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Editors’ Vox is a blog from AGU’s Publications Department.

In May 2023, a group of scientists gathered in Agros, Cyrpus, for an AGU Chapman Conference, “Hydrothermal Circulation and Seawater Chemistry: What’s the Chicken and What’s the Egg?” They discussed the role of hydrothermal fluxes in regulating ocean biogeochemistry and the Earth system. To share key finding from that conference—and other groundbreaking research on hydrothermal systems—the AGU book Hydrothermal Circulation and Seawater Chemistry: Links and Feedbacks came to life.

The latest volume in AGU’s Geophysical Monograph Series, this book explores on- and off-axis hydrothermal systems, boundary conditions such as climate and sedimentation history, and approaches for tracking oceanic processes. We asked the book’s Volume Editors about the latest methods and techniques, practical applications, and the future of the field of study.

In simple terms, what is hydrothermal circulation as it relates to seawater chemistry?

Hydrothermal circulation in this context is the flow of seawater through ocean crust.

Hydrothermal circulation in this context is the flow of seawater through ocean crust. It occurs at high-temperatures at mid-ocean ridges and at lower temperatures across much of the seafloor.

Along mid-ocean ridges, high-temperature (~400°C) reactions between seawater and crust turn the circulating seawater into hydrothermal fluids that become enriched in many elements such as potassium, calcium, and iron. Because these hot fluids are much less dense than cold seawater, the fluid flows rapidly upwards and vents at the seafloor. Cooling leads to precipitation of dissolved constituents forming chimney structures and particles carried by the fluid (the well-known “black smoke”). Other dissolved ions stay in solution and are added to the ocean, changing the composition of the seawater.

Hydrothermal circulation farther from mid-ocean ridges typically heats seawater by only 5–10°C and changes the chemical composition of the fluid much less than that of the hotter fluids. However, orders of magnitude more seawater circulates through low-temperature systems than high-temperature systems, making them equally important to seawater chemistry.

How do various boundary conditions impact hydrothermal processes in the ocean, and why is it important to study them?

The amount of fluid that flows through mid-ocean ridge hydrothermal systems depends on the geodynamic boundary conditions that control characteristics such as the average global rate of accretion of new oceanic crust and its thickness. These boundary conditions also control the types of rocks that make up the ocean crust, which impacts fluid–rock reactions and hence the composition of the fluid vented into the ocean. The composition and temperature of the deep seawater that circulates into the crust are also important boundary conditions controlling fluid–rock reactions and have both changed substantially over Earth’s history. For example, changes in the redox state of seawater over Earth’s history changed fluid–rock reactions and in-turn hydrothermal fluxes into the ocean.

What are the latest methods and techniques for studying hydrothermal circulation discussed in the book?

The book covers the latest methods and techniques for advancing various interdisciplinary fields focused on hydrothermal vents.

The book covers the latest methods and techniques for advancing various interdisciplinary fields focused on hydrothermal vents. For example, deploying novel instrumentation in the harsh conditions of high-pressure and high-temperature seafloor hydrothermal systems is improving studies of hydrothermal systems. Attaching instruments, such as Raman spectrometers and mass spectrometers, to seafloor cabled observatories will provide new insights into the dynamics of hydrothermal systems. Advancements in instrumentation will also significantly benefit the study of diffuse flow along mid-ocean ridges where fluids vent at tens of degrees Celsius rather than the much higher temperatures characteristic of “black smokers.” Diffuse venting circulates much more water, and probably more heat, than black smoker venting and yet has received only a fraction of the study. Scientists studying low-temperature seafloor hydrothermal systems distributed across the seafloor are starting to borrow methods used for studying continental weathering systems, which should lead to rapid progress being made in understanding these systems.

What practical applications do the studies presented in the book have for Earth system science?

