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This article was originally published on Undark. Read the original article.

1 June 2021 by James Dinneen and Alexander Kennedy.

A multimedia version of this story, with rich maps and data, is available here.

On May 19, 2020, a group of engineers and emergency officials gathered at a fire station in Edenville, Michigan to decide what to do about the Edenville Dam, a 97-year-old hydroelectric structure about a mile upstream on the Tittabawassee River. Over the preceding two days, heavy rains had swelled the river, filling the reservoir to its brim and overwhelming the dam’s spillway. The group was just about to discuss next steps when the radios went off, recalled Roger Dufresne, Edenville Township’s longtime fire chief. “That’s when the dam broke.”

“Up at the dam, Edenville residents watched as a portion of the eastern embankment liquified. Muddy water gushed through the breach. Over the next few minutes, that water became a torrent, snapping trees and telephone poles as it rushed past town, nearly submerging entire houses further downstream.

About 10 miles and two hours later, the flood wave bowled into a second aging dam, damaging its spillway, overtopping, and then breaching the embankment.

Al Taylor, then chief of the hazardous waste section within the state’s Department of Environment, Great Lakes, and Energy, was following the situation closely as the surge swept 10 miles further downstream into the larger city of Midland, where it caused a Dow Chemical Company plant flanking the river to shut down, and threatened to mix with the plant’s containment ponds. Taylor, who retired at the end of January, worried that contamination from the ponds would spill into the river. But that was just the first of his concerns.

In prior decades, Dow had dumped dioxin-laden waste from the plant directly into the river, contaminating more than 50 miles of sediment downstream — through the Tittabawassee, the Saginaw River, and the Saginaw Bay — with carcinogenic material. The contamination was so severe that the U.S. Environmental Protection Agency stepped in, and since 2012, worked with Dow to map and cap the contaminated sediments. In designing the cleanup, engineers accounted for the river’s frequent flooding, Taylor knew, but nobody had planned for the specific impacts of flooding caused by a dam failure.

An Undark investigation has identified 81 other dams in 24 states, that, if they were to fail, could flood a major toxic waste site and potentially spread contaminated material into surrounding communities.While the dramatic breach of the Edenville Dam captured national headlines, an Undark investigation has identified 81 other dams in 24 states, that, if they were to fail, could flood a major toxic waste site and potentially spread contaminated material into surrounding communities.

In interviews with dam safety, environmental, and emergency officials, Undark also found that, as in Michigan, the risks these dams pose to toxic waste sites are largely unrecognized by any agency, leaving communities across the country vulnerable to the same kind of low-probability, but high-consequence disaster that played out in Midland.

After the flooding subsided, Dow and state officials inspected the chemical plant’s containment ponds and found that, though one of the brine ponds containing contaminated sediment had been breached, there was no evidence of significant toxic release. Preliminary sediment samples taken downstream did not find any new contamination. The plant’s and the cleanup’s engineering, it seemed, had done its job.

“Dow has well-developed, comprehensive emergency preparedness plans in place at our sites around the world,” Kyle Bandlow, Dow’s corporate media relations director, wrote in an email to Undark. “The breadth and depth of these plans — and our ability to quickly mobilize them — enabled the safety of our colleagues and our community during this historic flood event.”

But things could have gone differently — if not in Midland, then somewhere else with a toxic waste site downstream of an aging dam less prepared for a flood. “As a lesson learned from this,” Taylor said, “we need to be aware of that possibility.”

In the United States, there are more than 90,000 dams providing flood control, power generation, water supplies, and other critical services, according to the National Inventory of Dams database maintained by the U.S. Army Corps of Engineers, which includes both behemoths like the Hoover Dam and small dams holding back irrigation ponds. Structural and safety oversight of these dams falls under a loose and, critics say, inadequate patchwork of state and federal remit.

A 2019 report from the Congressional Research Service (CRS), the nonpartisan research arm of the U.S. Congress, found roughly 3 percent of the nation’s dams are federally owned, including some of the country’s largest, with the rest owned and operated by public utilities, state and local governments, and private owners. The report estimated that half of all dams were over 50 years old, including many that were built to now obsolete safety standards. About 15 percent of dams in the Army Corps database lacked data on when they were built.

In addition to information on age and design, the Army Corps database includes a “hazard potential” used to describe the possible impact of a dam failure to life and property. In 2019, roughly 17 percent, or 15,629 dams, had a high hazard potential rating, indicating that a loss of human life was likely in the event of a dam failure. The number of high-hazard dams has increased in recent years due to new downstream development.

According to the CRS report, more than 2,300 dams in the database were both high-hazard and in “poor” or “unsatisfactory” condition during their most recent inspection. Due to security concerns that arose after the September 11 terrorist attacks, the report did not name these dams, though an investigation by The Associated Press in 2019 identified nearly 1,700 of them.

For all that is known about America’s aging dam infrastructure, however, little information exists about the particular hazards dams pose to toxic waste sites downstream.For all that is known about America’s aging dam infrastructure, however, little information exists about the particular hazards dams pose to toxic waste sites downstream. This is why regulators knew about problems with the Edenville Dam and knew about the Dow cleanup, but had not connected the dots.

