EOS

Earth Day was first celebrated on 22 April 1970. Gaylord Nelson, then a U.S. senator from Wisconsin, came up with the idea after he saw the aftermath of the massive oil spill off the coast of Santa Barbara, Calif., in 1969. Nelson wanted to have a “national teach-in on the environment.” Pete McCloskey, a conservation-minded Republican congressman from California, cochaired the event, which was organized by Denis Hayes, then a student at Harvard University’s John F. Kennedy School of Government. More than 20 million Americans across the country demonstrated for a healthy, sustainable environment. A few months later, President Richard Nixon proposed the creation of the Environmental Protection Agency, which was established by the end of the year.

Almost 50 years later, people all over the planet continue to celebrate Earth Day. This year its theme is “Save Our Species,” which draws attention to the rapid loss of biodiversity around the world caused by human activity, including climate change, deforestation, habitat loss, trafficking and poaching, unsustainable agriculture, and the proliferation of pollution and pesticides.

AGU’s podcast, Third Pod from the Sun, has several episodes focused on these very issues. . .

“Tracking Adorable Chainsaws”: Every summer, researchers at the National Oceanic and Atmospheric Administration’s Alaska Fisheries Science Center journey to the beaches of the Pribilof Islands to study northern fur seals and try to understand why their populations have been declining since the mid-1970s. . .

“Chasing Narwhals, Unicorns of the Sea”: University of Washington biologist Kristin Laidre travels to the Arctic to study animals many of us have seen only in pictures. She has successfully tracked down the elusive narwhal, seeking to understand how the loss of sea ice and the effects of climate change are altering Arctic ecosystems. . .

“Gunslingers of the Sea”: Ocean acoustics specialist Joe Haxel describes the myriad of animals that contribute to Earth’s underwater soundscape, including fish that growl and crabs that scratch their backs. Haxel discusses how he and his colleagues identified snapping shrimp by their characteristic racket and what their presence means for marine life along the Oregon coast. . . Check out these and other episodes and subscribe to the podcast. Consider applying for an AGU Celebrate 100 Grant for up to $10,000 to reimburse the expenses of your Earth Day event.

—Joshua Speiser (jspeiser@agu.org), Manager of Strategic Communications, AGU

Every 3 minutes. That’s how often an earthquake struck Southern California from 2008 to 2017, new research published in Science shows.

Scientists have discovered over 1.6 million previously unknown earthquakes, most of them tiny, by mining seismic records. These results, which constitute the most comprehensive earthquake catalog produced to date, reveal in detail how faults crisscross the Golden State and shed light on how one earthquake triggers others.

“Having a better earthquake catalog is just like having a better microscope,” said Robert Skoumal, a geophysicist at the U.S. Geological Survey in Menlo Park, Calif., not involved in this study. “We are able to take a closer look at the location of faults, how those faults rupture, and how they interact with each other.”

Small and Numerous

A tenet of earthquake science motivated Zachary Ross, a seismologist at the California Institute of Technology in Pasadena, and his collaborators: Earthquake catalogs are always incomplete. That’s because small earthquakes, many of which are too tiny to feel, are always lurking below the limit of detectability. And these little temblors are much more numerous than the building-toppling, highway-churning beasts that make headlines.

“For every magnitude unit you go down in size, you get about 10 times as many.”“For every magnitude unit you go down in size, you get about 10 times as many,” said Ross.

Ross and his colleagues used data from over 500 seismometers in the Southern California Seismic Network to tease out small, previously unrecorded earthquakes.

They used a technique called template matching, which involves using the seismic waveforms of known earthquakes as templates and then looking for matches in seismic data collected nearby.

“The shaking that’s recorded…will look almost the same,” said Ross. “They’re seeing all the same rocks as they’re traveling along.”

Down to the Noise

“We’re basically at the noise level of the instrumentation.”Ross and his team combed through a decade of seismic records using over 280,000 earthquakes as template events. They found over 1.6 million new earthquakes as small as magnitude 0.3. Such low levels of ground shaking can also be caused by construction-related vibrations, ocean waves, and nearby aircraft, said Ross.

“We’re basically at the noise level of the instrumentation.”

Using small differences in the arrival times of seismic waves from an earthquake, the scientists calculated the hypocenter of each new event. This information, along with an earthquake’s timing and magnitude, allowed Ross and his colleagues to assemble detailed maps of Southern California’s earthquakes.

Video by Caltech

The new earthquake catalog does a far better job of tracing fault lines and revealing how earthquakes trigger others compared with older records, said Ross.

—Katherine Kornei (@katherinekornei), Freelance Science Journalist

An unexpected acceleration in methane growth is threatening to negate or reverse efforts to stave off climate change by reducing carbon dioxide emissions. Although scientists don’t know where all the extra methane is coming from, it’s clear that drastically reducing emissions from man-made sources will be necessary to meet the goals of the 2015 Paris climate agreement, researchers say.

Methane is a powerful greenhouse gas with more than 25 times carbon dioxide’s ability to trap heat in the atmosphere over a 100-year period. It issues from many natural sources, such as microbes in the digestive tracts of cows and soggy wetlands, as well as man-made sources, such as natural gas wells, coal mines, and human-lit fires.

For thousands of years, levels of methane hovered below 1,000 parts per billion (ppb) in Earth’s atmosphere. When the Industrial Revolution began around 1750, however, atmospheric methane levels started to rise. Although the overall trend has been consistently upward, the rate of increase for methane emissions has accelerated and slowed fitfully since detailed measurement began in the 1980s. By the end of the 20th century methane growth had slowed, and it looked as if the amount in the air had stabilized. Then in 2007, growth began again.

In a new study, Nisbet et al. looked at air samples from around the globe and found that the growth rate of methane sped up yet again in 2014, leading to a total of 1,850 ppb in the air by 2018 and increasing quickly. That’s more than double preindustrial levels—and a much faster increase than scientists anticipated when world leaders came together in 2015 to sign the Paris Agreement, a commitment to limit global warming to well below 2°C.

It’s not clear whether the sudden increase in methane is from natural or man-made sources or whether the growth has been because of a reduction in the destruction of methane, but there is some evidence that natural sources have increased in response to climate warming. Because methane produced by microbes is richer in the carbon-12 isotope and thus lighter than methane emitted by the fossil fuel industry, which has more carbon-13, the team was able to use the gases’ distinct isotopic signatures to estimate the abundance of methane from both sources. The team found that the proportion of the lighter isotope has grown, reversing the trend of the past two centuries.

There are several potential explanations for the increase in lighter methane. One is an increase in emissions from tropical regions, where cattle are abundant and where warmer temperatures and increased flooding could be increasing methane emissions from wetlands. Another explanation is that the atmosphere is losing its ability to cleanse itself of methane with hydroxyl, which breaks methane down and is better at destroying the lighter gas. A third explanation is that a decrease in methane biomass burning, which is relatively rich in the heavier isotope, has camouflaged a rise in fossil fuel emissions.

Regardless of what’s causing the increase, the growth in methane makes it much more difficult to reach the goals of the United Nations Paris Agreement. Anthropogenic emissions have to be reduced sharply if we are to meet the Paris goal, the authors write. There are many possible ways to do this, including curbing leaks from natural gas wells, cutting coal use, and reducing the burning of tropical grassland and forests. (Global Biogeochemical Cycles, https://doi.org/10.1029/2018GB006009, 2019)

—Emily Underwood, Freelance Writer

Despite recent attempts to equip decision-makers with the practical, scientifically based data they need to help communities effectively adapt to climate change, few studies have examined the specific types of climate services these users require. This deficiency includes information about mean sea level rise, a crucial basis for informed decisions about local coastal adaptations.

