predicting tipping points

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ISMAEL ALONSO

Predicting Tipping PointsR I C H A R D B L A U S T E I N

ABANDONED BOATS AT THE FORMER SITE OF THE ARAL SEA

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T I P P I N G P O I N T S

At the time he attended the 2004 European Open Science Forum in Stockholm, German physi-

cist Hans Joachim Schellnhuber had been speaking out publicly since the late 1990s about climate change impacting what he termed “the Achilles heels of the Earth system.” Using science-speak, he talked of “switch-and-choke points” and “large scale discontinuities” to describe the possibility that key natural systems whose stability we take for granted—like the Atlantic Ocean’s jet stream and a mechanism called the thermohaline circulation, which circu-lates warm water globally—could abrupt-ly change under climate change stress.

Richard Blaustein is a science and environmental journalist based in Washington, D.C.

His Twitter is @richblaustein.

Not surprisingly, Schellnhuber was gain-ing adherents among some scientists, in-cluding within the Intergovernmental Panel on Climate Change (IPCC) of which he was a part, but hardly with the broader public that is so essential if action is to be taken on this issue. Indeed, it was not until the 2004 meeting that he signifi-cantly broadened this kind of reach with one phrase he tried out on BBC reporter, Alex Kirby, who was at the Stockholm conference and interviewed Schellnhuber. “I tried to explain to him the switch-and-choke-points of the Earth system—the ice changes, the thermohaline system, and he didn’t quite get it,” recalls Schellnhu-

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governments and international organiza-tions will at least develop the means to anticipate tipping points—when natural systems flip to harsher, altered states. In fact, in this decade, a robust university-based scientific discourse on predicting tipping points is debuting. Some are urg-ing that these investigations be broadened and that government networks prepare for a changing, less-familiar natural world.

PREDICTIVE SCIENCE

After the 2004 BBC interview, Schelln-huber received a little more governmental support for his efforts to build collabora-tions between British and German climate scientists. Some 36 scientists gathered the next year at the British Embassy in Berlin to discuss the science of climate change tipping points. British climate scientist Timothy Lenton, a friend and collaborator of Schellnhuber, played a prominent role in the Berlin meeting that now appears to be a watershed event. New collaborations, publications, and wider interest grew from the Berlin Embassy gathering.

Lenton, Schellnhuber, and five other authors began working on a paper that laid out the basic science of climate change tipping points, highlighting nine areas of concern, such as the altering of the Indi-an summer monsoon season, the Amazon rainforest switching over to savanna, and melting ice sheets in Greenland and Ant-arctica. “Tipping elements in the Earth’s climate system” even touched on early warning signals for these abrupt changes.

Lenton, now at the University of Exeter, is proud of that meeting and his ensuing re-search, but recalls that early time as a kind of a wilderness spell. Not only was there lit-tle public interest in climate change tipping points, the larger climate change scientific community was resistant. “It would be fair

ber. But “when I said we found practical ‘tipping points’ in the Earth system, then [with his article] this climate change ‘tip-ping points’ notion was born.”

A decade later, climate change tipping points are now perceived as a serious con-cern. Heat waves and drought throughout the world indicate that climate change is having an impact that will likely worsen. The global warming effects can be bizarre: some scientists attribute the recent winter cold snaps to global warming as the Arc-tic system is thrown out of balance, with winter Arctic weather spilling over into the lower latitudes—hence the growing fre-quency and intensity of the Polar Vortex. More ominous, however, are changes in the permafrost, which is already showing signs of melting in places like Alaska and Rus-sia, where incidents of thaw have upended property and public roads. It is a bad sign. Northern thawing would release massive amounts of methane, compounding the greenhouse gas problem. The changes are no less worrisome in the planet’s southern extremes. In 2002, in Antarctica, the Lars-en B Shelf, a 1,250 square mile area of ice larger than Rhode Island, collapsed and dis-integrated into the ocean, validating glaci-oligist John Mercer’s 1978 prediction that global warming would first impact Antarc-tica’s outreaching ice shelves. With an eerie sense of déjà vu in May 2014, two widely-reported scientific papers predicted the on-going loss and eventual collapse of glaciers in West Antarctica’s Amundsen Sea, caus-ing the sea level to rise three to 12 feet.

So far, climate change diplomacy has been ineffectual at addressing the global emissions problem, with only economic downturn having a large-scale mitigat-ing effect—hardly the best policy choice. However, an increasing number of scien-tists are now in preparation mode, hoping

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government, built upon existing govern-ment capacity for data collection and nat-ural disaster prediction. The early warning system idea includes new modeling and other means for foretelling climate change tipping points.

