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Aerosols are microscopic particles suspended in the atmosphere. Studying their characteristics and behavior is a rapidly developing area of atmospheric chemistry. A recent paper in Reviews of Geophysics focuses on aerosol mixing state, describing methods for measuring and modeling, as well as understanding the impacts on climate. Here, two of the authors of the paper give an overview of our understanding and recent developments in this scientific field.

What different sources of particles and atmospheric processes combine to form overall aerosol in the atmosphere?

There are countless sources for aerosols, but to keep it from being overwhelming, we can roughly categorize them as coming from combustion processes (soot, biomass burning, and industrial particles), from mechanical processes (mineral dust, sea salt, car brake dust), and forming directly from low-volatility gases.

The challenge we face is that particles are not simply emitted and remain the same through their lifetime.The challenge we face is that particles are not simply emitted and remain the same through their atmospheric lifetime. Instead, various gases condense on or evaporate from the particles, and new species can form within the particles. Also, particles randomly collide with each other and form aggregates.

Thus, understanding the aerosol at any given time resembles hitting a moving target. The experimental methods and modeling approaches discussed in our review try to get a handle on this moving target.

What is ‘aerosol mixing state’ and what are the main factors that influence it?

At its simplest, aerosol mixing state is trying to say which chemical species are mixed with each other in individual particles and understanding how that contributes to the overall aerosol.

There are two extremes to mixing state, external and internal mixing. An external mixture means that all particles consist of just one chemical species, meaning a soot particle is just soot, a dust particle is just dust, etc. An internal mixture on the other hand means that every particle contains all the species in the overall aerosol in the same proportions (so a soot particle may also contain organic carbon, sulfate, nitrate, etc.).

The challenge is that it is very rare to have a perfectly externally mixed or internally mixed aerosol and usually we are somewhere in between. Aerosol mixing state is evolving in the atmosphere and a reflection of the processes that act on the particles during their lifetime.

Evolution of aerosol mixing state of an aerosol that is transported in the atmosphere. The line graph at the top illustrates how aerosol mixing state changes qualitatively between a more or less internally mixed state and how different aerosol processes contribute to that change. Adding new types of aerosol particles makes the population more externally mixed (steps 2 and 4), while aerosol aging processes (step 3) or the addition of one dominate particle type (step 5) moves the population toward a more internally mixed state. DMS = dimethyl sulfide. Credit: Riemer et al. [2019], Figure 2 How could you describe this in simpler language for a non-expert?

I like to think about my son’s plate at dinner, with each bite he takes as an aerosol particle. His plate starts as externally mixed, with green beans, chicken, squash and rice all separated on his plate, just like the aerosol particles as they enter the atmosphere from different sources.

Within a few minutes some of the green beans are in the squash, while some bites contain chicken and green beans. Then we add barbeque sauce (representing secondary species), which ends up on pretty much everything.

The bites are not an internal mixture, which would be the equivalent of the plate being put in a blender, with each bite having the same amount of everything. Thus, the plate and bites are each unique, but neither fully externally nor internally mixed. Various methods and models help us understand that complexity.

What techniques are used to measure aerosol mixing state?

Scanning electron microscopy (SEM) image and energy dispersive X-ray spectroscopy (EDX) maps of particles and the elements contained within them from the Southern Oxidant and Aerosol (SOAS) field campaign in 2013. Each of the elemental map panels corresponds to two elements overlaid to show the elemental distributions from the SEM image. The following particle types are shown: (a–d) dust, (e–f) aged sea salt aerosol, and (g) primary biological aerosol. Credit: Amy Bondy

Any technique that can provide single-particle composition data can help us understand the mixing state of the atmospheric aerosol. The key is that a method has to be able to analyze in situ or collected samples particle-by-particle, so that the different distinct particle chemical compositions don’t get smeared together within an average.

The methods have been used the most to study aerosol mixing state each provide their own flavor of information: single particle mass spectrometry (molecules and fragments), electron microscopy with X-ray spectroscopy (elemental) and microspectroscopy (functional groups).

Researchers are constantly pushing to understand more about the physicochemical properties of individual particles, which are enabling us to probe even variations within particles in composition, surface properties, acidity, and more.

These advances are enabling us to learn more than ever about the mixing state of the overall aerosol.

What impact does aerosol mixing state have on the atmospheric properties that influence climate?

Aerosols influence climate because they impact the Earth’s radiative balance.Aerosols influence climate because they impact the Earth’s radiative balance. This can happen directly as the particles scatter or absorb sunlight, or indirectly as the particles provide nuclei that help form cloud droplets and ice particles, thereby changing the reflectivity and lifetime of clouds. The magnitude of these effects depends on the make-up of the individual particles and how they are assembled in a population.

For example, soot particles that are coated with organic material absorb light differently than a situation where soot and organics reside in separate particles. This changes the heating of the atmosphere where the aerosol resides. Similarly, the propensity to form cloud droplets or ice particles is different for coated soot particles compared to bare soot. These differences in the initial formation process of clouds propagate and determine how much sunlight clouds reflect back into space, a key process in determining the Earth’s energy budget.

What are some of the unresolved questions where additional research is needed?

