Army ants’ ‘living’ bridges span collective intelligence, ‘swarm’ robotics (PNAS)

By Morgan Kelly, Office of Communications

Columns of workers penetrate the forest, furiously gathering as much food and supplies as they can. They are a massive army that living things know to avoid, and that few natural obstacles can waylay. So determined are these legions that should a chasm or gap disrupt the most direct path to their spoils they simply build a new path — out of themselves.

Without any orders or direction, individuals from the rank and file instinctively stretch across the opening, clinging to one another as their comrades-in-arms swarm across their bodies. But this is no force of superhumans. They are army ants of the species Eciton hamatum, which form “living” bridges across breaks and gaps in the forest floor that allow their famously large raiding swarms to travel efficiently.

Researchers from Princeton University and the New Jersey Institute of Technology (NJIT) report for the first time that these structures are more sophisticated than scientists knew. The ants exhibit a level of collective intelligence that could provide new insights into animal behavior and even help in the development of intuitive robots that can cooperate as a group, the researchers said.

Ants of E. hamatum automatically form living bridges without any oversight from a “lead” ant, the researchers report in the journal Proceedings of the National Academy of the Sciences. The action of each individual coalesces into a group unit that can adapt to the terrain and also operates by a clear cost-benefit ratio. The ants will create a path over an open space up to the point when too many workers are being diverted from collecting food and prey.

“These ants are performing a collective computation. At the level of the entire colony, they’re saying they can afford this many ants locked up in this bridge, but no more than that,” said co-first author Matthew Lutz, a graduate student in Princeton’s Department of Ecology and Evolutionary Biology.

“There’s no single ant overseeing the decision, they’re making that calculation as a colony,” Lutz said. “Thinking about this cost-benefit framework might be a new insight that can be applied to other animal structures that people haven’t thought of before.”

The research could help explain how large groups of animals balance cost and benefit, about which little is known, said co-author Iain Couzin, a Princeton visiting senior research scholar in ecology and evolutionary biology, and director of the Max Planck Institute for Ornithology and chair of biodiversity and collective behavior at the University of Konstanz in Germany.

Previous studies have shown that single creatures use “rules of thumb” to weigh cost-and-benefit, said Couzin, who also is Lutz’s graduate adviser. This new work shows that in large groups these same individual guidelines can eventually coordinate group-wide, he said — the ants acted as a unit although each ant only knew its immediate circumstances.

“They don’t know how many other ants are in the bridge, or what the overall traffic situation is. They only know about their local connections to others, and the sense of ants moving over their bodies,” Couzin said. “Yet, they have evolved simple rules that allow them to keep reconfiguring until, collectively, they have made a structure of an appropriate size for the prevailing conditions.

“Finding out how sightless ants can achieve such feats certainly could change the way we think of self-configuring structures in nature — and those made by man,” he said.

Ant-colony behavior has been the basis of algorithms related to telecommunications and vehicle routing, among other areas, explained co-first author Chris Reid, a postdoctoral research associate at the University of Sydney who conducted the work while at NJIT. Ants exemplify “swarm intelligence,” in which individual-level interactions produce coordinated group behavior. E. hamatum crossings assemble when the ants detect congestion along their raiding trail, and disassemble when normal traffic has resumed.

The video below shows how E. hamatum confronted a gap they encountered on an apparatus that Lutz and Reid built and deployed in the forests of Barro Colorado Island, Panama. Previously, scientists thought that ant bridges were static structures — their appearance over large gaps that ants clearly could not cross in midair was somewhat of a mystery, Reid said. The researchers found, however, that the ants, when confronted with an open space, start from the narrowest point of the expanse and work toward the widest point, expanding the bridge as they go to shorten the distance their compatriots must travel to get around the expanse.

“The amazing thing is that a very elegant solution to a colony-level problem arises from the individual interactions of a swarm of simple worker ants, each with only local information,” Reid said. “By extracting the rules used by individual ants about whether to initiate, join or leave a living structure, we could program swarms of simple robots to build bridges and other structures by connecting to each other.

“These robot bridges would exhibit the beneficial properties we observe in the ant bridges, such as adaptability to local conditions, real-time optimization of shape and position, and rapid construction and deconstruction without the need for external building materials,” Reid continued. “Such a swarm of robots would be especially useful in dangerous and unpredictable conditions, such as natural disaster zones.”

