The dirt and water under Greenland will control the sea of the future
UNIVERSITY OF STOCKHOLM
PICTURE: HENNING ÅKESSON Show more CREDIT: HENNING ÅKESSON
The type of material beneath glaciers has a huge impact on how fast they slide towards the ocean. Scientists are faced with the challenging task of collecting data on this under-ice landscape, let alone how it can be accurately represented in models of future sea level rise.
“Choosing the wrong equations for the under-ice landscape can affect the predicted contribution to sea level rise just as much as several degrees warming,” says Henning Åkesson, who led a recently published study on the Petermann Glacier in Greenland.
Glaciers and ice sheets around the world are currently losing more than 700,000 Olympic pools every day. Glaciers are formed by the conversion of snow into ice, which later in summer is melted by the atmosphere or slides into the sea. With climate change, glaciers are breaking up and dropping icebergs into the ocean at an ever faster pace. How fast, exactly, depends a lot on the bed under all the ice. Glaciers hide a landscape beneath ice covered with rocks, sediments and water. A new study shows that the way we computer model this under-ice landscape means a lot to our predictions of future sea level rise. More precisely, how we incorporate the friction between the ground and the ice sliding over it into glacier models influences our predictions. This was discovered by a team of Swedish and American scientists when they were simulating the future of the Petermann Glacier, the largest and fastest glacier in northern Greenland.
Petermann is one of the few glaciers in the northern hemisphere with a remaining tongue of ice, a type of floating glacier extension that is mostly found in Antarctica, where they are called ice shelves. These floating extensions have been found to be exposed to the warm underground water flowing from the open ocean towards the glaciers. This happens both in Antarctica and in many fjords around Greenland, including the Petermann Fjord.
“Peterman has lost 40% of its floating ice tongue in the past decade. It still has a 45 km long tongue, but we found that a slightly warmer ocean than today would cause it to break up and retreat, ”says Henning Åkesson, postdoctoral fellow at Stockholm University who led the study.
Many glaciers in Greenland and Antarctica flow towards the ocean much faster than they did a few decades ago and therefore contribute more to global sea level rise. Scientists have therefore gone to great lengths to find out what is going on in these environments. This has led to new knowledge about the landscape beneath glaciers and the shape of the seabed where they drain. We also now know a lot more about what happens to the ice when glaciers meet the sea.
Still, the remote polar regions are notoriously difficult to study because of sea ice, icebergs, and often harsh weather. The landscape under the ice is a particular challenge because, honestly, it’s hard to measure something that’s covered by a kilometer of ice. Even in areas with known under-ice topography, it is difficult to describe its physical properties with mathematical equations. Computer models are therefore still somewhat in the dark when it comes to representing things like sediments, rocks, ponds and rivers under glaciers in the equations for describing the flow of ice. These equations ultimately form the basis of the IPCC’s models for estimating how fast glaciers will flow and how much the sea level will rise in the event of future global warming.
“As I said, choosing the wrong equations for the under-ice landscape can have the same effect on the contribution to sea level rise as several degrees warming,” says Åkesson.
“In fact, the predicted sea level rise for this Greenland glacier can quadruple, depending on how we represent the friction under the ice. We still don’t know which route is best, but our study shows that ice sheet models still have to make progress in this regard in order to improve our estimates of mass loss from Earth’s polar ice sheets. “