Image: Sarah Das Woods Hole Oceanographic Institution
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Ice sheet: continental glaciers that have joined together to cover the surrounding land in an area greater than 50,000 km². There are only three in the world: Greenland and Antarctica which has two: the WAIS and the EAIS (Fig. 1). The existence of these ice sheets are why we are still in the Quaternary Ice Age.
- Marine ice sheet: an ice sheet whose base is on ground below sea level. This makes it particularly vulnerable to undercutting by warming waters (Fig. 6).
- Outlet glacier: drains inland glaciers/ice sheets through gaps in the surrounding topography. If an outlet glacier reaches the coast (some terminate inland), it can become an:
- Ice shelf: a tidewater (coastal) glacier or ice sheet that flows down to a coastline and onto the ocean surface, where it floats.
- Grounding line: the point where the bottom or ‘basal’ side of a glacier leaves land and extends out over the ocean.
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- If the grounding line is below sea level, the glacier is prone to undercutting by increasingly warmer ocean waters.
- If land behind the grounding line slopes down inland instead of up, warm water can flow further underneath, destabilising the glacier even faster.
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
The scientific scramble to understand events being witnessed since 1986 was well underway. In 2008, a year before the 2009 IPCC Report stating that sea levels were not likely to be greatly affected, a staggering volume of ice on the leading edge (front) of the Jakobshaven Glacier (Fig. 1 map) was filmed as it carved spectacularly in just 75 minutes (Video 3). Then in 2010, a single iceberg 260km² x 213m deep broke off the Petermann Glacier (Fig. 1 map). By 2019, the amount of melting was finally being recognized by the IPCC:
Warming over Greenland in 2022 began much earlier than usual, following the trends shown in Figs. 5 & 6. This also accelerated melting permafrost and methane clathrates, adding greenhouse gases to the atmosphere. With each passing year, the rate of Arctic sea ice melting is accelerting, 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 from warming air temperatures (Fig. 7) and ‘bottom up’ melting from a warming ocean (Fig. 8) varies from place to place and over time.
“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. 7). This is causing around 34% of melting across Greenland. 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 (Videos 1 & 5) 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 5).
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. This is causing around 66% of melting across Greenland. 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 6). 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. 9).
“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’