To address this gap, Hinkel et al. utilize decision analysis to systematically identify which sea level rise data users most need and then determine whether these requirements can be met given current scientific knowledge. The results indicate that the kinds of information that would be most helpful to decision-makers depend upon both the context and the level of uncertainty each user can tolerate.

The team’s analysis identified several types of desired sea level rise information that are currently scientifically attainable. These include high- and low-end sea level rise scenarios created for a range of uncertainty tolerance levels as well as probabilistic predictions, which can support short-term decisions (prior to 2050) in locations where the reasons behind climate variability are relatively well understood. The team also determined that learning scenarios, which estimate what new information about sea level rise will become available in the future, could improve longer-term decisions.

By focusing on the needs of decision-makers as well as sea level rise data that are scientifically attainable, this study offers a noteworthy example of how to bridge the divide between the scientists who produce climate information and the decision-makers who use it. Because this approach can also be applied to other types of climate change mitigation and adaptation information, it offers a practical road map for codesigning the climate services upon which the health and safety of communities around the world will increasingly depend. (Earth’s Future, https://doi.org/10.1029/2018EF001071, 2019)

—Terri Cook, Freelance Writer

Scientists have long known that winds can generate internal gravity waves in the surface layer of Earth’s oceans. Because the kinetic energy of these waves peaks near the inertial frequency, these motions are called near-inertial oscillations (NIOs), and they provide part of the estimated 2 terawatts of energy required to sustain the global system of thermohaline circulation. But the exact contribution of wind power to these near-inertial motions and wind’s relative importance compared to tidal forces remain topics of vigorous debate.

Although a number of previous studies have estimated this wind power input using numerical models, the results have varied widely from 0.3 to 1.5 terawatts. Now, for the first time, Liu et al. have estimated the near-inertial input of wind power solely on the basis of observations. Using hourly ocean current measurements from surface drifters combined with satellite-derived surface wind measurements, the team calculated that from 1993 to 2016 the worldwide near-inertial wind power contribution was 0.3 to 0.6 terawatt. The researchers also found that the strongest flux of energy occurs between 30° and 60° latitudes during the winter season, when storms are the most prevalent.

The team’s revised estimates are considerably lower than those derived from numerical models, a fact the authors attribute to the models’ disregard of the effects of ocean currents on wind stress. Their sensitivity testing suggests this omission could overestimate local, near-inertial wind power by up to 120%.

By narrowing the estimated contribution of near-inertial wind power to the energy driving global ocean circulation, this study has the potential to reduce the uncertainties of parameterizing the effects of NIOs on thermohaline circulation and climate in numerical models. As such, this contribution is likely to provide a new benchmark for comparing future calculations. (Geophysical Research Letters, https://doi.org/10.1029/2018GL081712, 2019)

—Terri Cook, Freelance Writer

With greenhouse gas emissions unabated, scientists are increasingly concerned that different components of the Earth system will be stressed to their “tipping points,” defined as critical thresholds at which small perturbations can qualitatively alter the state or development of a system. The collapse of the Antarctic ice sheet, leading to dramatic sea level rise, is one example of a tipping point in the Earth system.

At the recent General Assembly of the European Geosciences Union (EGU), a group of European climate scientists introduced a new 4-year project that will urgently advance current understanding and identify tipping point thresholds that scientists say, if crossed, would be very dangerous for life on Earth.

A main goal for TiPES is for interdisciplinary collaboration to clarify theory and for modeling to explain the dynamics and the thresholds for climate change tipping points.Funded by the European Union’s Horizon 2020 program, Tipping Points in the Earth System (TiPES) already enlists more than 30 European scientists from more than 15 institutions.

“The most important thing is that this will have impact,” says Peter Ditlevsen, a University of Copenhagen climate scientist and the TiPES project lead and main coordinator. “This is the whole discussion of 1.5° warming or 2.0°—of what is safe. The short answer to that right now is we don’t know.”

Ditlevsen points out that Earth system tipping points are definitively found in the paleorecord, but modeling and theory do not adequately explain the abrupt changes.

“The current modeling is getting better but still does not explain processes that are strongly nonlinear,” he says.

According to Ditlevsen, key goals for TiPES are interdisciplinary collaboration to clarify theory and refining modeling to explain the dynamics and thresholds of climate change tipping points.

Identifying Tipping Point Components

Niklas Boers, a scientist at Germany’s Potsdam Institute for Climate Impact Research and TiPES associate coordinator, presented an outline of the TiPES project at an EGU session on 11 April.

Boers identified the six real-time tipping point concerns for which TiPES will provide critical threshold and system dynamics data: the Atlantic Meridional Overturning Circulation; the Arctic and Antarctic ice sheets and sea ice; the Amazon rain forest ecosystem; the South American and Asian monsoons; the Mediterranean region, with its risk of desertification; and the alpine regions, which are already experiencing dramatic melting glaciers.

“We have the mathematicians, we have the climate scientists, we even have people dealing with societal decision-making, so we really have a good group of people to do these tipping point challenges.”“I would be really happy if after 4 years we would be able to give more precise estimates of the critical values of the anthropogenic forcing, in terms of greenhouse gases released to the atmosphere, at which we would expect any of these major tipping points to tip,” Boers says.

TiPES is currently seeking to expand, and project leads anticipate having a full team in place in early September. An official launching ceremony for TiPES is set for mid-October at the Institut Henri Poincaré in Paris.

Anna von der Heydt, a climate scientist at the Institute for Marine and Atmospheric Research in Utrecht, Netherlands, and another TiPES associate coordinator, says she is excited that TiPES will both clarify key scientific questions, such as her current focus on the equilibrium climate sensitivity, and also enlist a cross-disciplinary approach to tackle tipping points.

“We bring together so many different disciplines,” von der Heydt says. “We have the mathematicians, we have the climate scientists, we even have people dealing with societal decision-making, so we really have a good group of people to do these tipping point challenges.”

—Richard Blaustein (@richblaustein), Science Writer

AGU received the very first Clean Energy DC Award to honor its commitment to sustainability through our newly renovated headquarters building.As the leader of an innovative, forward looking organization, I am proud to share that AGU received the very first Clean Energy DC Award to honor its commitment to sustainability through our newly renovated headquarters building, the first net-zero energy commercial renovation in Washington, D.C. This award was presented to AGU during the District Sustainability Awards ceremony on 17 April 2019.

Each year, the Washington, D.C., Department of Energy and Environment (DOEE) presents District Sustainability Awards to nonprofits, educational organizations, and private-sector businesses that support sustainable District goals, including energy and water conservation, green building and construction, healthy food access, solar energy production, storm water management, and sustainable waste management. This year, AGU was included in this distinguished class of honorees.

Being the first recipient of the Clean Energy DC Award is not only an honor but also a signal that our building is already an important achievement in sustainability.Being the first recipient of the Clean Energy DC Award is not only an honor but is also a signal that our building is already an important achievement in sustainability. This is just the beginning of our building’s legacy as AGU demonstrates that a building located on a tight urban footprint can operate on a net-zero energy basis, reduce its carbon footprint, and serve as a productive and healthy place to work and meet. Earlier this year, in recognition of this commitment to sustainability, Washington, D.C., mayor Muriel Bowser signed the Clean Energy DC Omnibus Amendment Act of 2018 at AGU’s renovated building. This historic piece of legislation will require electricity in the city to come from 100% renewable sources by 2032, among other sustainable initiatives and incentives.

The AGU community is incredibly grateful to receive this recognition from the DOEE and its director, Tommy Wells. AGU has appreciated our partnership with the District and its agencies to explore strategies to realize our net-zero energy goals. We now aspire to lead and serve as an example to others in how to implement sustainable solutions and technologies in their own building or renovation projects, and this award demonstrates the impact our building has already had in the local area.

