Warm Atlantic water is melting Greenland’s largest floating ice tongue

The Nioghalvfjerdsfjorden Glacier – also known as the 79° North Glacier – on the northeast coast of Greenland flows directly into a fjord, where it forms an 80-kilometre-long tongue of floating ice. Although the tongue hasn’t lost much length during the past several decades, it has grown thinner and thinner. A team from the Alfred Wegener Institute can now tell us why. By applying a computer-based model, they were able to show that warm water from the Atlantic flows into the European North Sea and ultimately into the cavern under the glacier tongue, where it melts the ice from below. Their study findings, which have just now been published in the journal Nature Communications, pave the way for more precise projections on the future of the Greenland Ice Sheet and increasing sea level rise from global warming.

Greenland’s massive ice sheet contains nearly 3 million cubic kilometres of water. If it were to melt completely, the global sea level would rise by more than 7 metres. Part of the ice sheet – the Northeast Greenland Ice Stream – flows into two major marine outlet glaciers on the country’s coast: the Nioghalvfjerdsfjorden Glacier (or 79NG) and the Zachariae Isstrom (or ZI). Here, the two glaciers flow into the Greenland Sea, where they formed two huge floating glacier tongues 20 years ago. While the ZI glacier lost its floating tongue back in the 2010s, ice from 79NG continues to flow toward the sea through a fjord, in a swathe roughly 20 kilometres wide and 80 kilometres long.

Why is this floating ice tongue, the largest left in Greenland – apparently – so stable? And which factors will determine its ultimate fate? “The tongue of 79NG is protected by its surroundings. In this regard, the topography of the fjord and its seafloor, as well as certain islands on the calving front, which effectively serve as anchoring points, are very important,” explains Claudia Wekerle, a physical oceanographer at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). “But from previous studies, we know that the ice lost roughly 30% of its thickness between 1999 and 2014, because – at least that’s what we assume – the melting rates on the underside increased significantly due to warm water inflow.” But when it comes to the cavern below the ice, until recently there had been only sporadic measurements of the current and ocean temperature. “Thanks to our high-resolution ocean model, now, for the first time, we were able to draw conclusions regarding water currents in the cavern.”

First author Claudia Wekerle and her team relied on the ocean model FESOM2 (Finite-Element/volumE Sea ice-Ocean Model), developed at the AWI. What sets the model apart: it can also simulate smaller ocean regions of interest in high resolution and therefore more realistically – in this case, the cavern beneath the 79NG’s ice tongue. To arrive at their findings, the team increased the model’s resolution to 700 metres for the cavern and its immediate vicinity. “By way of comparison: in our high-resolution Arctic model, the resolution is 4.5 kilometres, and the typical resolution of ocean models is roughly 25 kilometres or even worse. Thanks to this high resolution, FESOM2 can accurately reproduce the glacier’s topography. This is particularly important for the influx of warm Atlantic water, which flows into the cavern through a roughly 5-kilometre-wide trench.

“Using our model, we were able to determine the cause of the high melting rates on the underside of the floating ice tongue,” says the AWI researcher. “In this regard, there are two important factors.” First of all, due to global warming, more surface meltwater has reached the Greenland Ice Sheet in the past few decades, penetrating the ice. Part of the freshwater flows to the glacier’s grounding line – the point where the ice is no longer in contact with the ground and starts to float – and flows under the glacier into the cavern as a subglacial inflow. “Here, it intensifies the water’s circulation within the cavern, increasing the ice’s contact with water and therefore the melting on its underside.” In addition, over the past few decades the temperature in the Atlantic water layer on the northeast Greenland continental shelf has generally increased. This comparatively warm water stems from the Atlantic, flows through the Arctic Ocean, and circulates westwards in Fram Strait before reaching the continental shelf of northeast Greenland and finally 79NG. The warmer water flows into the cavern through a trench in the calving front and melts the underside of the ice tongue. “Our study determined that the higher ocean temperatures in the Atlantic water layer are what chiefly determine the melting rates, not the increased subglacial influx of meltwater.”

Armed with these findings, the experts can now take the next step: in further simulations, they plan to project the future development of 79NG in various climate scenarios. But one thing is already clear: if the ice tongue were to disappear completely, it would have far-reaching consequences for the stability of the land-based ice behind it, and for progressively rising sea levels. After all, today the Northeast Greenland Ice Stream flows seaward across the land much faster than it did just a few years ago. And this – as a study from 2022 shows – is a direct result of losing the ice tongue of the Zachariae Isstrom, to the south of 79NG. “That’s why, for reliable projections on sea level rise and other climate change impacts, it is essential to keep a close eye on and understand the Greenland Ice Sheet as a whole, and its contact regions with the ocean, which are critical to its future development,” says Claudia Wekerle. “And one of those key regions is the 79° North Glacier on the island’s northeast coast.”

Original publication:

Claudia Wekerle, Rebecca McPherson, Wilken-Jon von Appen, Qiang Wang, Ralph Timmermann, Patrick Scholz, Sergey Danilov, Qi Shu, and Torsten Kanzow: Atlantic Water warming increases melt below Northeast Greenland’s last floating ice tongue. Nature Communications. DOI: 10.1038/s41467-024-45650-z