Radhika Nagpal, a professor of computer science at Harvard University who studies robotics and self-organizing biological systems, said that the findings reveal that there is “something much more fundamental about how complex structures are assembled and adapted in nature, and that it is not through a supervisor or planner making decisions.”

Individual ants adjusted to one another’s choices to create a successful structure, despite the fact that each ant didn’t necessarily know everything about the size of the gap or the traffic flow, said Nagpal, who is familiar with the research but was not involved in it.

“The goal wasn’t known ahead of time, but ’emerged’ as the collective continually adapted its solution to the environmental factors,” she said. “The study really opens your eyes to new ways of thinking about collective power, and has tremendous potential as a way to think about engineering systems that are more adaptive and able to solve complex cost-benefit ratios at the network level just through peer-to-peer interactions.”

She compared the ant bridges to human-made bridges that automatically widened to accommodate heavy vehicle traffic or a growing population. While self-assembling road bridges may be a ways off, the example illustrates the potential that technologies built with the same self-assembling capabilities seen in E. hamatum could have.

“There’s a deep interest in creating robots that don’t just rely on themselves, but can exploit the group to do more — and self-assembly is the ultimate in doing more,” Nagpal said. “If you could have small simple robots that were able to navigate complex spaces, but could self-assemble into larger structures — bridges, towers, pulling chains, rafts — when they face something they individually did not have the ability to do, that’s a huge increase in power in what robots would be capable of.”

The spaces E. hamatum bridges are not dramatic by human standards — small rifts in the leaf cover, or between the ends of two sticks. Bridges will be the length of 10 to 20 ants, which is only a few centimeters, Lutz said. That said, E. hamatum swarms form several bridges during the course of a day, which can see the back-and-forth of thousands of ants. Many ants pass over a living bridge even as it is assembling.

Bridging a gap

Image courtesy of Matthew Lutz at Princeton University and Chris Reid at the University of Sydney.

“The bridges are something that happen numerous times every day. They’re creating bridges to optimize their traffic flow and maximize their time,” Lutz said.

“When you’re moving hundreds of thousands of ants, creating a little shortcut can save a lot of energy,” he said. “This is such a unique behavior. You have other types of ants forming structures out of their bodies, but it’s not such a huge part of their lives and daily behavior.”

The research also included Scott Powell, an army-ant expert and assistant professor of biology at George Washington University; Albert Kao, a postdoctoral fellow at Harvard who received his doctorate in ecology and evolutionary biology from Princeton in 2015; and Simon Garnier, an assistant professor of biological sciences at NJIT who studies swarm intelligence and was once a postdoctoral researcher in Couzin’s lab at Princeton.

To conduct their field experiments, Lutz and Reid constructed a 1.5-foot-tall apparatus with ramps on both sides and adjustable arms in the center with which they could adjust the size of the gap. They then inserted the apparatus into active E. hamatum raiding trails that they found in the jungle in Panama. Because ants follow one another’s chemical scent, Lutz and Reid used sticks and leaves from the ants’ trail to get them to reform their column across the device.

Lutz and Reid observed how the ants formed bridges across gaps that were set at angles of 12, 20, 40 and 60 degrees. They gauged how much travel-distance the ants saved with their bridge versus the surface area (in centimeters squared) of the bridge itself. Twelve-degree angles shaved off the most distance (around 11 centimeters) while taking up the fewest workers. Sixty-degree angles had the highest cost-to-benefit ratio. Interestingly, the ants were willing to expend members for 20-degree angles, forming bridges up to 8 centimeters squared to decrease their travel time by almost 12 centimeters, indicating that the loss in manpower was worth the distance saved.

Lutz said that future research based on this work might compare these findings to the living bridges of another army ant species, E. burchellii, to determine if the same principles are in action.

The paper, “Army ants dynamically adjust living bridges in response to a cost-benefit trade-off,” was published Nov. 23 by Proceedings of the National Academy of Sciences. The work was supported by the National Science Foundation (grant nos. PHY-0848755, IOS0-1355061 and EAGER IOS-1251585); the Army Research Office (grant nos. W911NG-11-1-0385 and W911NF-14-1-0431); and the Human Frontier Science Program (grant no. RGP0065/2012).