The AGU community should be proud of the efforts that went into making our headquarters a model for other buildings to help our society work toward a more sustainable city, country, and world.I would like to especially acknowledge the members of our AGU building staff team, including Janice Lachance, Mike Andrews, Matt Boyd, Emily Johnson, Cristine Gibney, Liz Landau, Beth Bagley, Ron Bennett, Sabina Sadirkhanova, Michelle Brown, and Beth Trimmer, for their incredible efforts that made our net-zero energy renovation a reality. The AGU community should be proud of the efforts that went into making our headquarters a model for other buildings to help our society work toward a more sustainable city, country, and world.



—Chris McEntee (agu_execdirector@agu.org), Executive Director/CEO, AGU

Tiny pieces of plastic debris—“microplastics”—have been found in the deep ocean and even in human stool samples.

Now, for the first time, this form of pollution has been spotted on a glacier. By carefully sifting through natural debris atop a glacier in the Italian Alps, scientists collected polyester, polypropylene, and polyethylene fragments and fibers. These microplastics, likely transported to their remote location by the wind or hikers, are evidence of the widespread influence of plastic pollution, the researchers conclude.

Clogs on a Glacier

Roberto Sergio Azzoni, an environmental scientist at the University of Milan, and his collaborators focused their research on Forni Glacier. This glacier, located about 100 kilometers north of Milan, is one of the country’s longest and a popular site for hikers and climbers. The researchers collected supraglacial debris—bits of soil, rock, and dust—from the top of the glacier.

They wore cotton clothing and wooden clogs during the fieldwork to avoid inadvertently leaving plastic behind, a real concern because outdoor clothing and hiking equipment often contain plastics.

Back in the laboratory, Azzoni and his colleagues extracted tiny pieces of plastic, most smaller than 50 micrometers in diameter, from the debris on the basis of their density.

The scientists found that microplastics were numerous, about 75 pieces per kilogram of dried glacial debris, the team estimated.

“That’s very much in the range of variability of plastic contamination observed in marine and coastal sediments,” Azzoni said in a press conference last week at the General Assembly of the European Geosciences Union in Vienna, Austria, where these results were presented.

These microplastics were likely shed by hikers’ clothing and gear or transported by the wind from nearby cities, the researchers propose. All in all, between 131 million and 162 million pieces of microplastics are likely lurking within the lower part of Forni Glacier, the team estimated.

Glaciers aren’t so immaculate after all, the researchers concluded, and that’s potentially bad news: Microplastics released by melting glaciers can contaminate the glacial runoff water that’s used by local communities for drinking and irrigation.

These future effects should be investigated and monitored, said David Jones, an environmental scientist at the University of Portsmouth in the United Kingdom not involved in the research. Although it’s not at all surprising to find microplastics on glaciers, it’s critical to determine the consequences of these contaminants, said Jones. “The question is what the impact is going to be.”

—Katherine Kornei (@katherinekornei), Freelance Science Journalist

European geoscientists recently called for integration and cooperation between member states of the European Union (EU) to benefit global scientific research and progress.

At a 10 April session at the European Geosciences Union (EGU) General Assembly 2019 in Vienna, Austria, scientific and political leaders spoke about mounting threats to scientific progress and how a lack of European unity could damage research and researchers alike.

The looming specter of the United Kingdom’s exit from the EU in October, the rise of populism in the United States and elsewhere, rampant proliferation of fake news, and growing attacks on scientific credibility could interrupt the EU’s “virtuous circle” of economic growth and scientific discovery, according to former Italian prime minister Mario Monti. Monti also served as a European commissioner from 1995 to 2004.

“We may witness an undoing of the formerly virtuous circle into a potentially vicious circle, where the forces at play—followership, short-termism, personal interest, rejection of competence, fake news, fake history, and social media—may bring…authoritarian or slightly more demagogic organizations of power at the national level,” Monti said during the session.

“The next victim, I’m afraid, is going to be you, that is, science,” Monti said, “because there was once upon a time, and there still is, a very virtuous cycle between Europe, European integration, and science.”

Following the conference session, the EGU Council released a statement saying that “the EGU firmly believes that threats to a united Europe are threats to scientific research.”“Populism and science are completely incompatible.”

Decrying Populism

“Populism and science are completely incompatible,” virologist Ilaria Capua said during her address. Capua was a member of the Italian Parliament from 2013 to 2016 and is a professor at the University of Florida in Gainesville. “Your decisions or opinions are more linked to your emotions and not to facts. And we know that science doesn’t work like this.”

Populistic politics tries to appeal to average citizens who feel that their concerns are neglected in favor of those of elite groups. This, in and of itself, isn’t bad, Monti said. “In most cases, [populists] point to really existing problems. I happen to believe that in 98% of the cases they come to wrong or impracticable or counterproductive solutions.”

Mario Monti speaking during a 2003 news conference in Brussels, Belgium, during his time as European commissioner. Credit: danacreilly, CC BY 2.0

For example, Monti said that populism promotes closing national borders and restricting the outward flow of information as economic conditions worsen domestically. However, doing so also restricts the flow of scientific information, limits researchers’ access to resources and equipment beyond their borders, and stymies scientific developments that might stimulate economic growth.

“To tackle the greatest challenges that we face such as antibiotic resistant bacteria, climate change, energy, food and water security, the scientific community within Europe needs to work together, pooling complementary skills, expertise and infrastructure, and share data and information within an open and unified environment,” according to the EGU statement.

Fighting Attacks on Facts and Science

“As a virologist,” Capua said, “I can tell you that I am very, very concerned of the next threat that is going to become viral. And this threat is the fake news threat for science.”

Capua cited the antivaccination movement, protests against necessary animal trials, and misleading information campaigns about disease outbreaks as examples of fake news that directly hinders scientific progress and puts people’s lives at risk.

Ilaria Capua is a virologist and former member of the Italian Parliament. Credit: Ilaria Capua

“There is an industry out there, ready to make noise about whatever they dislike,” she said. “And this industry has a very clear objective. And the objective is to change opinions and to make money.”

“Despite communication being very easy today, so we have unprecedented opportunities to communicate, misconception and fake news have never been so high as well,” said EGU president Alberto Montanari. “It’s a contradictory setting.”

Moreover, Monti and Capua explained, fake news catches on by using short and catchy—and also inaccurate—descriptions of scientific research, conclusions, or applications. Refuting those 5-second sound bites, Monti said, takes much longer and is not an effective method of defending the benefits of EU integration on science.

Capua has experience with how the fake news machine can personally affect researchers. In 2014, Capua learned that she was the target of a fake news conspiracy theory that accused her of deliberately causing international disease epidemics to profit from patented vaccines. Because of this attack, she faced invasive international investigation, damage to her scientific credibility, and the possibility of life imprisonment. She was cleared of all charges in 2016.

“We cannot lose our credibility. We cannot. We must not.”Her story, she said, is an extreme example of the attacks and gaslighting many climate scientists have faced for more than a decade.

“We were all brought together under one umbrella of European research,” she said. “We need to prepare because attacks will come and we need to develop strategies to maintain our credibility. And we need to find new ways to engage with the public.”

“We cannot lose our credibility. We cannot. We must not,” Capua urged.

Being Vocal Supporters of Integration

During the session, Günter Blöschl, a hydrologist at the Vienna University of Technology and a former EGU president, asked a question that is likely on many scientists’ minds: “What can we as average researchers do to foster integration in our daily work?”

“It’s very simple,” Monti replied. “Be yourself and tell surrounding people who you are and how the EU relates to you and the aspects in your [research] activity.”