Lately, in fact, there has been some con-siderable university-based exploration of early warning signals of abrupt change, fo-cusing on a phenom-enon called “criti-cal slowing down,” which occurs when a system takes longer and longer to recover from a disturbance.

Beginning this decade, a few re-searchers have shown that critical slowing down precedes a tip-ping point, such as when a microorgan-ism population rap-idly heads to extinction. The investigations raise the possibility that under climate change stress, some natural systems head-ing toward abrupt change would send off early warning signals.

In the not-too-distant future, critical slowing down might very well join tip-ping points and become part of the cli-mate change vernacular.

DEBATE OVER TIPPING POINTS

While these investigations have ad-vanced, some scientists have criticized the way the term “tipping point” has been used, saying it is applied in a vague or overreaching way. Indeed, tipping points, sometimes depicted as a global dominos crescendo, are not so closely linked, and their time frame varies, with some way off on the horizon. Lenton says he and his

to say there was a lot of skepticism from the conservative ranks of climate science to the whole concept of tipping points,” Lenton recalls. “We were sometimes kind of accused of alarmism to be even drawing attention to what were seen as high impact-low probability events.”

Recently, however, much has changed. Last spring the IPCC released a report where for the first time it spoke explic-itly of tipping points: “With increasing warming, some physical systems or eco-systems may be at risk of abrupt and ir-reversible change.” The report points out that coral reef and Arctic ecosystems are already undergoing irreversible changes. And it warns that tipping point risks “in-crease disproportionately” as temperature rises above 5.4 degrees Fahrenheit, “due to the potential for a large and irreversible sea-level rise from ice sheet loss.”

Just a few months earlier, the U.S. National Research Council, too, had made headlines by releasing a report on tipping points. University of Colorado climate scientist Jim White led the com-mittee of scientists that put together “Abrupt Impacts of Climate Change: An-ticipating Surprises,” and says he briefed agencies and foundations on the find-ings through much of 2014. The report gave an up-to-date assessment of tipping points, with mass species extinctions and critical water shortages among the points we will witness in the next few decades. Some fearful impacts not expected this century but highly probable after the 21st century include a critical decline of oceans’ oxygen levels, which would have serious toxic effects.

White’s group urged more work on tipping point early warning signals and proposed the creation of an Abrupt Change Early Warning System within the U.S.

“there’s no denying the concept of tipping points, and there are tipping points in ecosystems that are well-known in nature.”


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to Arctic summer ice loss in 2007 as huge. But 2008, Serreze recalls, was a year of sig-nificant ice recovery, and 2009 was also a summer with higher sea ice. “Then we start going down again,” Serreze says. “2012 was a record low, but then 2013 quite a recovery. Now all the way from record low to sixth lowest on the record, so it is going from really bad to a little better. It shows you have variability in the system.”

Arctic sea ice could snap back more resiliently in the summer if governments reduce greenhouse gas emissions and sta-bilize atmospheric levels. But the Arctic is getting warmer with massive ice loss when measured across recent decades. And gov-ernments have not shown any ability to sta-bilize—let alone reduce—greenhouse gas levels. The effects could be serious. Arctic scientist Julienne Stroeve points out that with reduced Arctic sea ice, a “huge ampli-fied warming” occurs in the Arctic. “We know, in theory, that with this change in temperature gradient between the pole and the equator, [come] changes in large scale atmospheric circulation patterns,” Stroeve says. More extreme weather for the Western Hemisphere is one very real possibility.

“That is not to say that there are not systems where there may be some kind of critical threshold, such as in permafrost,” Serreze clarifies.

Once permafrost warms up enough to form an unfrozen layer, a dynamic sets in causing the permafrost to dwindle rapidly. Kevin Schaefer, a permafrost expert, adds that permafrost thawing will dramatically compound the greenhouse gas problem, as permafrost contains twice as much carbon as currently in the atmosphere, with great potentiality for release. He points out that experts predict permafrost greenhouse gas emissions to begin mid-century. “They won’t be huge but they will continue for

Berlin colleagues clarified climate change tipping points as self-accelerating trans-formations set off when natural thresh-olds are crossed. He stresses that tipping points often infer geological timescales longer than human history and are not necessarily irreversible. Lenton admits there is “quite a bit of slightly sloppy use of the term in the scientific discourse, as well as in the wider discourse that will make some of the more precisely minded scientists uncomfortable.”

