By John Sullivan, School of Engineering and Applied Science
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.
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)
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.
Using a light-activated catalyst, researchers have unlocked a new pathway in nickel chemistry to construct carbon-oxygen (C-O) bonds that would be highly valuable to pharmaceutical and agrochemical industries.
“It was extraordinary to see the reaction go from zero to 91 percent yield just by adding a photocatalyst and switching on a light,” said David MacMillan, the James S. McDonnell Distinguished University Professor of Chemistry and principal investigator of the work published on August 12 in the journal Nature.
C-O coupling reaction scheme
The article reported the first general C-O cross-coupling reaction, which connects ring-shaped molecules, called aromatics, to alcohol-containing molecules, using a dual nickel-photoredox catalyst system. Extending nickel’s reach to C-O coupling reactions has great potential given the tremendous impact nickel chemistry has had on analogous C-C coupling reactions.
For the most part, these C-O cross-coupling reactions have been unattainable by traditional nickel catalysis. That’s because the final bonding-forming step—called reductive elimination—in which nickel excises itself to leave behind a C-O bond, is fundamentally unfavorable. By introducing a photocatalyst, the research team was able to remove a single electron from the key nickel intermediate to access an elusive oxidation state of nickel that can readily form the desired bond.
Using a photocatalyst to effectively expand the possible oxidation states of nickel has significant implications beyond this specific transformation. “We assume that it’s not just nickel chemistry that you can dramatically change, but other metals as well,” MacMillan said. “That’s very exciting position to be in.”
To confirm their understanding of how the catalysts worked together to promote the reaction, Valerie Shurtleff, a graduate student in the MacMillan lab and co-author on the paper, performed a series of mechanistic experiments.
Shurtleff synthesized a model nickel complex that mimicked the key bond-forming intermediate, a nickel compound bridging the two coupling partners. She found that without the presence of both light and photocatalyst, the complex was unable to form the product. Further electrochemical experiments confirmed that the model nickel complex was well within the range of molecules with which the photocatalyst could theoretically interact.
The new nickel-photocatalyst combination also offers a mild alternative to similar existing methods that employ palladium or copper catalysts and can access complementary coupling partners.
The MacMillan group has made many major contributions in the area of photoredox catalysis, but has only recently begun discovering the possibilities that arise from combining photoredox with other forms of catalysis, such as nickel. “There are so many different avenues to explore,” Shurtleff said. “We’re really just getting started.”
Terrett, J. A.; Cuthbertson, J. D.; Shurtleff, V. W.; MacMillan, D. W. C. “Switching on Elusive Organometallic Mechanisms with Photoredox Catalysis.” 2015, Nature.
This work was supported by financial support from the National Institute of General Medical Services (R01 GM093213-01).
By Raphael Rosen, Princeton Plasma Physics Laboratory
PPPL Scientist Elena Belova Photo Credit: Elle Starkman, PPPL
Creating controlled fusion energy entails many challenges, but one of the most basic is heating plasma – hot gas composed of electrons and charged atoms – to extremely high temperatures and then maintaining those temperatures. Now scientist Elena Belova of the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and a team of collaborators have proposed an explanation for why the hot plasma within fusion facilities called tokamaks sometimes fails to reach the required temperature, even as researchers pump beams of fast-moving neutral atoms into the plasma in an effort to make it hotter.
The results, published in June in Physical Review Letters, could lead to improved control of temperature in future fusion devices, including ITER, the international fusion facility under construction in France to demonstrate the feasibility of fusion power. This work was supported by the DOE Office of Science (Office of Fusion Energy Sciences).
The researchers focused on the puzzling tendency of electron heat to leak from the core of the plasma to the plasma’s edge. “One of the largest remaining mysteries in plasma physics is how electron heat is transported out of plasma,” said Jon Menard, program director for PPPL’s major fusion experiment, the National Spherical Tokamak Experiment-Upgrade (NSTX-U), which is completing a $94 million upgrade.
Belova hit upon a possible answer while performing 3D simulations of past NSTX plasmas on computers at the National Energy Research Scientific Computing Center (NERSC), in Oakland, California. She saw that two kinds of waves found in fusion plasmas appear to form a chain that transfers the neutral-beam energy from the core of the plasma to the edge, where the heat dissipates. While physicists have long known that the coupling between the two kinds of waves – known as compressional Alfvén waves and kinetic Alfvén waves (KAWs) – can lead to energy dissipation in plasmas, Belova’s results were the first to demonstrate the process for beam-excited compressional Alfvén eigenmodes (CAEs) in tokamaks.
Her simulations showed that when researchers try to heat the plasma by injecting beams of energetic deuterium, a form of hydrogen, the beams excite CAE waves in the plasma’s core. Those waves then resonate with KAW waves, which occur primarily at the plasma’s edge. As a result, the energy is transported from the injection site deep within the plasma to the plasma’s edge.
“Originally, when scientists found that the electron temperature wouldn’t go up with increased beam power, everybody assumed that the electrons were getting heated at the plasma’s center and then were somehow losing that heat,” Belova said. “Our explanation is different. We propose that part of the beam energy goes into CAEs and then to KAWs. The energy then dissipates at the plasma’s edge.”
The simulations provided a broad perspective. “In simulations you can look everywhere in a plasma,” Belova said. “In the experiments, on the other hand, you are very limited in what and where you can measure inside the hot plasma.”
Belova’s findings reflect the growing collaboration between theoretical and experimental research at the Laboratory. “Her results uncover a novel loss mechanism for electron energy that could be important for NSTX-U plasmas,” said Amitava Bhattacharjee, head of the Theory Department at PPPL.