Despite the very sophisticated measurement methods that are currently available, there is no single instrument that can capture aerosol mixing state in its entirety.Despite the very sophisticated measurement methods that are currently available, there is no single instrument that can capture aerosol mixing state in its entirety. We therefore need to use multiple instruments that simultaneously sample the same aerosol and combine them in a way that yields a complete picture of aerosol mixing state. How to exactly do this, and how to connect these measurements to model output is challenging – but necessary for building a quantitative understanding of the aerosol mixing state in the ambient atmosphere.

Another important unresolved question is how to predict aerosol climate impacts – that is the aerosol impact on cloud properties and the aerosol interaction with light – from a known aerosol mixing state. The largest unknowns presently are associated with the formation of ice particles.

—Nicole Riemer (email: nriemer@illinois.edu), University of Illinois at Urbana‐Champaign; and Andrew Ault, University of Michigan

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The hazardous, turbulent waters around Africa’s southern tip have sunk countless ships, but they also sustain plentiful fisheries, including abundant sardine and anchovy populations. Fish populations in the region have fluctuated sharply in the past, possibly because of changing ocean temperatures. Now, a study shows that shifting winds are the main driver of long-term temperature shifts in the shallow coastal waters, a finding that could improve fisheries management.

Two powerful ocean currents, the Agulhas Current and the Benguela Current, collide around the southernmost promontory of South Africa, creating the dangerous conditions that once earned the region the moniker “Graveyard of Ships.” The currents swirl over a shallow, triangular shelf called the Agulhas Bank, churning up nutrients that feed plankton blooms and a rich spawning ground for sardines and anchovies.

Fish populations in this area have fluctuated dramatically over the past century, but scientists aren’t entirely sure why. One possibility is changing temperatures along the coast: In 1996, for example, the anchovy population shifted rapidly east when temperatures around the Agulhas Bank dropped by 0.5°C. However, long-term observations of this ocean region are rare, making answers hard to find.

In the new study, Malan et al. used a computer simulation of atmospheric conditions, ocean currents, and wind patterns in the Agulhas Bank to investigate what factors might affect water temperatures from decade to decade. As they tweaked different variables in the model, they found that shifting wind belts, not ocean currents or heating from the atmosphere, were the most important driver of coastal temperatures.

As winds shift direction, their angle relative to the coastline changes. When the wind blows parallel to the coastline, cold, nutrient-dense water gets pushed up toward the ocean surface, a phenomenon called upwelling. When winds blow toward the shore, warm surface water is forced toward the coastline, causing temperatures to rise.

In the simulated model of the Agulhas Bank, altering wind direction forced the boundary of warm water—demarcated by the 17°C isotherm—to shift toward or away from the coastline by up to 100 kilometers, the team found.

The Southern Annular Mode, the north–south wobbling of a massive, westerly wind belt, has a profound impact on local wind patterns in this region. Understanding how large-scale wind belts may affect temperatures in the Agulhas Bank could help fisheries experts manage marine protected areas in the region, the scientists note. (Journal of Geophysical Research: Oceans, https://doi.org/10.1029/2018JC014614, 2019)

—Emily Underwood, Freelance Writer

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Far from being static features of the landscape, glaciers are dynamic rivers of ice, flowing and carving earth beneath them in a diverse range of rates. There are fast-flowing glaciers, slow or stagnant glaciers, and surging glaciers that periodically accelerate and slow down again.

Understanding glacier movement is essential for accurate modeling of future climate.

“Their physics are critical for our understanding sea level change,” says Zhongwen Zhan, a professor of geophysics at the California Institute of Technology, “because that is where you are draining the ice sheets into the oceans.”

It’s long been thought, Zhan notes, that liquid water at the base of glaciers might be acting as a lubricant, speeding glaciers up and along, but it is difficult to fully characterize what is taking place under many meters of ice. In a recent study published in Geophysical Research Letters, Zhan details the results of a new approach that offers a work-around.

Zhan’s insight was to make use of two seismological stations set up astride the surging Bering Glacier in Alaska. Zhan examined station data for a 12-year period, which included a surge that lasted from 2008 to 2010, measuring changes in the speed of background seismic waves as they passed through the glacier. He found that waves slowed down during the surge, indicating they were traveling though softer material—water rather than ice or rock.

“This sort of geological observation and seismological observation are coming together and showing the same phenomenon.”Zhan thinks that the bottom 10 or 20 meters of a glacier crack during a surge, with those cracks running perpendicular to the direction of the glacier’s flow. Water, he says, rather than simply pooling at the base of the ice, fills these cracks. Zhan measured two types of seismic waves, Rayleigh and Love waves, to reach this conclusion.

“You have waves oscillating in the vertical direction, and another one in the horizontal direction, they are polarized,” he says. “Based on those differences in behavior of the polarization of the wave, we found that we need to have the cracks aligned perpendicular to the ice movement.”

Zhan’s research matches field observations of ridges of fine sediment left at the front of retreating surging glaciers, hinting at just these types of transverse cracks at the glacier base. “This sort of geological observation and seismological observation are coming together and showing the same phenomenon,” he says.

Zhan’s work is a step forward both in understanding surging glaciers and in furthering techniques for future studies, according Timothy Bartholomaus, a glaciologist and assistant professor in the University of Idaho’s Department of Geological Sciences.

