While the GRACE satellites were active, their incredibly precise gravity measurements tracked a loss of about 280 billion tons of ice from Greenland each year. That’s glacial land ice that raises sea level as it flows into the ocean—and it’s vanishing at a remarkable clip. But just how remarkable is that clip? We don’t have such excellent measurements going back too far into Greenland’s history.
A new study led by the University of Buffalo’s Jason Briner takes this question on. We have lots of paleoclimate records of climate conditions in Greenland, the position of the ice on the landscape, and even changes in sediments carried into the sea by meltwater. None of that directly tells you how much ice was accumulating or disappearing. To put the pieces together and calculate that, you need to combine that data with a model.
The researchers used a high-resolution ice-sheet model simulating (roughly) the southwest quadrant of Greenland. There’s a good reason for that: 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-to-year growth or losses of the ice sheet here nicely mirror the Greenland-wide total. So simulate this area well, and at high resolution, and your numbers should scale to the whole ice sheet.
Using the paleoclimate data from ice cores, the researchers drive the model with temperature and precipitation changes over the last 12,000 years. This is the period known as the Holocene, encompassing warming out of the last ice age followed by the warmer and relatively stable interglacial climate. Once the model reaches 1850, it switches to monthly climate records. The model can also be rolled into the future, out to 2100, using common projections for different greenhouse gas emission scenarios.
Glaciers build curved ridges of piled rock at their end, so there are landscape markers preserving the position of the ice sheet’s edge over the Holocene. Each can be dated, telling you when the ice retreated to that point. The model’s simulation matches up with these data points nicely, just as it matches with our 20th-century observations.
So the model works. What does it say about how the modern melt compares to the past? Well, the rate of loss in this portion of Greenland from 2000 to 2018 is equivalent to about 6,100 billion tons 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 average out to 4,900 ±1,400 billion tons per century.
That means that the recent rate is already about equal to the fastest rate in the previous 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 where climate change halts at about 2°C of total warming, comparing it to a scenario of much higher greenhouse gas emissions that produce 4°C or more. Calculating the 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 surpassing anything in the last 12,000 years. And the researchers note that their model tends to simulate smaller future losses than some others do, so that might be conservative.
The researchers conclude, “Our results suggest that the rate of mass loss from the GIS [Greenland ice sheet] this century will be unprecedented in the context of natural GIS variability over the past 12,000 years, unless a low-carbon-emission scenario is followed.”
Lower emissions even than the 2ºC warming scenario, that is. For context, current emissions pledges would likely get us something around 3ºC warming this century.
Ice sheets have considerable inertia, slowly responding to warming and then shrinking for a very 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.