*[Enwl] Feature: Seven Tipping Points That Threaten Earth
ENWL.Bellona
7 Tipping Points That Could Transform Earth
* By Brandon Keim Email Author
* December 23, 2009 |
When the Intergovernmental Panel on Climate
Change issue its last report in 2007,
environmental tipping points were a footnote. A
troubling footnote, to be sure, but the science
was relatively new and unsettled. Straightforward
global warming was enough to worry about.
But when the IPCC meets in 2014, tipping points -
or tipping elements, in academic vernacular -
will get much more attention. Scientists still
disagree about which planetary systems are
extra-sensitive to climate shifts, but the
possibility can't be ignored.
"The problem with tipping elements is that if any
of them tips, it will be a real catastrophe. None
of them are small," said Anders Levermann, a
climate physicist at the Potsdam Institute for
Climate Impact Research in Germany.
Levermann's article on potential disruptions of
South Asia's monsoon cycles was featured in a
series of tipping element research reviews,
published December 8 in the Proceedings of the
National Academy of Sciences.
Also discussed were ocean circulation, polar
icecaps, Amazon rainforests, seafloor methane
deposits and a west African dustbowl. Each is
stressed by rising planetary temperatures. Some
are less likely than others to tip; some might
not be able to tip at all. Ambiguities,
probabilities a limited grasp of Earth's complex
systems are inherent to the science. But if any
tip, it will be an epic disaster.
Wired Science takes you on a tour.
Image: Earth's Eastern and Western Hemispheres/NASA.
----
Polar Sea Ice
Dwindling Arctic sea ice and crumbling Antarctic
ice sheets are now a common sight. Whether they
signal an impending tip, with rapid melts causing
Earth's seas to inundate heavily-populated
coastal plains, is debated.
The process appears to accelerate itself: Warming
ice melts, which exposes darker areas, causing
local temperatures to rise further. But in the
Arctic, another feedback may stabilize the ice,
wrote Max Planck Institute meteorologist Dirk
Notz in PNAS. Though most of the ice "will
disappear during summer," much of it will
re-freeze in the winter. Arctic sea ice loss "is
likely to be reversible if the climate were to
become cooler again."
But Notz is less optimistic about Antarctic sea
ice, its undersides heated by eddying Southern
Ocean currents. And the West Antarctic and
Greenland ice sheets have shrunk suddenly at
least twice in the last several million years, a
behavior that's backed up by climate models. It's
"well possible that a tipping point exists for a
possible collapse" for those sheets, wrote Notz.
It could "render the loss of ice sheets and the
accompanying sea-level rise unstoppable beyond a
certain amount of warming."
Image: NASA.
----
Amazon Rainforest
As one of Earth's great carbon sinks, the
replacement of Amazon jungles with savannah or
forest would drastically accelerate global
warming.
On their own, rising temperatures and changing
weather patterns would not trigger jungle
dieback, wrote researchers led by Oxford
University ecosystem scientist Yadvinder Malhi in
PNAS. But deforestation combined with intensified
dry seasons leaves forests vulnerable to fire,
producing more weather-altering deforestation.
"The dieback of the forests of East Amazonia in
the 21st century is far from inevitable but
remains a distinct possiblity," they wrote.
Image: NASA.
----
Bodélé Depression, Chad
Winds whipping across the Bodélé, a 10,000 square
mile Saharan plain covered by ancient lakebed
sediments, carry 700,000 tons of dust into the
atmosphere annually. It floats around the world,
blocking sunlight and lowering temperatures in
some regions, and causing rain and warming in
others. Saharan dust influences Atlantic
ecosystems, Caribbean coral reefs and the Amazon.
Its full effects are unknown.
Small atmospheric changes "could profoundly alter
the behavior of this feature," wrote Richard
Washington, a specialist in African weather
African weather specialist at Oxford University,
and colleagues in PNAS.
At one point in the last 10,000 years, dust
ceased to flow altogether from the Bodélé. That
doesn't seem to be our problem. "Although subject
to a great deal of uncertainty, some simulations
of the 21st century indicate the potential for a
substantial increase in dust production," wrote
the researchers.
Image: NASA.
----
South Asian Monsoons
Hundreds of millions of people depend on regular
monsoon rains to nourish their crop, but the
monsoons are historically capricious. In what is
now India and China, they've have changed
abruptly several times since the Last Ice Age
ended.
Levermann's studies suggest that monsoon systems
amplify themselves. Rainfall releases heat,
fueling winds that pull more moisture from the
seas, producing more rainfall. Small changes can
swell monsoons, or nip them in the bud.
The model is limited, but its simulations track
with history. "We have a long paleorecord for
precipitation, and you see that there was almost
a switch. The monsoon was either on, or it was
off, with very little in between," said
Levermann. Climate change can flip the switch,
but it's not the only cause. "If you turn a
forest into a desert, it reflects more sunlight
and makes it cooler. Strong air pollution
reflects sunlight, and can trigger an event. Both
exist in Indian and Chinese regions."
Image: flickrohit/Flickr.