“I think that it’s a very clever use of existing data and another really nice example of the ways by which people are applying techniques from seismology,” Bartholomaus says. “Over the last 5 or 10 years or so, there’s been a really major push by seismologists, oftentimes in collaboration with experts in other fields, to apply those same techniques to understand a whole range of other processes.”

An earlier study published in Geophysical Research Letters, for instance, used a network of 34 seismology stations in Antarctica to characterize high-frequency, wind-generated waves in ice shelves and correlated changes in the waves with a coming thaw.

Zhan would also like to extend his technique in future studies, perhaps adding additional seismological stations at different orientations across the glacier to better test his hypothesis.

“Our pair is aligned parallel to the cracks we are proposing. If we have a pair that’s perpendicular or 45° to the cracks, then we should see a change that’s very different,” he says. In the long term, with an array of sensors, a study could be like “the hospital people using X-ray or MRI [magnetic resonance imaging] to do tomography to show what the structure looks like and where something is changing.”

—Jon Kelvey (@JonKelvey), Science Writer

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When it comes to climate modeling, understanding the past is critical to predicting the future. Scientists use all manner of materials to reconstruct Earth’s past climate. Tree rings, ice cores, corals, bat guano, and even whale earwax can reveal clues about historical conditions.

Among the various climate reconstruction strategies currently in play, lake sediments stand out for their completeness and continuity. The deposits lining lake basins supply a trove of data on changes in air temperature, water temperature, and hydrology. The global lake data record spans hundreds to thousands of years; in some regions, lake sediment archives offer a cohesive view of lacustrine conditions dating back to the Last Glacial Maximum, approximately 21,000 years ago.

Unfortunately, paleoclimate data, like those scrounged from debris at the bottom of a lake, do not always produce a clean and coherent signal. Climatic variables like temperature and precipitation can often be masked and hard to constrain. In turn, such data are more difficult to incorporate into climate models, so scientists use proxy system models to translate climate model data into the same reference frame as paleoclimate data. This translation puts the two data types on a level playing field for an apples-to-apples comparison. These translations are essential for improving the physics that underpin climate forecasts.

In a new study, Dee et al. detail the first comprehensive proxy system model for lake archives, called PRYSM (Python, Proxy System Modeling). The model describes several components of a lake’s history, including the energy balance and hydrology of the system, the accumulation of leaf waxes or other physical and biological deposits, and sedimentation and compaction at the bottom of the water body. PRYSM also provides an accounting of errors in data sampling, dating, and analysis.

To validate the model, the authors used the Paleoclimate Modelling Intercomparison Project Phase III (PMIP3) to simulate the 20th century conditions in two African lakes, Lake Malawi and Lake Tanganyika. They then compared the results to data in the field. The comparison between the simulated and real-world data revealed that lake systems exert a confounding influence on the climate signal of interest (e.g., air temperature). For instance, the results showed that lake surface temperatures respond differently than air temperatures to changes in mean climate; therefore, changes in lake temperature may not adequately reflect changes in air temperature. The revelation has far-reaching implications for understanding how lakes may respond to human-caused climate change.

The lake model promises to improve interpretations of past climatic conditions and predictions of the future climate. The model will also help researchers answer questions like, What percent change in precipitation is needed to simulate a 150-centimeter change in lake level over 100 years?

PRSYM, now in its second version, is the first step toward a larger distribution platform for lake sediment data and proxy system models that describe them. Eventually, the authors envision a fully operational platform that can serve as a research tool and resource for the entire paleoclimate community. (Paleoceanography and Paleoclimatology, https://doi.org/10.1029/2018PA003413, 2018)

—Aaron Sidder, Freelance Writer

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There are several examples from icy and rocky worlds in our Solar System explored by spacecraft of volcanic processes either occurring or having occurred in the past. Volcanism is apparently a common process and displays a wide variety of forms. We have yet to explore, however, metal worlds (i.e., the exposed cores of differentiated planetesimals).

Abrahams and Nimmo [2019] seek to determine whether ancient metallic volcanism could have happened on a metal asteroid. Unfortunately, observations of metal asteroids are largely just points of light in the sky, tantalizingly unresolved. The authors thus make predictions that may enable exploration via variations seen in iron meteorites. In addition, this possibly sets the stage for potentially amazing discoveries when Psyche (the spacecraft) arrives at Psyche (the asteroid), as no one knows the form that metal volcanic constructs could take.

Citation: Abrahams, J. N. H., & Nimmo, F. [2019]. Ferrovolcanism: Iron volcanism on metallic asteroids. Geophysical Research Letters, 46. https://doi.org/10.1029/2019GL082542

—Andrew Dombard, Editor, Geophysical Research Letters

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Some of the world’s leading scientific experts in biodiversity delivered to Congress the message of a damning new report that up to 1 million species face extinction.

“The evidence is unequivocal: Biodiversity, which is important in its own right and essential for human well-being, is being destroyed by human activities at a rate unprecedented in human history.”They delivered that message despite efforts by several Republican members of Congress and their witnesses to belittle, dismiss, and drown out those findings at a 22 May congressional hearing.

