Orphan Elected Fellow of American Academy of Microbiology

Professor of Geobiology Victoria Orphan has been elected to the American Academy of Microbiology. Fellows are elected through a highly selective peer-review process to recognize scientific achievement and "original contributions that have advanced microbiology."

"It's a great honor to receive this award, and there's also a nostalgic component," Orphan says. "The first microbiology conference I attended was the American Academy of Microbiology meeting in New Orleans 20 years ago. This year, the location has cycled back to New Orleans, and that's where I'll be receiving this award. It has been a great journey."

For the past 20 years, Orphan has studied anaerobic marine microorganisms that live within the seafloor and breathe methane. Through their unusual metabolism, these organisms restrict the amount of methane that seeps into the ocean and atmosphere. Methane is a much stronger greenhouse gas than carbon dioxide, so understanding how it cycles through the oceans and atmosphere is an important component of modeling Earth's climate.

Recently, Orphan and her team discovered evidence that these microbes inhabit not only sediments on the ocean floor but also huge calcium carbonate mounds that can rise hundreds of feet above the seafloor. The mounds represent a previously unrecognized biological sink for methane that could be preventing large amounts of the potent greenhouse gas from reaching the atmosphere.

Orphan is one of 79 other microbiologists elected as fellows to the academy in 2015. She joins current fellows Jared Leadbetter, professor of environmental microbiology, and Dianne Newman, professor of biology and geobiology, and investigator at the Howard Hughes Medical Institute.

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Friday, April 10, 2015
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Friction Means Antarctic Glaciers More Sensitive to Climate Change Than We Thought

One of the biggest unknowns in understanding the effects of climate change today is the melting rate of glacial ice in Antarctica. Scientists agree rising atmospheric and ocean temperatures could destabilize these ice sheets, but there is uncertainty about how fast they will lose ice.

The West Antarctic Ice Sheet is of particular concern to scientists because it contains enough ice to raise global sea level by up to 16 feet, and its physical configuration makes it susceptible to melting by warm ocean water. Recent studies have suggested that the collapse of certain parts of the ice sheet is inevitable. But will that process take several decades or centuries?

Research by Caltech scientists now suggests that estimates of future rates of melt for the West Antarctic Ice Sheet—and, by extension, of future sea-level rise—have been too conservative. In a new study, published online on March 9 in the Journal of Glaciology, a team led by Victor Tsai, an assistant professor of geophysics, found that properly accounting for Coulomb friction—a type of friction generated by solid surfaces sliding against one another—in computer models significantly increases estimates of how sensitive the ice sheet is to temperature perturbations driven by climate change.

Unlike other ice sheets that are moored to land above the ocean, most of West Antarctica's ice sheet is grounded on a sloping rock bed that lies below sea level. In the past decade or so, scientists have focused on the coastal part of the ice sheet where the land ice meets the ocean, called the "grounding line," as vital for accurately determining the melting rate of ice in the southern continent.

"Our results show that the stability of the whole ice sheet and our ability to predict its future melting is extremely sensitive to what happens in a very small region right at the grounding line. It is crucial to accurately represent the physics here in numerical models," says study coauthor Andrew Thompson, an assistant professor of environmental science and engineering at Caltech.

Part of the seafloor on which the West Antarctic Ice Sheet rests slopes upward toward the ocean in what scientists call a "reverse slope gradient." The end of the ice sheet also floats on the ocean surface so that ocean currents can deliver warm water to its base and melt the ice from below. Scientists think this "basal melting" could cause the grounding line to retreat inland, where the ice sheet is thicker. Because ice thickness is a key factor in controlling ice discharge near the coast, scientists worry that the retreat of the grounding line could accelerate the rate of interior ice flow into the oceans. Grounding line recession also contributes to the thinning and melting away of the region's ice shelves—thick, floating extensions of the ice sheet that help reduce the flow of ice into the sea.

According to Tsai, many earlier models of ice sheet dynamics tried to simplify calculations by assuming that ice loss is controlled solely by viscous stresses, that is, forces that apply to "sticky fluids" such as honey—or in this case, flowing ice. The conventional models thus accounted for the flow of ice around obstacles but ignored friction. "Accounting for frictional stresses at the ice sheet bottom in addition to the viscous stresses changes the physical picture dramatically," Tsai says.

In their new study, Tsai's team used computer simulations to show that even though Coulomb friction affects only a relatively small zone on an ice sheet, it can have a big impact on ice stream flow and overall ice sheet stability.

In most previous models, the ice sheet sits firmly on the bed and generates a downward stress that helps keep it attached it to the seafloor. Furthermore, the models assumed that this stress remains constant up to the grounding line, where the ice sheet floats, at which point the stress disappears.

Tsai and his team argue that their model provides a more realistic representation—in which the stress on the bottom of the ice sheet gradually weakens as one approaches the coasts and grounding line, because the weight of the ice sheet is increasingly counteracted by water pressure at the glacier base. "Because a strong basal shear stress cannot occur in the Coulomb model, it completely changes how the forces balance at the grounding line," Thompson says.

