Viscous AND elastic: Glaciers are more “solid” than previously assumed

The massive Nioghalvfjerdsfjorden Glacier in northeast Greenland lies at the 79^th parallel North and is often referred to as “79°NG” for short. The colossus flows directly into the Greenland Sea and contains enough ice to raise the global sea level by ca. 1.1 metres if it melted completely. As a result of climate change, the ice loss at 79°NG has increased considerably. For example, the icebergs calving from it are getting larger and larger – including one larger than Paris (112 km²) in September 2020. 

“If we want to more precisely forecast the sea-level rise from ice loss, we have to duplicate the flows of glaciers like 79°NG as precisely as possible,” says Dr Julia Christmann, the study’s first author and a glaciologist at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). “To keep the amount of required computing power manageable, glaciers’ movements are often shown in a highly simplified form; the simulations show the ice as flowing. But glacier ice also has solid-body qualities that are virtually always ignored. Our study shows that exactly these qualities are very important, making it worthwhile to integrate them into simulations.”

Together with her international team of experts from Germany, Denmark and the USA, Christmann prepared a simulation of 79°NG that combines its “elastic” solid-body behaviour and its “viscous” flowing behaviour. In this regard, the subglacial water under the glacier is also taken into account, an aspect for which AWI glaciologist Dr Thomas Kleiner used the AWI Hydrological Model. To gauge how well their “viscoelastic” simulation matched with the real 79°NG, the experts compared the computer data with actual GPS movement data on the ice, gathered from an AWI field campaign and satellite-based remote sensing.

“We were able to show that the elastic component is important, for example, where the glacier flows into the ocean,” explains Christmann, who conducted her work as part of the BMBF-funded project GROCE (Greenland Ice Sheet Ocean Interaction). “Here, there’s nothing but seawater below the glacier, so it’s no longer in contact with the ground. High and low tides cause the floating sheet of ice to rise and fall. In addition, the ocean water pushes against the subglacial water under the ice on land, affecting the glacier’s flow speed. This tidal signal produces elastic deformations in the glacier as far as 10 kilometres inland from the grounding line, where the ice is still resting on the ground. Though tides’ remote effects on inland ice had previously been documented in the Antarctic, they had been largely ignored when it came to Greenland.”

Another surprising find: even beyond the reach of the tidal signal, far inland, deformation can be observed – wherever the glacier flows at a relatively high speed (more than 70 cm per day) over “mountains” and substantial swells in the ground below the ice. “This creates considerable tension and leads to elastic deformation in the ice,” says Christmann. “And exactly these high-strain sites in our model match the satellite data amazingly well. That’s exactly where, throughout Greenland, we can see huge fields riddled with crevasses. This makes it clear why glaciers can’t be accurately described without the solid-body component. Pure fluids don’t have any cracks or crevasses.”

The team surmises that both phenomena – the tidal signal and elastic deformation in inland ice – manifest on several outlet glaciers around the globe that are comparable to 79°NG. “Accordingly, it’s important to integrate the elastic component in our models, even if doing so makes them more complex,” says Prof Angelika Humbert, coordinator of the AWI study. “The solid-body characteristics also determine how quickly a glacier flows out to sea and how much ice it loses there in a warmer climate. As such, they can help us make more accurate projections of sea-level rise.”