To identify dams that might pose the most serious risk to toxic waste sites, Undark searched for dams in the national database that are both high-hazard and older than 50 years, the age after which many dams require renovations. To narrow our search, we selected dams that sit 6 or fewer miles away from and appear in satellite images to be upstream of an EPA-listed toxic waste site. Experts say that many dams would flood much farther than 6 miles.

We then filed requests under state and federal freedom of information laws with various agencies, including the Federal Energy Regulatory Commission, seeking dam inspection reports and the Emergency Action Plans (EAPs) that dam owners are typically required to prepare and maintain. Among other things, these plans usually include inundation maps, which model the area that would likely be flooded in a dam failure scenario.

The inputs for these models vary by state, and while some inundation maps were highly sophisticated, involving contingencies for weather and other variables, others were less so. In one Emergency Action Plan for a dam in Tennessee, the inundation zone was simply hand-drawn on a map with a highlighter (see above image). But whatever their quality, the maps represent dam officials’ best estimate of where large volumes of water will flow if a dam fails.

Undark successfully obtained inundation modeling information for 153 of the 259 dams identified in our search. For 63 dams, state and federal officials declined to provide or otherwise redacted pertinent inundation information, citing security concerns. For 31 dams, agencies said they did not have inundation maps prepared, or provided maps that were illegible or did not extend to the site. Despite improvement in recent years, about 19 percent of high-hazard dams still lacked plans as of 2018, according to the American Society of Civil Engineers.

With those maps, we then looked to see if any EPA-listed toxic waste sites fell within the delineated inundation areas. Because the precise boundaries of each toxic waste site are not consistently available, we followed the methodology of a 2019 Government Accountability Office analysis of flood risks to contaminated sites on the EPA’s National Priorities List — more commonly known as Superfund sites — which used a 0.2-mile radius around the coordinates listed by the EPA for each location.

For a number of dams for which we were not able to obtain inundation maps to review ourselves, dam regulators or owners confirmed that coordinates we provided for the toxic waste site fell within 0.2 miles of the dam’s inundation zone.

We focused our search on the nation’s highest priority cleanup sites, as indicated by a designation of Superfund (for non-operating sites) or RCRA (Resource Conservation and Recovery Act of 1976, for operating sites). We considered 5,695 of these sites, including both current and former sites. Types and levels of contamination vary widely across sites, as would the impact of any flooding.

“There are many situations across the country then with these dams where they don’t meet the current safety standards.”Using this methodology, we identified at least 81 aging high-hazard dams that could flood portions of at least one toxic waste site if they failed, potentially spreading contaminated material into surrounding communities and exposing hundreds or thousands of people — in the case of very large dams, many more — to health hazards atop significant environmental impacts. At least six of the dams identified were in “poor” or “unsafe” condition during their most recent inspection.

In many instances, state and local agencies responsible for dam safety and toxic waste have not accounted for this kind of cascading disaster, and so remain largely unprepared.

Undark shared this analysis with engineering and dam safety experts, who verified the methodology. Several suggested that the true number of dams that could flood toxic waste sites if they were to fail is almost certainly far greater, but because no agency tracks this particular hazard, the actual number remains unknown.

“There are many situations across the country then with these dams where they don’t meet the current safety standards …” said Mark Ogden, a civil engineer who reviewed the American Society of Civil Engineer’s 2021 Infrastructure Report Card section on dams, which gave U.S. dams a “D” grade. “And the fact that there could be these hazardous sites as part of that, just increases that concern and what the consequences of a failure might be.”

Though impacts would vary widely, environmental scientists and toxicologists interviewed by Undark suggested that a sudden, intense flood caused by a dam failure could spread contaminants from hazardous waste sites into surrounding communities. Even sites designed to withstand flooding might be impacted if the debris carried in floodwater managed to scour and erode protective caps, potentially releasing toxic material into the water, explained Rick Rediske, a toxicologist at Grand Valley State University in Michigan. In Houston in 2017, flooding from Hurricane Harvey eroded a temporary protective cap at the San Jacinto River Waste Pits Superfund site, exposing dioxins and other toxic substances.

Water could then move contaminants around the site and redeposit them anywhere in the floodplain, exposing people and ecosystems to health hazards, said Jacob Carter, a research scientist at the Union of Concerned Scientists, who formerly studied flooding hazards to contaminated sites for the EPA. Carter also pointed out that communities living nearest to toxic waste sites and so most vulnerable to these events tend to be low income and communities of color.

It’s possible that any toxic material would be diluted by the flood and new clean sediment, said Allen Burton, director of the Institute for Global Change Biology at the University of Michigan. But this, he emphasized, would be a best-case scenario.

“You have no way of predicting, really, how much of the bad stuff moved, how far it moved, how far it got out into the floodplain, what the concentrations are.”Generally, when there’s a massive flood like the one in Michigan, “it just moves the sediment everywhere downstream,” said Burton. “You have no way of predicting, really, how much of the bad stuff moved, how far it moved, how far it got out into the floodplain, what the concentrations are.” And regulated waste sites are just one source of potential contamination in a dam breach scenario, said Burton. Sediment behind dams is itself often contaminated after years of collecting whatever went into the river upstream.