Tsai says the idea of investigating the effects of Coulomb friction on ice sheet dynamics came to him after rereading a classic study on the topic by American metallurgist and glaciologist Johannes Weertman from Northwestern University. "I wondered how might the behavior of the ice sheet differ if one factored in this water-pressure effect from the ocean, which Weertman didn't know would be important when he published his paper in 1974," Tsai says.

Tsai thought about how this could be achieved and realized the answer might lie in another field in which he is actively involved: earthquake research. "In seismology, Coulomb friction is very important because earthquakes are thought to be the result of the edge of one tectonic plate sliding against the edge of another plate frictionally," Tsai said. "This ice sheet research came about partly because I'm working on both glaciology and earthquakes."

If the team's Coulomb model is correct, it could have important implications for predictions of ice loss in Antarctica as a result of climate change. Indeed, for any given increase in temperature, the model predicts a bigger change in the rate of ice loss than is forecasted in previous models. "We predict that the ice sheets are more sensitive to perturbations such as temperature," Tsai says.

Hilmar Gudmundsson, a glaciologist with the British Antarctic Survey in Cambridge, UK, called the team's results "highly significant." "Their work gives further weight to the idea that a marine ice sheet, such as the West Antarctic Ice Sheet, is indeed, or at least has the potential to become, unstable," says Gudmundsson, who was not involved in the study.

Glaciologist Richard Alley, of Pennsylvania State University, noted that historical studies have shown that ice sheets can remain stable for centuries or millennia and then switch to a different configuration suddenly.

"If another sudden switch happens in West Antarctica, sea level could rise a lot, so understanding what is going on at the grounding lines is essential," says Alley, who also did not participate in the research.

"Tsai and coauthors have taken another important step in solving this difficult problem," he says.

Along with Tsai and Thompson, Andrew Stewart, an assistant professor of atmospheric and oceanic sciences at UCLA, was also a coauthor on the paper, "Marine ice sheet profiles and stability under Coulomb basal conditions." Funding support for the study was provided by Caltech's President's and Director's Fund program and the Stanback Discovery Fund for Global Environmental Science.

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Caltech Professors Awarded 2015 Sloan Fellowships

Five Caltech faculty members have been named among the 2015 class of Sloan Research Fellows. The fellowships, awarded by the Alfred P. Sloan Foundation, honor "early-career scientists whose achievements and potential identify them as rising stars, the next generation of scientific leaders." This year, 126 young scientists were awarded fellowships in eight scientific and technical fields: chemistry, computer science, economics, mathematics, computational and evolutionary biology, neuroscience, ocean sciences, and physics. Candidates must be nominated by a department head or other senior researcher and are reviewed by a selection committee of three distinguished scientists in each field.

Viviana Gradinaru (BS' 05), an assistant professor of biology and the faculty director of the Beckman Institute Pilot Center for Optogenetics and CLARITY, received her fellowship in the area of neuroscience. The CLARITY technique, codeveloped by Gradinaru, is used to render tissues, organs, and even whole organisms transparent. Her research focuses on developing tools and methods for neuroscience as well as investigating the mechanisms underlying deep brain stimulation and its long-term effects on neuronal health, function, and behavior.

Mitchell Guttman, an assistant professor of biology, received the fellowship in the category of computational and evolutionary molecular biology. His work exploring unknown regions of the genome has led to the identification of genes that do not produce proteins, known as long noncoding RNAs (lncRNAs), which act as efficient administrators, gathering and organizing key proteins necessary for packaging genetic information and regulating gene expression. Guttman and his colleagues recently discovered that lncRNAs can shape chromosome structure to remodel the genome and pull in necessary target genes, unlike other proteins that must travel to their targets.

Gregg Hallinan, an assistant professor of astronomy, received his fellowship in the physics category. His group studies the universe at radio wavelengths, particularly examining the radio emissions produced by stars and their planets. His team recently completed construction of a new radio telescope at Caltech's Owens Valley Radio Observatory that can survey the entire sky instantaneously. This project aims to deliver the first detection of radio waves produced by the interaction of the magnetic field of an exoplanet—a planet outside our own solar system—with the stellar wind of its host star.

Heather Knutson, an assistant professor of planetary science, received the fellowship in the physics category. She studies the structure, chemistry, and atmospheric dynamics of extrasolar planets. These planets are often classified into broad categories based on their mass and radius. Knutson's research measuring exoplanet temperatures and characterizing atmospheric compositions adds detail to these classifications. She has helped develop many of the techniques that are now used to study exoplanet atmospheric dynamics.

Xinwen Zhu, an associate professor of mathematics, received the fellowship in the mathematics category. His research interests focus on geometric representation theory, in particular the geometric aspects of the Langlands program, a kind of "unified theory of mathematics" linking together many different mathematical fields of research. This research aims to provide a more intuitive visualization of prime numbers by relating the field to diverse topics such as geometry and quantum physics.

Also included among this year's class of fellows are six other Caltech alumni: Brandi Cossairt (BS '06), Jennifer A. Dionne (MS '05, PhD '09), Aaron Esser-Kahn (BS '04), Michael Kesden (PhD '05), Neal Mankad (PhD '10), and Stephanie Waterman (MS '02).

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