More Antarctica is either warming or cooling ... stuff.
April 2, 2002
Deciphering Contradictory Antarctic Climate Patterns
By KENNETH CHANG
Antarctica is experiencing some of the
fastest warming in the world. Antarctica
is cooling.
Some of its glaciers are thinning. Some are
thickening. Ice shelves are disappearing. More
sea ice is forming.
Scientists have reported all this in recent
months. It may all be true, even the
contradictory parts.
"Confusing, isn't it?" asked Dr. Eric Rignot, a
glacier expert at the NASA Jet Propulsion
Laboratory in Pasadena, Calif.
Dr. Peter T. Doran, a professor of earth and
environmental sciences at the University of
Illinois at Chicago, agreed. "It's a mixed bag of
signals."
The reason is that Antarctica is not a single,
simple place. At 5.4 million square miles, it is
one-third larger than the United States, and just
as the Midwest may experience a heat wave
while the Northeast is unusually cool, climate
does not move in lock step across Antarctica.
Those warning of dire consequences from
global warming and those playing down the
dangers of heat-trapping greenhouse gases can
both find pieces of data to support their views.
"People forget that it's a continent," said Dr.
David Vaughan, a glaciologist at the British
Antarctic Survey. "We don't expect everything
to be the same across Asia when climate
changes. It's the same thing there."
Antarctica's role in climate and the oceans is
largely a story of ice. Ninety percent of the
world's ice lies either on the continent, in ice
sheets that are on average 1.3 miles thick, or in
sheets that have flowed offshore to form
floating platforms of ice along the coast,
hundreds to thousands of feet thick. The
largest of these, the Ross Ice Shelf, covers 200,000 square miles, an area about
the size of France.
The third component of Antarctic ice is a thin layer of frozen ocean, or sea ice,
that grows and shrinks with the seasons. A few feet thick, sea ice covers one
million square miles of ocean in summer and grows to six million square miles in
winter, doubling the size of the continent.
News like the disintegration of an ice shelf the size of Rhode Island a month ago
conjures a vision that a warming world will lead to doom by drowning — not
from melting ice shelves, which like melting ice in a glass do not change water
levels, but from melting ice sheets sending their fresh water flowing toward the
sea. If all of Antarctica's ice sheets turned to water, the world's oceans would
deepen by more than 200 feet.
That will not happen. Annual temperatures in the Antarctic interior average minus
70 degrees or colder. Even a 10-degree temperature rise — greater than climate
models' worst-case predictions — would leave almost all of the ice frozen.
But ice sheets along the warmer coastal fringes of Antarctica are more vulnerable
to melting. Even modest sea-level rises could increase flooding of coastal lands
like Bangladesh, Florida and even Manhattan. Shifting ice could also divert the
ocean currents that circle the continent, possibly disrupting the global flow of
ocean water and altering the climate still further.
The changes in the Antarctic landscape do not have a single cause. Some are part
of the natural cycles of the continent. Some are probably delayed effects of the
end of the last ice age. Some may have been brought on by the warming trend of
the last century.
On the spindly peninsula that stretches out toward South America, temperatures
have risen rapidly, nearly 5 degrees Fahrenheit over the past 50 years, about 10
times as much as the average temperature rise worldwide.
The consequences have been quick and startling.
With summer temperatures now regularly rising above freezing, ice melts into
puddles on the top of ice shelves along the peninsula. The water flows down into
cracks in the ice, its weight forcing the cracks wider until large sections of the
shelf shatter with surprising quickness.
"There's no doubt these ice shelves are disappearing because of this warming
trend," said Dr. Rignot of NASA.
In 1995, researchers started noticing the disintegration of the Larsen Ice Shelf,
on the northeastern side of the Antarctic Peninsula, when the northernmost
section, known as Larsen A, shattered into shards. In 1998, the middle portion,
Larsen B, started shrinking, losing 1,000 square miles over four years.
Then, in February, a 1,250-square-mile section — larger than Rhode Island —
started splintering, and in just over a month, it was gone, sending billions of tons
of ice floating into the ocean to melt. Scientists expect the remaining nub of
Larsen B and C, the last section of the shelf, to fall apart in the coming years.
Although their destruction does not directly raise sea level, the shelves had acted
like door jambs against inland ice sheets. The sheets may now flow more quickly
to the sea; the new ice displaces water and raises sea levels, the way extra ice
cubes raise the water level in a glass.
A melting ice shelf is not necessarily a sign of human-induced global warming.
Ice shelves have grown and shrunk through the ages, mirroring the natural
cooling and warming of the climate.
In a core of sediments taken from the sea floor that was once covered by the
Larsen A Ice Shelf, researchers led by Dr. Eugene W. Domack, a professor of
geology at Hamilton College in Clinton, N.Y., found the tiny fossils of marine
algae. The finding indicates that this part of the ice shelf had been open water at
least once before. The shelf probably melted about 6,000 years ago in a previous
warm spell, Dr. Domack said, and remained open water until refreezing during
the Little Ice Age about 700 years ago, then remained frozen until it fell apart in
1995.