Read the abstract

Group living: For baboons intermediate size is optimal (PNAS)

New research reveals that intermediate-sized groups of baboons (50 to 70 individuals) exhibit optimal ranging behavior and low stress levels. Pictured is a group of wild baboons in East Africa. Credit: Beth Archie

New research reveals that intermediate-sized groups of baboons (50 to 70 individuals) exhibit optimal ranging behavior and low stress levels. Pictured is a group of wild baboons in East Africa. Credit: Beth Archie

By Gregory Filiano, Stony Brook University

Living with others can offer tremendous benefits for social animals, including primates, but these benefits could come at a high cost. New research from a project that originated at Princeton University reveals that intermediate-sized groups provide the most benefits to wild baboons. The study, led by Catherine Markham at Stony Brook University and published in the journal, Proceedings of the National Academy of Sciences, offers new insight into the costs and benefits of group living.

In the paper titled “Optimal group size in a highly social mammal,” the authors reveal that while wild baboon groups range in size from 20 to 100 members, groups consisting of about 50 to 70 individuals (intermediate size) exhibit optimal ranging behavior and low physiological stress levels in individual baboons, which translates to a social environment that fosters the health and well-being of individual members. The finding provides novel empirical support for an ongoing theory in the fields of evolutionary biology and anthropology that in living intermediate-sized groups has advantages for social mammals.

“Strikingly, we found evidence that intermediate-sized groups have energetically optimal space-use strategies and both large and small groups experience ranging disadvantages,” said Markham, lead author and an assistant professor in the Department of Anthropology at Stony Brook University. “It appears that large, socially dominant groups are constrained by within-group competition whereas small, socially subordinate groups are constrained by between-group competition and/or predation pressures.”

The researchers compiled their findings based on observing five social wild baboon groups in East Africa over 11 years. This population of wild baboons has been studied continuously for over 40 years by the Amboseli Baboon Research Project. They observed and examined the effects of group size and ranging patterns for all of the groups. To gauge stress levels of individuals, they measured the glucocorticoid (stress hormone) levels found in individual waste droppings.

“The combination of an 11-year data set and more intensive short-term data, together with the levels of stress hormones, led to the important finding that there really is a cost to living in too small a group,” said Jeanne Altmann, the Eugene Higgins Professor of Ecology and Evolutionary Biology, Emeritus and a senior scholar at Princeton University. Altmann is a co-director of the Amboseli Baboon Research Project and co-founded the project in 1971 with Stuart Altmann, a senior scholar in the Department of Ecology and Evolutionary Biology at Princeton.

“The cost of living in smaller groups is a concern from a conservation perspective,” Jeanne Altmann said, “Due to the fragmentation of animal habitats, many animals will be living in smaller groups. Understanding these dynamics is one of the next things to study.” The research was supported primarily by the National Science Foundation and the National Institute on Aging.

Markham, who earned her Ph.D. at Princeton University in 2012 with Jeanne Altmann as her thesis adviser, explained that regarding optimal group sizes for highly social species the key to the analysis is how are trade-offs balanced, and do these trade-offs actually result in an optimal group size for a social species.

She said that their findings provide a testable hypothesis for evaluating group-size constraints in other group-living species, in which the costs of intra- and intergroup competition vary as a function of group size. Additionally, their findings provide implications for new research and a broader understanding of both why some animals live with others and how many neighbors will be best for various species and situations.

The research was conducted in collaboration with Susan Alberts, a professor of biology at Duke University and co-director of the Amboseli Baboon Research Project, and with Laurence Gesquiere, a former postdoctoral researcher at Princeton who is now a senior research scientist working with Alberts. Altmann and Alberts are also affiliated with the Institute for Primate Research, National Museums of Kenya.

Additional support was provided by the American Society of Primatologists, the Animal Behavior Society, the International Primatological Society, and Sigma Xi.

Article courtesy of Stony Brook University.

Read the abstract.