“The evidence is unequivocal: Biodiversity, which is important in its own right and essential for human well-being, is being destroyed by human activities at a rate unprecedented in human history,” Robert Watson, immediate past chair of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), testified before the House Committee on Natural Resources’ Subcommittee on Water, Oceans, and Wildlife.

In a recent report, IPBES found that the global rate of extinction “is already at least tens to hundreds of times higher than the average rate over the past 10 million years and is accelerating.”

“The loss of biodiversity is not only an environmental issue, but an economic, development, social security, moral, and ethical issue,” Watson testified.

The report found that many species could become extinct within decades, largely because of human activities. The report states that “goals for conserving and sustainably using nature and achieving sustainability cannot be met by current trajectories, and goals for 2030 and beyond may only be achieved through transformative changes across economic, social, political and technological factors.”

IPBES released a summary of its peer-reviewed report on 6 May and plans to release the full report soon.

Responding to Charges of “Exaggerated Claims”

Rep. Tom McClintock (R-Calif.), the ranking Republican member of the subcommittee, characterized the IPBES report as “the latest contribution to apocalyptic predictions.”

Patrick Moore, chairman of the board of the CO2 Coalition and a witness at the hearing, testified that “the highly exaggerated claims of the IPBES are not so much out of concern for endangered species as they are a front for a radical political, social, and economic ‘transformation’ of our entire civilization.”

The nonprofit CO2 Coalition promotes positive contributions of carbon dioxide and pushed for the United States to withdraw from the Paris climate accord.

However, Watson sharply disputed the charge, saying that the numbers in the IPBES document are drawn from two distinct lines of evidence, including an independent analysis and a straight extrapolation of the International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species. Watson added that IUCN has endorsed the IPBES analysis.

Moore, who is an ecologist and a policy adviser to The Heartland Institute, a free market think tank based in Arlington Heights, Ill., also stated in his testimony, “As with the manufactured ‘climate crisis,’ they are using the specter of mass extinction as a fear tactic to scare the public into compliance. The IPBES itself is an existential threat to sensible policy on biodiversity conservation.”

Marc Morano, editor of ClimateDepot.com and a prominent climate change denier, according to the DeSmog blog, criticized Watson, whom he sat next to at the witness table. “[Watson] says it’s our last chance to save the planet. These are the words of a salesman, a science bureaucrat, not a disinterested…” Morano never finished that sentence because subcommittee chair Rep. Jared Huffman (D-Calif.) interrupted and told Morano to direct testimony to him.

Later in the hearing, Huffman castigated Morano, saying, “I don’t know what inspires someone to make a career of trolling scientists or monetizing contrarian ideology on the YouTube and Ted Talk circuit. But it’s just a very different kind of conversation than the science-based conversation I think many of us would like to try to have.”

“Shadowy Stuff”

Huffman criticized Republicans for choosing Morano and Moore as their witnesses.

“There’s a narrative around here [in Congress] that Republicans are coming around on science and climate. Look no further than the witnesses they continue to dredge out of the fever swamp for these subcommittee hearings, and you’ll see that they’ve got a long way to evolve,” Huffman told Eos in an interview. “This is shadowy stuff, and we see it week after week: instead of scientists, people from these junior varsity think tanks that they keep dredging up. Apparently, witnesses from QAnon and Infowars”—a conspiracy theory group and website, respectively—”were unavailable, and so this is what we get.”“It’s just a choice that the Republicans keep making in these hearings. Instead of serious policy conversations, serious science conversations, they want to do politics.”

Morano “brought a provocative, almost like a World Wrestling type of ethos to his testimony. There were very few facts and certainly no science. He admitted that, at the outset, he’s a political science guy, and he’s a former staffer to Jim Inhofe,” Huffman told Eos, referring to the Oklahoman senator who authored a book entitled The Greatest Hoax: How the Global Warming Conspiracy Threatens Your Future. “[Morano] is here to throw bombs. It’s just a choice that the Republicans keep making in these hearings. Instead of serious policy conversations, serious science conversations, they want to do politics.”

Huffman said the way to counter the attacks on science is to present expert witnesses. “Well, look, we just presented you with four of the world’s leading scientists, for God’s sake. That’s probably where you start in countering that,” he said. “Then people can weigh for themselves: Do they want to consider the overwhelming weight of the world’s best scientists or the shadowy junior varsity think tank from people that used to work for Jim Inhofe?”

“Today, We Are That Asteroid.”

The last time there was a mass extinction “it happened because an asteroid hit the planet. Today, we are that asteroid.”After the hearing, another witness said he was disappointed in the “antiscience view” expressed by Morano and Moore. “It’s hype. It’s bombast. It’s all of this stuff that is not based in reality,” Jacob Malcom, director of the Center for Conservation Innovation for Defenders of Wildlife, a Washington, D.C.–based conservation group, told Eos. “I don’t think it will ever go away. But as long as it stays marginalized, I think more and more people will see it for what it is.”

Malcom, at the hearing, said that the last time there was a mass extinction “it happened because an asteroid hit the planet. Today, we are that asteroid.”

Watson, who chaired the Intergovernmental Panel on Climate Change from 1997 to 2002, told Eos that perhaps Republicans chose Morano and Moore as their witnesses “because we have said”—in the IPBES report—“that climate change and biodiversity must be dealt with together.”

