While the GRACE satellites were active, their incredibly accurate gravity measurements tracked a loss of about 280 billion tons of ice from Greenland each year. It̵
A new study led by Jason Briner of the University of Buffalo addresses this question. We have many paleoclimatic records of the climatic conditions in Greenland, the location of the ice on the landscape and even changes in the sediments transferred to the sea from the molten water. None of that directly tells you how much ice is accumulating or disappearing. To collect the pieces and calculate this, you need to combine this data with a model.
The researchers used a high-resolution ice model simulating (approximately) the southwestern quadrant of Greenland. There is a good reason for this: the ice sheet melts mostly before it reaches the ocean here, making it the simplest area to simulate. As we track things, the annual growth or losses of the ice sheet here reflect well the total for the whole of Greenland. So simulate this area well and with high resolution and your numbers should be scaled to the entire ice roof.
Using ice core paleoclimate data, researchers have been managing the model with changes in temperature and precipitation over the past 12,000 years. This is the period known as the Holocene, covering the warming of the last ice age, followed by a warmer and relatively stable interglacial climate. After the model reached 1850, it switched to monthly climate records. The model could be launched in the future, by 2100, using common forecasts for different greenhouse gas emission scenarios.
Glaciers form curved ridges of accumulated rocks at their ends, so there are landscape markers that maintain the position of the edge of the ice sheet above the Holocene. Anyone can be dated by telling you when the ice has receded so far. The simulation of the model coincides well with these data points, just as it coincides with our observations of the 20th century.
So the model works. What does it say about how the modern melt compares to the past? Well, the rate of loss in this part of Greenland from 2000 to 2018 is about 6 100 billion tons per century. Greenland ice also receded rapidly between 10,000 and 7,000 years, then stabilized and slowly increased. The five centuries during which the fastest retreat was observed amounted to an average of 4900 ± 1400 billion tons per century.
This means that the recent rate is now approximately equal to the fastest rate in the last 12,000 years.
But Greenland is not stabilizing in a world that continues to warm, of course, and the rate of loss is expected to increase. The researchers looked at a future scenario in which climate change stops about 2 ° C from total warming, comparing it to a scenario with much higher greenhouse gas emissions that produce 4 ° C or more. Calculating the average loss rates for the 21st century, they find a period of 8,800 to 35,900 billion tons lost per century for this area – far surpassing anything in the last 12,000 years. And researchers note that their model tends to simulate smaller future losses than some others, so this may be conservative.
The researchers conclude: “Our results suggest that the rate of mass loss from GIS [Greenland ice sheet] this century will be unprecedented in the context of the natural variability of GIS over the last 12,000 years, unless a low-carbon scenario is followed. “
Lower emissions even than the 2 ° C warming scenario, ie. In context, current emissions promises are likely to warm us around 3ºC this century.
Ice sheets have considerable inertia, reacting slowly to warming and then shrinking for a very long time. Regional climate variability may also affect short-term trends in Greenland. However, the expected consequences of our rapid warming experiment on the planets seem clear.
Nature, 2020. DOI: 10.1038 / s41586-020-2742-6 (For DOI).