University of Maryland-Baltimore landscape ecologist Erle C. Ellis has criti-cized the whole Earth global tipping point scenario, pointing out that the Earth’s bio-sphere lacks the requisite interconnectiv-ity for a tipping point response of global magnitude. Ellis is anything but a climate change skeptic, and he says that human-caused climate change will have serious ecological impacts. “There’s no denying the concept of tipping points, and there are tipping points in ecosystems that are well-known in nature,” says Ellis. But he points out that there are many types of responses to stress “for systems as complex as ecosys-tems and social-ecological systems,” and that the ecological flips that do occur will probably be varied. Because of this discon-tinuity, Ellis emphasizes that scientists and policy makers should concentrate on local and regional impacts.

Similarly, glaciologist Mark Serreze, di-rector of the National Snow and Ice Data Center, finds the term tipping point too loosely used, especially about Arctic sum-mer ice. “That’s the issue, how you frame it,” Serreze says. “The term tipping point has not been well defined.” In the Arctic, says Serreze, “what we’ve seen is that you can have periods of rapid ice loss, but it is not a threshold so to speak—it is not really a tipping point.” Specifically, Serreze points

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This tipping point frontline becomes better grounded when it sticks to a sys-tem-by-system approach. In fact, with this tight focus, the science of early warning signals is advancing.

A BACKGROUND IN ECOLOGY

In 1978, ecologist C.S. Holling and col-leagues changed the way ecologists view nature when they looked at Douglas fir for-ests prone to spruce budworm infestations. The forests had two states. In one, there

several centuries,” says Schaefer. “And once you turn them on, you can’t turn them off. So it is a classic tipping point.”

While there are some points of dis-agreement, the science of tipping points is progressing with new knowledge gained. White, for example, says that just in the past 10 years, the understanding of gla-cier dynamics has expanded dramatically. Schellnhuber is heartened and calls tip-ping points the “next frontier of climate change research.”

ACTION STEPS• U.S. government agencies need to begin talks on the Abrupt Change Early Warning

System (ACEWS). This would not be expensive and builds on leveraging in-place capacities. New modeling, monitoring devices, and placements (such as additional Gulf Stream monitoring posts) should be encouraged. The White House Office of Science and Technology and its linked President’s Council of Advisors on Science and Technology are strong forums for early and open discussions on the ACEWS. The ACEWS should be advanced with international collaborations in mind. Other nations should also take action for early warning systems.

• Greater support for investigations on tipping points and critical slowing down, especially in national labs, is needed. A good example of an experiment to be supported and also as an inspiration for different ecosystems experiments is the United States Department of Energy’s northern boreal project, the Spruce and Peatland Responses Under Climate and Environmental Change (SPRUCE) experiment.

• International cooperation and collaboration on tipping point cataloguing, such as on the most vulnerable ecological areas, should commence. From there, the international community should develop models and systems for early warning systems of particular tipping points with the largest impacts, such as monsoons, as Tim Lenton suggests.

• Most importantly, there must be a commitment to mitigate. Nothing better addresses the threat of tipping points as mitigation, reducing greenhouse gas emissions, and subsequently lowering atmospheric greenhouse gas levels. While a global program to reduce greenhouse gas emissions remains elusive, support should be urgently given to technological and natural terrestrial carbon (ecosystem restoration, reforestation) efforts that pull carbon from the atmosphere. The recent opening of the ASU Center for Negative Carbon Emissions is promising.

—Richard Blaustein


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system like a lake, you could have those tipping points,” says Scheffer. He also be-gan to work through the theory and math-ematics of tipping points, which he notes were not too sophisticated. “We were not thinking of early warning signals in those days,” Scheffer explains.

Around the same time, Stephen Car-penter was working on lake die-off at the University of Wisconsin, trying to frame water pollution problems in a way that would help policy makers. “As a natural scientist, I knew that there were lots of phenomena in living systems that have big jumps,” says Carpenter.

Together, Carpenter, Scheffer, and a few colleagues became well known for their work on nature’s abrupt changes. In 2003, they started thinking about how to pre-dict these tipping points, and were drawn to the phenomenon known in physics and mathematics as “critical slowing down.”