“Thanks to the army of European scientists, the scientific advancements of Europe are one of the main products of European integration.”Blöschl told Eos that after the session, Monti added that scientists should also be vocal about the benefits of an integrated Europe for their research. Blöschl said, “My reaction to this is that in simple answers there is often a lot of truth.”

“I believe it is very important to promote Europe with the positives that Europe achieves,” Monti said. “Thanks to the army of European scientists, the scientific advancements of Europe are one of the main products of European integration.”

“And, of course,” he added, “good education is of the essence because otherwise, electors will not make [informed] use of their electoral power, which may correspond to the political system delivering what they really care for.”

What’s at Stake

“The economic arguments are clear,” the EGU Council stated. “For every euro invested in research and innovation, the return into the economy is multiplied by between a factor 6 to 8. Beyond the simple economic principles, it is also widely recognised that European Framework programmes provide a unique and critical mechanism for fostering and enabling trans-national collaboration on research and innovation.”

“This is what European research does. It brings together an immense strength, love, and passion that we have in Europe for science.”Capua agrees. “What does European research do?” she asked. “It creates teams. It creates fantastic teams of people who worked together in the same place, or in another place in Europe, or in another place in the world.”

“This is what European research does,” she said. “It brings together an immense strength, love, and passion that we have in Europe for science. And it brings diversity. And this is what is empowered by our European research programs.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

During sporadic eruptions in the solar atmosphere, such as solar flares and coronal mass ejections (CMEs), very energetic solar particles are produced. These particles can reach Earth quickly, sometimes within a quarter of an hour, and can significantly alter radiation levels in the atmosphere and even at ground levels.

These intense events, known as ground level enhancements (GLEs), are potentially hazardous for aircraft electronics and passengers, particularly those flying over the poles, where there is less protection from Earth’s magnetic shield. In a new study, Mishev and Usoskin calculated the radiation levels passengers would be exposed to during a typical commercial flight over the pole, simulating the radiation levels present throughout a GLE.

When solar particles hit Earth’s atmosphere, they induce nucleonic-electromagnetic-muon showers and cause an increase in secondary particles bombarding the ground and detected by the Global Neutron Monitor Network, a system of ground-based monitoring stations around the world. When such an increase is recorded simultaneously by at least two monitors and a spaceborne probe, a GLE alert is set.

A map of global radiation doses at 40,000 feet (12 kilometers) during GLE 72 on 10 September 2017. Here μSv.h-1 means microsieverts per hour. Credit: Mishev and Usoskin, 2018, https://doi.org/10.1029/2018SW001946

To determine how much radiation commercial airline passengers and crew are likely to be exposed to during a GLE, the team examined data from one such event, which occurred on 10 September 2017. During the week leading up to that date, several solar flares and CMEs were observed, and a flare with a CME on 10 September triggered a globally observed event called GLE 72, detected by the monitor network. Using the network’s data, the team modeled likely radiation exposure during two transcontinental polar flights at several cruise flight altitudes ranging from 30,000 to 50,000 feet (about 9 to 15 kilometers).

In the worst-case scenario, in which the airplanes took off close to the onset of the GLE and maintained a high cruise altitude of 40,000 feet (12 kilometers), passengers on a flight from Helsinki, Finland, to Osaka, Japan, would have received a roughly 90-microsievert dose of radiation, the team found. A flight from Helsinki to New York would have received a slightly higher dose, around 110  microsieverts.

Such levels are far below an average American’s annual radiation exposure of 1 millisievert. But they remain above typical background radiation and could pose a cumulative health risk for aircraft crew and pilots, who already receive roughly triple the average yearly dose of radiation. Radiation can also upset or damage the sensitive electronics aboard commercial aircraft, underscoring the importance of preparing for severe space weather. (Space Weather, https://doi.org/10.1029/2018SW001946, 2018)

—Emily Underwood, Freelance Writer

As our knowledge of Earth’s geometry has become more precise, we have come to realize that our planet is not a uniform sphere, as was previously believed. Earth’s rotation flattens it into an ellipsoid that is wider around the equator than around the poles. Heavy mineral deposits, deep-sea trenches, and magma reservoirs alter the distribution of mass, distorting Earth’s gravitational field on regional scales. The source of the largest equipotential gravitational field distortion in the world, a 106-meter anomaly in the Indian Ocean, remains a mystery, so an Indian research group went to sea last year to gather clues.

One way of describing Earth’s irregular shape, the geoid, is a hypothetical equipotential surface. That is, the geoid is what the shape of Earth’s surface would be if oceans covered the whole planet and if there were no winds and tides to ruffle the surface—just Earth’s rotation and the forces of gravity. Regional deviations of the geoid from an idealized hydrostatic ellipsoid, known as geoid anomalies, can be high geoid (positive) or low geoid (negative). Positive anomalies indicate a dense concentration of mass, like recently subducted oceanic slab. Negative anomalies indicate regions of less dense materials—a reservoir of hot magma, for example—beneath the surface. Extreme geoid anomalies are interesting because they imply a significant shift in the subsurface geodynamic conditions.

This expedition, the first of its kind, aims to unravel the source of this geoid anomaly—the largest in the world.Geophysical studies over the past few decades have found an extremely low geoid anomaly in the Indian Ocean. This low-gravity region, which shows up as a 106-meter “dent” in the geoid, is referred to as the Indian Ocean Geoid Low (IOGL) [Sreejith et al., 2013; Ghosh et al., 2017]. Why is Earth’s mass in this region so low? The dominant hypotheses are based on seismological investigations [Čadek and Fleitout, 2006; Reiss et al., 2017] and viscoelastic modeling [Ghosh et al., 2017]. However, because of inherent methodological limitations and an almost complete lack of offshore seismological observations from this region, the mystery of this perplexing anomaly remains unsolved.

Last year, a research team at the National Centre for Polar and Ocean Research (NCPOR) in India led an extensive long-term deployment of broadband ocean bottom seismometers (OBSs) in the IOGL region. This expedition, the first of its kind, aims to unravel the source of this geoid anomaly—the largest in the world.

The scientific team deploys an ocean bottom seismometer (OBS) from the deck of ocean research vessel (ORV) Sagar Kanya. Credit: Rahul Mavi Hypotheses and Theories

The low-geoid estimates in the Indian Ocean span a vast area to the south of the Indian subcontinent. Mathematically speaking, these estimates are dominated by very long wavelength (>3,000 kilometers) anomaly components [Sreejith et al., 2013]. The most plausible explanation so far is that anomalous lower mantle convection, weakly coupled to shallow crustal plate motions, causes the large geoidal undulations [Chase, 1979].

Previous researchers have put forth several distinct theories underlying the IOGL’s existence. These include structural undulation in the core-mantle boundary [Negi et al., 1987], seismic low-velocity anomalies in the upper mantle [Rao and Kumar, 2014], and subducted slabs of oceanic origin that collected in a “slab graveyard” in the lower mantle during the Mesozoic era [Spasojevic et al., 2010; Simmons et al., 2015].

The IOGL could be a response to an extended mass anomaly with multiple sources.Numerical modeling supported by global seismic tomography results provides new insight into the possible source and the mantle geodynamics beneath the Indian Ocean [Ghosh et al., 2017]. This model shows a low-density anomaly between the upper and middle mantle (~300–900 kilometers in depth) that migrates from an African deep mantle plume toward the northeast, driven by movement of the Indian tectonic plate.

Another view indirectly relates the geoid anomalies to intraplate deformation zones that are a surface manifestation of the mantle convection processes. These zones, which include the Central Indian Ocean Deformation Zone between the Indian and Australian plates, are associated with large-scale faults, folds, high heat flow, and seismic activity [Mishra, 2014]. Whether these intense deformation zones within the lithosphere really contribute to such a large-wavelength geoid anomaly is still under debate.

