Scientists shed light on Greenland glacier melt – and what it means for sea level rise
A new study estimates how much Greenland’s glaciers could raise sea level under a warming climate – and it could be as much as 8.5 cm by 2100. This is less than some previous models suggest, but not insignificant for calculating total sea level rise.
Satellite data suggests that over the last 20 years, the Greenland ice sheet has lost 140 billion tonnes of ice each year. As ice sheets melt, water that was previously held on land is added to the ocean, causing sea levels to rise.
As well as warmer air directly melting the surface of the ice sheet, glaciers are an important part of the picture. Glaciers move ice from the ice sheet to the sea, and react quickly to changes in atmospheric conditions.
This sensitivity makes glaciers very visible indicators of climate change. But unravelling the different ways they’re affected by rising temperature has proven challenging, which means the contribution of Greenland’s glaciers to sea level rise is not well understood.
Ways to lose ice
In a new study published today in Nature, scientists used a detailed computer model to simulate all the ways glaciers can lose ice. We often hear about glaciers retreating, but meltwater can loosen up the ice so that glaciers flow quicker, discharging more ice to the ocean. Meltwater can also cause the ice to fracture – a process known as calving.
The paper looks in detail at four of Greenland’s major glaciers, all of which have lost ice rapidly over the last few decades. Once the scientists had tested their model against the glaciers’ past behaviour, they used it to forecast how ice loss might contribute to sea level rise up to 2200 under different emissions scenarios.
The team of scientists placed GPS systems on the edge of glaciers to monitor how far they retreat over time. Credit: Dirk van As
Future loss
Under a medium emissions scenario, the new data shows all four glaciers continue to lose ice up to 2200 – although at varying rates. Together, they contribute between 9 and 13 mm to sea level rise by 2100 and 19 to 30 mm by 2200.
Perhaps unsurprisingly, the rate of loss was faster under a higher emissions scenario. Under the IPCC’s new RCP8.5 scenario, all four glaciers together contributed 11 to 18 mm to sea level rise by 2100 and 29 to 49 mm by 2200.
The researchers found that the rate of ice loss was not constant over the time period. Rather than being a smooth process, changes in the amount of ice discharged to the ocean occur as sudden, episodic events.
The scientists found that such events happened less and less over time – because of the way their occurrence changes the shape of the channel the glacier calves out in the rock.
This means the new estimate of the glaciers’ contribution to sea level is lower than some earlier model predictions, which assumed such sudden events continued at the same pace over the coming centuries.
Substantial sea level rise
That’s not to say Greenland’s glaciers won’t be making an important contribution to sea level rise.
The four glaciers in the new study only drain about 22 per cent of the total area of Greenland. While the scientists acknowledge there are likely to be variations between different glaciers, they scale the original estimate up to estimate the likely contribution from glaciers over the whole ice sheet – and the figure is between 40 and 85 mm by 2100.
Add that to existing estimates of ice loss from both ice sheets – which scientists say could be about 30 cm by 2100 – and that’s some substantial sea level rise by the end of the century.