Understanding the Earth system requires a quantitative understanding of the controls on global biogeochemical cycles. Currently, most models of global biogeochemical cycles either ignore hydrothermal systems or assume they do not change over time. However, there is now copious evidence that the fluxes of elements into and out of seawater due to hydrothermal systems are dependent on environmental conditions (e.g., climate, seawater chemistry) and hence do change over time. Furthermore, there can be feedbacks between the environment and the hydrothermal fluxes. This book will help people modeling the Earth system to better incorporate hydrothermal systems into biogeochemical models. In turn, the results of such models will become more robust.

Octopi brooding their eggs in warm water venting from the ocean crust near Davidson Seamount underwater volcano at a depth of 3,200 meters. Credit: Chad King / OET, NOAA

Where do you see the study of hydrothermal systems heading in the next 10 years?

The book features many exciting directions that the study of hydrothermal systems will hopefully take in the next decade. For example, there is a pressing need for more intense study of seafloor weathering until we understand it as well as we understand continental weathering. Expanding the availability of novel instruments that can be deployed at hydrothermal vents at mid-ocean ridges will advance the understanding of hydrothermal systems (e.g., during volcanic eruptions that are currently poorly understood). Vast potential also exists to better incorporate hydrothermal processes into Earth system models. Finally, a growth area for future research will be the role of hydrothermal systems on exoplanets and in the search for other habitable bodies. For example, NASA’s ongoing Clipper Mission to Jupiter’s moon Europa will hopefully enrich our understanding of the role of hydrothermal activity in controlling the habitability of this body.

How is the book organized?

After an introductory chapter, the next six chapters address hydrothermal processes at mid-ocean ridges. They consider both black smoker and diffuse flow systems, as well as the impact of these systems on the water column above mid-ocean ridges. The methods used to study hydrothermal systems, both in the lab and field, are also covered. An example of the fingerprint of changes in axial hydrothermal processes through changing seawater chemistry is discussed next. This is followed by three chapters about low-temperature hydrothermal systems that discuss how much more we have to discover about these systems. The last four chapters address the role of oceanic hydrothermal systems on planetary scale processes, both on Earth and other rocky bodies. They discuss how global-scale models work, how hydrothermal processes can be incorporated in such models, and how hydrothermal systems might work on other rocky bodies.

Who is the intended audience of the book? 

The audience for the book is intended to be broad—anyone interested in oceanic hydrothermal systems and/or ocean chemistry. People who are new to the field can use the book to get up-to-speed on ongoing interdisciplinary research in this area. This includes new graduate students or experienced researchers who have not previously considered the role of oceanic hydrothermal system in ocean chemistry. The book can also act as a starting point for researchers who develop global biogeochemical cycle models and who want to incorporate hydrothermal fluxes into these models. Finally, the book will appeal to people interested in planetary habitability and the role that hydrothermal systems may play in making other rocky bodies habitable, or the role hydrothermal systems may have played in nurturing early life on Earth.

Hydrothermal Circulation and Seawater Chemistry: Links and Feedbacks, 2025. ISBN: 978-1-394-22915-4. List price: $225 (hardcover), $180 (e-book)

Chapter 1 is freely available. Visit the book’s page on Wiley.com and click on “Read an Excerpt” below the cover image.

—Laurence A. Coogan (lacoogan@uvic.ca; 0000-0001-7289-5120), University of Victoria, Canada; Alexandra V. Turchyn (0000-0002-9298-2173), University of Cambridge, United Kingdom; Ann G. Dunlea (0000-0003-1251-1441), Woods Hole Oceanographic Institution, United States; and Wolfgang Bach (0000-0002-3099-7142), University of Bremen, Germany

Editor’s Note: It is the policy of AGU Publications to invite the authors or editors of newly published books to write a summary for Eos Editors’ Vox.

Citation: Coogan, L. A., A. V. Turchyn, A. G. Dunlea, and W. Bach (2025), Hydrothermal circulation and its impact on the Earth system, Eos, 106, https://doi.org/10.1029/2025EO255036. Published on 3 December 2025. This article does not represent the opinion of AGU, Eos, or any of its affiliates. It is solely the opinion of the author(s). Text © 2025. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

Editors’ Highlights are summaries of recent papers by AGU’s journal editors. Source: Journal of Geophysical Research: Space Physics

Earth is surrounded by rings of energetic particles called radiation belts. The inner belt can sometimes be populated by megaelectron volt (MeV) energetic electrons during particularly strong solar storms. When moved by electromagnetic waves, these energetic particles can rain into the atmosphere.

