Image: Sarah Das Woods Hole Oceanographic Institution
As with Antarctica, it had been assumed that the Greenland ice sheet was too large to be de-stabilised by just a degree or two of warming. The 2009 IPCC 4th Assessment Report stated that sea levels were not likely to be greatly affected by melting glaciers, either from Antarctica, Greenland, or anywhere else one Earth, in the twenty-first century.
However, the scientific scramble to understand events that were being seen in real life since 1986, that failed to be predicted by the IPCC’s climate models was well underway. By the early 2000s, it was clear that Greenland was losing far more ice than it gained each year, all of it going into the ocean. In 2008, a year before the 2009 IPPC report, a staggering volume of ice on the leading edge (front) of the Jakobshaven Glacier on the south west coast (Fig. 1 map) was filmed as it carved spectacularly in just 75 minutes (Video 2). Then in 2010, a single iceberg 260km² x 213m deep broke off the Petermann Glacier (Fig. 1 map) in north Greenland. By 2019, the amount of melting was finally being recognized by the IPCC:
“Summer melting of the Greenland Ice Sheet (GIS) has increased since the 1990s to a level unprecedented over at least the last 350 years, and two-to-five-fold faster than pre-industrial levels” – IPCC, 2019
Warming over Greenland in 2022 once began much earlier than usual, following the trends shown in Figs. 4 & 5). This also accelerated melting permafrost and methane clathrates, adding greenhouse gases to the atmosphere. With each passing year, the that Arctic sea ice is melting ins increasing, further destabilizing the jetstream and changing the world’s climate.
How can it possibly melt this fast? The processes: warm air + warmer ocean
Note: the processes described below are the same as those described on the Antarctica page. The degree to which ‘top down’ melting bottom (Fig. 6) and ‘bottom up’ melting (Fig. 7) contributes varies from place to place and over time. Across Greenland, on average, top-down warming may be the dominant factor:
“Rapid ice loss from the Greenland ice sheet since 1992 is due in equal parts to increased surface melting and accelerated ice flow. The latter is conventionally attributed to ocean warming, which has enhanced submarine melting of the fronts of Greenland’s marine-terminating glaciers. Yet, through the release of ice sheet surface meltwater into the ocean, which excites near-glacier ocean circulation and in turn the transfer of heat from ocean to ice, a warming atmosphere can increase submarine melting even in the absence of ocean warming.
“…we show that in south Greenland, variability in submarine melting was indeed governed by the ocean, but, in contrast, the atmosphere dominated in the northwest. At the ice sheet scale, the atmosphere plays a first-order role in controlling submarine melting and the subsequent dynamic mass loss.” – Slater and Strano, October 2022
The Zwally effect: top down melting from warm air melts ice into giant meltwater lakes on the surface of glaciers (Fig. 6). Thanks to their much lower albedo, like the ocean, the dark pools absorb more heat than the surrounding ice, causing more warming and hence further melting in a feedback effect. When this happens on ice shelves, the water finds crevasses in the ice, whereupon it drains down moulins (Video 4) that it scours out like a drill into the heart of glacier. Until the late 1990s it was assumed this water would re-freeze inside the glacier. Instead, through hydrofracturing, the weight and warm temperature of the water the moulin to widen as it drops, fracturing the ice at depth.
When this happens on marine glaciers or ice shelves (ie, sitting on the ocean), when the water reaches the base of the ice, the ice shelf is effectively turned into Swiss cheese and rapidly breaks up.
Whereas when this happens to glaciers sitting on land the outcome is different. If the glacier is on land that slopes downhill inland , when the meltwater reaches the bottom of the glacier, it lifts the glacier and/or joins with ocean water that has reached this point, adding to the melting and undercutting from below.
Where the glacier is on land that slopes downhill towards the ocean, the meltwater lubricates the glacier like a water slide, making it flow faster, which in turn opens or widens more crevasses, allowing yet more meltwater lakes to drain, and so on in a feedback effect. When this warm buoyant freshwater reaches the submerged terminus (front) of the glacier, it scours it from below, then shoots hundreds of metres up the terminus. In some instances it erupts at the surface in a churning jaccuzi-like swirl of mud and ice (Video 4).
Bottom up melting from relatively warm water originating from the Irminger Sea near Iceland, eats away at the underside of ice shelves (in some places over 2km deep), thinning them from below. Continued undercutting allows more water to travel further under the ice shelf, eroding it and thinning it until it’s detached from the grounding line and the ice begins to float.
The Jakobshavn effect now comes into play (Video 5). As the thinning glacier become more buoyant, it floats at the calving front. And this means it’s forced to move up and down with the tides, effectively bending it up and down. These forces travel up the length of the glacier, ultimately assisting the leading edge to snap at the weakest point. Additionally, because the glacier is thinner at the front, the slope is steeper, so the glacial ice behind it speeds up due to gravity, allowing huge volumes of ice to surge downstream and into the sea (Fig. 8).
“Each year since 2015, the (NASA) team has dropped about 250 probes into the ocean around the edge of Greenland. They’ve found the toasty water—up to 10°C or more—nosed up to the end of glaciers around the island most of the time in most of the places…
“As they flew low over the leading edge of the massive Helheim glacier, aiming to drop a probe through a hole in the in the mélange of giant bergs floating at the glacier’s snout, they saw water roiling up through the hole “like a bubbling cauldron,” says Willis. When the probe pinged back data, it showed a warm wall of water extending straight down 2,000m to the bottom of the fjord: A solid wall of water ready to melt the glacier.” – National Geographic, 2019
“A small imbalance of forces caused by some perturbation can cause a substantial non-linear response.” – Prof. Terry Hughes, ‘The Jakobshavn Effect’
