his team. Even if the world stopped all greenhouse gas emissions today, these ice masses would melt. This in turn will lead to a sea level rise of at least 27 centimetres, which is already certain and can no longer be stopped. That would be enough water to flood the entire surface of the United States eleven meters.
"But this is an extremely conservative minimum value," emphasizes Box. "Realistically, we have to expect that this value will more than double in this century." Assuming that global warming continues for the time being, the imbalance between ice gain and loss will continue to increase and with it an ever larger part of the Greenland's ice sheet are irreversibly heading towards melting. In their study, the scientists calculated, among other things, what would happen if the summer of 2012, which was particularly warm in Greenland, was used as a benchmark for further developments. According to their calculations, ten percent of the Greenland ice sheet and a volume of 314,000 cubic kilometers of ice would then be thawed. This then irreversible loss of ice would raise global sea levels by 78 centimeters.
In all scenarios, the glaciers in south-west and west Greenland are the most affected because they are deeper and there is less snowfall. With them, the imbalance between ice gain and ice loss is already very pronounced, as Box and his colleagues report. The ice in the central east of Greenland, on the other hand, is still in equilibrium and is not contributing to sea-level rise – at least not yet. "But this region would also be susceptible to future ice loss if a warm climate like that of 2012 continues," the scientists explain. For Greenland's future development in the 21st century, this would be a bleak prognosis.
Source: Jason Box (Geological Survey of Denmark and Greenland, Copenhagen) et al., Nature Climate Change, doi: 10.1038/s41558-022-01441-2