Contamination can also come from more mundane sources in the floodplain, like wastewater treatment plants or the oil canisters in people’s basements that get swept into floodwaters, said Burton. “The fish downstream,” he quipped, “don’t care if contaminants came from your garage or Dow Chemical.”

Undark’s investigation found that state and local governments often have not prepared for the flooding that could occur at toxic waste sites in the event of a dam failure.

Emporia Foundry Incorporated, a federally-regulated hazardous waste site in Greensville County, Virginia, provides a representative example. It falls within the inundation zone of the 113-year-old Emporia Dam, which is a hydroelectric dam partially owned by the city and located on the Meherrin River, just over one mile west of the foundry site.”

The foundry, which once manufactured manhole covers and drain grates, includes a landfill packed with byproducts containing lead, arsenic, and cadmium. The landfill was capped in 1984, and in 2014, a second cap was added nearer to the river as a buffer against flooding. As in Midland, cleanup engineers accounted for flooding within the 100-year floodplain, but according to a spokesperson from the Virginia Department of Environmental Quality, they did not account for flooding from a dam failure.

The Emporia Dam inundation map shows that if the dam were to fail during a severe storm, the entire foundry site could be flooded, potentially disintegrating the cap and spreading contaminants across the floodplain. However, the site would not be flooded in the event of a “sunny day” failure.

More than 3,000 people live within a mile of the Emporia Foundry site, around 75 percent of whom are Black, according to EPA and 2010 census data.

Wendy C. Howard Cooper, director of Virginia’s dam safety program, explained that her program’s mandate is to define a dam’s inundation zone and inform local emergency managers of any immediate risks to human life and property — not to identify toxic waste sites and analyze what might happen to them during a flood. “That would be a rabbit hole that no one could regulate,” Howard Cooper said. She added that local governments should be familiar with both the dams and the contaminated sites inside their borders and should have proper emergency procedures in place.

This turned out not to be true in Greensville County, where the program coordinator for emergency services, J. Reggie Owens, told Undark he was unaware of the potential for the foundry site to flood if the Emporia Dam were to fail. The site is “not even in the floodplain,” he said. “It’s never been put on my radar by DEQ or anyone else.”

A similar pattern emerged in other states. In Rhode Island, for instance, our search identified eight dams. One of these, the 138-year-old Forestdale Pond Dam, was considered “unsafe” during its most recent inspection.

Located in the town of North Smithfield, the dam is immediately adjacent to the Stamina Mills Superfund site, which once housed a textile mill that spilled the toxic solvent trichloroethylene into the soil. Another area on the site was used as a landfill for polycyclic aromatic hydrocarbons, sulfuric acid, soda ash, wool oil, plasticizers, and pesticides.

A few years after trichloroethylene was detected in groundwater in 1979, the site received a Superfund designation from the EPA. According to the federal agency, construction for the site cleanup — which involved removing the contaminated soil from the landfill and installing a groundwater treatment system — was completed in 2000 and accounted for a 100-year flood, but it did not account for flooding due to a dam failure.

According to EPA and census data, more than 2,500 people lived within a mile of Stamina Mills as of 2010, and Forestdale Pond is not the only dam that could pose a threat.

In fact, the site sits within the inundation zones of two other high hazard dams identified by Undark. A failure of either of these dams on the Slatersville Reservoir could cause a domino effect of dam failures downstream, according to Rhode Island dam safety reports, all leading to flooding at Stamina Mills.

When asked to comment on possible flood risks to the Superfund site, the EPA responded that the only remaining remedy at Stamina Mills, the groundwater treatment system, would not be affected if Forestdale Pond Dam were to fail. EPA made no reference to the larger Slatersville Reservoir dams less than two miles upstream.

Spokespersons at the Rhode Island dam safety office and the state office responsible for hazardous waste had not considered that a dam failure could flood any of the sites identified by Undark, including Stamina Mills.

By building engineered structures or taking other resiliency measures, the most hazardous waste sites can be designed to withstand flooding, explained Carter, who recently co-authored a report on climate change and coastal flooding hazards to Superfund sites. But in order to prepare for floods, Carter said, flooding hazards have to be recognized first, whether they come from rising seas, increasing storm surge, or, as in these cases, dams.

“They could have looked at that dam and said, ‘Oh, it gets a D minus for infrastructure. This thing could break.’”“They could have looked at that dam and said, ‘Oh, it gets a D minus for infrastructure. This thing could break,’” said Burton, referring to the Edenville Dam. “So in the future, it would be smart of EPA to require the principal party who’s responsible for the cleanup to look at the situation to see if it actually could happen.”

One step that could make that process much easier is for dam inundation zones to be regularly included in FEMA’s publicly available flood risk maps, which show the 100-year floodplain and other flood risks to communities, said Ogden. A lack of available data on dam inundations — sometimes the result of security concerns — presents a major obstacle, said a FEMA spokesperson, but plotting inundation zones on commonly-used flood risk maps would ensure communities and agencies are aware of and can respond to dam hazards.