Considering these hypotheses and the broad nature of the anomaly, the IOGL could be a response to an extended mass anomaly with multiple sources. Therefore, local and regional seismic velocity models become critically important to highlight the multiple wavelength sources of this colossal geoid anomaly in the Indian Ocean.

Imaging Deep Structures Beneath the Indian Ocean

To understand and narrow down the gap between the dynamics of materials beneath the surface and its surface manifestation as the IOGL geoid anomaly, NCPOR started a large-scale seismological array deployment in the Indian Ocean. As a pilot project, in May 2018, NCPOR deployed 17 passive broadband OBSs. These sensors will record continuous time series data of seismic events for 1 year. The array extends laterally from the foci of the nearly circular IOGL anomaly to its southern extent (Figure 1).

Fig. 1. An OBS array (black triangles) was deployed in a large geoid anomaly (blue area) in the Indian Ocean in May 2018. Black lines represent major tectonic boundaries. Green triangles and red squares represent other regional seismological stations around the IOGL region.

The stations are spaced at distances of approximately 100 kilometers along the OBS profile. The OBS systems are equipped with four-component sensors (one in the vertical direction, two in the horizontal direction, and a hydrophone) and a digitizer with a dynamic range of –3 decibel points at 120 seconds, taking 100 samples per second.

The scientific team (a) tests the releaser and (b) assembles the OBS before releasing it into the deep parts of Indian Ocean. Credit: Lachit Ningthoujam Our Focus and Potential Applications

We hope to explain the key factors that make the Indian Ocean geoid anomaly different from the geoid anomalies in other parts of the world.The prime objective of our experiment is to image the deep mantle structures and their relationship with the geoid low anomaly in the Indian Ocean. We hope to explain the key factors that make the Indian Ocean geoid anomaly different from the geoid anomalies in other parts of the world. Such findings would offer several types of opportunities to geoscientists researching deep-ocean mantle dynamics.

For example, the ancient Tethys Ocean began to close as the Indian continent separated from the African, Antarctic, and Australian continents during the Late Cretaceous to early Paleocene, leading to the opening of the Indian Ocean. Researchers believe that during this period, slabs of oceanic origin subducted into a slab graveyard in the lower mantle [Spasojevic et al., 2010], which possibly contributed to the IOGL. We hope that the seismological data from the OBSs in the Indian Ocean will be able to resolve the uncertainties associated with the hypothesis that the subducted Tethyan plate seized beneath the Indian plate.

We foresee that seismological data acquired during the deployment period of our project will not only benefit the researchers working in solid Earth science beneath the unexplored parts of oceanic plates but will also help to quantify deep-ocean wave dynamics. For example, one research group at the Institut de Physique du Globe de Paris in France is currently attempting to extract signals from seismic network data that could model ocean wave dynamics using a beam-forming approach.

The U.S. Geological Survey (USGS) Global Seismographic Network shows 290 earthquakes with magnitudes at or greater than 5.5 recorded between May and December 2018 within an arc distance of 20° to 120° from the center of the OBS array that we deployed in May 2018. We are using these earthquakes to model the theoretical ray paths between sources and stations. Modeling theoretical ray paths gives quite a good sense of the resolution we can expect using teleseismic earthquake data from this experiment (Figure 2). Local earthquakes are not common in the central Indian Ocean, but with the help of our OBS array, we might be able to look into the local seismicity around the IOGL region.

Fig. 2. Locations of teleseismic earthquakes (red stars) recorded by the Global Seismographic Network, USGS, from May 2018 to December 2018 (top). Theoretical ray paths of P phase waves between the locations of OBS stations and the teleseismic earthquakes ranging between 20° and 120° arc distance from the center of the array (bottom). The latitudinal (bottom left) and longitudinal (bottom right) cross sections show the convergence boundary of the dense ray path (dashed lines) up to 1,900 and 600 kilometers of depth, respectively.

The seismological data retrieved from the IOGL experiment will be under a moratorium for 3 years. After that, it will be made available to the geoscience community and can be accessed on request through the portal of India’s National Centre for Seismology.

View from the deck of the ORV Sagar Kanya during the pilot phase of OBS deployments in the Indian Ocean. Credit: Lachit Ningthoujam Acknowledgments

We thank the director of NCPOR, Goa, for his sustained support in the successful deployment of passive OBSs. The project is funded by the Ministry of Earth Science, government of India, through research grant MoES/P.O.(Seismo)8(11)-Geoid/2012 (dated 15 November 2013). For the locations of land-based seismometers, the authors thank the National Centre for Seismology, Delhi, and Indian Institute of Science Education and Research, Pune. We acknowledge USGS for providing global earthquake locations. The authors gratefully acknowledge all scientific, fieldwork, and logistical help provided by participants of the IOGL project.

The accurate distinction of the original source of water masses in the central Arctic basin provides a key indicator as to how the circulation and trends of this climatically important region might be changing. In the past, the Arctic waters of Pacific Ocean origin have been distinguished from those of Atlantic Ocean origin primarily through the distribution of nutrient concentration. For example, high silicate and high phosphate relative to nitrate are generally indicative of Pacific water sources. However, recent changes to sea ice cover within the Arctic have resulted in the local denitrification of waters such that they become geochemically very similar to the Pacific waters. Alkire et al. [2019] demonstrate the utility of the semi-conservative tracer NO (nitrate and dissolved oxygen) to more accurately identify the fronts associated with water mass contributions of Pacific origin (red dashed line in figure) than identification based on traditional tracers (blue dotted lines).

Citation: Alkire, M. B., Rember, R., & Polyakov, I. [2019]. Discrepancy in the identification of the Atlantic/Pacific front in the central Arctic Ocean: NO versus nutrient relationships. Geophysical Research Letters, 46. https://doi.org/10.1029/2018GL081837

—Janet Sprintall, Editor, Geophysical Research Letters

Scientists have long known that Earth and Mercury have metallic cores. Like Earth, Mercury’s outer core is composed of liquid metal, but there have only been hints that Mercury’s innermost core is solid. Now, in a new study, scientists report evidence that Mercury’s inner core is indeed solid and that it is very nearly the same size as Earth’s solid inner core.

Some scientists compare Mercury to a cannonball because its metal core fills nearly 85 percent of the volume of the planet. This large core—huge compared to the other rocky planets in our solar system—has long been one of the most intriguing mysteries about Mercury. Scientists had also wondered whether Mercury might have a solid inner core.

The findings of Mercury’s solid inner core, published in AGU’s journal Geophysical Research Letters, help scientists better understand Mercury but also offer clues about how the solar system formed and how rocky planets change over time.

Size comparison of Mercury and Earth. Credit: NASA

“Mercury’s interior is still active, due to the molten core that powers the planet’s weak magnetic field, relative to Earth’s,” said Antonio Genova, an assistant professor at Sapienza University of Rome who led the research while at NASA Goddard Space Flight Center in Greenbelt, Maryland. “Mercury’s interior has cooled more rapidly than our planet’s. Mercury may help us predict how Earth’s magnetic field will change as the core cools.”

To figure out what Mercury’s core is made of, Genova and his colleagues had to get, figuratively, closer. The team used several observations from NASA’s MESSENGER mission to probe Mercury’s interior. The researchers looked, most importantly, at the planet’s spin and gravity.

The MESSENGER spacecraft entered orbit around Mercury in March 2011 and spent four years observing this nearest planet to our Sun until it was deliberately brought down to the planet’s surface in April 2015.

Scientists used radio observations from MESSENGER to determine Mercury’s gravitational anomalies (areas of local increases or decreases in mass) and the location of its rotational pole, which allowed them to understand the orientation of the planet.

