A large chunk of the Pine Island Glacier has broken free today, the media announced. The giant iceberg is estimated to cover an area of roughly 6,000 km2. About a quarter the size of Wales in the United Kingdom.
Glaciologists have been monitoring the event.
A one trillion tonne iceberg – one of the biggest ever recorded – has calved away from the Larsen C Ice Shelf in Antarctica. The calving took place between Monday 10th July and Wednesday 12th July 2017, when a 5,800 km2 section of Larsen C finally broke away.
The Midas Project at Swansea University has followed the evolution of the iceberg most closely.
The Larsen C Ice Shelf floats on the ocean at the edge of the Antarctic Peninsula, holding back the flow of glaciers that feed into it.
It is between 200 and 600 metres thick.
Researchers from Project MIDAS have been monitoring the rift in Larsen C for years, following the collapse of the Larsen A ice shelf in 1995 and the sudden break-up of the Larsen B shelf in 2002.
They observed rapid advances of the rift in January, May and June, which increased its length to over 200 kilometres and left the iceberg hanging on by a thread of ice just 4.5 km (2.8 miles) wide.
The team used a technique called satellite radar interferometry (SRI). SRI allows the impact of very small changes in ice shelf geometry to be detected, and the rift tip to be monitored with precision.
Environmental scientists had been following the development of a large crack in the Larsen’s ice shelf for over a decade.
Since 2014, the rift’s propagation had accelerated, making an imminent calving even likelier.
The over 200-metre-thick tabular iceberg is not expected to move very far or very fast in the short term. The iceberg is not expected to be an immediate danger to shipping.
However, a “berg” of such a large size will need to be monitored.
The giant white wanderer is on its way.
The development of the rift over the last year was monitored using data from the European Space Agency and NASA.
A NASA satellite observed the iceberg today while passing over a region, known as the Larsen C Ice Shelf.
The American space agency’s Aqua MODIS satellite’s infrared sensor spied on clear water in the rift between the shelf and the berg on today.
Aqua MODIS images in the thermal infrared at a resolution of 1 km.
The water is warmer relative to the surrounding ice and air – both of which are at sub-zero celsius temperature.
In recent weeks, the rift was barely visible in these data. Now, the signature appears so clearly that it must have opened considerably along its whole length.
The breakthrough was also confirmed by NASA’s Suomi VIIRS instrument.
The European Sentinel-1 satellite radar system – part of the European Copernicus Space Component – also acquired recent imagery to confirm the break. Sentinel-1 is a radar imaging system capable of acquiring images regardless of cloud cover, and throughout the current winter period of polar darkness.
The Sentinel array can detect any changes in the giant block’s motion relative to the shelf.
Dynamics of Ice Shelves
An ice shelf is a thick slab of ice, attached to a coastline and extending out over the ocean as a seaward extension of the grounded ice sheet.
Ice shelves range in thickness from about 50 to 600 metres. Some shelves can persist for thousands of years.
Ice shelves periodically calve icebergs.
Ice shelves fringe the continent of Antarctica, and occupy a few fjords and bays along the Greenland and Ellesmere Island coasts. An ice shelf occupying a fjord is sometimes called an ice tongue.
At their seaward edge, ice shelves periodically calve icebergs. Some can be the size of a small European country or U.S. state.
Because they are exposed to both warming air above and warming ocean below, ice shelves and ice tongues respond more quickly than ice sheets or glaciers to rising temperatures.
Warmer temperatures can destabilise the system by increasing glacier flow speed and – even more dramatically – by disintegrating the ice shelf.
Without a shelf to slow its speed, the glacier accelerates.
The Larsen Ice Shelf
The Larsen Ice Shelf is a long, fringing ice shelf situated in the Northwest part of the Weddell Sea, extending along the East coast of the Antarctic Peninsula, from Cape Longing to the area Southward of Hearst Island.
