(Southern Ocean interface with Pine Island Glacier as seen during the second week of January, 2014. Note the ocean has already invaded substantially land-ward pushing the glacial coastline back by between seven and ten miles. Image source: Lance-Modis)
Among Antarctica’s most vulnerable ice shelves, the Pine Island Glacier (PIG) is a massive feature. It rests on sloped terrain that is mostly below sea level even as it spills out into the southern ocean through a nearby bay, calving great icebergs that then slowly ride out, like floating ice faerie castles, into the stormy seas. In total, the shelf covers 160,000 square miles, an area two thirds the size of Great Britain.
The Pine Island Glacier is vulnerable for many reasons. It rests on sloped land that tilts it toward the warming seas. Much of it rests below sea level, making its underbelly open to the assaults of the upwelling currents of a rapidly warming ocean. As portions of the under-structure melt, the glacier becomes buoyant, floating on surface waters subject to waves, winds and currents which adds further stress to inland structures.
A few anchors held the great glacier in place over the millenia. The great pressure of ice pushing down shoved the glacier deep into the underlying Earth, for the most part, sticking it in place as it only slowly ground toward the sea.
But now these anchoring features are disintegrating, the warming waters rushing in from underneath, lubricating the ice bottom. The slope, the gravity, the long tongues of ice entering the ocean are all coming into play. The great ice sheet is in motion. A motion that scientists now conclude will not stop until the entire glacier collapses into the heating waters.
Rumors of Glacial Demise
That the Pine Island Glacier was one of Antarctica’s most sensitive to human warming has long been well known to scientists. The geographic features surrounding the glacier, the relatively high angle of slope tipping the glacier toward the ocean, and the large section of the ice shelf below sea level all attracted interest, questions and research.
By the mid 1990s, records of massive melt coming from the Pine Island Glacier began, with upwards of 10 cubic kilometers of ice observed to be lost each year. With ice loss rates continuing to increase, more efforts focused on determining the glacier’s ultimate fate. By the mid to late 2000s, average net ice loss rates were over 20 cubic kilometers per year.
(The July 2013 calving of the Pine Island Glacier as shown in a Lance-Modis satellite shot.)
At about the same time, in 2001, 2007, and 2013, three great icebergs calved off of Pine Island. These were massive bergs, averaging over 2000 square kilometers in size. Though large iceberg calving from the Pine Island Glacier was historically typical, the size and frequency of these amazing events were enough to raise eyebrows and add to already rampant speculation that the Glacier may well be headed toward an inexorable collapse.
Ocean’s Impact on Basal Melt Discovered
By 2010, studies were beginning to come in showing that the Pine Island Glacier was experiencing a rapid melt from underneath. Warming deep ocean currents were upwelling from the Amundsen Sea to erode the glacier’s base. Ice loss from this basal melt was estimated to be even greater than that observed through the increasingly rapid motion of the glacier and related large ocean calving events.
(Image source: Nature)
Basal melt was also shown to be undermining the glacier, pushing deeper and deeper beneath the ice shelf and driving ocean water further into the continent. The mechanism for this increased basal melt came directly from a human warming of the deep ocean surrounding Antarctica. Accelerated deep ocean warming was coming more and more into play as human atmospheric heating transferred through the ocean surface and into the depths.
In the Antarctic, a massive pool of warm water developed in the depths surrounding the continent. The warmer water gathered beneath a fresher, colder layer that kept a lid on the warmth, forcing it toward the bottom. But near the continents, the dynamics of ocean currents and coastal mixing brought this warm water up to contact the coast and, in this case, the base of the Pine Island Glacier.
A Nature Geoscience study led by Dr. Adrian Jenkins found progressive basal melt due to the action and heat transfer of this warm, upwelling water (see image above). The evidence collected seemed grim. It appeared that the Pine Island Glacier may well be in the first stages of disintegration. But more comprehensive study was needed before conclusions could be drawn.
Prognosis: Irreversible Collapse
By 2013, enough information had been collected to start making model runs to determine the ice sheet’s ultimate fate. And, recently, three teams of scientists took up the task. The results of these model runs were stark. They showed that, no matter what, Pine Island’s Glacier was probably suffering from the early stages of an irreversible collapse.
(Glacial velocity map of Antarctica. Note the very high velocity of the Pine Island and adjacent Thwaites glaciers. Image source: Antarctic Glaciers)
In the new Nature study entitled “Retreat of Pine Island Glacier Controlled by Marine Ice Sheet Instability” the authors applying these models found that the glacier had “been kicked and it’s just going to keep on rolling for the foreseeable future.”
Dr Hilmar Gudmundsson, one of the study’s authors in a recent interview with BBC noted:
“Even if you were to reduce melt rates, you would not stop the retreat. We did a number of model runs where we allowed PIG to retreat some distance back, and then we lowered the melt rates in our models. And despite doing that, the grounding line continued to retreat. You can talk about external forcing factors, such climate and ocean effects, and then there are internal factors which are the flow dynamics. What we find is that the internal dynamics of flow are such that the retreat is now self-sustaining.”
In other words, even if the climate somehow miraculously cooled or if the warming ocean somehow managed to melt less ice at the base of the Pine Island Glacier, the glacier would still ultimately destabilize and collapse.
This is hard news, as it has implications for the rest of West Antarctica and, ultimately, about 25 feet worth of sea level rise now locked in the ice. As noted above, the Pine Island Glacier is a massive section of West Antarctica. It is responsible for the draining of about 20% of this section of the continent’s Ice and is one of the primary barriers preventing rapid sea level rise. It is the first domino to start falling. But other dominoes sit in series behind it.
The beginning of PIG’s catastrophic collapse will also likely have major implications for Antarctica’s net ice loss. Gudmundsson’s group found that average melt rates from the Pine Island Glacier are expected to more than quadruple over the next 20 years, increasing to over 100 cubic kilometers of ice loss each year. Total sea level contribution from the Pine Island Glacier alone could be as much as 10 millimeters over the same period, according to model assessments.
This is a large contribution from just one ice sheet. A contribution that is not yet accounted for in global climate simulations for sea level rise. And we have yet to take into account potential additions from other Antarctic melt sources like the adjacent Thwaites glacier or the large glaciers that drain into the Ross Ice Shelf.
In short, if Pine Island has reached the point of no return, then the rest of West Antarctica may well be soon to follow.
Retreat of Pine Island Glacier Controlled by Marine Ice Sheet Instability
Observations Beneath Pine Island Glacier in West Antarctica and Implications for its Retreat
Pine Island Glacier Retreat “Irreversible”
Ocean Warming Shown to Melt Ice Sheets From Below