Some states, including Rhode Island, have already made inundation zones, Emergency Action Plans, and inspection reports for the dams they regulate publicly available online. In South Carolina, following a 2015 event when heavy rains caused 50 dams to fail, dam inundations for the most hazardous state-regulated dams were made publicly available. Though no state agency tracks hazardous waste sites within dam inundation zones, Undark was able to identify three dams in South Carolina which could flood a hazardous waste site in the state using this resource.

In California, inundation zones for the state’s most hazardous dams were made available following a 2017 dam failure scare at the Oroville Dam, the tallest dam in the country, which led to the evacuation of more than 180,000 people.

Using this resource, Undark identified four dams which would flood at least one hazardous waste site in California. These included the Oroville Dam, which could flood at least one current and one former Superfund site if it were to fail.

According to the EPA, neither of those sites downstream of the Oroville Dam had considered the possibility of flooding due to dam failure prior to the failure scare. Even so, commented EPA, due to the “extraordinary volume of water” that would flood the sites if the Oroville Dam were to fail, “it is not feasible to alter the existing landfills and groundwater remedy infrastructure to protect against the potential failure of the Oroville Dam.”

In order to fix the nation’s dams, the first step is to spread awareness about the importance of dams and the hazards they pose to people and property.In order to fix the nation’s dams, the first step is to spread awareness about the importance of dams and the hazards they pose to people and property, said Farshid Vahedifard, a civil engineer at Mississippi State University who co-authored a recent letter in Science on the need to proactively address problematic dams. “The second thing is definitely we need to invest more.”

According to the Association of State Dam Safety Officials, the fixes necessary to rehabilitate all the nation’s dams would cost more than $64 billion; rehabilitating only the high hazard dams would cost around $22 billion. Meanwhile, the $10 million appropriated by Congress in 2020 for FEMA’s high hazard dam rehabilitation program are “kind of a drop in the bucket for what’s really needed,” said Ogden.

Indeed, state dam safety programs report a chronic lack of funds for dam safety projects, both from public sources and from private dam owners unable or unwilling to pay for expensive repairs. In Michigan, both dams that failed were operated by a company called Boyce Hydro, which received years of warnings from dam safety regulators that there were deficiencies.

Lee Mueller, Boyce Hydro’s co-manager, told Undark that the company made numerous improvements to the dams over the years. After losing revenue when the Federal Energy Regulatory Commission (FERC) revoked the company’s hydroelectric permit, however, it was unable to fund repairs that might have prevented the dam failures.

“Regarding the Edenville Dam breach, the subject of the State of Michigan’s governance and political policy failures and the insouciance of the environmental regulatory agencies are the subject of on-going litigation and will be more thoroughly detailed in the course of those legal proceedings,” Mueller wrote in an email.

“The state of Michigan knew about this,” said Dufresne, the Edenville fire chief. State regulators, he says, should have insisted that the company pay for the badly needed repairs. “They needed to push him,” said Dufresne, referring to Mueller. More than half of all dams in the U.S. are privately owned.

Without the funding to match the problem, members of the state dam safety community have looked to non-typical sources of funding, says Bill McCormick, chief of the Colorado dam safety program. In Eastern Oregon for example, the 90-year old Wallowa Lake Dam — which Undark found would flood the former Joseph Forest Products Superfund site if it were to fail — was slated last year for a $16 million renovation to repair its deteriorating spillway and add facilities for fish to pass through. But the plans have stalled since the Covid-19 pandemic has reduced Oregon’s lottery revenues, which were funding most of the project.

“If we start getting much bigger storms, then that itself will lead to a higher probability of overtopping and dam failure.”The challenges facing U.S. dams are also exacerbated by climate change, say dam safety experts, with more frequent extreme weather events and more intense flooding expected in parts of the country adding new stresses to old designs. “If we start getting much bigger storms, then that itself will lead to a higher probability of overtopping and dam failure,” said Upmanu Lall, director of the Columbia Water Center at Columbia University and co-author of a recent report on potential economic impacts of climate-induced dam failure, which considered how the presence of hazardous waste sites might further amplify damages. The report also outlines how in addition to more extreme weather, factors like changes in land use, sediment buildup, and changing frequencies of wet-dry and freeze-thaw cycles all can contribute to a higher probability of dam failure.

Several state dam safety programs contacted by Undark said they are planning for climate change-related impacts to dam infrastructure, though according McCormick, the Colorado dam safety chief, his state is the only one with dam safety rules which explicitly account for climate change. New rules that took effect in January require dam designs “to account for expected increases in temperature and associated increases in atmospheric moisture.”

“We were the first state to take that step, but I wouldn’t be surprised if others follow that lead,” McCormick said.

No deaths were reported in the Michigan flooding, but more than 10,000 residents had to be evacuated from their homes and the disaster likely caused more than $200 million in damage to surrounding property, according to a report from the office of Michigan Gov. Gretchen Whitmer. Restoring the empty reservoirs, as well as rebuilding the two dams, could cost upwards of $300 million, according to the Four Lakes Task Force, an organization that had been poised to buy the dams just before they failed.