“I would have hoped that the Republicans would have chosen two very good scientists who could have debated the merits of the IPBES report rather than clearly someone who’s just a straight climate denier,” Watson said, referring to Morano.

Why People Should Care About Biodiversity Loss

Watson told Eos that “the evidence is strongly behind this report” and that it is “the most heavily peer-reviewed document you could imagine.” The report, which assessed about 15,000 articles and responded to about 15,000 comments, was prepared by 145 expert authors and is the most comprehensive document ever prepared about biodiversity.

Watson said that the American public should care about the IPBES report for several reasons. “First, we shouldn’t destroy nature. Every religion in the world says we shouldn’t destroy nature. Nature is important,” he told Eos. “But more important than that, you could argue, is it is the substance behind food security, water security, it does control our climate in part, it does control pollination, it does control storm surges. These are things that affect everyday Americans. If we continue to lose biodiversity, if we continue to fragment our ecosystems, then human well-being will indeed suffer. This is something the average American should care about.”

Trump Administration “Going in the Opposite Direction”

At the hearing, Huffman said that “scientists have been ringing the alarm bell for years” about threats to biodiversity. However, Huffman criticized what he said are efforts by the Trump administration to increase the risk to species, including the administration’s attempts to weaken the federal Endangered Species Act, its efforts to expand oil and gas activities in the Arctic National Wildlife Refuge and elsewhere, and its plan to withdraw from the Paris climate accord.

“All of these are things we can do something about, but we are not yet on track to slow the extinction crisis. We need to do more.”“Despite overwhelming evidence that we have an extinction crisis on our hands, the Trump administration is going in the opposite direction to appease special interests and big donors,” he said.

The IPBES report states that the direct drivers of change in nature that have the largest global impact currently are, in order, changes in land and sea use, exploitation of organisms, climate change—with the report warning that future impacts of climate change on biodiversity and ecosystem functioning are projected to become more pronounced over the coming decades—pollution, and the invasion of alien species.

“All of these are things we can do something about, but we are not yet on track to slow the extinction crisis,” Huffman said. “We need to do more.”

—Randy Showstack (@RandyShowstack), Staff Writer

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Coastal habitats bear the brunt of global environmental change. Rising sea levels, intense shoreline development, and pollution all contribute to worldwide habitat losses on the order of 1% to 7% per year. Cumulatively, nearly half of the world’s wetlands, mangroves, and seagrass habitats have eroded away over the past several decades.

Coastal ecosystems serve as vital carbon sinks and storm buffers while also providing critical habitat for countless species, both rare and abundant. To protect and maintain both natural and developed shorelines, scientists and landscape planners need reliable models on how water, sediment, and vegetation interact in coastal environments.

For years, so-called sediment transport models relied on measurements of bed shear stress; however, recent studies indicate these models underestimate the amount of material transported through plant-laden waterways. Specifically, the models do not account for the turbulence plants create in flowing water. In response to these shortcomings, Yang and Nepf propose a new framework for modeling sediment transport along vegetated shorelines and floodplains.

The authors developed an alternative method that relies on turbulent kinetic energy to predict the rate of sediment transport in vegetated zones. They conducted experiments in a 1-meter-wide by 10-meter-long flume that recirculated water and sediment while they varied the amounts of model plants and water velocity in the flume.

Upon successfully developing a model for vegetated regions, the authors tested their work in conditions lacking vegetation, scenarios that the bed shear stress model typically captures well. The results of the follow-up experiments suggest that the turbulence models also successfully predict sediment movement in unvegetated channels; in some cases, the new model worked even better than the standard model.

When taken together, the results from both the vegetated and bare-channel experiments indicated that turbulence is a better universal predictor of sediment flow than bed shear stress. The findings represent a significant advance in the field of coastal hydrology and geomorphology. The newly developed model should improve predictions of sediment transport and retention in both vegetated and unvegetated regions while improving restoration and planning in coastal environments. (Geophysical Research Letters, https://doi.org/10.1029/2018GL079319, 2018)

—Aaron Sidder, Freelance Writer

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Eos: Earth and Space Science News by Michael J. Mcphaden, Agus Santoso, .. - 3d ago

The El Niño–Southern Oscillation (ENSO) is the strongest year-to-year climate fluctuation on the planet. It is spawned in the tropical Pacific Ocean, but its societal and environmental impacts are felt worldwide. The character of ENSO, which is a naturally occurring phenomenon alternating between warm (El Niño) and cold (La Niña) phases, depends on the background climatic conditions in which it develops.

The climate conditions associated with ENSO are changing as the planet warms through unabated atmospheric greenhouse gas emissions, raising questions about whether anthropogenic greenhouse forcing has affected ENSO already, or will in the future. These questions have been debated for nearly 30 years, but they take on greater urgency as the manifestations of climate change become ever more apparent.

Sixty research scientists from around the world gathered to address these questions at a symposium held at CSHOR in Australia.

Participants considered suggestions from the instrumental record and paleoproxy data—primarily of tropical Pacific sea surface temperature—that climate change has affected the observed ENSO cycle.

The evidence was deemed to be inconclusive.