From North to South, the Larsen Ice Shelf is divided by scientists into segments occupying distinct embayments along the coast:
- Larsen A (the smallest),
- Larsen B,
- Larsen C (the largest)
- Larsen D
- Larsen E, F and G (much smaller, further South)
After several decades of warming and years of gradual retreat, the Larsen A ice shelf disintegrated in January 1995.
At the time, the breakup pattern, in which roughly 1,500 km2 suddenly disintegrated rapidly into small sliver-shaped icebergs, appeared to indicate a new style of ice shelf response to pronounced climate warming.
Larsen B was stable for at least 10,000 years, essentially the entire Holocene period since the last glacial period. In contrast, Larsen A was absent for a significant part of that period, reforming about 4,000 years ago.
Between 31 January 2002 and March 2002, the Larsen B sector partially collapsed and parts broke up.
The size of that berg was 3,250 km2 (1,250 miles2) of ice x 220 m (720 feet) thick, an area comparable to the American state of Rhode Island.
Despite its great age, the Larsen B was clearly in trouble at the time of the collapse.
With warm currents eating away the underside of the shelf, it had become a “hotspot of global warming“. It took just three weeks for the ice sheet to break, due to the powerful effects of water.
Ponds of melt-water formed on the surface during the near 24 hours of daylight in the summertime, then the water flowed down into cracks and, acting like a multitude of wedges, levered the shelf apart. Other potential factors in the break-up were the higher ocean temperatures and the decline of the ice of the peninsula.
The Larsen B ice shelf disintegrated in February of 2002.
A 2015 study predicted that the remaining Larsen B ice-shelf would disintegrate by 2020, based on observations of faster flow and rapid thinning of glaciers in the area.
The Larsen C Ice Shelf itself has been known to spawn bigger icebergs. The Larsen C shelf is a mass of floating ice formed by glaciers that have flowed down off the Eastern side of the Antarctic Peninsula into the ocean. On entering the water, their buoyant fronts lift up and join together to make a single protrusion.
The Larsen D Ice Shelf is located between Smith Peninsula in the South and Gipps Ice Rise in the North. It is considered to be stable.
Over the past fifty years, Larsen D has advanced where the other ice shelves George VI, Bach, Stange, and Larsen C have all retreated. However, this gain has been relatively small in comparison to the retreat of the others.
A recent survey of Larsen D measured it at 22,600 km2. There is fast ice along the entire front, making it difficult to interpret the ice front because the semi-permanent sea ice varies in thickness and it may be nearly indistinguishable from shelf ice.
Many of Larsen’s progeny can get wound up in a gyre in the Weddell sea or be dispatched North by sea currents into the Southern Ocean, and even the South Atlantic.
A good number of icebergs from this sector can end up being caught on the shallow continental shelf around the British overseas territory of South Georgia where they gradually wither away and melt.
The majority of ice shelves are fed by inland glaciers.
Together, an ice shelf and the glaciers feeding it, can form a stable system, with the forces of outflow and back pressure balanced.
The calving of icebergs at the forward edge of the Larsen ice shelf is a natural behaviour. Over time, the ice shelf maintains a steady equilibrium.
The ejection of icebergs is one way that it balances the accumulation of mass from snowfall and the input of more ice from the feeding glaciers on land.
However, scientists think Larsen C is now at its smallest extent since the end of the last ice age some 11,700 years ago.
About 10 other shelves further to the North along the Peninsula have either collapsed or greatly retreated in recent decades.
Larsen A and Larsen B, the two nearby smaller shelves, disintegrated around the turn of the century. A warming global climate very probably had a role in their demise.
Scientists believe the Larsen C ice shelf is its smallest since the last Ice Age.
But Larsen C today does not yet look like its siblings.
The signs we saw at Larsen A and B are not being observed yet.
The thinning we saw for Larsen A and B – we’re not seeing. According to most glaciologists, there is no evidence for large volumes of surface melt-water on the order of what would be needed to hydro-fracture the ice shelf.
Business as usual for the Larsen C shelf.