In contrast, the Four Lakes Task Force, which now owns the dams, planned to spend about $35 million to acquire and repair those dams and an additional two dams prior to the breach. Boyce Hydro declared bankruptcy in July and now faces numerous lawsuits related to the flooding. FERC is coordinating with officials in Michigan on investigations into the dam failures, and has fined Boyce Hydro $15 million for failing to act on federal orders following the incident.

Dufresne, the Edenville fire chief, watched for years as political and financial challenges prevented the dams on the Tittabawassee from getting fixed. His advice for any other community dealing with a problematic dam: Call your state representatives, tell them, “Hey you need to investigate this.”

By August, life in Midland County was slowly getting back to normal. “Some of the people started putting their houses back together. The businesses are trying to figure out what to do next,” said Jerry Cole, the fire chief of Jerome Township, located south of Edenville.

At the Edenville Dam, neat houses looked out over a wide basin of sand-streaked mud where the impounded lake used to be. Near the bottom, where the river was still flowing through the gap in the fractured dam, a group of teenagers lounged on inner tubes, splashing around.

“It just amazes me that this actually happened here,” said Dufresne.

James Dinneen is a science and environmental journalist from Colorado, based in New York.

Alexander Kennedy is a software engineer specializing in data visualization.

This article was originally published on Undark. Read the original article.

Although scientists often assume that random variations in scientific data fit symmetrical, bell-shaped normal distributions, nature isn’t always so tidy. In some cases, a skewed distribution, like the log-normal probability distribution, provides a better fit. Researchers previously found that primary production by ocean phytoplankton and carbon export via particles sinking from the surface are consistent with log-normal distributions.

In a new study, Cael et al. discovered that fluxes at the seafloor also fit log-normal distributions. The team analyzed data from deep-sea sediment traps at six different sites, representing diverse nutrient and oxygen statuses. They found that the log-normal distribution didn’t just fit organic carbon flux; it provided a simple scaling relationship for calcium carbonate and opal fluxes as well.

Uncovering the log-normal distribution enabled the researchers to tackle a longstanding question: Do nutrients reach the benthos—life at the seafloor—via irregular pulses or a constant rain of particles? The team examined the shape of the distribution and found that 29% of the highest measurements accounted for 71% of the organic carbon flux at the seafloor, which is less imbalanced than the 80:20 benchmark specified by the Pareto principle. Thus, although high-flux pulses do likely provide nutrients to the benthos, they aren’t the dominant source.

The findings will provide a simple way for researchers to explore additional links between net primary production at the ocean surface and deep-sea flux. (Geophysical Research Letters, https://doi.org/10.1029/2021GL092895, 2021)

—Jack Lee, Science Writer

Are the processes that generate planetary habitability in our solar system common or rare elsewhere? Answering this fundamental question poses an enormous challenge.

For example, observing Earth-analogue exoplanets—that is, Earth-sized planets orbiting within the habitable zone of their host stars—is difficult today and will remain so even with the next-generation James Webb Space Telescope (JWST) and large-aperture ground-based telescopes. In coming years, it will be much easier to gather data on—and to test hypotheses about the processes that generate and sustain habitability using—“Earth cousins.” These small-radius exoplanets lack solar system analogues but are more accessible to observation because they are slightly bigger or slightly hotter than Earth.

Here we discuss four classes of exoplanets and the investigations of planetary materials that are needed to understand them (Figure 1). Such efforts will help us better understand planets in general and Earth-like worlds in particular.

Fig. 1. Shown here are four common exoplanet classes that are relatively easy to characterize using observations from existing telescopes (or telescopes that will be deployed soon) and that have no solar system analogue. Hypothetical cross sections for each planet type show interfaces that can be investigated using new laboratory and numerical experiments. CO2 = carbon dioxide, Fe = iron, H2O = water, Na = sodium. What’s in the Air?

Atmospheres are now routinely characterized for Jupiter-sized exoplanets. And scientists are acquiring constraints for various atmospheric properties of abundant smaller worlds.On exoplanets, the observable is the atmosphere. Atmospheres are now routinely characterized for Jupiter-sized exoplanets. And scientists are acquiring constraints for various atmospheric properties of smaller worlds (those with a radius R less than 3.5 Earth radii R⨁), which are very abundant [e.g., Benneke et al., 2019; Kreidberg et al., 2019]. Soon, observatories applying existing methods and new techniques such as high-resolution cross-correlation spectroscopy will reveal even more information.

For these smaller worlds, as for Earth, a key to understanding atmospheric composition is understanding exchanges between the planet’s atmosphere and interior during planet formation and evolution. This exchange often occurs at interfaces (i.e., surfaces) between volatile atmospheres and condensed (liquid or solid) silicate materials. For many small exoplanets, these interfaces exhibit pressure-temperature-composition (P–T–X) regimes very different from Earth’s and that have been little explored in laboratory and numerical experiments. To use exoplanet data to interpret the origin and evolution of these strange new worlds, we need new experiments exploring the relevant planetary materials and conditions.

