The calving front of Helheim Glacier in Greenland.
(Knut Christianson)
From WashingtonPost by Chris Mooney
For many scientists studying Antarctica, and particularly the vulnerable West Antarctic ice sheet, a major
new study significantly
increasing expectations for sea-level rise is the culmination of a
large body of prior research — combined with alarming recent
observations.
The study, just published in Nature, is based on an
improved understanding of past warm eras in Earth’s history that
featured much higher seas.
By creating advanced computer simulations of
how Antarctica’s ice melts and flows — ones that can accurately capture
sea level during these eras — the current study was also able to project
considerable sea-level rise in the relatively near future.
In effect, it found that we’re about to start repeating the past.
The
research stated that for a very high-carbon-emissions scenario, melting
ice from Antarctica alone could cause seas to rise 1.14 meters (3.74
feet), give or take 36 centimeters, by 2100 — and vastly more, more than
15 meters (over 50 feet), by 2500.
The world has recently taken steps
to try to avoid such a high-emissions pathway — steps whose
effectiveness remains to be seen — but even in a more moderate emissions
scenario, the study found the Antarctic contribution could be 58
centimeters (nearly two feet), give or take 28 centimeters, and close to
six meters (nearly 20 feet) by 2500.
According to a new study, high levels of
greenhouse gas emissions could cause oceans to rise by close to two
meters in total (over six feet) by the end of the century, and more than
13 meters (42 feet) from Antarctica alone by 2500.
(Nature, Rob
DeConto, David Pollard)
That would be in addition to contributions from melting mountain
glaciers, swelling seas caused by warming itself, and losses from
Greenland.
The smaller ice sheet in Greenland is melting even faster
than Antarctica and contains up to six meters (20 feet) of potential
sea-level rise, including about
two meters in vulnerable areas where marine-based ice is in contact with the ocean.
So
how did scientists reach this conclusion?
Through a series of key steps
that weave together modern observations and a better understanding of
the ancient Earth.
The Thwaites Glacier in West Antarctica. (AFP via NASA)
Alarm about West Antarctica.
The first major sign
that consensus projections about sea-level rise might be too
conservative came in 2014, when two groups of scientists published
research suggesting that the marine-based glaciers of West Antarctica
had begun an irreversible retreat, as a result of warm water reaching
the bases of glaciers such as the enormous Thwaites (which is larger
than Pennsylvania, and over a mile thick in places) and melting them
from below.
Thwaites and other West Antarctic glaciers not only
are perched deep below the sea surface but also rest on seabeds that get
deeper as you move inland, meaning that as they move downhill, they
will present taller and thicker fronts to the ocean, and ice could flow
out of them still faster.
Researchers have dubbed this condition a
“Marine Ice Sheet Instability,” and other recent research has suggested
that similar alignments exist in the far-larger ice sheet of East
Antarctica, as well.
In light of these observations, the community of West Antarctic scientists has recently called for an
urgent international research mission
to the remote region to gather new data about the Thwaites Glacier, in
particular.
The reason is that there is still a lack of understanding of
much about Thwaites, including the precise topography of the seafloor
beneath it.
Better mapping of this terrain could validate, or
potentially refute, some of the results of the most recent computer
modeling study, in which a major retreat of Thwaites drives a key part
of the result.
The new study, said Robin Bell, an Antarctic
expert at Columbia University’s Lamont-Doherty Earth Observatory,
“points a laser pointer at where we need to go” to do more research.
“Right
now we do not know if the topography under the Thwaites Glacier is a
smooth ramp sloping back to some of the thickest ice in West Antarctica
or if it is a lumpy terrain,” Bell said.
“We just do not know, and it
matters for models like this.”
The giant boulders of Eleuthera in the Bahamas
have sparked debate
among scientists about their origin.
(Charles
Ommanney/The Washington Post)
The troubling message from the Earth’s past.
At the
same time, eyeing places such as Thwaites and fearing the potential for
far greater sea-level rise than currently forecast by groups such as the
United Nations’ Intergovernmental Panel on Climate Change, scientists
have been closely examining the ancient past for clues about what Earth,
and its ice sheets, are actually capable of.
For instance, the
last interglacial period (sometimes called the Eemian), between 130,000
and 115,000 years ago, featured a sea-level high stand that is believed
to have been 6 to 9 meters above current levels.
The Eemian has drawn
major attention lately because events at that time, including claims of
enormous waves moving very large boulders in the Bahamas, have fueled
the prominent climate researcher James Hansen’s
particularly dire ideas
about polar melt, rising seas, the shutdown of ocean circulations and
worsening storms.
In other words, scientists are looking more and more
at this particular period, not all that long ago in geologic time, as a
possible analogue for our own.
The problem has been that computer
models that simulate Antarctica in different climates have, until
lately, been unable to capture these past sea-level high stands.
“We
were not [able] to get more than a meter of sea-level rise out of the
ice sheet in the last interglacial,” said the University of
Massachusetts at Amherst’s Rob DeConto, who authored the new study with
David Pollard of Pennsylvania State University.
But what about the way ice moves and changes was missing from models?
The calving front of Jakobshavn Glacier in Greenland. (Brennan Linsley/AP)
The key message from Greenland.
