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While the GRACE satellites were active, their incredibly precise gravity measurements tracked a loss of approximately 280 billion tons of ice from Greenland each year. That’s glacial land ice that raises the sea level as it flows into the ocean, and it is fading at a remarkable rate. But only how remarkable is that clip? We don’t have such excellent measurements that go back too far into Greenlandic history.
A new study led by Jason Briner of the University at Buffalo addresses this question. We have many paleoclimatic records of the climatic conditions in Greenland, the position of the ice in the landscape, and even the changes in sediment carried out to sea by meltwater. Nothing of that directly tells you how much ice was accumulating or disappearing. To put the pieces together and calculate it, you must combine that data with a model.
Digital ice
The researchers used a high-resolution ice sheet model that simulated (roughly) the southwestern quadrant of Greenland. There’s good reason for this: The ice sheet mostly melts before reaching the ocean here, making it the simplest area to simulate. Since we’ve been tracking things, the year-over-year growth or loss of the ice sheet here reflects the Greenland total very well. So simulate this area well and in high resolution, and your numbers should scale to the entire ice sheet.
Using data from the paleoclimate of the ice cores, the researchers drive the model with changes in temperature and precipitation over the past 12,000 years. This is the period known as the Holocene, encompassing the warming of the last ice age followed by the warmer and relatively stable interglacial climate. Once the model reaches 1850, it switches to monthly weather records. The model can also be implemented in the future, up to 2100, using common projections for different greenhouse gas emission scenarios.
Glaciers form curved ridges of rock stacked at their tip, so there are landscape markers that preserve the position of the ice sheet’s edge over the Holocene. Each can be dated, telling you when the ice was removed at that point. The model simulation matches these data points very well, as does our 20th century observations.
So the model works. What does it say about how modern melting compares to the past? Well, the rate of loss in this part of Greenland between 2000 and 2018 equates to about 6.1 billion tonnes per century. Greenland ice also retreated rapidly between 10,000 and 7,000 years ago, after which it stabilized and slowly grew. The five centuries that saw that fastest retreat averaged 4.900 ± 1.4 billion tons per century.
That means the recent rate is already roughly equal to the fastest rate for the previous 12,000 years.
It is melting
But Greenland is not stabilizing in a world that continues to warm, of course, and the loss rate is expected to increase. The researchers looked at a future scenario in which climate change stops at about 2 ° C of total warming, comparing it to a scenario of much higher greenhouse gas emissions producing 4 ° C or more. Calculating average rates of loss for the 21st century, they find a span of 8,800 to 35,900 billion tons lost per century for this area, far exceeding anything in the last 12,000 years. And the researchers note that their model tends to simulate smaller future losses than some others, so it could be conservative.
The researchers conclude: “Our results suggest that the mass loss rate of the GIS [Greenland ice sheet] this century will be unprecedented in the context of the natural variability of GIS over the past 12,000 years, unless a low-carbon scenario is followed. “
Lower emissions even than the 2ºC warming scenario, ie. For context, current emissions promises would likely lead to a warming of around 3ºC this century.
Ice sheets have considerable inertia, respond slowly to heating, and then shrink for a long time. Regional climate variability can also have an impact on short-term trends in Greenland. But even so, the expected consequences of our rapid planetary warming experiment seem clear.
Nature, 2020. DOI: 10.1038 / s41586-020-2742-6 (About DOIs).