Each planet spins on an axis, also known as the pole. Mercury spins much more slowly than Earth, with its day lasting about 58 Earth days. Scientists often use tiny variations in the way an object spins to reveal clues about its internal structure. In 2007, radar observations made from Earth revealed small shifts in Mercury’s spin, called librations, that proved some of the planet’s core must be liquid-molten metal. But observations of the spin rate alone were not sufficient to give a clear measurement of what the inner core was like. Could there be a solid core lurking underneath, scientists wondered?

An artist’s concept of the interiors of Earth, Mars and Earth’s moon. New research shows Mercury has a solid inner core like Earth does. Credit: NASA/JPL-Caltech

Gravity can help answer that question. “Gravity is a powerful tool to look at the deep interior of a planet because it depends on the planet’s density structure,” said Sander Goossens, a researcher at NASA Goddard and co-author of the new study.

As MESSENGER orbited Mercury over the course of its mission and got closer and closer to the surface, scientists recorded how the spacecraft accelerated under the influence of the planet’s gravity. The density structure of a planet can create subtle changes in a spacecraft’s orbit. In the later parts of the mission, MESSENGER flew about 120 miles above the surface, and less than 65 miles during its last year. The final low-altitude orbits provided the best data yet and allowed for Genova and his team to make the most accurate measurements about the internal structure of Mercury yet taken.

Genova and his team put data from MESSENGER into a sophisticated computer program that allowed them to adjust parameters and figure out what the interior composition of Mercury must be like to match the way it spins and the way the spacecraft accelerated around it. The results showed that for the best match, Mercury must have a large, solid inner core. They estimated that the solid, iron core is about 1,260 miles (2,000 kilometers) wide and makes up about half of Mercury’s entire core (about 2,440 miles, or nearly 4,000 kilometers, wide). In contrast, Earth’s solid core is about 1,500 miles (2,400 kilometers) across, taking up a little more than a third of this planet’s entire core.

“We had to pull together information from many fields: geodesy, geochemistry, orbital mechanics and gravity to find out what Mercury’s internal structure must be,” said Erwan Mazarico, a planetary scientist at NASA Goddard and co-author of the new study.

The fact that scientists needed to get close to Mercury to find out more about its interior highlights the power of sending spacecraft to other worlds, according to the researchers. Such accurate measurements of Mercury’s spin and gravity were simply not possible to make from Earth. New discoveries about Mercury are practically guaranteed to be waiting in MESSENGER’s archives, with each discovery about our local planetary neighborhood giving us a better understanding of what lies beyond.

“Every new bit of information about our solar system helps us understand the larger universe,” Genova said.

Soil: It helps feed the world, but could it also help our efforts to keep it cool?

Soil is a store for carbon and moisture, and changing the way it is managed could help mitigate or even counteract global warming, according to two studies presented at the recent European Geosciences Union General Assembly in Vienna, Austria.

No-Till Farming

Hannah Cooper of the University of Nottingham in the United Kingdom is investigating the effect of no-till farming on the amount of carbon that is captured by the soil. No-till farming is currently used on about 10% of arable land worldwide.

Cooper took cores from 80 conventionally farmed fields in the United Kingdom’s East Midlands region and from no-till fields right next to those. Some hadn’t been under the plow for a few years; others hadn’t been plowed for up to 15 years.

After 1–5 years, Cooper found the nontilled soil was less porous than tilled soil, and carbon content was about the same.

After 5 years, she found water and roots had an easier time penetrating nontilled soil, and it contained more carbon. The carbon was also increasingly bound in organic compounds, such as ethers and aromatics, which are less readily released into the atmosphere.

“The tillage of arable land is probably the biggest civil engineering operation on the planet, year by year. And yet the scientific basis for why we do it and what benefit it derives is very vague.”Combining these data with the release by the soil of nitrous oxide, another greenhouse gas, Cooper concluded that the emissions from no-till soil had a global warming potential that was almost 6 times lower than that of tilled soil.

Her results were met with a bit of skepticism by Dani Or, a soil scientist and environmental physicist at the Swiss Federal Institute of Technology in Zurich, who was not involved with the study.

“I would say that no-till has tremendous ecological justification, and when it works, it is actually a good thing. The problem is that it is not a solution for all climates, or all soils, or all crops,” he told Eos. “I’m sure their work is very good. But the climate in the U.K. and the climate in the Sahel are quite different—there is a danger of generalization.”

On the other hand, Or said, “People have been plowing their field to change the structure from the dawn of civilization. The tillage of arable land is probably the biggest civil engineering operation on the planet, year by year. And yet the scientific basis for why we do it and what benefit it derives is very vague.”

Radical Climate Modeling Around Irrigation Practices

Whether soil is tilled or not tilled, the climate might benefit enormously by irrigating as much as possible, diverting all available water for that purpose, said Thomas Raddatz, a meteorologist of the Max Planck Institute for Meteorology in Hamburg, Germany.

Raddatz is not really proposing that, he reassured his listeners at the conference, but he did it in a computer model of the climate to see what effect irrigation may have on the climate now and in the future.

In Raddatz’s experiment, in a model of the world not yet burdened by human-triggered greenhouse gas emissions, he diverted all available water on all land masses to reservoirs, from which it was gradually released onto the local soil. To do all that, 41,000 cubic kilometers of water were needed each year, 50 times the amount used for irrigation today.

Surprisingly, Raddatz told Eos, diverting all that water didn’t mean that rivers stop flowing.

Attention must be paid to the climate-related consequences of policies that involve irrigation.“You bring the water to the surface of the land, this enhances infiltration, and after some time you have more drainage again, and you pump this water back to the reservoir. So you cycle it probably several times until it is evaporated to the atmosphere. And even then, for large parts of the Northern Hemisphere, you still keep it likely on the continent, because you also enhance precipitation.”

The global effects of this radical piece of geoengineering would be impressive. The evaporating water takes heat from the surface, causing a 2.1°C cooling over land. Once in the air as vapor, the water acts as a greenhouse gas but also ends up in clouds that radiate energy into space as infrared radiation. On balance, there is a global cooling of 1.1°C and an increase of 2.5 million square kilometers in sea ice in the Arctic. Raddatz also notes a strengthening by 15% of the Meridional Overturning Circulation, the current in the North Atlantic that has a strong influence on Earth’s climate and is thought to be vulnerable to global warming.

Raddatz said attention must be paid to the climate-related consequences of policies that involve irrigation. This concern is motivated by runs of his model in which only some parts of the world were irrigated.

“If the EU decides to have a massive irrigation program, to increase crop yields, to grow biofuels to reach carbon targets, to develop rural areas, they may conclude it is cost-effective. So over decades, you increase irrigation,” Raddatz explains. “But it turns out this decreases the precipitation in the Sahel by 100 to 200 millimeters per year. Then we have a large catastrophe there. And all these 300 million people living there will try to come to the EU. So no one profits. We should care about this, before we do it.”

—Bas den Hond (bas@stellarstories.com), Freelance Journalist

Massive algae blooms are staining Greenland’s ice sheets pink, green, and brown, darkening the ice and causing it to absorb more solar radiation. Recent studies have shown that these blooms play an even bigger role⁠ than dust and black carbon in melting the ice sheet, which holds enough water to raise the ocean by 6 meters (20 feet).  Now, a new satellite survey reveals the algae’s spread from space.

Greenland ice has large, dark swathes of dust, microbes, and other impurities that reduce the ice’s albedo, the amount of solar radiation it reflects back to space. Whereas a pure, blindingly white surface has an albedo of 1, bare ice has an albedo from 0.5 to 0.7, reflecting about 50%–70% of solar energy. As dirt and microbes darken the ice, they lower its albedo even further, causing the ice to melt faster.