A. Catherine Markham, Laurence R. Gesquiere, Susan C. Alberts and Jeanne Altmann. Optimal group size in a highly social mammal. Proceedings of the National Academy of Sciences. Published online before print October 26, 2015, doi: 10.1073/pnas.1517794112 PNAS October 26, 2015

Scientists predict cool new phase of superionic ice (Nature Communications)

by Tien Nguyen, Department of Chemistry

Uranus as viewed by Voyager 2 in 1986 (NASA/JPL-Caltech)

Uranus (NASA/JPL-Caltech)

Scientists have predicted a new phase of superionic ice, a special form of ice that could exist on Uranus and Neptune, in a theoretical study performed by a team of researchers at Princeton University.

“Superionic ice is this in-between state of matter that we can’t really relate to anything we know of — that’s why it’s interesting,” Salvatore Torquato, a Professor of Chemistry who jointly led the work with Roberto Car, the Ralph W. ‘31 Dornte Professor in Chemistry. Unlike water or regular ice, superionic ice is made up of water molecules that have dissociated into charged atoms called ions, with the oxygen ions locked in a solid lattice and the hydrogen ions moving like the molecules in a liquid.

Published on August 28 in Nature Communications, the research revealed an entirely new type of superionic ice that the investigators call the P21/c-SI phase, which occurs at pressures even higher than those found in the interior of the giant ice planets of our solar system. Two other phases of superionic ice thought to exist on the planets are body-centered cubic superionic ice (BCC-SI) and close-packed superionic ice (CP-SI).

Each phase has a unique arrangement of oxygen ions that gives rise to distinct properties. For example, each of the phases allows hydrogen ions to flow in a characteristic way. The effects of this ionic conductivity may someday be observed by planetary scientists in search of superionic ice. “These unique properties could essentially be used as signatures of superionic ice,” said Torquato. “Now that you know what to look for, you have a better chance of finding it.”

Salvatore Torquato (left) and Roberto Car (right)

Salvatore Torquato (left) and Roberto Car (right)

Unlike Earth, which has two magnetic poles (north and south), ice giants can have many local magnetic poles, which leading theories suggest may be due to superionic ice and ionic water in the mantle of these planets. In ionic water both oxygen and hydrogen ions show liquid-like behavior. Scientists have proposed that heat emanating outward from the planet’s core may pass through an inner layer of superionic ice, and through convection, create vortices on the outer layer of ionic water that give rise to local magnetic fields.

By using theoretical simulations, the researchers were able to model states of superionic ice that would be difficult to study experimentally. They simulated pressures that were beyond the highest possible pressures attainable in the laboratory with instruments called diamond anvil cells. Extreme pressure can be achieved through shockwave experiments but these rely on creating an explosion and are difficult to interpret, Professor Car explained.

The researchers calculated the ionic conductivity of each phase of superionic ice and found unusual behavior at the transition where the low temperature crystal, in which both oxygen and hydrogen ions are locked together, transforms into superionic ice. In known superionic materials, generally the conductivity can change either abruptly (type I) or gradually (type II), but the type of change will be specific to the material. However, superionic ice breaks from convention, as the conductivity changes abruptly with temperature across the crystal to close-packed superionic transition, and continuously at the crystal to P21/c-SI transition.

As a foundational study, the research team investigated superionic ice treating the ions as if they were classical particles, but in future studies they plan to take quantum effects into account to further understand the properties of the material.

Read the article here:

Sun, J.; Clark, B. K.; Torquato, S.; Car, R. “The phase diagram of high pressure superionic ice.Nature Communications, Published online August 28, 2015.

This work was supported by the National Science Foundation (DMS-1065894) and the US Department of Energy (DE-SC0008626 and DE-SC0005180).



More rain leads to fewer trees in the African savanna (PNAS)

Lone tree on savanna

More rain on African savanna leads to fewer trees, a Princeton study found. (Credit PEI)

by Angela Page for the Princeton Environmental Institute

In 2011, an influx of remote sensing data from satellites scanning the African savannas revealed a mystery: these rolling grasslands, with their heavy rainfalls and spells of drought, were home to significantly fewer trees than researchers had previously expected given the biome’s high annual precipitation. In fact, the 2011 study found that the more instances of heavy rainfall a savanna received, the fewer trees it had.

