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.

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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.

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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.

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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.

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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.

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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

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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.

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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:

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.