Studying Earth cousin exoplanets can help us probe the delivery and distribution of life-essential volatile species—chemical elements and compounds like water vapor and carbon-containing molecules, for example, that form atmospheres and oceans, regulate climate, and (on Earth) make up the biosphere. Measuring abundances of these volatiles on cousin worlds that orbit closer to their star than the habitable zone is relatively easy to do. These measurements are fundamental to understanding habitability because volatile species abundances on Earth cousin exoplanets will help us understand volatile delivery and loss processes operating within habitable zones.

For example, rocky planets now within habitable zones around red dwarf stars must have spent more than 100 million years earlier in their existence under conditions exceeding the runaway greenhouse limit, suggesting surface temperatures hot enough to melt silicate rock into a magma ocean. So whether these worlds are habitable today depends on the amount of life-essential volatile elements supplied from sources farther from the star [e.g., Tian and Ida, 2015], as well as on how well these elements are retained during and after the magma ocean phase.

Different types of Earth cousin exoplanets offer natural solutions that can ease volatile detection.Volatiles constitute a small fraction of a rocky planet’s mass, and quantifying their abundance is inherently hard. However, different types of Earth cousin exoplanets offer natural solutions that can ease volatile detection. For example, on planets known as sub-Neptunes, the spectroscopic fingerprint of volatiles could be easier to detect because of their mixing with lower–molecular weight atmospheric species like hydrogen and helium. These lightweight species contribute to more puffed-up (expanded) and thus more detectable atmospheres. Hot, rocky exoplanets could “bake out” volatiles from their interiors while also heating and puffing up the atmosphere, which would make spectral features more visible. Disintegrating rocky planets may disperse their volatiles into large, and therefore more observable, comet-like tails.

Let’s look at each of these examples further.

Unexpected Sub-Neptunes

About 1,000 sub-Neptune exoplanets (radius of 1.6–3.5 R⨁) have been confirmed. These planets, which are statistically about as common as stars, blur the boundary between terrestrial planets and gas giants.

A warm, Neptune-sized exoplanet orbits the red dwarf star GJ 3470. Intense radiation from the star heats the planet’s atmosphere, causing large amounts of hydrogen gas to stream off into space. Credit: NASA/ESA/D. Player (STScI)

Strong, albeit indirect, evidence indicates that the known sub-Neptunes are mostly magma by mass and mostly atmosphere by volume (for a review, see Bean et al. [2021]). This evidence implies that an interface occurs, at pressures typically between 10 and 300 kilobars, between the magma and the molecular hydrogen (H2)-dominated atmosphere on these planets. Interactions at and exchanges across this interface dictate the chemistry and puffiness of the atmosphere. For example, water can form and become a significant fraction of the atmosphere, leading to more chemically complex atmospheres.

Improved molecular dynamics calculations are needed to quantify the solubilities of gases and gas mixtures in realistic magma ocean compositions (and in iron alloys composing planetary cores, which can also serve as reservoirs for volatiles) over a wider range of pressures and temperatures than we have studied until now. These calculations should be backed up by laboratory investigations of such materials using high-pressure instrumentation like diamond anvil cells. These calculations and experiments will provide data to help determine the equation of state (the relationship among pressure, volume, and temperature), transport properties, and chemical kinetics of H2-magma mixtures as they might exist on these exoplanets.

Fig. 2. Ranges of plausible conditions at the interfaces between silicate surface rocks and volatile atmospheres on different types of worlds are indicated in this pressure–temperature (P-T) diagram. Conditions on Earth, as well as other relevant conditions (critical points are the highest P-T points where materials coexist in gaseous and liquid states, and triple points are where three phases coexist), are also indicated. Mg2SiO4 = forsterite, an igneous mineral that is abundant in Earth’s mantle.

Because sub-Neptunes are so numerous, we cannot claim to understand the exoplanet mass-radius relationship in general (in effect, the equation of state of planets in the galaxy) without understanding interactions between H2 and magma on sub-Neptunes. To understand the extent of mixing between H2, silicates, and iron alloy during sub-Neptune assembly and evolution, we need more simulations of giant impacts during planet formation [e.g., Davies et al., 2020], as well as improved knowledge of convective processes on these planets. Within the P-T-X regimes of sub-Neptunes, full miscibility between silicates and H2 becomes important (Figure 2).

Beyond shedding light on the chemistry and magma-atmosphere interactions on these exoplanets, new experiments may also help reveal the potential for and drivers of magnetic fields on sub-Neptunes. Such fields might be generated within both the atmosphere and the magma.

Hot and Rocky

Hot, rocky exoplanets experience high fluxes of atmosphere-stripping ultraviolet photons and stellar wind, but whether they retain life-essential elements like nitrogen, carbon, and sulfur is unknown.From statistical studies, we know that most stars are orbited by at least one roughly Earth-sized planet (radius of 0.75–1.6 R⨁) that is irradiated more strongly than our Sun’s innermost planet, Mercury. These hot, rocky exoplanets, of which about a thousand have been confirmed, experience high fluxes of atmosphere-stripping ultraviolet photons and stellar wind. Whether they retain life-essential elements like nitrogen, carbon, and sulfur is unknown.