This paradox may finally have a solution due to new work from Princeton University recently published in the Proceeding of the National Academy of Sciences. In the study, researchers use mathematical equations to show that physiological differences between trees and grasses are enough to explain the curious phenomenon.

“A simple way to view this is to think of rainfall as annual income,” said Xiangtao Xu, a doctoral candidate in David Medvigy’s lab and first author on the paper. “Trees and grasses are competing over the amount of money the savanna gets every year and it matters how they use their funds.” Xu explained that when the bank is full and there is a lot of rain, the grasses, which build relatively cheap structures, thrive. When there is a deficit, the trees suffer less than grasses and therefore win out.

To establish these findings, Xu and his Princeton collaborators Medvigy, assistant professor in geosciences, and Ignacio Rodriguez-Iturbe, professor of civil and environmental engineering, created a numerical model that mimics the actual mechanistic functions of the trees and grasses. “We put in equations for how they photosynthesize, how they absorb water, how they steal water from each other—and then we coupled it all with a stochastic rainfall generator,” said Xu.

Whereas former analyses only considered total annual or monthly rainfall, understanding how rainfall is distributed across the days is critical here, Xu said, because it determines who will win in a competition between grasses and trees for the finite resource of water availability.

The stochastic rainfall generator draws on rainfall parameters derived from station observations across the savanna. By coupling it with the mechanistic equations describing how the trees and grasses function, the team was able to observe how the plants would respond under different local climate conditions.

The research team found that under very wet conditions, grasses have an advantage because they can quickly absorb water and support high photosynthesis rates. Trees, with their tougher leaves and roots, are able to survive better in dry periods because of their ability to withstand water stress. But this amounts to a disadvantage for trees in periods of intense rainfall, as they are comparatively less effective at utilizing the newly abundant water.

“We put realistic rainfall schemes into the model, then generated corresponding grass or tree abundance, and compared the numerical results with real-world observations,” Xu said. If the model looked like the real-world data, then they could say it offered a viable explanation for the unexpected phenomenon, which is not supported by traditional models—and that is exactly what they found. They tested the model using both field measurements from a well-studied savanna in Nylsvley, South Africa and nine other sites along the Kalahari Transect, as well as remote sensing data across the whole continent. With each site, the model accurately predicted observed tree abundances in those locations.

The work rejects the long held theory of root niche separation, which predicts that trees will outcompete grasses under intense rainfall when the soil becomes saturated, because their heavy roots penetrate deeper into the ground. “But this ignores the fact that grasses and trees have different abilities for absorbing and utilizing water,” Xu said. “And that’s one of the most important parts of what we found. Grasses are more efficient at absorbing water, so in a big rainfall event, grasses win.”

“Models are developed to understand and predict the past and present state — they offer a perspective on future states given the shift in climatic conditions,” said Gaby Katul, a Professor of Hydrology and Micrometeorology in the Nicholas School of the Environment at Duke University, who was not involved in the research. “This work offers evidence of how shifts in rainfall affect the tree-grass interaction because rainfall variations are large. The approach can be used not only to ‘diagnose’ the present state where rainfall pattern variations dominate but also offers a ‘prognosis’ as to what may happen in the future.”

Several high profile papers over the last decade predict that periods of intense rainfall like those described in the paper will become more frequent around the globe, especially in tropical areas, Xu said. His work suggests that these global climate changes will eventually lead to diminished tree abundance on the savannas.

“Because the savanna takes up a large area, which is home to an abundance of both wild animals and livestock, this will influence many people who live in those areas,” Xu said. “It’s important to understand how the biome would change under global climate change.”

Furthermore, the study highlights the importance of understanding the structure and pattern of rainfall, not just the total annual precipitation—which is where most research in this area has traditionally focused. Fifty years from now, a region may still experience the same overall depth of precipitation, but if the intensity has changed, that will induce changes to the abundance of grasses and trees. This, in turn, will influence the herbivores that subsist on them, and other animals in the biome — essentially, affecting the entire complex ecosystem.

Xu said it would be difficult to predict whether such changes would have positive or negative impacts. But he did say that more grasses mean more support for cows and horses and other herbivores. On the other hand, fewer trees mean less CO2 is captured out of the atmosphere, as well as diminished habitat for birds and other animals that rely on the trees for survival.

