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Nature: Human Warming Now Pushing Entire Greenland Ice Sheet into the Ocean
(Leading
edge of the accelerating Zachariae Ice Stream meets the warming and
increasingly ice free ocean on August 20 of 2013. Satellite image
source: Lance-Modis.)
17
March, 2014
Greenland
— a vast store of ice three kilometers tall at its center and the
final remnant of the Northern Hemisphere’s great glaciers of the
last ice age has now begun what is likely an unstoppable rush to the
sea. For according to a new report in Nature
Climate Change,
the last stable region of glacial ice along the Greenland coastline
is now accelerating through one of the ice sheet’s largest and
deepest outlets — the Zachariae Ice Stream.
Zachariae,
the last domino to fall
The
Zachariae Ice Stream is a vast river of ice in the northeast section
of Greenland. It terminates in two outlets through a broad and deep
ice-choked bay facing the Fram Strait.
Throughout
the 1990s and into the early 2000s, both warmer air and ocean water
temperatures at the margins of Greenland began to speed up and
destabilize glaciers all along Greenland’s southern, eastern and
western coasts. But the northern glaciers remained relatively
stalwart, continuing the rate of seaward motion observed over
previous decades.
Then,
starting in 2003, something ominous began to happen. A combination of
sea ice loss, warming air and ocean temperatures began to affect the
northern edge of the great ice sheet. Its speed of forward motion
through its outlet bays began to increase. By 2012, the great
glaciers were dumping 10 billion tons, or roughly ten cubic
kilometers of ice into the ocean every single year. In just nine
years the Zachariae Ice Stream had retreated a total of 20 kilometers
toward the heart of Greenland. By comparison, the Jakobshavn Ice
Stream, known to be Greenland’s fastest and located in South
Greenland, has retreated 35 kilometers over the past 150 years.
(Greenland
Ice Sheet velocity map as of 2010. Measures in red are in the range
of 1,000+ meters per year. Note that ice sheet velocity is fastest at
the glacial outlet face and that rapid ice sheet velocity now extends
far into interior Greenland. The Zacharie ice stream, the most recent
to show rapid acceleration, is indicated by the letter Z in the upper
right hand corner of the image. Note how this ice stream plunges deep
into the heart of Northern Greenland and that a rapid flow is now
established all the way to the ice sheet’s core. Image credit:
Joughin, I., B. Smith, I. Howat, and T. Scambos.)
An
ice stream drains an ice sheet the same way a river drains a
watershed. So long as ice stream flow and rate of glacial recharge at
the top of the glacier remains about equal, the ice sheet retains
stability. But if the rate of ice stream flow exceeds the rate of
recharge the glacier is said to have begun a difficult to reverse
process called destabilization.
The
initiation of the great Zachariae Ice Stream’s destabilization is
ominous for a number of reasons. First, it means the entire Greenland
Ice Sheet has, as of the early 2000s, began a plunge into the ocean
that is likely unstoppable. For once the great and massive glaciers
of Greenland start to move, gravitational inertia sets in and even a
radical cooling of the climate may not halt the surge.
Furthermore,
the Zacharie Glacier drains 16 percent of the entire Greenland ice
sheet alone. And finally, Zachariae stretches deep into the heart of
Greenland, extending seven hundred kilometers inland and taking hold
of Greenland’s massive central glaciers in its now accelerating
ocean-ward draw.
“Northeast Greenland is very cold. It used to be considered the last stable part of the Greenland ice sheet,” explained the study’s lead investigator Michael Bevis of The Ohio State University in a recent press release. “This study shows that ice loss in the northeast is now accelerating. So, now it seems that all of the margins of the Greenland ice sheet are unstable.”
(Greenland
Ice Sheet ice surface elevation change in meters per year from
2003-2006, 2006-2009 and 2009-2012 respectively. Note the elevation
loss of greater than 3 meters per year in Northeast Greenland near
the Zachariae Ice Stream’s outlet in the final frame. Image
source: Nature.)
The
recent study involved the use of GPS modules scattered over top of
the Greenland ice sheet to measure both glacial speed and altitude
loss. This finding is the most recent study provided by the
group’s network dubbed GNET.
