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The Increasingly Fragile Pine Island Glacier Just Calved Again

The point where the Pine Island and Thwaites glaciers meet the sea serve as a back-stop restraining most of the great ice flows of West Antarctica. If those backstops were to fail, ocean water would flood inland along a reverse slope and generate a massive and swift out-rush of ice that would ultimately raise the world’s oceans by about 3 meters. And, lately, the evidence is mounting that the backstops are failing.

At Thwaites, just south of the neighboring Pine Island Glacier (PIG), recent research found that the ocean was flooding inland beneath that enormous ice sheet at a rate of up to 400 meters per year. But to the north, there is indication of trouble at the ice surface.

Back to Back Calving Events

Just last September, a massive 100 square mile ice berg calved off the Pine Island Glacier. The event was significant in that it marked the first major retreat of the glacial front in the face of an advancing ocean. Pine Island had already sped up. But the calving face withdrawal inland appeared to mark a new phase for the large glacier.

(Sentinel 1 satellite observations show a rapidly moving Pine Island Glacier calving off another large ice berg. Meanwhile, considerable damage appears to have been done to the glacial front.)

Now, just 7 months later, PIG is calving again. A large, approximately 6 kilometer long, 1 kilometer wide, chunk appears to have broken off into the Southern Ocean and shattered. Meanwhile, to the north and south along the glacial front, rifts appear to have formed.

This recent calving event is significant for a number of reasons. The first is that it’s happening just months after a recent large break-off during 2017. Other recent calving events at Pine Island occurred during 2001, 2007, and 2013. The present 2017-2018 events are back-to-back. The second reason is that the splintering appears to indicate a more fragile ice face. An impression reinforced by the concordant formation of rifts spreading away from the calving zone. The third is that the satellite imagery suggests Pine Island Glacier is moving quite rapidly (Recently, this rate of motion has been 1-2 km per year. However, it’s reasonable to question whether the glacier is continuing to speed up).

Conditions in Context

Present global warming due to fossil fuel burning has now forced the world into a range of temperatures between 1.0 and 1.21 degrees Celsius above 1880s averages. This boundary is similar to that of the lower range of the Eemian 120,000 years ago when oceans where 10-20 feet higher than they are today.

(The tall ice cliffs composing the Pine Island Glacial front have become increasingly fragile and fast moving as they enter the warming Southern Ocean and as that warming water continues to invade inland. Image source: Commons, Pine Island Glacier Calving Front, NASA.)

Under present greenhouse gas forcing and planned emissions, additional warming is in store. Climate models produced by Dr. Michael E Mann indicate that we are likely to hit the 1.5 C global temperature boundary some time between 2027 and 2031 on the current emissions pathway. This predicted warming is significant because analysis of past climates appears to indicate a risk of more rapid rates of sea level rise when global temperatures rise to a range between 1.5 to 2.5 C above past base line averages (see meltwater pulse 1 A).

Since the 1990s, the global rate of sea level rise has proceeded at roughly 3.3 mm per year with an apparent acceleration to around 3.6 to 4.1 mm per year during the 2010 to present time period. Given observed ice sheet instability in West Antarctica, in East Antartica, and in Greenland, there is a serious risk that this rate of rise will continue to accelerate over the coming years and decades. The key question of concern is how much and how soon.

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Gigantic Iceberg Disintegrates as Concern Grows Over Glacier Stability, Sea Level Rise

The stability of a key Antarctic glacier appears to have taken a turn for the worse as a large iceberg that broke off during September has swiftly shattered. Meanwhile, scientists are concerned that the rate of sea level rise could further accelerate in a world forced to rapidly warm by human fossil fuel burning.

(Iceberg drifting away from the Pine Island Glacier rapidly shatters. Image source: European Space Agency.)

This week, a large iceberg that recently calved from West Antarctica’s Pine Island Glacier rapidly and unexpectedly disintegrated as it drifted away from the frozen continent. The iceberg, which covers 103 square miles, was predicted to drift out into the Southern Ocean before breaking up. But just a little more than two months after calving in September, the massive chunk of ice is already falling apart.

The break-off and disintegration of this large berg has caused Pine Island Glacier’s ice front to significantly retreat. From 1947 up until about 2015, the glacier’s leading edge had remained relatively stable despite significant thinning as warmer water began to cut beneath it. But since 2015, this key West Antarctic glacier has begun to rapidly withdraw. And it now dumps 45 billion tons of ice into the world ocean each year.

(Glaciers like Pine Island balance on a geological razor’s edge. Because they sit on a reverse slope, it only takes a relatively moderate amount of ocean warming to precipitate a rapid collapse. These collapses have happened numerous times in the past when the Earth warmed. Now, human-forced climate change is driving a similar process that is threatening the world’s coastal cities. Image source: Antarctic Glaciers.)

The present rate of melt is enough to raise sea levels by around 1 millimeter per year. That’s not too alarming. But there’s concern that Pine Island Glacier will speed up, dump more ice into the ocean and lift seas by a faster and faster rate.

Pine Island Glacier and its sister glacier Thwaites together contain enough water to raise seas by around 3-7 feet. The glacier sits on a reverse slope that allows more water to flood inland, exposing higher and less stable ice cliffs as the glacier melts inland. If the glacier melts too far back and the ice cliffs grow too high, they could rapidly collapse — spilling a very large volume of ice into the ocean over a rather brief period of time. As a result, scientists are very concerned that Pine Island could swiftly destabilize and push the world’s oceans significantly higher during the coming years and decades.

No one is presently predicting an immediate catastrophe coming from the melt of glaciers like Pine Island. However, though seemingly stable and slow moving, glacial stability can change quite rapidly. Already, sea level rise due to melt from places like Greenland and Antarctica is threatening many low-lying communities and nations around the world. So the issue is one of present and growing crisis. And there is very real risk that the next few decades could see considerable further acceleration of Antarctica’s glaciers as a result of human-forced warming due to fossil fuel burning.

Dr Robert Larter, a marine geophysicist at British Antarctic Survey, who has researched Pine Island Glacier in his work with the Alfred Wegener Institute, recently noted to Phys.org:

“If the ice shelf continues to thin and the ice front continues to retreat, its buttressing effect on PIG will diminish, which is likely to lead to further dynamic thinning and retreat of the glacier. PIG already makes the largest contribution to  of any single Antarctic glacier and the fact that its bed increases in depth upstream for more than 200 km means there is the possibility of runway retreat that would result in an even bigger contribution to sea level.”

CREDITS:

Hat tip to Colorado Bob

Hat tip to Erik Friedrickson

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