What the model does offer is an entry point for better policies and decisions to help communities adapt to future changes. “It’s just like with the weather,” Xu said. “If you don’t read the weather report, you have to take what nature gives you. But if you know in advance that it will rain tomorrow, you know to bring an umbrella.”

This work was supported by the Princeton Environmental Institute and the Andlinger Center for Energy and the Environment at Princeton University.

Read the abstract.

Xiangtao Xua, David Medvigy, and Ignacio Rodriguez-Iturbe. Relation between rainfall intensity and savanna tree abundance explained by water use strategies. Published online September 29, 2015, doi: 10.1073/pnas.1517382112. PNAS October 5, 2015.

Long-sought chiral anomaly detected in crystalline material (Science)

By Catherine Zandonella, Office of the Dean for Research

A study by Princeton researchers presents evidence for a long-sought phenomenon — first theorized in the 1960s and predicted to be found in crystals in 1983 — called the “chiral anomaly” in a metallic compound of sodium and bismuth. The additional finding of an increase in conductivity in the material may suggest ways to improve electrical conductance and minimize energy consumption in future electronic devices.

“Our research fulfills a famous prediction in physics for which confirmation seemed unattainable,” said N. Phuan Ong, Princeton’s Eugene Higgins Professor of Physics, who co-led the research with Robert Cava, Princeton’s Russell Wellman Moore Professor of Chemistry. “The increase in conductivity in the crystal and its dramatic appearance under the right conditions left little doubt that we had observed the long-sought chiral anomaly.”

The study was published online today in the journal Science.


This sketch illustrates the concept of handedness, or chirality, which is found throughout nature. Most chemical structures and many elementary particles come in right- and left-handed forms. Source: Princeton University

The chiral anomaly – which describes how elementary particles can switch their orientation in the presence of electric and magnetic fields – stems from the observation that right- and left-handedness (or “chirality” after the Greek word for hand) is ubiquitous in nature. For example, most chemical structures and many elementary particles come in right- and left-handed forms that are mirror images of each other.

Early research leading up to the discovery of the anomaly goes back to the 1940s, when Hermann Weyl at the Institute for Advanced Study in Princeton, New Jersey, and others, discovered that all elementary particles that have zero mass (including neutrinos, despite their having an extremely small mass) strictly segregate into left- and right-handed populations that never intermix.

A few decades later, theorists discovered that the presence of electric and magnetic fields ruins the segregation of these particles, causing the two populations to transform into each other with observable consequences.

This field-induced mixing, which became known as the chiral anomaly, was first encountered in 1969 in work by Stephen Adler of the Institute for Advanced Study, John Bell of the European Organization for Nuclear Research (CERN) and Roman Jackiw of the Massachusetts Institute of Technology, who successfully explained why certain elementary particles, called neutral pions, decay much faster — by a factor of 300 million — than their charged cousins. Over the decades the anomaly has played an important if perplexing role in the grand quest to unify the four fundamental forces of nature.

The prediction that the chiral anomaly could also be observed in crystals came in 1983 from physicists Holger Bech Nielsen of the University of Copenhagen and Masao Ninomiya of the Okayama Institute for Quantum Physics. They suggested that it may be possible to detect the anomaly in a laboratory setting, which would enable researchers to apply intense magnetic fields to test predictions under conditions that would be impossible in high-energy particle colliders.

Recent progress in the development of certain kinds of crystals known as “topological” materials has paved the way toward realizing this prediction, Ong said. In the crystal of Na3Bi, which is a topological material known as a Dirac semi-metal, electrons occupy quantum states which mimic massless particles that segregate into left- and right-handed populations.

To see if they could observe the anomaly in Na3Bi, Jun Xiong, a graduate student in physics advised by Ong, cooled a crystal of Na3Bi grown by Satya Kushwaha, a postdoctoral research associate in chemistry who works with Cava, to cryogenic temperatures in the presence of a strong magnetic field that can be rotated relative to the direction of the applied electrical current in the crystal. When the magnetic field was aligned parallel to the current, the two chiral populations intermixed to produce a novel increase in conductivity, which the researchers call the “axial current plume.” The experiment confirmed the existence of the chiral anomaly in a crystal.