The past 40 years have witnessed three extreme El Niños (1982–1983, 1997–1998, and 2015–2016) unlike any comparable period in the nearly 150-year-long instrumental record. However, 150 years is too short a period to unambiguously determine a climate change effect, considering natural variability and data reliability before 1950.

Regardless of whether the ENSO cycle has been affected already or will be affected in the future, the effects of ENSO today appear to be compounded by climate change.Paleoproxies can provide much longer records, with some studies finding that 20th-century ENSO variance is higher than in the distant past. However, paleoproxy records are relatively limited in geographical distribution, and their interpretation is complicated by the convolution of biological, geochemical, and physical factors not related to climate, leaving large uncertainties.

For future projections, scientists rely on climate models whose performance has been improving with regard to representation of ENSO, although the climate sensitivity varies widely across models and systematic errors persist. However, the latest modeling studies coupled with theoretical understanding suggest that under the usual emission scenarios, occurrences of strong ENSO events may increase by the end of the 21st century.

Regardless of whether the ENSO cycle has been affected already or will be affected in the future, the effects of ENSO today appear to be compounded by climate change simply because of the superposition of ENSO conditions on a warmer background state. This became most evident during the extreme 2015–2016 El Niño, which coincided with record-breaking cyclone activity in the tropical Pacific, an unprecedented global coral bleaching event, and extreme disruption of ecosystems and fisheries in the central Pacific (linked to record high sea surface temperatures there).

The ENSO Science Symposium participants discussed the need to reduce uncertainty in ENSO predictions and long-term projections, which will require ongoing efforts to sustain satellite and in situ climate observing systems, expand the database of paleoreconstructions, and improve models. There is likewise a need for better understanding of climate interactions across ocean basins, across timescales spanning decadal variability, and interactions with biogeochemistry. Significant progress has been made on many of these topics, but more work remains to be done.

The symposium was followed by a 2-day coordination workshop on an AGU monograph for the AGU Centennial titled ENSO in a Changing Climate, which will cover the latest science on ENSO dynamics, effects, prediction, and future projections.

Acknowledgments

CSHOR sponsored the symposium and the book workshop. CSHOR is a joint research center between the Qingdao National Laboratory for Marine Science and Technology and the Commonwealth Scientific and Industrial Research Organisation. This is Pacific Marine Environmental Laboratory (PMEL) contribution 4951.

—Michael J. McPhaden (michael.j.mcphaden@noaa.gov), PMEL, National Oceanic and Atmospheric Administration, Seattle, Wash.; Agus Santoso, Australian Research Council (ARC) Centre of Excellence for Climate Extremes and Climate Change Research Centre, University of New South Wales, Sydney, Australia; and CSHOR, CSIRO Oceans and Atmosphere, Hobart, Tas., Australia; and Wenju Cai, CSHOR, CSIRO Oceans and Atmosphere, Hobart, Tas., Australia; and Key Laboratory of Physical Oceanography/Institute for Advanced Ocean Studies, Ocean University of China and Qingdao National Laboratory for Marine Science and Technology, Qingdao, China

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Throughout the 19th and early 20th centuries, the concept of linear systems—in which any changes in output are directly proportional to modifications made to their inputs—dominated the thinking and methodology used to study the physical sciences. Following the end of World War II, however, advances in observational techniques and the development of increasingly powerful computational devices began to alter the way scientists formulate physical problems and solve them mathematically.

During the past half century, these breakthroughs led to a rapid expansion in nonlinear approaches to studying the physical sciences, a development that can only be characterized as revolutionary. The evolution of nonlinear concepts, which describe the cause-and-effect relationships in most natural systems, has significantly increased the range of inquiries geoscientists are able to address. Still, only a small number of nonlinear methodologies in the discipline currently exist, according to a recent paper by Ghil.

The author highlights a small selection of key achievements that aptly illustrate the importance of nonlinear concepts in the geosciences. These include novel insights into fluid dynamics, such as the role of multiple large-scale flow patterns in the ocean and atmosphere, which greatly improved long-range weather forecasting; applications related to geophysical turbulence and stochastic dynamical systems; the development of vacillation theory, which led to the theory of strange attractors; and the concept of networks, whose applications include modeling aspects of climate dynamics such as the changes in sea surface temperature patterns associated with the El Niño–Southern Oscillation.

By offering a broad overview of the development and application of nonlinear concepts across the geosciences, the author affords researchers from numerous disciplines an opportunity to reflect on the importance of nonlinearity for understanding geological and geophysical phenomena.

The logical next step, the author argues, is to apply the ideas he presents to the problem of prediction, which will serve as a crucial test of geoscientists’ physical and mathematical understanding of the natural world. (Earth and Space Science, https://doi.org/10.1029/2019EA000599, 2019)

—Terri Cook, Freelance Writer

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The American West, while steeped in mythology, is also a region that depends heavily on science for its long-term livability—and perhaps no one was quicker to realize that than John Wesley Powell. A Civil War veteran and an indefatigable explorer, Powell landed on the national stage in 1869, after an expedition he led became the first to navigate the Colorado River’s path through the Grand Canyon.

In the decades that followed, Powell would argue that careful, democratic management of water resources in the West must be a crucial component of its development and that a pattern of settlement and land cultivation based on the 19th century status quo would prove unsustainable.

