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Evidence & Impacts: Greenland melting

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Greenland melting

Summary

“We knew this past summer [2019] had been particularly warm in Greenland, melting every corner of the ice sheet, but the numbers are enormous.’ – Prof. Isabella Velicogna, UCI Earth system science and JPL senior scientist.

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Summary

“We knew this past summer [2019] had been particularly warm in Greenland, melting every corner of the ice sheet, but the numbers are enormous.’ – Prof. Isabella Velicogna, UCI Earth system science and JPL senior scientist.

Collapsing glaciers: it’s not just warm air doing the melting, the ocean is warming too.

Note: the processes described below are the same as those described on the Antarctica page. The degree to which ‘top down’ and ‘bottom up’ (Fig. 4) melting contributes to iceberg carving and/or disintegration of ice shelves varies from place to place and over time, due to rapidly changing circumstances (Fig. 5).

Video 1: How the ice sheet flows to outlet glaciers along Greenland’s coast
Video 2: The spectacular ‘top down’ melting across Greenland’s ice cap

As with Antarctica, until relatively recently it had been assumed that the Greenland ice sheet was too large to be destabilised by a relatively small amount 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 failed to be predicted by the IPCC’s climate models, had been underway since 1986. By the early 2000s, it was evident 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 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 3). Then in 2010, a single iceberg 260km² x 213m deep broke off the Petermann Glacier (Fig. 1 map) in north Greenland.

Video 3: 4 min. extract from the documentary ‘Chasing Ice’. The footage has not be slowed; it’s just that the sheer scale of the icebergs breaking off Jakobshavn Glacier, which drains about 40% of Greenland’s icecap (Fig. 1 map) which gives that impression.

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. 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. 4).

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 waterup to 10°C or morenosed 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

Fig. 4: 'Bottom up' melting from an increasingly warmer ocean water has cause the leading edge of Jakobshaven Glacier to rapidly retreat. The ice is normally stabilized by sitting on the seafloor. As warm ocean currents eat away at the base, the ice thins, lifts away from the seafloor, and becoming unstable, breaks, losing its ability to act as a brake on the flow of ice from the ice cap inland.
Fig. 4: ‘Bottom up’ melting from an increasingly warmer ocean water has cause the leading edge of Jakobshaven Glacier to rapidly retreat. The ice is normally stabilized by sitting on the seafloor. As warm ocean currents eat away at the base, the ice thins, lifts away from the seafloor, and becoming unstable, breaks, losing its ability to act as a brake on the flow of ice from the ice cap inland.

The Zwally effect: top down melting from warm air melts ice into giant meltwater lakes (Figs. 5 & 6) on the surface of glaciers. 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 1) that it scours out like a drill. Until the late 1990s it was assumed this water would re-freeze inside the glacier. Instead, through hydrofracturing, the weight of the water widens the moulin as it drops.

When this happens on marine glaciers or ice sheets (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.

When this happens to glaciers sitting on land the outcome is different. If the glacier is on land that slopes downhill inland (Fig. 4), 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 the it, shooting 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 2).

A small imbalance of forces caused by some perturbation can cause a substantial non-linear response.”Prof. Terry Hughes, ‘The Jakobshavn Effect’

Or as Prof. Jason Box puts it: “There are too many variables that determine exactly when a glacier calves. A single cracking event could conceivably be triggered by a seagull, acting like the straw that broke the camel’s back.”

Video 4: ‘Bottom up’ cooling. Between 2016 and 2017, Jakobshavn Glacier grew slightly and the rate of mass loss slowed because the ocean temperatures in the region cooled. In spite of this, melting across Greenland continued to accelerate as air temperatures increased and high pressure weather systems stalled, melting vast areas of the ice sheet. By 2019, the warm water had also returned.
Fig. 5: Meltwater lakes and dark snow dramatically reduce the albedo effect, so heat is absorbed, which promotes further melting. (Image: Eli Kintisch).
Fig. 6: From ‘The great Greenland meltdown’. Click image to read the Science magazine story (image: V. Altounian/Science)

“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-fivefold faster than pre-industrial levels” IPCC, 2019

Warming over Greenland in 2019 and 2020 began much earlier than usual (Fig. 7 & 8). In addition to melting the Greenland ice cap, these high temperatures are of considerable concern in terms of melting permafrost (Fig. 3) and methane clathrates adding greenhouse gasses to the atmosphere, and rapidly disappearing Arctic sea ice, which is changing the world’s climate.

Fig. 7: Arctic temperature anomalies across the Arctic 13 May 2020. The temperatures are in Centigrade. Image: Computer model simulation (Karsten Haustein)/ Washington Post, 14 May 2020)
Fig. 8: Click on this screen grab to be taken to NASA’s National Snow and Ice Data Centre (NSIDC). Here you can select any year from 1979 to yesterday’s date (New Zealand is ahead by 18hrs, and there can be a delay over the weekend), to see the extend of melting on the Greenland ice cap. There are instrucitons on the landing page explaining how to do this. Note: the site does not operate properly using Internet Explorer.
Video 5: Arctic Report Card 2021

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