Although scientists have set up a number of field stations to study the algal blooms, the sites are scattered and can’t reveal the extent of species such as those in the class Zygnematophyceae, which turn ice brownish gray. In a new study, Wang et al. used data from the Sentinel satellites—Earth-observing satellites launched by the European Space Agency—to quantify algal blooms in the Greenland ice during the summers of 2016 and 2017. Although most satellites don’t typically capture the narrow spectral signals reflected by algal pigments, the team was able to obtain these from Sentinel and tease them apart from dirt and soot, which also darken ice.

Observations from the Sentinel-3 satellite in 2016 and 2017 show widespread algal blooms. Credit: Wang et al., 2018, https://doi.org/10.1029/2018GL080455; data from Copernicus Sentinel-3/ESA

The satellite data revealed widespread algal blooms in July and August, a finding that aligns with the timing of algal proliferations detected at field sites, the team reports. By tracking how algal blooms increase meltwater from the ice sheet’s surface, satellite-based algae mapping could help scientists fine-tune predictions of global sea level rise, they write. (Geophysical Research Letters, https://doi.org/10.1029/2018GL080455, 2018)

—Emily Underwood, Freelance Writer

Extreme precipitation events, which fall into the 99th percentile of daily events, have increased across the contiguous United States since the 1950s in response to rising temperatures. But despite assertions by the climate community that increasing precipitation extremes inevitably lead to higher flood magnitudes, multiple studies have demonstrated that this has not been the case.

In a recent commentary, Sharma et al. suggest a number of reasons for this dichotomy, in the process highlighting the complex relationship between changes in precipitation and flooding. The authors argue that a number of factors, including less soil moisture at storm onset, shifts in atmospheric circulation that reduce storm durations, smaller snowpacks, and earlier snowmelt, can all decrease the magnitude of floods even when the atmosphere holds more moisture as temperatures warm.

Because a substantial amount of uncertainty remains regarding the connections between changes in precipitation and in floods across a wide range of storm, catchment, and hydrologic conditions, the team asserts that a more collaborative approach between the hydrologic and atmospheric communities is needed to advance our understanding of how floods may change in the future. Given the societal implications, the authors argue that better comprehending this relationship and making it more understandable to the climate community and policy makers should constitute a new grand challenge for the hydrologic community. (Water Resources Research, https://doi.org/10.1029/2018WR023749, 2018)

—Terri Cook, Freelance Writer

Atmospheric gravity waves are a type of buoyancy wave that, on breaking or becoming unsteady, deposit their energy and momentum into the mean atmospheric flow driving atmospheric circulation. These waves are ubiquitous in the atmosphere. They are caused by sources mainly in the lower atmosphere; for example, storms, wind flow over mountains, and perturbations in the Polar Vortex. They are an important dynamical means of coupling throughout the atmosphere.

Global climate models do not accurately capture these waves. This can result in modelled temperatures being too cold, and wind speeds too fast in the Polar Regions. Although observations of these waves are needed to constrain their modelled representation, in the Polar Regions there is a paucity of observations, especially of the short-horizontal wavelength waves that have been shown to carry the most momentum.

The Antarctic Gravity Wave Instrument Network (ANGWIN) started as an idea between a few Antarctic scientists who were studying mesospheric (~ 87 km altitude) gravity waves using all-sky airglow imagers to share analysis techniques and to study the gravity wave field at sites around Antarctica. Through the collaboration of different countries and deployment of different instrumentation, ANGWIN has grown to include modelling work and observations across different atmospheric “spheres”, in both Polar Regions.

https://eos.org/wp-content/uploads/2019/04/mesospheric-bore-gravity-wave.mp4

A type of gravity wave called a mesospheric bore, captured using an all-sky OH airglow imager located at Halley, Antarctica. The wave structure can be clearly seen entering the picture from the top right and progressing across the field of view. Details about these kinds of waves can be found in Nielsen et al. [2006]. Credit: ANGWIN community

At the third ANGWIN workshop (in 2016) the large range of ANGWIN activities and results from collaborations became fully apparent. Atmospheric Gravity Wave Science in the Polar Regions and First Results From ANGWIN is a joint special collection of JGR: Atmospheres and JGR: Space Physics which presents the main results from this workshop.

This special collection contains a range of observational and modelling results concerning gravity wave studies throughout the whole atmosphere (from the ionosphere to the troposphere) and across both Polar Regions.

Wave formations seen in tropospheric clouds above Rothera, Antarctica. Credit: Tracy Moffat-Griffin

Many of the papers are single site studies, which are either case studies or climatologies. These use a range of ground-based instrumentation (i.e. radars, radiometers, all-sky airglow imagers, lidars and radiosondes), satellites (i.e. NASA AIM measurements) and modelling work (e.g. a gravity wave ray tracing model, the Horizontal Wind Model and Unified Model). These papers illustrate how different the gravity wave field is across many sites and that the influence of the different gravity wave sources varies considerably, not just with season but also with latitude and longitude.

A highlight of the special issue is research that has resulted directly from early ANGWIN discussions: application of the same analysis software to different locations of airglow all-sky imager data. The work by Matsuda et al. [2017] has provided a clear example of how data and analysis software sharing can yield new insights into a field of research.

Gravity waves seen in OH airglow captured by all-sky imagers at Halley VI station (left) and King Sejong Station (right), Antarctica. On the right, the Milky Way can be seen in addition to the banded structure of a gravity wave. Credit: ANGWIN community

They used four Antarctic sites and developed a new method of analysis (The M-transform) that can be applied to these airglow data. The results show how the phase velocity of mesospheric gravity waves varies at each site and how gravity wave power varies with latitude.

ANGWIN is pursuing more collaborative research through data and software sharing.Moving forward ANGWIN is pursuing more collaborative research through data and software sharing.

By using the same analysis techniques on similar datasets we can build a more robust understanding of the variations and properties of the gravity waves field across the Polar Regions.

—Tracy Moffat-Griffin (email: tmof@bas.ac.uk), British Antarctic Survey, UK; Mike Taylor, Utah State University, USA; Takuji Nakamura, National Institute of Polar Research, Japan; Damian Murphy, Australian Antarctic Division, Australia; Jose Valentin Bageston, National Institute for Space Research, Brazil; and Geonhwa Jee, Korea Polar Research Institute, South Korea

  Deep in the Sahara desert, ancient rock paintings depict a verdant world full of elephants, cattle, giraffe, hippos, and antelope. The people who created these images lived in North Africa when the now hyperarid Sahara was a very different place. About 11,000 years ago, the desert turned green. Grass grew on the dunes, lakes filled dry depressions, grassland animals moved in, and people followed. It was the most recent African Humid Period, or Green Sahara.

During the last Green Sahara, the region received 10 times the rain that falls in the desert now, according to a 2017 study published in Science Advances, led by Jessica Tierney, a paleoclimatologist at the University of Arizona. The study also found evidence of a 1,000-year pause in the Green Sahara conditions 8,000 years ago, during a time when people abandoned permanent settlements in the area.

Mud cores taken off the coast of West Africa indicate the Sahara’s wet period ended abruptly about 5,000 years ago.Tierney studies Earth’s past climate through mud, laid down on the ocean floor over the millennia and pulled up in long columns from the depths with big shipborne drills. These mud columns stretch backward in time as they extend deeper underground, preserving chemical and biological signatures of the climate at the time the mud became mud.

Mud cores taken off the coast of West Africa indicate the Sahara’s wet period ended abruptly about 5,000 years ago. Within a few hundred years, the landscape dried drastically. As the sands took back the Sahara, people congregated along the Nile Valley, and the Egyptian civilizations arose.