Under the Larsen B, however, the researchers found no algae remains in the
sediments, indicating that this shelf had remained intact since it formed during the
last full-fledged ice age, more than 10,000 years ago, until its demise last month.
Dr. Theodore A. Scambos, a glaciologist at the National Snow and Ice Data
Center at the University of Colorado, said the long life of Larsen B "makes you
think there's something particularly unusual about this warming" — perhaps
evidence that the warming has been brought on by artificial emissions of carbon
dioxide and other greenhouse gases that trap heat in the atmosphere.
Almost at the same time of Larsen B's demise, further to the south, an iceberg
the size of Delaware broke off the floating part of the Thwaites Glacier. A few
months earlier, parts of the Ross Ice Shelf broke off. But these two events,
scientists say, are not unusual. As glaciers flow into the water, the ice shelves
grow until some point when a large iceberg breaks off, and then the process is
repeated. The part of the Ross Ice Shelf where the iceberg broke away is now
the same smaller size it was when the explorers Robert F. Scott and Ernest
Shackleton observed it at the start of the 20th century.
The melting of the Larsen and other peninsula ice shelves will also not have
worldwide repercussions. The amount of ice on the peninsula is relatively small,
potentially contributing only a small rise in sea level.
The rest of Antarctica shows no signs of widespread warming. In an article in
the journal Nature in January, Dr. Doran of the University of Illinois and his
colleagues reported that temperatures in the McMurdo Dry Valleys, a rocky,
ice-free area west of the Ross Ice Shelf, had cooled about 2 degrees Fahrenheit
from 1979 to 1998. Extrapolating that data with other temperature measurements
in other parts of Antarctica from the past 35 years, they concluded that
Antarctica as a whole has cooled, too.
Scientists at NASA Goddard Space Flight Center have also reported that satellite
measurements show that sea ice now covers about 2 percent more area around
Antarctica than it did two decades ago, another suggestion of recent cooling.
Because climate naturally warms and cools on cycles lasting for decades, the
observed Antarctic cooling does not disprove the idea of human-induced global
warming. Scientists cannot say yet whether the cooling is a short-term blip
masking a long-term warming trend or a long-term trend contrary to their
predictions. "It may still warm," Dr. Doran said. "It's not really making up its
mind yet."
In a comparison of 17 computer models of world climate, all predict global
warming will kick in over Antarctica, and most indicate temperatures in the
interior of the continent will rise faster than in the rest of the world, said Dr.
Benjamin D. Santer, an atmospheric scientist at Lawrence Livermore National
Laboratory. "There is a common warming signal," he said.
Because ice reflects most light and heat back into space, the exposed water will
absorb more heat as sea ice melts, and further add to the warming.
Counterintuitively, global warming would actually lower sea levels at first. In
warmer temperatures, evaporation of ocean water increases and more snow falls,
more than offsetting the melting ice at the edges. But over the longer term —
perhaps centuries, perhaps thousands of years — prolonged warmth in
Antarctica would add to the ocean depths.
Of particular worry is West Antarctica, where most of the bedrock lies below
sea level. That makes the ice sheet on top more vulnerable to warming waters
lapping at its edges. The worry is not entirely theoretical. Scientists have found
algae remains beneath the West Antarctic ice far inland from the present ocean, a
sign that the ice sheet had entirely melted at some time in the last two million
years. But the fossil evidence gives little hint how quickly the melt occurred or its
cause.
Scientists do not have a good sense of the current trends, because until a few
years ago, data came from only a few ground-based weather stations. With
satellites, scientists now keep close watch of changes to Antarctica as they
occur. "We can get images of what happened yesterday on the computer," said
Dr. Rignot. "That changes everything for glaciologists."
In December, Dr. Rignot reported on satellite measurements indicating that the
Pine Island and Thwaites glaciers in West Antarctica were thinning and speeding
up, losing about 36 cubic miles in the last decade. A month later, other
researchers, including Dr. Ian R. Joughin, an engineer at the Jet Propulsion
Laboratory with an office near Dr. Rignot's, reported a change with opposite
effect nearby. Two of the ice streams that flow in the Ross Ice Shelf have
slowed, they said, and that area of Antarctica is gaining mass.
Neither the slowdown in the Ross ice streams nor the speed-up of the Pine Island
and Thwaites glaciers results from climate changes, at least not recent ones. One
Ross ice stream stopped flowing 150 years ago.
Even if the Antarctic warms in coming years, some glaciers will not be affected
for a long time. Ice does not conduct heat well, and a rise of a few degrees in the
air would take thousands of years to affect a glacier base a mile away, where it
could lubricate the flow.
The thinning of the Pine Island and Thwaites glaciers "may be a direct impact of
global warming that happened 20,000 years ago," said Dr. Robert Bindschadler, a
glaciologist at Goddard. "It may be only now that Antarctica is getting around to
its full-fledged response to that."
In Antarctica, climate change sometimes takes its time.
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