“One of the key findings in the experiment is that the intermixing leads to a charge current, or axial current, that resists depletion caused by scattering from impurities,” Ong said. “Understanding how to minimize the scattering of current-carrying electrons by impurities — which causes electronic devices to lose energy as heat — is important for realizing future electronic devices that are more energy-efficient. While these are early days, experiments on the long-lived axial current may help us to develop low-dissipation devices.”

The research was supported by the National Science Foundation, the Army Research Office and the Gordon and Betty Moore Foundation.

Read the abstract or paper.

The paper, “Evidence for the chiral anomaly in the Dirac semimetal Na3Bi,” was published online in the journal Science by Jun Xiong; Satya K. Kushwaha; Tian Liang; Jason W. Krizan; Max Hirschberger; Wudi Wang; Robert J. Cava; and N. Phuan Ong.

Grey Swans: Rare but predictable storms could pose big hazards (Nature Climate Change)

By John Sullivan, School of Engineering and Applied Science

Grey Swan events

Toward the end of this century (project here for the years 2068 to 2098) the possibility of storm surges of eight to 11 meters (26 to 36 feet) increases significantly in cities not usually expected to be vulnerable to tropical storms, such as Tampa, Florida, according to recent research in the journal Nature Climate Change.

Researchers at Princeton and MIT have used computer models to show that severe tropical cyclones could hit a number of coastal cities worldwide that are widely seen as unthreatened by such powerful storms.

The researchers call these potentially devastating storms Gray Swans in comparison with the term Black Swan, which has come to mean truly unpredicted events that have a major impact. Gray Swans are highly unlikely, the researchers said, but they can be predicted with a degree of confidence.

“We are considering extreme cases,” said Ning Lin, an assistant professor of civil and environmental engineering at Princeton. “These are relevant for policy making and planning, especially for critical infrastructure and nuclear power plants.”

In an article published Aug. 31 in Nature Climate Change, Lin and her coauthor Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology, examined potential storm hazards for three cities: Tampa, Fla.; Cairns, Australia; and Dubai, United Arab Emirates.

The researchers concluded that powerful storms could generate dangerous storm surge waters in all three cities. They estimated the levels of devastating storm surges occurring in these cities with odds of 1 in 10,000 in an average year, under current climate conditions.

Tampa Bay, for example, has experienced very few extremely damaging hurricanes in its history, the researchers said. The city, which lies on the central-west coast of Florida, was hit by major hurricanes in 1848 and in 1921.

The researchers entered Tampa Bay area climate data recorded between 1980 and 2005 into their model and ran 7,000 simulated hurricanes in the area. They concluded that, although unlikely, a Gray Swan storm could bring surges of up to roughly six meters (18 feet) to the Tampa Bay area. That level of storm surge could dwarf those of the storms of 1848 and 1921, which reached about 4.6 meters and 3.5 meters respectively.

The researchers said their model also indicates that the probability of such storms will increase as the climate changes.

“With climate change, these probabilities can increase significantly over the 21st century,” the researchers said. In Tampa, the current storm surge likelihood of 1 in 10,000 is projected to increase to between 1 in 3,000 and 1 in 1,100 by mid-century and between 1 in 2,500 and 1 in 700 by the end of the century.

The work was supported in part by Princeton’s Project X Fund, the Andlinger Center for Energy and the Environment’s Innovation Fund, and the National Science Foundation.

Read the abstract.

Ning Lin & Kerry Emanuel, “Grey swan tropical cyclones,” Nature Climate Change (2015); doi:10.1038/nclimate2777

On warmer Earth, most of Arctic may remove, not add, methane (ISME Journal)


McGill Arctic Research Station during late-spring at Expedition Fjord, Axel Heiberg Island, Nunavut, Canada. (Photo by Nadia Mykytczuk, Laurentian University)

By Morgan Kelly, Office of Communications

In addition to melting icecaps and imperiled wildlife, a significant concern among scientists is that higher Arctic temperatures brought about by climate change could result in the release of massive amounts of carbon locked in the region’s frozen soil in the form of carbon dioxide and methane. Arctic permafrost is estimated to contain about a trillion tons of carbon, which would potentially accelerate global warming. Carbon emissions in the form of methane have been of particular concern because on a 100-year scale methane is about 25-times more potent than carbon dioxide at trapping heat.