Green Saharas recur because of a wobble in Earth’s axis of spin, called axial precession, combined with Earth’s less than perfectly circular orbit around the Sun. The axis of Earth’s spin tilts about 23° from perpendicular with respect to Earth’s orbit around the Sun. This tilt creates seasons, bringing summer to the Northern Hemisphere when the North Pole leans toward the Sun and winter when it angles away. But the axis also rotates slowly, like an unbalanced spinning top. This rotation takes about 26,000 years.

A run of wet years 800 years ago may have aided Genghis Khan in his conquest of Southeast Asia.When precession brings Northern Hemisphere summer into alignment with the closest point of Earth’s orbit to the Sun, the extra warmth juices the African monsoon and brings water to the desert. So the Green Sahara returns about every 20,000 years, unless other climatic patterns, like large-scale glaciation, intervene.

Tierney is interested in how past climate changes, like the Green Sahara, may have motivated human migrations. Even small climatic shifts can have wide-ranging consequences for ecosystems and the people who depend on them. A run of wet years 800 years ago may have aided Genghis Khan in his conquest of Southeast Asia. Drought in Mesoamerica 1,000 years ago may have precipitated the collapse of the Mayan civilization. Humans have lived with change throughout our time on Earth, from small droughts to the advance and retreat of the great ice age glaciers. Dry and cold conditions may even have motivated the major diaspora out of Africa 60,000 years ago, Tierney suggests in a 2017 study published in Geology.

In this Centennial episode of Third Pod from the Sun, she reveals the secrets of the mud, how humans may have weathered climate swings of the past, and what the past can tell us about our warming world.

—Liza Lester (@lizalester), Contributing Writer

Hurricane Maria dropped more rain on Puerto Rico than any storm to hit the island since 1956, a feat due mostly to the effects of human-caused climate warming, new research finds.

A new study analyzing Puerto Rico’s hurricane history finds 2017’s Maria had the highest average rainfall of the 129 storms to have struck the island in the past 60 years. A storm of Maria’s magnitude is nearly five times more likely to form now than during the 1950s, an increase due largely to the effects of human-induced warming, according to the study’s authors.

“What we found was that Maria’s magnitude of peak precipitation is much more likely in the climate of 2017 when it happened versus the beginning of the record in 1950,” said David Keellings, a geographer at the University of Alabama in Tuscaloosa and lead author of the new study in AGU’s journal Geophysical Research Letters.

Previous studies have attributed Hurricane Harvey’s record rainfall to climate change, but no one had yet looked in depth at rainfall from Maria, which struck Puerto Rico less than a month after Harvey devastated Houston and the Gulf Coast. Extreme rainfall during both storms caused unprecedented flooding that placed them among the top three costliest hurricanes on record (the other being Hurricane Katrina in 2005).

The new study adds to the growing body of evidence that human-caused warming is making extreme weather events like these more common, according to the authors.

“Some things that are changing over the long-term are associated with climate change – like the atmosphere getting warmer, sea surface temperatures increasing, and more moisture being available in the atmosphere – together they make something like Maria more likely in terms of its magnitude of precipitation,” Keellings said.

Constructing a History of Rain

José Javier Hernández Ayala, a climate researcher at Sonoma State University in California and co- author of the new study, is originally from Puerto Rico and his family was directly impacted by Hurricane Maria. After the storm, Hernández Ayala decided to team up with Keellings to see how unusual Maria was compared to previous storms that have struck the island.

Comparison of lights at night in Puerto Rico before (top) and after (bottom) Hurricane Maria. Credit: NOAA

The researchers analyzed rainfall from the 129 hurricanes that have struck Puerto Rico since 1956, the earliest year with records they could rely on. They found Hurricane Maria produced the largest maximum daily rainfall of those 129 storms: a whopping 1,029 millimeters (41 inches) of rain. That places Maria among the top 10 wettest hurricanes to ever have hit United States territory.

“Maria is more extreme in its precipitation than anything else that the island has ever seen,” Keellings said. “I just didn’t expect that it was going to be so much more than anything else that’s happened in the last 60 years.”

Keellings and Hernández Ayala also wanted to know whether Maria’s extreme rain was a result of natural climate variability or longer-term trends like human-induced warming. To do so, they analyzed the likelihood of an event like Maria happening in the 1950s versus today.

They found an extreme event like Maria was 4.85 times more likely to happen in the climate of 2017 than in 1956, and that change in probability can’t be explained by natural climate cycles.

At the beginning of the observational record in the 1950s, a storm like Maria was likely to drop that much rain once every 300 years. But in 2017, that likelihood dropped to about once every 100 years, according to the study.

Infrared satellite loop of Maria passing east of the Dominican Republic on September 21, after leaving Puerto Rico. Credit: NOAA

“Due to anthropogenic climate change it is now much more likely that we get these hurricanes that drop huge amounts of precipitation,” Keellings said.

The findings show human influence on hurricane precipitation has already started to become evident, according to Michael Wehner, a climate scientist at Lawrence Berkeley National Laboratory in Berkeley, California, who was not connected to the new study. Because so much of Maria’s damage was due to flooding from the extreme amount of rain, it is safe to say that part of those damages were exacerbated by climate change, Wehner said.

“Extreme precipitation during tropical cyclones has been increased by climate change,” he said. “Not all storms have a large amount of inland flooding, of freshwater flooding. But of those that do, the floods are increased to some extent by climate change.”

The Moon is pockmarked with impact craters from collisions with meteorites and asteroids, some as big as 1,000 kilometers in diameter. These massive impact craters contain three or more concentric rings, a mysterious feature that has long intrigued scientists interested in how Earth’s early surface and those of other planets evolved. Now a new study, in which scientists simulated an asteroid bigger than New York City slamming into a Moon-like object, explores how such rings form.

Billions of years ago, Earth looked a lot like the Moon: riddled with craters from impacts with asteroids and other space debris. Many of these craters have been erased or eroded by the atmosphere and the water flowing over Earth’s surface, so scientists must look to the atmosphereless Moon to reconstruct how different crater features are created.

Researchers already know a lot about how relatively small, simple impact craters form. When a projectile hits its target, it transfers its kinetic energy to the planet or moon, creating powerful shock waves that ripple through the rock. The projectile simultaneously melts and vaporizes, launching molten and solid rock called ejecta hundreds of kilometers away. Then, the remaining ejecta rings the crater site and slumps inward, forming a smooth bowl.

On the Moon, this process seems to hold for craters smaller than 20 kilometers in diameter. As impact craters get bigger, however, they grow more complex, eventually forming multiple concentric rings. For example, one of the Moon’s most famous impact craters—the nearly 1,000-kilometer-wide basin Orientale—has three distinctive, bull’s-eye-like rings that have long confounded scientists.

To get a fresh perspective on this complex crater structure, Johnson et al. took advantage of data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission: two washing machine–sized spacecraft that orbit the Moon and produce a high-resolution map of its gravitational field. Using this new, 10-kilometer-scale data, the authors were able to build a high-resolution computer model of a 64-kilometer-diameter asteroid hurtling into a Moon-like object at 15 kilometers per second.

The team found that the dominant hypothesis for how concentric rings form in impact craters, known as ring tectonic theory, appears to be correct. In this hypothesis, rings are formed as rock flows inward during crater collapse, dragging the base of the lithosphere—a planet’s or moon’s rigid, outermost rock shell—and creating a distinctive pattern of faults in the rock, forming rings.

By tweaking different variables within the simulation, the team discovered that factors such as the interior temperature of the Moon, the strength of the lithosphere, and the thickness of its crust affect ring locations and spacing. Importantly, they were able to reproduce the approximate spacing and offset of Orientale’s rings, bolstering both the model’s credibility and ring tectonic theory itself, the authors report. (Journal of Geophysical Research: Planets, https://doi.org/10.1029/2018JE005765, 2018)

—Emily Underwood, Freelance Writer