No longer considered an extreme event! Mega Icebergs are wreaking havoc in the Southern Ocean

Mega-icebergs dump billions of gallons of freshwater into the oceans. Image credit: Merco Press

Scientists have sent out an urgent message to the world. We need to study West Antarctica's glaciers, and in particular the Thwaites glacier, fast.  "It is hard to find tipping points in physical systems likely to be activated in the near future, with the possible exception of Thwaites,” Richard Alley, a glaciologist at Penn State University. Our understanding of predicting sea level rise depends on the infusion of resources per the National Academies of Sciences, Engineering and Medicine. There is now widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith, and Kohler glaciers.

Until recently, the glaciers and the ice sheets of Antarctica have been in harmony. They had formed a stable system with forces of outflow and back pressure in balance for hundreds of thousands of years.  Warming ocean water and changing ocean and wind currents have upset this balance by increasing the flow of the land based glacier and by causing the decay of the ice sheet itself.  A weakened, deteriorating and calving ice shelf is no longer able to slow the speeding of the glacier's ice into the sea. Inland basins connect Thwaites Glacier to other major glaciers in the region and it is the accelerating ice from Antarctica's inland that will raise sea levels. The calving ice shelves are introducing fresh melt water to the ocean, but they are not adding to sea level rise as a floating object will displace the same volume of water as it weighs.

The massive B-15 iceberg that calved off the Ross Ice Shelf in 2000 eventually broke into several bergs that created navigation problems for ships supplying McMurdo Research Station in Antarctica in 2004. Josh Landis took this picture of the edge of the Rhode-Island-sized B-15A iceberg in 2001. (Photo courtesy Josh Landis, National Science Foundation).  

(Listen to ice of B15 shattering (recorded from 700 miles away) when the mega berg slams into a shoal in the Ross sea causing it  to split into several pieces.)

Antarctica Glaciers reports on the mechanisms of ice shelf collapse:

There are several reasons why ice shelves disintegrate rapidly rather than slowly and steadily shrinking. Ice shelves collapse in response to long term environmental changes, which cause on-going thinning and shrinking. When certain thresholds are passed, catastrophic ice shelf disintegration through iceberg calving is initiated. Before collapse, ice shelves first undergo a period of long-term thinning and basal melting, which makes them vulnerable.

1. Long term thinning and basal melting

Long-term thinning from surface and basal melting preconditions the ice shelf to collapse. Negative mass balances on tributary glaciers can lead to thinning of the glaciers and ice shelves. The highest rates of thinning are where relatively warm ocean currents can access the base of ice shelves through deep troughs[9,10]. Ice-shelf structure seems to be important, with sutures between tributary glaciers resulting in weaker areas of thinner ice, which are susceptible to rifting.

snip 2. Surface melting and ponding

Increased atmospheric temperatures lead to surface melting and ponding on the ice surface. Catastrophic ice-shelf collapsed tend to occur after a relatively warm summer season, with increased surface melting[12]. Based on the seasonality of ice shelf break up, and the geographic distribution of ice shelf collapse near the southerly-progressing -9°C isotherm, it appears that surface ponding is necessary for ice-shelf collapse[12]. This meltwater melts downwards into the ice shelf, causing fractures and leading to rapid ice-berg calving[5, 12]. Increased surface meltwater also leads to snow saturation, filling crevasses with water and increasing hydrostatic pressures. Brine infiltration can also cause crack over deepening.

snip 3. Plate bending and tidal flexure

However, meltwater ponding alone does not explain rapid ice-shelf fragmentation. We need to invole a third process. Bending at the frontal margin of the ice shelf as a result of tidal flexure may cause small cracks to form parallel to the ice front. When subject to the above conditions (thinning with abundant surface water), a threshold may be passed, causing rapid ice shelf disintegration[12].

When icebergs are formed through the above mechanisms, long, thin icebergs are formed at the ice front. These icebergs will capsize as they are thinner than they are deep. Iceberg capsize releases gravitational potential energy and increases tensile stress on the ice shelf. This may lead to a cascade of fragmentation, capsize, and iceberg break up

Once these monster icebergs calve from an ice shelf they do not move away from the continent to the open ocean as there is no path for them due to their enormity. Instead, they immediately begin to scour the ocean floor by bouncing and crashing their way along the shallow sea floor killing 99% of every thing they came in contact with. Eventually they crumble and shatter into smaller pieces. It is these "smaller" icebergs that get swept into the currents known as the Weddell-Scotia Confluence heading towards the Atlantic where they ground in remote locations such as South Georgia Island's shallow continental shelf to melt and crumble. The shallow shoal at South Georgia Island is known as the Mega Iceberg graveyard.

BBC News reports on how the mega icebergs are interfering with the Krill population:

These shrimp-like creatures follow the same currents as the bergs and are a vital source of food to many of the island's animals, including its penguins, seals and birds. In years when there are few krill at South Georgia, the predators that eat them will suffer poor breeding success. In really bad years, the beaches of South Georgia can be littered with dead pups and chicks, Professor Murphy says.

"When that berg was sat on the shelf, it was directly in the path of areas that we would normally think of being the main inflow areas for the krill," he told BBC News.

Wired notes impacts to penguin populations when a mega iceberg blocks access to the open ocean requiring the exhausted birds to attempt to swim around the iceberg to locate food: What they can’t deal with very effectively is a combination of shifting sea ice and mega-icebergs, which until recently happened infrequently enough to be considered an extreme event. From 2001 to 2005, during the time the team was studying the penguins, two enormous icebergs – each measuring tens of miles on a side – collided with the Ross ice shelf near the penguins’ breeding grounds at Cape Crozier, Ross Island. The bergs blocked the normal flow and formation of sea ice and messed with the ocean’s primary production of food. Unable to adjust their behavior to cope with these mega-bergs, the penguins and their chicks suffered (though perhaps not as much as Emperor penguins, who lost half their breeding adults). So, it seems that the birds’ ability to deal with variations in sea ice coverage has a limit, and giant blocks of ice are a real problem. That’s not good because guess what happens when the globe and its oceans warm? More mega-icebergs. Iceberg stranded on a South Georgia Island shoal. Image Credit: M  Brandon

This 18 mile-long crack in the Pine Island Glacier could produce an iceberg 800 square kilometers in size. Take the pilot's seat on a computer-generated tour of the feature which surprised its discoverers in 2011.