He couldn’t have had a more unreceptive audience. Elected officials, industry titans, and even fellow scientists wanted a narrative that better supported the westward march of “progress,” narrowly defined.

Fast-forward 150 years, and Powell’s 19th century appeals are making modern headlines on the strength of their perception and foresight. Even while western states lead the nation in population and economic growth—Arizona, Colorado, Nevada, and Utah were among the top five states with the fastest growing gross domestic product between 2016 and 2017—drought conditions that have persisted for decades have left parched cities under constant threat of water emergencies.

On the Colorado River, the country’s two largest reservoirs—Lake Mead on the border between Nevada and Arizona and Lake Powell (so named for John Wesley) on the border between Arizona and Utah—are being drained faster than they can replenish. The effects of climate change are only adding to the pressure on limited water supplies.

In Powell’s account of his explorations, published in 1895 as Canyons of the Colorado, he describes the river’s waters emptying “as turbid floods into the Gulf of California.” Today, only in very wet years does the river reach the ocean. Meanwhile, communities that rely on the Colorado for water, including sprawling metropolitan areas like Phoenix, Denver, and Los Angeles, are facing the possibility of having their supply cut off or severely limited in a future that’s moving alarmingly nearer.

Unforeseen Potential

It’s tempting, then, to imagine how the West might have evolved had Powell’s vision for its development been implemented, rather than shunned, a century and a half ago.

What if Congress, undeterred by the siren song of American expansion, had listened to the call of the pragmatic?

Would L.A. be a backwater?

Would Tucson even appear on the map?

Would the Colorado still rush freely to the gulf from its headwaters in the Rocky Mountains?

Speculation about what might have been is complicated by society’s shifting priorities and values, as well as by technology. Powell, for his part, envisioned much smaller communities dispersed over the western landscape.

“One of the big things that would’ve happened if we’d listened to Powell is that we…would have responded earlier to the information about global climate change.”“One thing that he didn’t anticipate [was] the degree to which we would accumulate western society in big, urban complexes,” says Jack Schmidt, the Janet Quinney Lawson Chair in Colorado River Studies at Utah State University and former chief of the U.S. Geological Survey’s (USGS) Grand Canyon Monitoring and Research Center.

Powell, Schmidt says, might not have imagined that these urban complexes “would have these tentacles that extended way out into the distant landscape [or] the degree to which these big urban centers would be maintained by these really long canals…these really complicated electricity transmission systems that bring in power from distant coal-fired and nuclear and hydroelectric dam facilities.”

Although Powell’s vision of small communities was largely focused on irrigated agriculture, water management, he thought, would be developed at a more local scale. This would, among other benefits, help to hedge against the uncertainties of climate variation. When, for instance, the 1922 Colorado River Compact apportioned shares of Colorado River water to seven states, it was during a particularly wet period, leading to overestimated water allocations.

“One of the big things that would’ve happened if we’d listened to Powell is that we…would have responded earlier to the information about global climate change,” says John F. Ross, author of The Promise of the Grand Canyon: John Wesley Powell’s Perilous Journey and His Vision for the American West. “He was a great proponent of America’s potential; he just wanted to do it in a way that was sensible to what was on the ground.”

When the West Was Young

The Union Army major—who was injured while fighting in the Battle of Shiloh—conquered the unpredictable 1,600-kilometer route with one arm, a small fleet of wooden rowboats, and a cobbled-together team of nine willing but inexperienced adventurers.In 1869, as the post–Civil War United States was knitting itself back into a union, the sparsely settled expanse of states and territories that stretched between the 100th meridian and the Pacific coast was still a great unknown for many Americans. That year, Ulysses S. Grant was inaugurated as the 18th president of the United States, Wyoming became the first U.S. state or territory to grant women’s suffrage, and a spike driven at Promontory Summit in Utah connected the country’s first transcontinental railroad.

To many communities in the East and Midwest, the newly accessible West was brimming with possibility and scarcely tapped resources. John Wesley Powell, then a professor of geology at what is now Illinois State University, had identified an opportunity as he contemplated the last blank spot on the map of the continental United States: the Colorado Plateau. Today it’s an area made up of eight national parks—including Arches, Zion, and Grand Canyon—and nearly two dozen national monuments, historic sites, and recreation areas. In Utah alone, the region’s five national parks brought in 15.2 million visitors in 2017.

But in the late 19th century it was an altogether different story. When Powell undertook the 3-month descent of the Colorado River in the name of science, the journey was considered by some to be all but suicidal. Still, the Union Army major—who was wounded while fighting in the Battle of Shiloh—conquered the unpredictable 1,600-kilometer route with one arm, a small fleet of wooden rowboats, and a cobbled-together team of nine willing but inexperienced adventurers (all white men).

Powell’s expedition down the Colorado was honored by the U.S. Postal Service on the expedition’s centenary in 1969. Credit: U.S. Bureau of Engraving and Printing, Smithsonian National Postal Museum

The expedition departed from Wyoming’s Green River City on 24 May 1869, with 10 months’ worth of supplies, an optimistic collection of scientific tools, and, among some of the men, hopes of finding a fortune. Four men would eventually abandon the expedition, one at the first opportunity and three others less than 2 days before the remaining team successfully emerged from the Grand Canyon. (Those three men were never seen or heard from again.)