Feinland and Blum [2025] show that periodic signatures of relativistic electron rain observed by satellites can be used to better predict when and where they might happen in the future. The authors find that these high-energy electrons usually came into the inner belt quickly after solar storms and gradually rained out over the course of a few weeks. During particularly quiet solar conditions, there were no detectable high-energy electrons in this region at all. These results are important to incorporate into models of the radiation belts, to better characterize and predict the high radiation environment in near-Earth space.

Citation: Feinland, M. A., & Blum, L. W. (2025). Lightning-induced precipitation as a proxy for inner belt MeV electron decay times. Journal of Geophysical Research: Space Physics, 130, e2025JA034258. https://doi.org/10.1029/2025JA034258

—Viviane Pierrard, Editor, JGR: Space Physics

Text © 2025. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

A new study highlights how Indigenous leadership, science and business can unite to protect coastal ecosystems while building long-term environmental and cultural knowledge.

A recent study published in Earth and Planetary Science Letters is the first to directly link earthquakes to climate change-induced glacial melt. Scientists analyzed 15 years of seismic activity in the Grandes Jorasses—a peak that is part of the Mont Blanc massif between Italy and France—to better understand this association. This massif is one of the more seismologically active areas in the region, and examining how climate change may affect earthquakes there could prove useful in preparing for them.

New research shows how the combination of extreme climate events, sea-level rise and land subsidence could create larger and deeper floods in coastal cities in future.

SummaryAccurately mapping the distribution of natural methane hydrates is crucial for understanding their role in climate change and predicting the risks associated with hydrate dissociation. Attenuation shows great potential for remote hydrate detection, yet its behavior and underlying mechanisms are still not well understood. We conducted laboratory experiments to synthesize high-saturation methane hydrate in unconsolidated sands and measure attenuation based on ultrasonic waveforms. The resulting attenuation showed an unexpected decreasing trend during hydrate formation, contradicting previous studies in sands, where attenuation generally increases with hydrate saturation. Theoretical modeling suggests that attenuation is jointly controlled by hydrate and free gas. The gas reduction in pores due to hydrate formation substantially suppresses the attenuation induced by gas-bubble oscillation, and is therefore thought to be responsible for the observed attenuation reduction. By comparison, hydrate effects are relatively weak and strongly frequency-dependent. The discrepancy between our results and previous studies arises primarily from the distinct attenuation behavior across different ranges of gas content. Our samples fall within a relatively low gas content range, where attenuation is particularly sensitive to gas, highlighting its impact. These findings contribute new insights into the attenuation characteristics and mechanisms due to the coexistence of hydrate and gas in sediments.

A new study shows that during drought, it's not how hot or how dry it is that determines gas emissions from plants—but how quickly conditions change. This discovery reshapes our understanding of the relationship between drought, vegetation, and air pollution.

The Fertile Crescent, a boomerang-shaped region spanning modern-day Middle Eastern countries, is considered the cradle of civilization and where farming first emerged. But little is known about how climate change influenced early societies in this part of the world. Now, new research into ancient climate history is shedding light on how farming and civilization began. And the insights are coming from an analysis of a stalagmite in a cave in Kurdistan.

Researchers at University of Tsukuba and the Meteorological Research Institute have identified how atmosphere–ocean interactions in the midlatitudes reinforce the East Asian winter monsoon (EAWM). During strong monsoon seasons, cold air outbreaks from the Eurasian continent cool the midlatitude western North Pacific (WNP). This oceanic cooling, in turn, alters atmospheric circulation in a manner that further intensifies the monsoon.

University of Alberta geochemists have discovered a missing piece to one of the great mysteries of science—the origin of life on Earth.

A new review has revealed that the Southern Annular Mode (SAM), the Southern Hemisphere's most influential climate driver, is now in its most positive state in more than 1,000 years. If greenhouse gas emissions continue to rise, this positive state is projected to persist throughout the 21st century, with long-term implications for Antarctica and the Southern Ocean.

