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Scientists:
Warming Ocean, Upwelling to Make an End to Antarctica’s Vast Pine
Island Glacier
(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)
16
January, 2014
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.
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.
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.
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