----
The Gulf Stream
Formally known as the Atlantic meridional
overturning circulation, or AMOC, the Gulf Stream
starts in the Gulf of Mexico and follows the
eastern contour of North America before flowing
to northern Europe and western Africa. Sudden
slowdowns in the circuation occurred repeatedly
during the last Ice Age. They "were associated
with large and abrupt changes in surface
climate," wrote Potsdam Institute climatologists
Matthias Hofmann and Stefan Rahmstorf in PNAS
Argument exists over whether slowdowns are
primarily wind-driven, or could be caused by an
influx of fresh water from melting ice sheets. In
its last report, the IPCC put the risk of Gulf
Stream slowdown during the 21st century at 10
percent. The true figure could be higher, or
lower. "Model deficiences make a risk assessment
for AMOC changes very difficult at present and
require urgent research attention," wrote Hofmann
and Rahmstorf.
Image: Models showing AMOC weakening from fresh
water influx/National Center for Atmospheric
Research
----
Seafloor Methane
Between 700 trillion and 10,000 trillion tons of
methane hydrate, a powerful greenhouse gas, are
trapped in the seafloor sediments where they've
accumulated over millions of years. If the planet
heats by 5.4 degrees Fahrenheit, well within the
range of warming possible if greenhouse gas
pollution levels remain high, seafloors could
heat enough to release a small but significant
fraction of the gases.
Methane bubbling slowly into the atmosphere could
raise planetary temperatures by a full degree
Fahrenheit for as much as 10,000 years. According
to researchers led by University of Chicago
geoscientist David Archer, methane-caused warming
would persist even if fossil fuel emissions
subsided.
"The modeling of methane hydrate is frankly in
its infancy," but it seems "robust to conclude"
that mankind could "melt a significant fraction
of the methane hydrates in the ocean," they wrote.
Image: Methane plumes rising from the Arctic
Ocean floor/National Oceanography Centre,
Southampton.
----
The Future
"What features establish the identity of a face;
what distortions erase that identity beyond
recognition?" asked Hans Schellnhuber, director
of the Potsdam Institute for Climate Research and
climate change advisor to German chancellor
Angela Merkel, in PNAS.
By Earth's face, Schellnhuber means the
environmental conditions that prevailed for most
of the last several thousand years. If there's
one dominant theme to the tipping element
reviews, it's that Earth's face is prone to what
he calls "singular transformations." They've
happened before.
Whether they will happen again, with mankind on
board, is the "cardinal question of earth systems
analysis [and] sustainability science," wrote
Schellnhuber.
How admittedly uncertain models should influence
international climate policy is an open question.
Levermann counsels caution.
"If you entered a plane and the captain said into
the speaker, 'There's a 10 percent chance this
plane will crash,' you wouldn't stay in it," said
Levermann. "This is the framework we have to
think about when we talk about tipping elements."
Image: An ensemble of global temperature
predictions contained in the last IPCC
report/IPCC.
See Also:
* Scientists Seek Warning Signs for Catastrophic Tipping Points
* 9 Environmental Boundaries We Don't Want to Cross
* Mediterranean Is Scary Laboratory of Ocean Futures
* Could Methane Trigger a Climate Doomsday Within a Human Lifespan
* How to Slow Climate Change for Just $15 Billion
Citations: "Tipping elements in the Earth
System." By Hans Joachim Schellnhuber.
Proceedings of the National Academy of Sciences,
Vol. 106 No. 49, December 8, 2009.
"Dust as a tipping element: The Bodélé
Depression, Chad." By Richard Washington,
Christel Bouet, Guy Cautenet, Elisabeth
Mackenzie, Ian Ashpole, Sebastian Engelstaedter,
Gil Lizcano, Gideon M. Henderson, Kerstin
Schepanski, and Ina Tegen. Proceedings of the
National Academy of Sciences, Vol. 106 No. 49,
December 8, 2009.
"Basic mechanism for abrupt monsoon transitions."
By Anders Levermann, Jacob Schewe, Vladimir
Petoukhov, and Hermann Held. Proceedings of the
National Academy of Sciences, Vol. 106, No. 49,
December 8, 2009.
"On the stability of the Atlantic meridional
overturning circulation." By Matthias Hofmann and
Stefan Rahmstorf. Proceedings of the National
Academy of Sciences, Vol. 106 No. 49, December 8,
2009.
"The future of ice sheets and sea ice: Between
reversible retreat and unstoppable loss." By Dirk
Notz. Proceedings of the National Academy of
Sciences, Vol. 106 No. 49, December 8, 2009.
"Ocean methane hydrates as a slow tipping point
in the global carbon cycle." By David Archer,
Bruce Buffett, and Victor Brovkin. Proceedings of
the National Academy of Sciences, Vol 106. No.
49, December 8, 2009.
"Exploring the likelihood and mechanism of a
climate-change-induced dieback of the Amazon
rainforest." By Yadvinder Malhi, Luiz E. O. C.
Aragão, David Galbraith, Chris Huntingford, Rosie
Fisher, Przemyslaw Zelazowski, Stephen Sitch,
Carol McSweeney, and Patrick Meir. Proceedings of
the National Academy of Sciences, Vol. 106 No.
49, December 8, 2009.
Brandon Keim's Twitter stream and reportorial
outtakes; Wired Science on Twitter. Brandon is
currently working on a book about ecosystem and
planetary tipping points.
http://www.wired.com/wiredscience/2009/12/tipping-elements/
*** NOTICE: In accordance with Title 17 U.S.C.
Section 107, this material is distributed,
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Sent: Friday, December 25, 2009 11:52 PM
Subject: Feature: Seven Tipping Points That Threaten Earth
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