Two new publications remap the understanding of reverse weathering in the scientific community. The Dauphin Island Sea Lab's Senior Marine Scientist, Dr. Jeffrey Krause, played a key role in both projects, which include several collaborating institutions.

Since 2020, the Barry Landslide in Alaska's Prince William Sound has been outfitted with instruments monitoring seismic signals from the area, as researchers hope to catch a destructive, tsunami-generating landslide before it starts.

The Serengeti is one of the most diverse ecosystems on Earth. The massive savanna stretches more than 30,000 square kilometers across Tanzania and southwestern Kenya, and conservation sites, including national parks and a United Nations Educational, Scientific and Cultural Organization World Heritage Site, mark its significance as one of the world’s last intact large-animal migration corridors.

Life in the Serengeti is shaped by interactions between herbivores, vegetation, fire, and rain. Every year, millions of wildebeest, zebras, and gazelles hoof it across the savanna for their great migration, an 800-kilometer loop through the Serengeti and Kenya’s adjacent Maasai Mara game reserve. The iconic migration is dictated by rainfall, with herbivores following the green grass brought by the rainy season.

New research documenting the far-reaching impact of increasing rainfall on the Serengeti will be presented on Monday, 15 December, at AGU’s Annual Meeting. Megan Donaldson, a postdoctoral researcher at Duke University’s Nicholas School of the Environment, and her colleagues will share how vegetation is consumed by both grazing herbivores and fire in the Serengeti and how that consumption is reflected in the landscape. Studies like Donaldson’s are emerging as an important area of research for scientists assessing how climate change will affect the closely intertwined biotic and abiotic components in tropical grassland ecosystems around the world.

“For now, we’re just looking at how those interactions are feeding back to each other, how increased rainfall is affecting the dynamics between vegetation, herbivores, and fire,” said Donaldson.

Rainfall, Fuel, and Food

Rainfall controls how much grass grows in the Serengeti: When rainfall is intense, grasses grow quickly.

That growth is consumed in two primary ways: by fire as fuel and by herbivores as food.

Fire can eradicate excess vegetation, which is why a previous rainy season in the Serengeti might be a reliable predictor for how much land will burn there in the near future.

More than 30 species of large herbivores consume vegetation in the Serengeti, each with its own ecological niche.

“Some are constantly on the move, others are residents, some are grazers, some browsers, others are mixed feeders, and they range in size from the minuscule dik-dik to the massive elephant. They all thrive together by seeking out seasonal sources of water and feeding differentially on the rich diversity and abundance of grasses, shrubs, and trees,” said Monica Bond, a wildlife biologist at the University of Zurich who was not part of the recent study.

Herbivores consume vegetation at a much slower rate than fire does. Under normal conditions, grazing herbivores keep grass levels low enough to reduce the spread of fire across large areas. But it can take several seasons for animal populations to adjust to differences in food availability, so as rainfall totals increase and cause explosive growth in savanna vegetation, herbivores are unable to maintain their ability to minimize the fuel available for wildfires.

In the new research, Donaldson and her colleagues examined weather station and camera trap data from sites inside Serengeti National Park in Tanzania.

In particular, the researchers tracked how recent shifts in the Indian Ocean Dipole caused rainfall totals to increase across the Serengeti. The Indian Ocean Dipole is a weather pattern similar to the El Niño–Southern Oscillation phenomenon that spawns El Niño or La Niña conditions in the Pacific. It alters wind, rain, and temperature conditions in East Africa. Between 2019 and 2024, mean rainfall totals in the Serengeti were 268 millimeters higher than in the period from 1999 through 2003.

The researchers found that within the park, rainfall was not uniform. “There’s a rainfall gradient. You get low rainfall in the south and high rainfall in the north,” said Donaldson.

In the northern Serengeti, surplus rainfall supported such rapid growth of grass that herbivore consumption had little influence on reducing the amount of fuel available for wildfires.

