The evidence of climate change

The evidence of climate change

Today, the Greenland ice sheet is losing mass about six times faster than it was just a few decades ago, whatever tenuous balance that existed before, long since upended. Between 2005 and 2016, melt from the ice sheet was the single largest contributor to sea level rise worldwide.”National Geographic, 2019

(Image: Sarah Das / Woods Hole Oceanographic Institution)

Dangerous tipping points: Greenland

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.

  • Greenland’s ice sheet is the second-largest in the world behind Antarctica, covering 1.71 million km2; ~79% of its surface area.
  • It contains ~10% of the frozen freshwater on Earth, the equivalent of ~6-7m of sea-level rise if it all melted.
  • Greenland is losing more ice than it gains each year (from snow) and the loss is accelerating: in 2004, ice loss contributed 0.5mm to sea levels annually. By 2014 that rate had doubled (Video 1). In 2018, it tripled. In 2019, ice loss contributed 2.2mm in 2 months, in spite of its largest glacier, Jakobshaven, growing due to a regionally cooling ocean current (Video 3). Overall it lost an average of 1 million tonnes of ice per minute in 2019.
  • Extremely high and sustained summer temperatures are due to an accelerating loss of Arctic sea ice destabilising the jetstream, promoting dry sunny summers and less snowfall in winter-spring
  • ‘Dark ice’soot and algae growing on the ice (Figs. 5 & 6)—is reducing the albedo and enhancing surface warming.
  • Warming ocean currents are a key driver in the retreat and accelerated flow of glaciers into the ocean.
Fig. 1: Greenland showing the thickness of the ice sheet. Much of the area coloured green along the edges has permanent snow cover (not ice) generally less than 10m thick. The Jakobshavn Isbræ glacier catchment is shown as an overlay in the south (Background image: Wikipedia; drainage area: Cooper et al, 2016).
Fig. 1: Greenland showing the thickness of the ice sheet. Much of the area coloured green along the edges has permanent snow cover (not ice) generally less than 10m thick. The Jakobshavn Isbræ glacier catchment is shown as an overlay in the south (Background image: Wikipedia; drainage area: Cooper et al, 2016).
Fig. 2: Greenland without ice. The centre of the island is below sea level, largely due to the weight of the ice pushing the crust down into the mantle (isostacy).(Image: NSIDC)
Fig. 3: In spite of the ice cap, large areas of the ground beneath the ice sheet has thawed (red). (Image: Jessie Allen/ NASA Earth Observatory)

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 each effect contributes to iceberg carving and/or disintegration of ice shelves varies enormously from place to place. In Greenland, ‘top down’ melting may be the largest driver for some glaciers but not others over time (Fig. 5).

Glacier glossary:

  • 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 an 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. 3). The WAIS is a marine ice sheet (Figs. 1 & 2).
  • 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. Thwaites and Pine Island Glaciers are outlet glaciers with ice shelves.
  • Grounding line: the point where the bottom or ‘basal’ side of a glacier leaves land and extends out over the ocean.
    • 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 (Fig. 3). Thwaites Glacier, now considered the most unstable (Video 3) is grounded below sea level, and much of the the land behind the grounding line slopes down.

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. The year before the IPPC report, a staggering volume of ice on the leading edge (front) of the Jakobshaven Glacier 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: From ‘The great Greenland meltdown’. Click image to read the Science magazine story (image: V. Altounian/Science)
Fig. 6: Meltwater lakes and dark snow dramatically reduce the albedo effect, so heat is absorbed, which promotes further melting. (Image: Eli Kintisch. Greenland, 2017).

Warming over Greenland in 2020 began much earlier than usual (Fig. 7). 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.

“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

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)

Explainers

The Albedo Effect:

Clean ice and snow have a very high albedo, that is, they reflect up to 90% of solar radiation back into space. The ocean is much darker, so it has a very low albedo, reflecting only about 6% of the incoming solar radiation and absorbing the other 94%, warming it much faster than the snow and ice (Fig. 8). This feedback effect then leads to more warming, then more melting, and so on.

Fig. 8: The Albedo Effect. Clean ice reflects about 90% of the sunlight that strikes it. Dark ocean water only reflects about 6%. This aerial photo shows a small portion of A-68, the iceberg that broke of Larsen C Ice Shelf in 2017 (Photo: NASA/ Nathan Kurtz).
Fig. 8: The Albedo Effect. Clean ice reflects about 90% of the sunlight that strikes it. Dark ocean water only reflects about 6%. This aerial photo shows a small portion of A-68, the iceberg that broke of Larsen C Ice Shelf in 2017 (Photo: NASA/ Nathan Kurtz).

Relatively warm water:

The world’s oceans have absorbed around 93% of global warming to date, and are heating up 40% faster on average than the IPCC estimated in 2013.

Sea levels not greatly affected by melting glaciers:

Glaciers and marine ice sheets that sit on water do not contribute to sea level rise when they melt; they are simply changing from a solid to a liquid while still in the ocean. However, while frozen, this large volume of water is concentrated in relatively small locations (primarily the centre of Greenland and the West Antarctic Ice Sheet). As it melts, the water is re-distributed across all of the world’s oceans.  Moreover, sea levels will actually drop slightly around Greenland and Antarctica if the ice sheets all melt, because the gravitational pull of all that ice mass declines. How much this and other factors contribute to sea levels rising, is covered here.

Secondly, ice shelves over water that are anchored to bedrock act as dams or buttresses that hold back ice sheets that are entirely on the land. As these marine ice shelves break up, there is nothing to stop ice sheets from the land to flow down into the ocean unimpeded. Along with direct melting on the surface, this adds to rising sea levels.


The 2009 IPCC report:

The report acknowledged that Greenland was most likely responsible for much of the 4m rise in sea levels ~125,000 years ago during the Eemian. Back then, temperatures were 1-2°C warmer that pre-Industrial levels, and CO2 in the atmosphere was 280ppm. Today, temperatures have passed 1.1°C and atmospheric CO2 is ~410ppm and climbing fast. There is a lag time between rising temperatures and rising sea levels, but uncertainty around what that lag time will be, resulted in the IPCC not including them as potential tipping points until 2018 and 2019. By then, the Paris Accord, which was based on the IPCC 2013 report, did not consider tipping points, had been signed.

Simply put, countries are currently working to keep warming under 1.5°C without consideration of what might happen as tipping points are exceeded.


The ‘Zwally Effect’:

The idea was proposed by Jay Zwally when researching the sudden acceleration of the Jakobshavn Isbræ glacier in Greenland in 1998 and 1999.

References and further reading