Even as the scientists were struggling to get the equations to work, real world events were hinting at a possible solution.
Take, for instance, the Jakobshavn Glacier of Greenland, which may be the
fastest retreating major glacier in
the world and one that holds the potential for 0.6 meters (nearly two
feet) of sea-level rise, according to University of California at Irvine
glaciologist Eric Rignot. Jakobshavn’s flow speed increased greatly
after the
loss of its floating ice shelf, an extension of the glacier out over open water in the fjord where it lies, in the early 2000s.
Ice
shelves provide stability because they tend to freeze onto landscape
features, such as islands, the sides of fjords or the seafloor — and
thus provide back-pressure on glaciers, holding them in place.
However,
at Jakobshavn, and at the
Larsen B ice shelf
on the Antarctic peninsula — part of which dramatically disintegrated
in 2002 — ice shelves are vulnerable.
Warm waters can eat into them from
below, and at the same time,
surface melting can lead to pools of water that fill crevasses on these shelves and weaken them, widening surface cracks.
After
losing its ice shelf, Jakobshavn now terminates in a tall, sheer ice
cliff extending above the surface of the water, with the vast bulk of
the glacier below the water.
There is no stabilizing shelf any longer.
Another Greenland glacier, Helheim, is in a similar state (see the image
at the top of this post).
And Antarctica’s Crane Glacier, which was
previously held back by the Larsen B ice shelf, is as well.
But
this presents a problem.
What these glaciers have in common is that they
have quite tall cliffs above the water, approaching 100 meters in
height.
In each case, below the water is much more ice — potentially
nine times as much.
But that ice is partly held in place by the ocean
itself.
It’s the part above the water that matters, because there are
reasons to believe that ice, not being very strong as a material, can’t
maintain a cliff above around 100 meters in height, about 330 feet,
without collapse.
“With the fantastic observations of Jakobshavn, Helheim and other
Greenland cliffs, the whole picture started coming together,” Richard
Alley, a glaciologist at Penn State who has published with DeConto and
Pollard, said of the revelations regarding ice-cliff collapse.
Indeed,
one key implication of the research is that, as more major glaciers
lose ice shelves, we could be moving into a new world where the dynamics
of ice cliffs matter more and more.
Another major Greenland glacier,
Zachariae Isstrom
in the northeast, has also now lost the vast majority of its ice shelf
and features a 75-meter-high cliff, scientists reported last year.
As
the glacier retreats further, “the height of the calving cliff will
increase from its current 75 m to enhance the risk of ice fracture,” the
researchers noted.
West Antarctica’s glaciers could create much bigger cliffs.
Greenland
may seem vast, but it’s actually far smaller than Antarctica and
contains much less ice. The reason this matters is that in Antarctica,
and most immediately West Antarctica, there is the possibility to create
ice cliffs much taller than 100 meters high (above sea level), because
we are talking about marine-based glaciers with far more than 1,000
meters (or one kilometer) of thickness overall in their bulkiest parts.
“For
this effect to be important, the ice would have to be about a kilometer
thick,” said Knut Christianson, a glaciologist at the University of
Washington in Seattle.
“In some places, it gets to be significantly
deeper than that. The interior of West Antarctica, some areas are more
than two kilometers below sea level, more than three kilometers thick.”
Thus, in the new ice-sheet model, two new processes are included —
“hydrofracture,” in which ice shelves collapse because of meltwater on
their surfaces, and “cliff collapse,” in which too-tall ice cliffs also
fail and crumble.
Meanwhile, the model also runs “ensembles,” in which
large numbers of simulations with slightly altered physics are tested
out — but only model runs capable of accurately simulating sea-level
rise for past eras such as the Eemian are ultimately included.
Add it all together — the past, and the alarming present — and sure
enough, scientific understanding produces the possibility of major ice
retreat from Antarctica, in this century and beyond.
And it’s important
to emphasize that there are reasons to think that the real world could
possibly outstrip even this study.
“Their model is not a worst
case, as they do place a limit on the rate of retreat in the model that
does not come directly from the physics,” Alley said.
In an interview,
DeConto explained that the model does not allow Antarctica’s ice to
retreat as quickly as Jakobshavn’s ice currently is retreating, for
instance.
A feedback with the ocean?
And there’s another
potentially missing factor. Large ice loss from Antarctica would pour
enormous amounts of fresh water into the Southern Ocean, in the form of
water and also vast icebergs that will begin to melt.
A much noted but
controversial climate-change catastrophe study by former NASA researcher
James Hansen used the concept of large meltwater injections like this
to outline a feedback mechanism that leads to still more ice loss —
meltwater stratifies the polar ocean with cold water on top and warmer
water below.
The warmer water then causes more melting of ice sheets.
The
two studies intersect: The new research finds that the ocean would be
gaining, in some extreme scenarios, more than a sverdrup of fresh water
as melting really advances, which translates into a flow of about
84 billion tons daily.
“The amount of meltwater going into the Southern Ocean in these
scenarios is more than ocean modelers even think about when they think
about doing a crazy freshwater forcing experiment,” DeConto said.
In
this scenario, not only would seas rise rapidly, but there could be
other major effects.
No wonder, then, that the new research, by squaring
the past with the present, raises deep and troubling new questions.
Links :