Posted here on 19/6/2014
Scientists Warn of Rising Oceans From Polar Melt
From the New York Times website.
A
large section of the mighty West Antarctica ice sheet has begun falling
apart and its continued melting now appears to be unstoppable, two
groups of scientists reported on Monday May 12, 2014. If the findings hold up, they
suggest that the melting could destabilize neighboring parts of the ice
sheet and a rise in sea level of 10 feet or more may be unavoidable in
coming centuries.
Global
warming caused by the human-driven release of greenhouse gases has
helped to destabilize the ice sheet, though other factors may also be
involved, the scientists said.
The
rise of the sea is likely to continue to be relatively slow for the
rest of the 21st century, the scientists added, but in the more distant
future it may accelerate markedly, potentially throwing society into
crisis.
Two
scientific papers released on Monday by the journals Science and
Geophysical Research Letters came to similar conclusions by different
means. Both groups of scientists found that West Antarctic glaciers had
retreated far enough to set off an inherent instability in the ice
sheet, one that experts have feared for decades. NASA called a telephone
news conference Monday to highlight the urgency of the findings.
The
West Antarctic ice sheet sits in a bowl-shaped depression in the earth,
with the base of the ice below sea level. Warm ocean water is causing
the ice sitting along the rim of the bowl to thin and retreat. As the
front edge of the ice pulls away from the rim and enters deeper water,
it can retreat much faster than before.
In
one of the new papers, a team led by Eric Rignot, a glaciologist at the
University of California, Irvine, used satellite and air measurements to document
an accelerating retreat over the past several decades of six glaciers
draining into the Amundsen Sea region. And with updated mapping of the
terrain beneath the ice sheet, the team was able to rule out the
presence of any mountains or hills significant enough to slow the
retreat.
“Today
we present observational evidence that a large sector of the West
Antarctic ice sheet has gone into irreversible retreat,” Dr. Rignot said
in the NASA news conference. “It has passed the point of no return.”
Weddell
Sea
South Pole
WEST
ANTARCTICA
Amundsen
Sea
500 MILES
Those
six glaciers alone could cause the ocean to rise four feet as they
disappear, Dr. Rignot said, possibly within a couple of centuries. He
added that their disappearance will most likely destabilize other
sectors of the ice sheet, so the ultimate rise could be triple that.
A separate team led by Ian Joughin of the University of Washington studied one of the most important glaciers, Thwaites,
using sophisticated computer modeling, coupled with recent measurements
of the ice flow. That team also found that a slow-motion collapse had
become inevitable. Even if the warm water now eating away at the ice
were to dissipate, it would be “too little, too late to stabilize the
ice sheet,” Dr. Joughin said. “There’s no stabilization mechanism.”
The
two teams worked independently, preparing papers that were to be
published within days of each other. After it was learned that their
results were similar, the teams and their journals agreed to release the
findings on the same day.
The new finding appears to be the fulfillment of a prediction
made in 1978 by an eminent glaciologist, John H. Mercer of the Ohio
State University. He outlined the vulnerable nature of the West
Antarctic ice sheet and warned
that the rapid human-driven release of greenhouse gases posed “a threat
of disaster.” He was assailed at the time, but in recent years,
scientists have been watching with growing concern as events have
unfolded in much the way Dr. Mercer predicted. (He died in 1987.)
A NASA animation shows glacier changes detected in the highly dynamic Amundsen Embayment of West Antarctica.
Credit NASA/Goddard Space Flight Center
Scientists
said the ice sheet was not melting because of warmer air temperatures,
but rather because relatively warm water that occurs naturally in the
depths of the ocean was being pulled to the surface by an
intensification, over the past several decades, of the powerful winds
that encircle Antarctica.
And
while the cause of the stronger winds is somewhat unclear, many
researchers consider human-induced global warming to be a significant
facto. The winds help to isolate Antarctica and keep it cold at the
surface, but as global warming proceeds, that means a sharper
temperature difference between the Antarctic and the rest of the globe.
That temperature difference provides further energy for the winds, which
in turn stir up the ocean waters.
Some scientists believe the ozone hole
over Antarctica — caused not by global warming but by an entirely
different environmental problem, the human-caused release of
ozone-destroying gases — may also be adding energy to the winds. And
natural variability may be contributing as well, though scientists do
not believe it is the primary factor.
The
global sea level has been rising since the 19th century, but Antarctica
so far has been only a small factor. The biggest factor to date is that
seawater expands as it warms.
This “airborne” tour was created from a small
portion of the images collected during a flight over the Pine Island
Glacier crack on Oct. 26, 2011.
Credit NASA/Goddard Space Flight Center
But
the melting from both Greenland and Antarctica is expected to be far
more important in the future. A United Nations scientific committee, the
Intergovernmental Panel on Climate Change,
has warned that the global sea level could rise as much as three feet
by the end of this century if stronger efforts are not made to control
greenhouse gases. The new findings suggest the situation is likely to
get far worse in subsequent centuries.
The
effects will depend in part on how much money future governments spend
to protect shorelines from a rising sea. Research published in 2012
found that a rise of less than four feet would inundate land on which
some 3.7 million Americans live today. Miami, New Orleans, New York and
Boston are all highly vulnerable.
Richard
B. Alley, a climate scientist at Pennsylvania State University who was
not involved in the new research but has studied the polar ice sheets
for decades, said he found the new papers compelling. Though he had long
feared the possibility of ice-sheet collapse, when he learned of the
new findings, “it shook me a little bit,” Dr. Alley said.
He
added that while a large rise of the sea may now be inevitable from
West Antarctica, continued release of greenhouse gases will almost
certainly make the situation worse. The heat-trapping gases could
destabilize other parts of Antarctica as well as the Greenland ice
sheet, potentially causing enough sea-level rise that many of the
world’s coastal cities would eventually have to be abandoned.
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