However, new research led by Princeton University researchers and published in The ISME Journal in August suggests that, thanks to methane-hungry bacteria, the majority of Arctic soil might actually be able to absorb methane from the atmosphere rather than release it. Furthermore, that ability seems to become greater as temperatures rise.

The researchers found that Arctic soils containing low carbon content — which make up 87 percent of the soil in permafrost regions globally — not only remove methane from the atmosphere, but also become more efficient as temperatures increase. During a three-year period, a carbon-poor site on Axel Heiberg Island in Canada’s Arctic region consistently took up more methane as the ground temperature rose from 0 to 18 degrees Celsius (32 to 64.4 degrees Fahrenheit). The researchers project that should Arctic temperatures rise by 5 to 15 degrees Celsius over the next 100 years, the methane-absorbing capacity of “carbon-poor” soil could increase by five to 30 times.

The researchers found that this ability stems from an as-yet unknown species of bacteria in carbon-poor Arctic soil that consume methane in the atmosphere. The bacteria are related to a bacterial group known as Upland Soil Cluster Alpha, the dominant methane-consuming bacteria in carbon-poor Arctic soil. The bacteria the researchers studied remove the carbon from methane to produce methanol, a simple alcohol the bacteria process immediately. The carbon is used for growth or respiration, meaning that it either remains in bacterial cells or is released as carbon dioxide.

First author Chui Yim “Maggie” Lau, an associate research scholar in Princeton’s Department of Geosciences, said that although it’s too early to claim that the entire Arctic will be a massive methane “sink” in a warmer world, the study’s results do suggest that the Arctic could help mitigate the warming effect that would be caused by a rising amount of methane in the atmosphere. In immediate terms, climate models that project conditions on a warmer Earth could use this study to more accurately calculate the future methane content of the atmosphere, Lau said.

“At our study sites, we are more confident that these soils will continue to be a sink under future warming. In the future, the Arctic may not have atmospheric methane increase as much as the rest of the world,” Lau said. “We don’t have a direct answer as to whether these Arctic soils will offset global atmospheric methane or not, but they will certainly help the situation.”

The researchers want to study the bacteria’s physiology as well as test the upper temperature threshold and methane concentrations at which they can still efficiently process methane, Lau said. Field observations showed that the bacteria are still effective up to 18 degrees Celsius (64.4 degrees Fahrenheit) and can remove methane down to one-quarter of the methane level in the atmosphere, which is around 0.5 parts-per-million.

“If these bacteria can still work in a future warmer climate and are widespread in other Arctic permafrost areas, maybe they could regulate methane for the whole globe,” Lau said. “These regions may seem isolated from the world, but they may have been doing things to help the world.”

From Princeton, Lau worked with geoscience graduate student and second author Brandon Stackhouse; Nicholas Burton, who received his bachelor’s degree in geosciences in 2013; David Medvigy, an assistant professor of geosciences; and senior author Tullis Onstott, a professor of geosciences. Co-authors on the paper were from the University of Tennessee-Knoxville; the Oak Ridge National Laboratory; McGill University; Laurentian University in Canada; and the University of Texas at Austin.

The research was supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research (DE-SC0004902); the National Science Foundation (grant no. ARC-0909482); the Canada Foundation for Innovation (grant no. 206704); the Natural Sciences and Engineering Research Council of Canada Discovery Grant Program (grant no. 298520-05); and the Northern Research Supplements Program (grant no. 305490-05)

Read the abstract.

M.C.Y. Lau, B.T. Stackhouse, A.C. Layton, A. Chauhan, T. A. Vishnivetskaya, K. Chourey, J. Ronholm, N.C.S. Mykytczuk, P.C. Bennett, G. Lamarche-Gagnon, N. Burton, W.H. Pollard, C.R. Omelon, D.M. Medvigy, R.L. Hettich, S.M. Pfiffner, L.G. Whyte, and T.C. Onstott. 2015. An active atmospheric methane sink in high Arctic mineral cryosols. The ISME Journal. Article published in print August 2015. DOI:10.1038/ismej.2015.13.