John Wesley Powell at age 35, the year he led the first expedition down the Colorado River and through the Grand Canyon. Credit: USGS

Although Powell’s scientific ambitions for the expedition were largely scuttled by the demands of survival, the widely heralded trip would help to launch his decades-long career as a geologic surveyor, shrewd political player, and government administrator. His recommendations to Congress would be instrumental in the creation of the U.S. Geological Survey, and he would later serve a dozen years as its director while also leading the Smithsonian’s Bureau of Ethnology and helping to found the Cosmos Club and the National Geographic Society.

But it was Powell’s unswaying advocacy for land and water management in the West that would prove to be one of his most remarkable legacies.

A Watershed Idea

It was the railroad that made it possible for Powell and his team to launch the expedition from the banks of the Green River. The conveniently located station at Green River City meant that Powell could easily bring his boats and supplies by train. But the technology that benefited Powell’s plans in 1869 would also facilitate the idealistic expansion that he would ultimately spend the latter part of his career warning against.

The completion of the transcontinental railroad was especially timely for a nation in pursuit of Manifest Destiny, which disregarded the realities of climate and the native peoples who occupied the land in favor of spreading American industrialism and progress from coast to coast. Politicians, speculators, and homesteaders were eager to exploit the promise of the West’s seemingly endless resources and would be quick to deny the hard truth that lives and livelihoods depended on one all-important ingredient: water.

In his 1879 Report on the Lands of the Arid Region of the United States, with a More Detailed Account of the Lands of Utah, with Maps, Powell foresaw the consequences of applying American optimism—and opportunism—in a part of the country where annual rainfall measured below 50 centimeters a year. He warned that there wasn’t enough water to support large-scale farming or the rapid settlement of federal lands stimulated by the Homestead Act of 1862. In addition, the costs of establishing effective irrigation systems threatened to keep control out of the hands of small farmers.

Certain conditions, Powell said, had to be met to develop the region successfully, including the identification of irrigable areas and local control of dam and irrigation projects.

It was a position Powell would refuse to abandon.

As director of the USGS in 1890, Powell presented this map of western watersheds (drainage districts) to Congress. Credit: USGS

While testifying before a congressional committee in 1890, when he was head of the USGS, Powell deployed a unique visual aid: a map that divided the western states and territories into a series of drainage districts.

On first viewing, it’s a surprising example of 19th century cartography, made all the more striking with rich colors and irregular, organic-looking boundaries that contrast sharply with the boxy borders we’re familiar with today.

But the schematic didn’t hold water, so to speak, with a nation determined to grow and expand. The outlook of the nation was invested in myths that encouraged development and defied science, whereas Powell, Schmidt says, lacked the tolerance for pursuing such myths, including the widely held belief that “rain follows the plow.”

In 1902, the year Powell died, Congress passed the Reclamation Act to “reclaim” the arid region for agriculture and settlement.

“That set the stage for this really large-scale water development in the West that almost defied the functioning of the watershed from an ecological perspective,” says Sandra Postel, founder and director of the Global Water Policy Project and author of Replenish: The Virtuous Cycle of Water and Prosperity.

Today, “the river is really operated more according to needs for hydropower, flood control, irrigation, and water supply,” Postel says. “You couldn’t have cities like Las Vegas and Los Angeles and Phoenix and Tucson without this extra water.”

In the Same Boat

Powell “introduced the idea that arid cultures either stood or fell…not on the absolute amount of water, but on how equitably—politically and economically—the system divided that resource.”Although deeply unpopular at the time, today, it’s apparent that Powell’s insistence on viewing the West’s water problem with scientific objectivity was a forward-thinking approach. Now science is taking a leading role in helping to reclaim the region for the environment while facilitating ways for a growing population to live there sustainably.

Powell believed that “science is a process of continually improving the details of our understanding of natural processes, and he would be very proud of the role of science in informing river management and protection,” says Schmidt.

According to Ross, Powell set the stage for the type of conversation we should be having about our natural resources. “He introduced the idea that arid cultures either stood or fell…not on the absolute amount of water, but on how equitably—politically and economically—the system divided that resource,” he says.

This boat, the Emma Dean, carried Powell and his characteristic chair down the Colorado in 1872. Credit: National Park Service

And what lessons can be taken from Powell as the West moves forward?

Says Ross, “We’re seeing this kind of bioregionalism now, where decisions are made not by the federal government but on a more local, or regional, basis—[which is] really the only way to work out these very knotty issues.”

Postel says that successful restoration often involves collaboration, such as conservationists working with farmers to find solutions to water management issues.

“If we get smarter about using and managing water, we can do better with what we’ve got than we’re currently doing,” she says.

As the challenges and accomplishments of western settlement continue to ebb and flow, Powell’s influence still lingers.

Like Postel, Schmidt believes that the key to water management in the West is in working together as a watershed community. “In a sense, that’s an idea of Powell’s that still exists today. It’s just that the community that we call the watershed includes the entire Colorado River basin. It includes every one of the seven states, all sitting around the table together.”

—Korena Di Roma Howley (korenahowley@gmail.com), Freelance Journalist

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