The Greenland Ice Sheet is melting more than previously believed

Published 08-06-2021

New research published in Nature Communications shows that the Greenland Ice Sheet is melting considerably from below due to underground heat, friction and melt water. This new knowledge should be used when we estimate the mass loss of the Greenland Ice Sheet.

Basal melt
The Greenland Ice Sheet is losing around 254 Gt per year, and currently the basal melt constitutes 8 % of this. Photo: Lars Holst Hansen

Since the beginning of the 2000s, the Greenland Ice Sheet has been one of the biggest sources of rising sea levels. But the extent to which the Greenland Ice Sheet is melting from below (called ‘basal melt’) is not always included in the estimations of the mass loss from the Greenland Ice Sheet.  

Now, for the first time, an international group of researchers, led by The Geological Survey of Denmark and Greenland (GEUS), has estimated the Greenland Ice Sheet basal melt and the results are published in Nature Communications. 

They conclude that the current basal melt discharge is approx. 21,4 Gt per year, which is equivalent to 50 % of the loss of ice from Jakobshavns Isbræ – Sermeq Kujalleq, one of the fastest Greenlandic glaciers. 

 “Our results show that it is possible to estimate the basal melt and that it is an important component in the Greenland Ice Sheet mass balance. The Greenland Ice Sheet is losing around 254 Gt per year, and currently the basal melt constitutes 8 % of this. This number should be included in the estimations of the mass loss as a whole in order for us to know just how much the Greenland Ice Sheet is actually melting,” says Dr. Nanna B. Karlsson, who was in charge of the research and is a senior scientist at GEUS. 

Important knowledge 

Today, the mass loss of the Greenland Ice Sheet is partly determined by a method, which shows how much snow falls on the ice, and compares it to how much of it leaves the Greenland Ice Sheet as icebergs and surface melt. The so-called input-output method also makes it possible to predict future changes. 

 “Until now, the input-output method has completely neglected the Greenland Ice Sheet basal melt. But it is important that we consider all parts of the ice sheet, otherwise we are limited in our understanding of the consequences of climate changes and the complex interaction between the Greenland Ice Sheet and the climate system,” says Nanna B. Karlsson. 

The researchers estimate that basal melt increased in the first decade of the 2000s, and as the Arctic gets warmer, the team expects that the Greenland Ice Sheet basal melt will continue to increase, which may worsen the present mass loss tendencies, change the water circulation in the fjord systems, affect the nutrient balance and increase the calving of icebergs. 

Geothermal energy, melt water and friction 

There are three causes of basal melt. Firstly, the geothermal flux (heat from the underground), which creates the basal melt when the ice touches the warmer Greenlandic mountains. This heat does not change over time. 

The other source of basal melt is surface melt water, which is warmer than the ice – and the warmer the climate becomes, the more melt water will flow through the ice sheet from the surface to the bottom, and on its way the water will melt out more ice. Thus, the overall mass loss becomes greater when surface melting increases as that will lead to more basal melt. And if our climate gas emissions continue the way they are now, this source of basal melt may increase five to seven times before the year 2100. 

The third cause is friction between the ice and the bedrock, which generates heat when the ice slides over the bedrock on its way to the fiords where the ice calves as icebergs. The researchers estimate that friction is responsible for approximately half the basal melt right now. 

“If the glaciers continue to accelerate and larger parts of the Greenland Ice Sheet move towards the coast, the frictional heat will increase as well, which will lead to a rise in the Greenland Ice Sheet basal melt,” says Nanna B. Karlsson. 

Basal melt
There are three causes of basal melt. Firstly, the geothermal flux (heat from the underground), which creates the basal melt when the ice touches the warmer Greenlandic mountains. The other source is surface melt water, which is warmer than the ice. The third cause is friction between the ice and the bedrock, which generates heat when the ice slides over the bedrock on its way to the fiords. Figure: Jacob Lind Bendtsen/GEUS

Self-reinforcing effect 

Increased acceleration of the Greenland Ice Sheet and thereby increased basal melt due to friction might also greatly affect fiord processes and ice-ocean interaction. 

In the winter, basal melt caused by friction will be the primary source of subglacial discharge from the Greenland Ice Sheet and we do not yet know the consequences of any increased melting during the winter. This will lead to more freshwater in the fiords in the winter, and studies indicate that this may lead to melting of the glacier fronts in the winter. 

Thus, increased basal melt will probably further increase the mass loss from glaciers. An effect that may be compared to the consequences of rising sea temperatures. The contribution of the Greenland Ice Sheet to sea-level rise is expected to become markedly larger if the basal melt discharge increases. 

Nanna Bjørnholt Karlsson
Senior Researcher
Glaciology and Climate

Read the article

Read the scientific article ‘A First Constraint on Basal Melt-water Production of the Greenland Ice Sheet’ published in the journal Nature Communications.

The researchers are from GEUS, Université Grenoble Alpes, NASA, University of St. Andrews and University of Oxford.  

Basal melt
The numbers show the total basal melt discharge for each sector in gigaton. The blue color shows how much water moves underneath the ice. The darker the blue, the more water. Figure: Nanna B. Karlsson/GEUS