On these hot, rocky exoplanets—and potentially on Venus as well—atmosphere-rock or atmosphere-magma interactions at temperatures too high for liquid water will be important in determining atmospheric composition and survival. But these interactions have been only sparingly investigated [Zolotov, 2018].

Many metamorphic and melting reactions between water and silicates under kilopascal to tens-of-gigapascal pressures are already known from experiments or are tractable using thermodynamic models. However, less well understood processes may occur in planets where silicate compositions and proportions are different than they are on Earth, meaning that exotic rock phases may be important. Innovative experiments and modeling that consider plausible exotic conditions will help us better understand these planets. Moreover, we need to conduct vaporization experiments to probe whether moderately volatile elements are lost fast enough from hot, rocky planets to form a refractory lag and reset surface spectra.

Exotic Water Worlds?

Water makes up about 0.01% of Earth’s mass. In contrast, the mass fraction of water on Europa, Ceres, and the parent bodies of carbonaceous chondrite meteorites is some 50–3,000 times greater than on Earth. Theory predicts that such water-rich worlds will be common not only in habitable zones around other stars but even in closer orbits as well. The JWST will be able to confirm or refute this theory [Greene et al., 2016].

If we could descend through the volatile-rich outer envelope of a water world, we might find habitable temperatures at shallow depths [Kite and Ford, 2018]. Some habitable layers may be cloaked beneath H2. Farther down, as the atmospheric pressure reaches 10 or more kilobars, we might encounter silicate-volatile interfaces featuring supercritical fluids [e.g., Nisr et al., 2020] and conditions under which water can be fully miscible with silicates [Ni et al., 2017].

We still need answers to several key questions about these worlds. What are the equilibria and rates of gas production and uptake for rock-volatile interfaces at water world “seafloors”? Can they sustain a habitable climate? With no land, and thus no continental weathering, can seafloor reactions supply life-essential nutrients? Do high pressures and stratification suppress the tectonics and volcanism that accelerate interior-atmosphere exchange [Kite and Ford, 2018]?

Relative to rock compositions on Earth, we should expect exotic petrologies on water worlds.As for the deep interiors of Titan and Ganymede in our own solar system, important open questions include the role of clathrates (compounds like methane hydrates in which one chemical component is enclosed within a molecular “cage”) and the solubility and transport of salts through high-pressure ice layers.

Experiments are needed to understand processes at water world seafloors. Metamorphic petrologists are already experienced with the likely pressure-temperature conditions in these environments, and exoplanetary studies could benefit from their expertise. Relative to rock compositions on Earth, we should expect exotic petrologies on water worlds—for example, worlds that are as sodium rich as chondritic meteorites. Knowledge gained through this work would not only shed light on exoplanetary habitability but also open new paths of research into studying exotic thermochemical environments in our solar system.

Magma Seas and Planet Disintegration

Some 100 confirmed rocky exoplanets are so close to their stars that they have surface seas of very low viscosity magma. The chemical evolution of these long-lived magma seas is affected by fractional vaporization, in which more volatile materials rise into the atmosphere and can be relocated to the planet’s dark side or lost to space [e.g., Léger et al., 2011; Norris and Wood, 2017], and perhaps by exchange with underlying solid rock.

Magma planets usually have low albedos, reflecting relatively little light from their surfaces. However, some of these planets appear to be highly reflective, perhaps because their surfaces are distilled into a kind of ceramic rich in calcium and aluminum. One magma planet’s thermal signature has been observed to vary from month to month by a factor of 2 [Demory et al., 2016], implying that it undergoes a global energy balance change more than 10,000 times greater than that from anthropogenic climate change on Earth. Such large swings suggest that fast magma ocean–atmosphere feedbacks operate on the planet.

To learn more about the chemical evolution and physical properties of exoplanet magma seas, we need experiments like those used to study early-stage planet formation, which can reveal information about silicate vaporization and kinetics under the temperatures (1,500–3,000 K) and pressures (10−5 to 100 bars) of magma planet surfaces.

Exoplanets and exoplanetesimals that stray too close to their stars are destroyed—about five such cases have been confirmed. These disintegrating planets give geoscientists direct views of exoplanetary silicates because the debris tails can be millions of kilometers long [van Lieshout and Rappaport, 2018]. For disintegrating planets that orbit white dwarf stars, the debris can form a gas disk whose composition can be reconstructed [e.g., Doyle et al., 2019].

To better read the signals of time-variable disintegration, we need more understanding of how silicate vapor in planetary outflows condenses and nucleates, as well as of fractionation processes at and above disintegrating planets’ surfaces that may cause observed compositions in debris to diverge from the bulk planet compositions.

Getting to Know the Cousins

Investigating Earth cousins will illuminate the processes underpinning habitability in our galaxy and reveal much that is relevant for understanding Earth twins.In the near future, new observatories like JWST and the European Space Agency’s Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL, planned for launch in 2029) will provide new data. When they do, and even now before they come online, investigating Earth cousins will illuminate the processes underpinning habitability in our galaxy and reveal much that is relevant for understanding Earth twins.