CRITICAL SLOWING DOWN

Since then, a small circle of researchers have generated a vigorous discourse on the signals of an approaching tipping point and focused on critical slowing down. In a natural system, such as an ecosystem or local precipitation region, the occur-rence of critical slowing down is a signal of imminent tipping point change. Criti-cal slowing down can be thought of as a loss of resiliency with major implications. The slowing down refers to a system tak-ing more time to stabilize itself from a for-merly minor perturbation. Even if changes in recovery appear minor to the human eye, there are statistical and mathematical ways to establish that a system is in a criti-cal slowing down mode. For example, a deprived organism’s weight gain may look fine, but subtleties in rate of return indi-cate otherwise. There are statistical stan-

is enough foliage to attract mainly birds who keep spruce budworms in check; and in the other, pests defoliate the forest and thrive until they just about eat themselves out of house-and-home, giving space for natural predators to rebound. The spruce budworm study also went into much de-tail explaining the mathematics of the cy-clical pest outbreaks.

The Douglar fir study is fundamental to understanding tipping points. It showed that stresses can abruptly tip a stable eco-system over to its “alternative state.” A pris-tine lake can unexpectedly flip to become a turbid and lifeless body of water; the flow-ing and dynamic thermohaline circulation can become stagnant, a phantom of itself, which in geological time it has done.

Scientists, such as lake ecologists Mar-ten Scheffer and Stephen Carpenter, have carried on from these early studies, exam-ining the abrupt transitions found in na-ture. After his university training, Scheffer worked on turbid lakes in the late 1980s at the Netherlands Institute for Inland Water Management and Waste Water Treatment. Scheffer says he learned in those early days that a few European biologists concluded that lakes were “trapped in a turbid state by some feedback mechanisms” and dem-onstrated “you could tip those lakes back” with a sort of “shock therapy” of removing certain abundant fish. Scheffer started to experiment on nearby turbid lakes, where flourishing carp and breen species fed on food in sediments, stirring up more tur-bidity while foraging. By drastically re-moving the carp and breen species, Schef-fer and colleagues were able to flip the lakes back to their clear state with a new assemblage of fish. Vegetation took hold on the lake bottom, keeping the sediment in place. “In the end, it turned out we could convincingly show that in a complex

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biomass indicating weakening recovery, which could in turn be directly linked with critical slowing down, which her team measured on six occasions until the bacteria went extinct.

“This decrease in recovery rate really started early, far from when the popula-tion collapsed,” Veraart says. “This critical slowing down starts early. You can’t really tell when you look at the population, but if you look at data of recovery rates you would see the resiliency decreasing.

Scheffer says it’s well-established that chemical systems send off critical slowing down signals and believes that this experi-ment led by Veraart and the other microor-ganism critical slow-ing down studies are proof-of-concept for critical slowing down in living sys-tems. “Altogether we showed that, yes, in living systems you can indeed see these theoretically predicted phenom-enon,” says Scheffer. “It is somehow sur-prising how univer-sal it can be.”

Scheffer says the potential for this and other early warning methods is great. “It helps with the tool kit we have to under-stand which coral reefs, which lakes, tropi-cal rain forests, which grass ecosystem or boreal forests are close to a tipping point and which are further away…In many cas-es you can protect the system…Like with a lake we can control the nutrients and make the ecosystem more resilient against the expected effects of climate change.”

“I think our experiment showed you really have to monitor an ecosystem’s

dards that guide the researchers who look at critical slowing down.

The investigations are varied and far-reaching. For example, Lenton found evi-dence of critical slowing down in some points in the Earth’s tumultuous geologi-cal history. For example, he has described critical slowing as occurring in the Young-er Dryas cooling period—a 1,300-year pe-riod of cold climatic conditions between 10,800 and 9,500 B.C.E. that took place within a larger warmer period. In fact, the Younger Dryas snap might have been linked to a temporary collapse of the large-scale ocean circulation. Similarly, Scheffer has begun a Netherlands Earth System Science Center project studying past and future climate change tipping points that includes a focus on critical slowing down.

The most concrete part of the critical slowing down research is the work done by researchers who have set up biology experiments to test for critical slowing down. There have been fewer than a doz-en of these investigations, most involving microorganisms in a lab setting. How-ever, their results are published in the top scientific journals with findings that shed light on how the natural world responds to subtle and profound stresses.

Over a period of one month, Annelies Veraart, a microecologist now with the Netherlands Institute of Ecology, and col-leagues subjected photosynthesizing bac-teria to increasing light. At a point a light threshold was reached and the bacteria, who communally shade one another dur-ing tolerable stresses, were damaged, their self-shading ceased, and they began dying rapidly. The group propelled ultimately into complete death, indeed extinction. Veraart and colleagues were able to col-lect data of critical slowing down by cor-relating the light stresses with declines in

critical slowing down is still a frontrunner as the one generic test for detecting when natural systems approach their thresholds.