In the typically drier south, however, herbivores were able to keep grasses short enough to slow the buildup of fuel.

But during periods of increased rainfall, Donaldson explained, “we see that those feedbacks are quicker. You’re getting fuel buildup much quicker, and you need all the [animal] migrants to come through that system to have any effect on fire.”

Untangling a Complex Ecosystem

Between 2019 and 2024, fire size in the Serengeti increased, but the increase was more complex than “more fuel feeding more fires.”

“The number of fires necessarily isn’t changing; it seems to be staying stable,” explained Donaldson. “We’re not seeing this very strong correlation between increased rainfall and increased fire. What is driving that? Why are we seeing that? And what are herbivores doing to that? Those are the things we’re trying to tease apart right now.”

“Because the Serengeti is one of the few intact biologically functioning ecosystems left on the planet, it makes for a perfect natural laboratory.”

Future work from Donaldson and her colleagues will further researchers’ understanding of how the Serengeti’s four major players—herbivores, biomass, fire, and rainfall—connect.

“Because the Serengeti is one of the few intact biologically functioning ecosystems left on the planet, it makes for a perfect natural laboratory to study complex ecological interactions and how these are affected by climate change,” said Bond. “This research has important implications for fire management and thus for wildlife conservation in this ecologically critical landscape. It is incredible the research that they have done here in fostering understanding of how this system works.”

—Rebecca Owen (@beccapox.bsky.social), Science Writer

Citation: Owen, R. (2025), Tracing fire, rain, and herbivores in the Serengeti, Eos, 106, https://doi.org/10.1029/2025EO250444. Published on 2 December 2025. Text © 2025. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

Our Sun is about halfway through its life, which means Earth is as well. After a star exhausts its hydrogen nuclear fuel, its diameter expands more than a hundredfold, engulfing any unlucky planets in close orbits. That day is at least 5 billion years off for our solar system, but scientists have spotted a possible preview of our world’s fate.

Elderly stars just get hungry.

Using data from the TESS (Transiting Exoplanet Survey Satellite) observatory, astronomers Edward Bryant of the University of Warwick and Vincent Van Eylen of University College London compared systems with stars in the main sequence of their lifetimes—fusing hydrogen, like the Sun—with post–main sequence stars closer to the end of their lifetimes, both with and without planets.

“We saw that these planets are getting rarer [as stars age],” Bryant said. In other words, planets are disappearing as their host stars grow old. The comparison between planetary systems with younger and older stars makes it clear that the discrepancy does not stem from the fact that the planets weren’t there in the first place: Elderly stars just get hungry.

“We’re fairly confident that it’s not due to a formation effect,” Bryant explained, “because we don’t see large differences in the mass and [chemical composition] of these stars versus the main sequence star populations.”

Complete engulfment isn’t the only way giant stars can obliterate planets. As they grow, giant stars also exert increasingly larger tidal forces on their satellites that make their orbits decay, strip them of their atmospheres, and can even tear them apart completely. The orbital decay aspect is potentially measurable, and this is the effect Bryant and Van Eylen considered in their model for how planets die.

“We’re looking at how common planets are around different types of stars, with number of planets per star,” Bryant said. Bryant and Van Eylen identified 456,941 post–main sequence stars in TESS data and, from those, found 130 planets and planet candidates with close-in orbits. “The fraction [of stars with planets] gets significantly lower for all stars and shorter-period planets, which is very much in line with the predictions from the theory that tidal decay becomes very strong as these stars evolved.”

Astronomers use TESS to find exoplanets by looking for the diminishment in light as they pass in front of their host stars, a miniature eclipse known as a transit. As with any exoplanet detection method, transits are best suited to large, Jupiter-sized planets in relatively small orbits lasting less than half of an Earth year, sometimes much less. So these solar systems aren’t much like ours in that respect. Studying planets orbiting post–main sequence stars poses additional challenges.

“If you have the same size planet but a larger star, you have a smaller transit,” Bryant said. “That makes it harder to find these systems because the signals are much shallower.”

