The outlet glaciers of the Greenland Ice Sheet are receding, calving more icebergs, and flowing faster. Further in land, the ice sheet is thinning, and there is more surface melt.
The Greenland Ice Sheet is drained by outlet glaciers that flow through deep fjords to the ocean. In the image below, you can see an outlet tidewater glacier (a glacier that ends in the ocean). The tidewater glacier is melting into the ocean (you can see the plumes of sediment-laden meltwater coming away from the ice margin if you look carefully), and calving icebergs.
In recent years, the amount of melt, and the amount of calving of ice bergs, of these Greenland outlet glaciers, has increased.
Credits: NASA/JPL-Caltech.
Explore Greenland Glacier Recession yourself
In this app by Dr James Lea from the University of Liverpool, you can explore the recession of some outlet glaciers of the Greenland Ice Sheet yourself. In Google Earth Engine, you can compare satellite images of glaciers from the 1990s and 2021.
Desktop: http://tiny.cc/GreenlandChangeDesktop
Mobile: http://tiny.cc/GreenlandChangeMobile
You can also explore the changing positions of Greenland outlet glaciers on Polar Portal.
Greenland glacier recession and ice sheet mass losses
The Greenland Ice Sheet is currently receding, and the rate of ice loss from Greenland is accelerating. Prior to 1980, the Greenland Ice Sheet had a slightly positive mass balance (gaining more snow than is lost by melting ice each year). From 1980 to 1990, this switched, and the ice sheet lost 51 ± 17 billion tonnes of ice per year (Gt/yr).
From 1990 to 2000, the mass loss was 41 ± 17 Gt/year, and from 2000 to 2010 it was 187 ± 17 Gt/yr. From 2010 to 2018, it was losing 286 ± 20 Gt/yr (Mouginot et al., 2019). This means that there has been a six-fold increase in mass loss since the 1980s (Mouginot et al., 2019).
In total, since 1972, the Greenland Ice Sheet has contributed 13.7 ± 1.1 mm to global sea level rise. Half of this sea level contribution was in the period of 2010-2018.
Another study estimated that between 1994 and 2017, the Greenland Ice Sheet lost 3.8 trillion tonnes of ice (Slater et al., 2021). Slater et al. (2021) also found an acceleration of the rate of global ice loss (by 57%) since the 1990s, owing to increased losses from mountain glaciers, Antarctica, Greenland and Antarctic ice shelves.
What is causing these changes?
In Greenland, around half of the mass loss is now due to meltwater runoff during warm summers (Slater et al., 2021). Since the year 2000,increasing surface melt has accounted to a larger proportion of the ice sheet mass loss (Mouginot et al., 2019).
The remaining ice loss is due to increased calving of icebergs at the edges of the largest ice streams draining the ice sheet (like Jakoshavn Isbrae and Humboldt Glacier). This increased calving means that the amount of ice being lost from the ice sheet to the ocean is increasing, and ice flow is increasing (Howat et al., 2008). The increase in ice discharge at these outlet glaciers has been related to a warming of the subsurface waters around the ice sheet from the end of the 1990s onwards (Mouginot et al., 2021).
As a result of these two processes, the Greenland Ice Sheet is thinning, both at the ice sheet margins and within individual glacier catchments (Slater et al., 2021).
The retreat and acceleration of the ocean-terminating outlet glaciers in Greenland has been attributed to changing ocean circulation (Wood et al. 2021, Rignot et al., 2021). The warm Atlantic Waters are increasingly penetrating into the fjords (Wood et al., 2021). Outlet glaciers with deep fjords, such as Humboldt Glacier (Rignot et al., 2021), are particularly vulnerable to the incursion of this warmer ocean water. Glaciers that terminate in colder, shallower fjords are conversely retreating much more slowly (Wood et al., 2021).
End of Nordvestfjord off the calving area of Daugaard-Jensen-Glacier, Scoresby Sund fjord system, East Greenland. In the fjord the German research vessel POLARSTERN during curise ARK-X/2. Source: Hannes Grobe 20:14, 16 December 2007 (UTC) (Wikimedia Commons)
Why is the ocean warming?
The warming of the subsurface waters has been caused by spreading of ocean heat from the subpolar gyre during a transition of the North Atlantic Oscillation from a high positive to a low to negative phase (Wood et al., 2021). This expanded the North Atlantic subpolar gyre, meaning that there is more ocean heat flux, and warming the subsurface waters on the continental shelf around Greenland.
Different glaciers behave differently
The ways in which individual glaciers are responding to oceanic and atmospheric warming are controlled by their different sensitivities. Overall, glaciers in the NW are most sensitive to ocean forcing, and glaciers in the SE are especially sensitive to summer air temperature (Fahrner et al., 2021).
For glaciers that terminate in the ocena (tidewater glaciers), glacier retreat is faster when there is a wide, deep trough, especially if the fjord becomes deeper as it nears the glacier terminus (Catania et al., 2018).