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ods to evaluate the resilience of ecosystems in a wide range of settings and to compare the resilience of different ecosystems.” However, he cautions that much more research is needed. “So far I think our ex-periment is the only large scale field experi-ment to demonstrate this phenomenon.”

“It is obvious critical slowing down should be one of the tests of whether you are approaching a tipping point,” says Schellnhuber who nevertheless believes it is conceivable that a small, apparently-stable ecosystem could be upended by a larger, en-compassing landscape undergoing change. So when the larger system tips over, it car-ries with it the smaller bit of nature, with no advance warning. “In the end, you have to understand the system and calculate the safe space,” Schellnhuber says.

SURPRISING HOPE

Know your natural systems. That is the first axiom for tipping point early warn-ing signals. And with this orientation the science of tipping points has gotten off the ground since Schellnhuber’s first 2004 BBC interview. Moreover, important sci-entific centers, such as the U.S. National Research Council and the IPCC have placed tipping points on their top list of concerns. Scientists are beginning to look at ecosystems’ vulnerabilities and the early warning signals they send off, which could provide the space for some local or region-al action to ease problems before they be-come broader.

And, notwithstanding caveats, criti-cal slowing down is still a frontrunner as the one generic test for detecting when natural systems approach their thresh-olds. Both Scheffer and Lenton, along with their colleagues, say that critical slowing down does not fit every situation, but it might very well be that first across-the-

health.” Veraart says. “These early warning signals might show which ecosystems are most fragile and perhaps allow you to do something” to protect them.

At Peter and Paul Lakes at the Uni-versity of Notre Dame Environmental Research Center, which extends over parts of northern Minnesota and Wisconsin, Carpenter and his team explored for three years whether abrupt changes in a lake’s food chain would send out early warning signals. With Paul Lake set as the control, at the experiment’s start, Peter Lake had 39 largemouth bass. In mid-2008, Car-penter and team added 12 largemouth bass. They then added 15 more on both the 169th and 203rd days of 2009. Car-penter explains they had to add the bass slowly to get good statistical results. “We were going to have to be careful not to shoot past the threshold too fast.”

As the experiment got underway, Car-penter says that the small prey fishes—pumpkinseeds, golden shiners, fathead minnows, dace, brook stickleback, and central mudminnow—kept to the lake’s periphery to avoid being eaten. But their behavior changed a lot, and “their numbers began to dwindle, too, as the predators be-gan to chip away on them.” Soon, zooplank-ton grew in size as they were not being eaten by the prey fish. As the zooplankton grew, the chlorophyll, which were the focus for critical slowing down, changed just as Carpenter’s models predicted. Over two and a half years, the frightened prey fish in the lake’s periphery embodied one ecological state about to give way to another. When the tipping point occurred in 2010, the food chain dramatically shifted from one being dominated by plankton-eating fish to one ruled by fish-eating fish.

Carpenter says his Peter Lake experi-ment “suggests we might have some meth-

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agrees and is especially happy about the early warning system idea.

Moreover, for Lenton and for White, the time is not too late, and they feel there is still opportunity for mitigation—actu-ally lowering greenhouse gas emissions and then bringing down atmospheric levels. “I hope we do get together and do mitigate,” says Lenton. “Then we are essentially cast-ing off a whole range of future temperature rises in which there lurk probably several tipping points. So we might not eliminate all possibility of tipping points, but at least we cut out a chunk of them.”

Predicting tipping points is what Len-ton has focused on most intensively recent-ly, and he feels that this new frontier is not only compelling but offers great promise. It just needs some broader support. “I honestly think we can do something use-ful here to help societies on this planet and help reduce the risk that these events pose,” Lenton says. “For that to happen, it needs to get some social-political momen-tum behind it, for people to believe it is possible and to want to see us try.” l

board-measurement for predicting tipping points for our planet.

Lenton, for example, is especially con-cerned about predicting changes in the monsoon season and believes that the fast monsoon dynamic might very well send out critical slowing signals. In fact, Len-ton and colleagues, in 2014, conducted an analysis of monsoons in a past ice age and also in computer modeling and found signs of critical slowing down. Progress is urgent, Lenton says, because an abrupt change for the monsoons “is the most wor-risome tipping point potentially in terms of human impact.”

White also believes that in a political culture where climate change is a partisan issue, the proposal for an Abrupt Change Early Warning System, which he calls the ACEWS, could be a unifier. White says the ACEWS “is about resilience, which people can get behind” and that “it stands a decent chance of cutting across ideologi-cal lines, political lines, and motivating, if not change, at least then the knowledge that’s needed to effect change.” Lenton

predicting tipping points

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