Comparison with Previous Calving Events
The iceberg, to be named A68, weighs more than a trillion tonnes (1,000,000,000,000 metric tonnes). Its volume is twice that of Lake Erie, one of the American Great Lakes.
The new Larsen iceberg is probably in the top 10 biggest ever recorded. Still, it is no match for some of the “monster-bergs” that have been previously witnessed in the Antarctic.
The largest of such events observed in the satellite era was an object called B-15. It broke away from the Ross Ice Shelf in 2000.
B-15 measured some 11,000 km2. Six years on, fragments of this super-berg still persisted and passed by New Zealand.
Among the top 10 biggest ice calving events.
In 1956, a US Navy icebreaker encountered an object of roughly 32,000 km2. Bigger than Belgium. Unfortunately, no satellites were in existence at the time to follow up and verify the observation.
An object measuring some 9,000 km2 came away in 1986.
Thawing Glaciers and Rising Sea-Level
Action of Melt-water
Warm summer temperatures and an impermeable surface that prevents water from being absorbed lead to the formation of melt ponds on the ice shelf. This melt-water fills small surface cracks.
Depending on the amount of water and the depth of a crack, the water can deepen the crack and eventually wedge through the ice shelf.
The formation of melt ponds depends most upon summer temperatures. Although a single warm summer cannot lead to collapse, a series of warm summers can transform permeable snow into impermeable ice, allowing melt ponds to form during subsequent warm summers.
The same process occurs on glaciers in warm climates. Here the effect does not disintegrate the ice shelf, as much as it accelerates the glacier flow. Even when the temperature of interior glacial ice remains below freezing, the melt-water can percolate through the glacier to its base and decrease friction between the glacial ice and the underlying rock.
This is a seasonal phenomenon. With a stable ice shelf in place, glacier acceleration ends with the warm summer temperatures.
However, if the ice shelf shatters, this may change. Recent studies cited evidence for warm ocean waters thinning ice shelves from underneath.
Basal melting appears associated with increased rifting and calving of large iceberg chunks (much larger than in the disintegration case).
Ocean water just a degree or two above freezing carries immense potential for melting ice.
In 2002, Rignot and Jacobs calculated that for every 0.1°C above freezing point, ocean water circulating under the shelf could melt 10 metres of ice per year. While ocean water in the Antarctic and Arctic is warming slowly, a more important effect appears to be more frequent circulation of warm waters in ice shelf areas due to wind and current changes.
A critical feature of an ice shelf is its grounding line – the point where the underside of the ice shelf detaches from land and floats on the ocean water.
If an ice shelf retreats to the grounding line, the shelf’s shape changes.
More ice protrudes above the water line, and the ocean water exerts little buoyant pressure on the ice. As a result, the flow of the glacier encounters very little resistance.
In the 18 months that followed the Larsen Ice Shelf disintegration, the glaciers feeding that ice shelf accelerated between three- to eight-fold.
The iceberg was already floating before it calved away, so the new calving will have no immediate impact on sea level. Nevertheless, the calving of this iceberg leaves the Larsen C Ice Shelf reduced by over 12% in area, and the Antarctic Peninsula landscape changed forever.
Ice Sheet Regrowth
Although the remaining ice shelf will continue naturally to regrow, Swansea researchers have previously shown that the new configuration is potentially less stable than it was prior to the major rift in the Pine Island Glacier.
There is a risk that Larsen C may eventually suffer the same fate as Larsen B, which disintegrated in 2002 following a similar rift-induced calving event in 1995.
After the Larsen B Ice Shelf disintegration, nearby glaciers in the Antarctic Peninsula accelerated up to eight times their original speed over the 18 months that followed. Similar losses of ice tongues in Greenland have caused speed-ups of two to three times the flow rate in just one year.
Another big shift in global climate means a shift towards hotter global temperatures. Researchers will be looking to see how the Antarctic ice shelf responds in the coming years, to see how well it maintains a stable configuration, and determine if its calving rate changes.