However, though the stars in the sample data have a much greater surface area, they are comparable in mass to the Sun, and that’s what matters most, the researchers said. A star with the same mass as the Sun will go through the same life stages and die the same way, and that similarity is what helps reveal our solar system’s future.

“The processes that take place once the star evolves [past main sequence] can tell us about the interaction between planets and host star,” said Sabine Reffert, an astronomer at Universität Heidelberg who was not involved in the study. “We had never seen this kind of difference in planet occurrence rates between [main sequence] and giants before because we did not have enough planets to statistically see this difference before. It’s a very promising approach.”

Planets: Part of a Balanced Stellar Breakfast

Exoplanet science is one of astronomy’s biggest successes in the modern era: Since the first exoplanet discovery 30 years ago, astronomers have confirmed more than 6,000 planets and identified many more candidates for follow-up observations. At the same time, the work can be challenging when it comes to planets orbiting post–main sequence stars.

One tricky aspect of this work is related to the age of the stars, which formed billions of years before our Sun. Older stars have a lower abundance of chemical elements heavier than helium, a measure astronomers call “metallicity.” Observations have found a correlation between high metallicity and exoplanet abundance.

“A small difference in metallicity…could potentially double the occurrence rate.”

“A small difference in metallicity…could potentially double the occurrence rate,” Reffert said, stressing that the general conclusions from the article would hold but the details would need to be refined with better metallicity data.

Future observations to measure metallicity using spectra, along with star and planet mass, would improve the model. In addition, the European Space Agency’s Plato Mission, slated to launch in December 2026, will add more sensitive data to the TESS observations.

Earth’s fiery fate is a long way in the future, but researchers have made a big step toward understanding how dying stars might eat their planets. With more TESS and Plato data, we might even glimpse the minute orbital changes that indicate a planet spiraling to its doom—a grim end for that world but a wonderful discovery for our understanding of the coevolution of planets and their host stars.

—Matthew R. Francis (@BowlerHatScience.org), Science Writer

Citation: Francis, M. R. (2025), Planet-eating stars hint at Earth’s ultimate fate, Eos, 106, https://doi.org/10.1029/2025EO250448. Published on 2 December 2025. Text © 2025. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

Editors’ Highlights are summaries of recent papers by AGU’s journal editors. Source: GeoHealth

Heatwaves can disrupt many parts of daily life, including access to essential healthcare services. Dey et al. [2025] evaluate how heatwaves are related to where women in India give birth.

The authors analyze data from over 200,000 births during 2019-2021 and find that during periods of heatwaves, women were more likely to deliver at home instead of in a health facility. This association was stronger for warmer regions, regions without government programs supporting facility-based births, and non-Hindu populations. The study indicates that extreme heat may create barriers to healthcare services (e.g., difficulty traveling or strained health services), which makes it challenging to reach a hospital in time for delivery. This brings a major concern because giving birth at home without a skilled medical attendant may lead to higher health risks for both the mother and the newborn.

As the frequency and intensity of heatwaves increases under climate change, these findings emphasize the urgent need for early warning systems and stronger healthcare support to protect vulnerable mothers and newborns.

Citation: Dey, A. K., Dimitrova, A., Raj, A., & Benmarhnia, T. (2025). Heatwaves and home births: Understanding the impact of extreme heat on place of delivery in India. GeoHealth, 9, e2025GH001540. https://doi.org/10.1029/2025GH001540

—Lingzhi Chu, Associate Editor, GeoHealth

Text © 2025. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

When hurricanes or strong storms sweep up the United States' East Coast and meet the shores of the country's largest estuary, Chesapeake Bay, the familiar pattern of storm activity gets a little more complicated. A new study, published in the Journal of Geophysical Research: Oceans, shows that water levels inside the bay can spike far more dramatically than along the open ocean, raising flood risks for coastal and inland communities.

Major storms are spreading industrial contaminants across entire neighborhoods, raising concerns about future well-being, especially in communities of color, according to new research from Rice University and the University of Alberta.

A new algorithmic framework that can predict flooding could help save lives and reduce the devastation as climate change drives more intense and unpredictable rainfall.