By Guy Paxman and Bethan Davies
The Greenland Ice Sheet is a huge mass of ice on the island of Greenland. In places, the ice is more than 3 km thick, and there are more than two thousand trillion tonnes of ice. If all the ice melted, this would be enough to raise global sea levels by more than seven and half metres!
This ice sheet has existed in Greenland for more than 3 million years, but today, the ice is melting. This fresh water can raise global sea levels, but can also alter oceanic circulation and change the marine biochemistry.
Understanding the topography of the ice underneath Greenland is important, because it influences the pattern of ice loss around Greenland.
The ice flow of the Greenland Ice Sheet
In Greenland, the ice flows towards the sea through a network of ice streams and glaciers. Glaciers that flow through low coastal valleys are particularly vulnerable to warming of the ocean, and could be relatively easily destabilised in the future.
In contrast, other parts of the ice sheet are more resilient, because they are hemmed in by large mountains, which act as barriers that stabilise the ice sheet.
The map below (Aschwanden et al., 2016) shows observed (a) and modelled (b) ice-surface velocity for the Greenland Ice Sheet, with the fast-flowing ice streams numbered and named.
Ice-penetrating radar to visualise the glacier bed
Greenland is the world’s largest island, but – as you can see from the map – 80% of it is covered by the ice sheet. The land surface that is hidden under the ice has remained mysterious for a long time. Even today, we have mapped the surface of Mars and parts of Pluto in better detail than the topography underneath the Greenland Ice Sheet.
Scientists use radar to help map and visualise the bed beneath the ice sheet. The reflections in the image below show the bed of the ice sheet in a cross section of the ice sheet. Ice-penetrating radar is when we transmit a radio wave and listen for the echo reflected off layers within and beneath the ice.
We can use many radar lines to create a three-dimensional model of the bed of the ice sheet. Radar is the most widely used tool to measure an ice-sheet’s thickness. Information collated over many years and many surveys from many international teams is used to generate these maps.
Subglacial topography in Greenland
The image below is an up-to-date map of the landscape underneath the ice. High topography is represented by the dark green and brown colours, while low topography is represented by the light green and blue colours. Greenland is ringed by a chain of mountains, which rise to more than 3000 metres high. The centre of Greenland is much lower and flatter. It looks like a huge bowl. In fact, you can see from the blue colours that the land beneath the middle of the ice sheet is lying below sea level.
The reason this land is so low is because of the ice sheet. There is such a large mass of ice sitting on Greenland that it pushes the Earth’s crust down into the mantle below; a process called ‘isostasy’. If the ice sheet were to be removed, the land would slowly rebound, in total by as much as 800 metres. The same process has been happening in Scotland since the ice melted at the end of the last ice age.
The subglacial landscape is also a window into Greenland’s deep past. If we look beneath the ice sheet, we see great networks of valleys that were formed not by ice, but by running water. These features were probably incised by rivers at a time when there was much less ice in Greenland. These features have survived for so long because large parts of the ice sheet are so cold and slow-moving that they protect the topography from the usual effects of erosion driven by wind, water, and sliding ice. The ice sheet has literally frozen the landscape in time.
The video below shows a 460 mile long canyon underneath the Greenland Ice Sheet, running from near the centre of the island northward to the fjord of Petermann Glacier.
Researchers have also found sediments below the ice sheet that were not covered by ice as recently as 400,000 years ago. These records tell us that there have been times in the past when the Greenland Ice Sheet was smaller than it is today. They provide vital clues as to what Greenland might have looked like at times in the distant past when Earth experienced natural climate warming. This information is so valuable because it helps us to predict how the ice sheet will respond to climate change during the 21st Century and beyond.
Further reading
Aschwanden, A., Fahnestock, M. & Truffer, M. Complex Greenland outlet glacier flow captured. Nat Commun 7, 10524 (2016). https://doi.org/10.1038/ncomms10524
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