Belova plans to run more simulations to determine whether the mechanism she identified is the primary process that modifies the electron heating profile. She will also look for ways in which physicists can avoid this wave-induced change in the profile. In the meantime, she is driven by her desire to learn more physics. “We want to understand how these waves are excited by the beam ions,” she said, “and how to avoid them in the experiments.”
PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.
Belova, E.V., N.N. Gorelenkov, E.D. Fredrickson, K. Tritz and N. A. Crocker. “Coupling of Neutral-Beam-Driven Compressional Alfvén Eigenmodes to Kinetic Alfvén Waves in NSTX Tokamak and Energy Channeling.” Physical Review Letters. Published June 29, 2015. DOI: 10.1103/PhysRevLett.115.015001
By Catherine Zandonella, Office of the Dean for Research
Researchers at Princeton University have for the first time directly calculated the rate at which water crystallizes into ice in a realistic computer model of water molecules. The simulations, which were carried out on supercomputers, provide insight into the mechanism by which water transitions from a liquid to a crystalline solid.
Understanding ice formation adds to our knowledge of how cold temperatures affect both living and non-living systems, including how living cells respond to cold and how ice forms in clouds at high altitudes. A more precise knowledge of the initial steps of freezing could eventually help improve weather forecasts and climate models, as well as inform the development of better materials for seeding clouds to increase rainfall.
The researchers looked at the process by which, as the temperature drops, water molecules begin to cling to each other to form a blob of solid ice within the surrounding liquid. These blobs tend to disappear quickly after their formation. Occasionally, a large enough blob, known as a critical nucleus, emerges and is stable enough to grow rather than to melt. The process of forming such a critical nucleus is known as nucleation.
To study nucleation, the researchers used a computerized model of water that mimics the two atoms of hydrogen and one atom of oxygen found in real water. Through the computer simulations, the researchers calculated the average amount of time it takes for the first critical nucleus to form at a temperature of about 230 degrees Kelvin or minus 43 degrees Celsius, which is representative of conditions in high-altitude clouds.
“The main significance of this work is to show that it is possible to calculate the nucleation rate for relatively accurate models of water,” said Haji-Akbari.
Cubic ice is made of double-diamond cages, each of which contains 14 water molecules arranged into seven interconnected six-member rings.Hexagonal ice is made of hexagonal cages, each of which contains 12 water molecules arranged into two six-membered rings that sit on top of each other.
In addition to calculating the nucleation rate, the researchers explored the origin of the two different crystalline shapes that ice can take at ambient pressure. The ice that we encounter in daily life is known as hexagonal ice. A second form, cubic ice, is less stable and can be found in high-altitude clouds. Both ices are made up of hexagonal rings, with an oxygen atom on each vertex, but the relative arrangement of the rings differs in the two structures.
“When water nucleates to form ice there is usually a combination of the cubic and hexagonal forms, but it was not well-understood why this would be the case,” said Haji-Akbari. “We were able to look at how the shapes of ice blobs change during the nucleation process, and one of the main findings of our work is to explain how a less stable form of ice is favored over the more stable hexagonal ice during the initial stages of the nucleation process.” (See figure below.)
Debenedetti added, “What we found in our simulations is that before we go to hexagonal ice we tend to form cubic ice, and that was very satisfying because this has been reported in experiments.” One of the strengths of the study, Debenedetti said, was the innovative method developed by Haji-Akbari to identify cubic and hexagonal forms in the computer simulation.
Computer models come in handy for studies of nucleation because conducting experiments at the precise temperatures and atmospheric conditions when water molecules nucleate is very difficult, said Debenedetti, who is Princeton’s Class of 1950 Professor in Engineering and Applied Science and Dean for Research. But these calculations take huge amounts of computer time.
Haji-Akbari found a way to complete the calculation, whereas previous attempts failed to do so. The technique for modeling ice formation involves looking at computer-simulated blobs of ice, known as crystallites, as they form. Normally the technique involves looking at the crystallites after every step in the simulation, but Haji-Akbari modified the procedure such that longer intervals of time could be examined, enabling the algorithm to converge to a solution and obtain a sequence of crystallites that eventually led to the formation of a critical nucleus.
Using a computer model to explore how water molecules connect and nucleate into ice crystals, the researchers found that two types of ice compete for dominance during nucleation: cubic ice (blue) which is less stable, and hexagonal ice (red), which is stable and forms the majority of ice on Earth. Nucleation occurs when water molecules come together to form blobs (pictured above), which grow over time (left to right). Eventually hexagonal ice wins out (not shown). The researchers found that adding new cubic features onto an existing crystalline blob gives rise to nuclei that are more spherical, and hence more stable. In contrast, adding hexagonal features tends to give rise to chains of hexagonal cages that make the nucleus less spherical, and hence less stable.
Even with the modifications, the technique took roughly 21 million computer processing unit (CPU) hours to track the behavior of 4,096 virtual water molecules in the model, which is known as TIP4P/Ice and is considered one of the most accurate molecular models of water. The calculations were carried out on several supercomputers, namely the Della and Tiger supercomputers at the Princeton Institute for Computational Science and Engineering; the Stampede supercomputer at the Texas Advanced Computing Center; the Gordon supercomputer at the San Diego Supercomputer Center; and the Blue Gene/Q supercomputer at the Rensselaer Polytechnic Institute.
Debenedetti noted that the rate of ice formation obtained in their calculations is much lower than what had been found by experiment. However, the computer calculations are extremely sensitive, meaning that small changes in certain parameters of the water model have very large effects on the calculated rate. The researchers were able to trace the discrepancy, which is 10 orders of magnitude, to aspects of the water model rather than to their method. As the modeling of water molecules improves, the researchers may be able to refine their calculations of the rate.
The research was funded by the National Science Foundation (Grant CHE-1213343) and the Carbon Mitigation Initiative at Princeton University.
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