Greenland:
An Archipelago with an Ice Sheet on Top
In
understanding why Greenland’s ice sheets are likely to continue to
flow into the ocean, it is useful to look at Greenland in its
geological context. For Greenland is essentially an island
archipelago with an enormous ice sheet sitting atop it. As such,
great channels and fractures run deep into the heart of this frozen
plcae.
Many of these fractures are below sea level, providing ocean
waters access further and further inland as Greenland melts.
Furthermore, the high elevation of the ice sheet creates a kind of
gravitational inertia that continuously drives glaciers toward the
sea. Only friction from the anchoring ground beneath the glaciers,
Greenland’s cold climate, and the chill of the surrounding airs and
oceans provided Greenland with enough ice recharge while slowing the
ice sheets enough to keep it stable during the last interglacial
period (Holocene).
Now, human warming is pulling that plug by
flushing melt water to the ice sheet’s base, creating warmer ocean
water invasions at the ice edge and creating conditions by which warm
air increasingly both overrides the ice sheet edge and invades into
the ice sheet interior.
It
is in this context that we should consider the relative positions of
ice stream fronts, grounding lines, and co-joining sea level as ice
sheets continue their flow toward the ocean:
(Comparison
of Northern Greenland ice face coming into contact with a floating
glacier in transect 1 to the deep reaching Zacharie Ice Stream
over-riding below sea level land masses in transect 2. Note that sub
land elevation beneath the ice surface is below sea level for a
stretch more than 150 kilometers inland along the base of the
Zacharie Ice Stream. Meanwhile, the ice surface slope and elevation
provide seaward momentum even in regions that are near or slightly
above sea level. The change in velocity of various sections of ice
flow are shown in the colored spaghetti lines. Image source: Nature.)
Conditions
in the Context of Human-Caused Climate Change
Current
atmospheric conditions now provide enough greenhouse gas forcing to
destabilize ice sheets in both Greenland and West Antarctica. These
levels, at
around 400 ppm CO2 and at around at least 425 CO2e when taking into
account all the additional negative and positive forcing from
aerosols and other greenhouse gasses,
were enough in past climates to send these immense regions of ice
plunging into the world ocean and to raise sea levels by between 15
and 115 feet over the course of centuries (Greenland alone contains
enough water locked in its ice sheets to raise sea levels by about 23
feet). All of the Greenland ice sheet and large sections of West
Antarctica are now undergoing the first stages of a similar
destabilization.
(Topographical
map of an archipelago-like Greenland without its overlaying ice
sheet. Glacial outflows are likely to be most intense toward the
northeast, northwest and southeast. Image source: Commons.)
Today’s
pace of sea level rise is currently 3.2 millimeters each year or a
little more than a foot each century. Of this total, fully 1/6th is
now being contributed by Greenland. But with inertia and
gravitational forces now taking hold as massive ice sheets
destabilize, and with human-caused warming continuing to ramp up, it
is likely that we can expect both the Greenland ice sheet’s
contribution and the pace of sea level rise to rapidly accelerate.
As
the great ice sheets sped toward the oceans at the end of the last
ice age, the pace of sea level rise hit as high as 10 feet each
century. With the pace of human warming now about 30 times faster
than at the last ice age’s fall, we may well eventually witness
something even outside this difficult to understand context.
Even
more ominous is the fact that greenhouse gas forcing levels that are
enough to destabilize and then melt all the world’s ice sheets,
eventually raising seas by about 250 feet, arrive as soon as the next
few decades once CO2 (or equivalent) forcing levels hit between 500
and 600 parts per million value. The current rate of emission gets us
there within about 20 years. But, unfortunately, that rate of
emission is still rising even as amplifying feedbacks from
terrestrial carbon stores in both the Arctic and the tropics loom.
In
essence, it looks more and more, from the point of ice sheet
stability, like we’ve probably at least locked in a Heinrich
type event and
will be well on our way to initiating total ice sheet loss over the
coming two decades.
Links:
Hat
tip to Colorado
Bob
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