Study shows topography is a key factor in controlling ice flow
By Bob Berwyn
FRISCO — Greenland’s swift outflow glaciers are sensitive to warming air and ocean temperatures, but a new study published in the journal Nature indicates that the recent acceleration of glacial flow isn’t continuing at a linear rate.
The shape of the ground and seafloor beneath the glaciers is crucial in determining how they respond to climate change — how fast they move and how much they will contribute to sea level rise in coming decades.
“What we are saying is that we shouldn’t extrapolate the rate of the last 10 years into the future … If you study these glaciers separately, they show different behavior,” said lead author Dr. Faezeh Nick, of the Université Libre de Bruxelles, describing the work done on the Petermann, Kangerdlugssuaq, Helheim and Jakobshavn Isbræ glaciers. Together, they drain about 22 percent of the Greenland ice sheet.
The model was developed within the EU-funded ice2sea program. It shows that, with a mid-range temperature increase of 2.8 degrees Celsius by 2100, the four glaciers will raise sea level by about 2cm to 3cm by the year 2200, which is less than estimates based solely on the extrapolation of current trends.
If temperatures warm 4.5 degrees Celsius by 2100, the projected losses increase by more than 50 percent, producing a cumulative sea level rise of about 3cm to 5cm by 2200.
Based in part on the new research, a revised overall estimate of sea level rise contribution from Greenland now stands at about 6.5 to 18.3 cm by 2100, but there are large uncertainties associated with that estimate, which doesn’t calculate important variations in the geometry of individual outlet glacier systems.
Nick emphasized that the study does not predict an overall slowdown in the loss of Greenland’s ice mass. The study does offer a more nuanced view of how the glaciers’ response to climate change is shaped by local topography.
She explained, for example, that the Petermann Glacier has a long ice shelf, making it sensitive to ocean warming. The flow rate of the Petermann Glacier has been rather steady. Calving of two huge icebergs in 2010 and 2012 raised concerns about the glacier’s stability but didn’t result in an acceleration of the flow rate.
Based on the new understanding of topographic factors, the Petermann Glacier is projected to lose mass mainly through surface melt through about 2100. After that, underwater melting is likely to become a more significant factor, causing a retreat of the grounding line.
By contrast, the Helheim and Kangerdlugssuaq glaciers are more responsive to reductions in sea ice and increased cracking due to more surface melt, “with little response to submarine melt or basal and lateral lubrication,” according to the article published in the journal Nature.
Glaciers that empty into an area with little sea ice move faster, which expands crevasses, generating a feedback loop that can accelerate ice loss. It also works the other way, Nick said, explaining that, if a glacier calves a huge amount of icebergs, it can cool the surrounding water and slow the movement of the glacier, at least for a while.
The Jakobshavn Isbræ glacier is reacting to the reduction in sea ice and is also sensitive to underwater melting, while the Petermann Glacier doesn’t seem to be affected by changes at its terminus, but is more affected by underwater melting near the grounding line.
“We know that the breaking off of icebergs from glaciers is influenced by climate, but this is the first time we’ve been able make projections of how the most important glaciers in Greenland will be affected by future climate change,” said
Professor David Vaughan, who works at the British Antarctic Survey in Cambridge and is head of the ice2sea programme.