From sub-Neptunes, for example, we can learn about volatile delivery processes. From hot, rocky planets, we can learn about atmosphere-interior exchange and atmospheric loss processes. From water worlds, we can learn about nutrient supplies in exoplanetary oceans and the potential habitability of these exotic environments. From disintegrating planets, we can learn about the interior composition of rocky bodies.

Laboratory studies of processes occurring on these worlds require only repurposing and enhancing existing experimental facilities, rather than investing in entire new facilities. From a practical standpoint, the scientific rewards of studying Earth cousins are low-hanging fruit.

Acknowledgments

We thank the organizers of the AGU-American Astronomical Society/Kavli workshop on exoplanet science in 2019.

New modes of aerial operations are emerging in the urban environment, collectively known as Advanced Air Mobility (AAM). These include electrically propelled vertical takeoff and landing aerial vehicles for infrastructure surveillance, goods delivery, and passenger transportation. However, ultra-fine weather and turbulence guidance products are needed that contribute to safe and efficient deployment of these activities. In fact, initial testing/demonstration exercises are planned to occur in the very near future, thus the timely and relevant nature of the present work.

To enable successful operation of these new aerial operations in the urban environment, the meteorological community must provide relevant guidance to inform and support these activities. Muñoz-Esparza et al. [2021] demonstrate how seasonal, diurnal, day-to-day, and rapidly evolving sub-hourly meteorological phenomena create unique wind and turbulence distributions within the urban canopy. They showcase the potential for efficient ultra-fine resolution atmospheric models to understand and predict urban weather impacts that are critical to these AAM operations.

Citation: Muñoz-Esparza, D., Shin, H., Sauer, J. et al. [2021]. Efficient GPU Modeling of Street-Scale Weather Effects in Support of Aerial Operations in the Urban Environment. AGU Advances, 2, e2021AV000432. https://doi.org/10.1029/2021AV000432

—Donald Wuebbles, Editor, AGU Advances

Global Climate Models (GCMs) suffer from the tropical rainfall bias, with double peaks on both sides of the equator rather than just north of the equator, known as the double Inter-Tropical Convergence Zone (ITCZ) bias. The tropical mean state bias limits the fidelity of GCMs in projecting the future climate. Much effort has gone into improving this double ITCZ bias, but it has not been alleviated since the early days of model development.

Baldwin et al. [2021] suggest that a significant portion of the double ITCZ bias originates from low biases in Central American orography in models. Orographic peaks are often smoothed out in models that use observed orography averaged onto model grid. Elevation of Central American orography is demonstrated to reduce the double ITCZ bias as the northeastern tropical Pacific becomes warmer owing to blocked easterlies. The study offers a simple and computationally inexpensive yet physically based method for improving pervasive double ITCZ bias.

Citation: Baldwin, J., Atwood, A., Vecchi, G. and Battisti, D. [2021]. Outsize Influence of Central American Orography on Global Climate. AGU Advances, 2, e2020AV000343. https://doi.org/10.1029/2020AV000343

For more than five decades, satellites orbiting Earth have recorded and measured different characteristics of the land, oceans, cryosphere, and atmosphere, and how they are changing. Observations of planet Earth from space are a critical resource for science and society. With the planet under pressure from ever-expanding and increasingly intensive human activities combined with climate change, observations from space are increasingly relied upon to monitor and to inform adaptation and mitigation activities to maintain food security, biodiversity, water quality, and responsiveness to disasters.

A new cross-journal special collection, The Earth in Living Color, aims to provide a state-of-art and timely assessment of how advances in remote sensing is revealing new insights and understanding for monitoring our home planet.  We encourage papers that cover the use of imaging spectroscopy and thermal infrared remote sensing to observe and understand the Earth’s vegetation, coastal aquatic ecosystems, surface mineralogy, snow dynamics, and volcanic activity. These may range from architecture studies that determine spaceborne measurement objectives, to papers on algorithm development, calibration and validation, and modeling to support traceability. Papers can be submitted either to Journal of Geophysical Research: Biogeosciences or Earth and Space Science.

The special collection is associated with the NASA Surface Biology and Geology Designated Observable (SBG), and will document:

how SBG will meet science and applications measurement objectives; how international partnerships (with the European Space Agency’s Copernicus Hyperspectral Imaging Mission (CHIME) and Land Surface Temperature Monitoring mission (LSTM) and with the Centre National d’Études Spatiales (CNES) and Indian Space Research Organization’s (ISRO) Thermal infraRed Imaging Satellite for High-resolution Natural resource Assessment mission (TRISHNA) will improve revisit times; describe new developments in atmospheric correction, surface reflectance retrievals, and algorithms; and detail synergies with other NASA Decadal Survey missions.

SBG leverages a rich heritage of airborne imaging spectroscopy that includes the AVIRIS and PRISM instruments, and thermal imagers such as HYTES and MASTER, as well space-based observations from pathfinder missions such as HYPERION, and current missions, including ECOSTRESS, PRISMA, DESIS, and HISUI.

Satellite measurements represent very large investments and the United States and space agencies around the globe organize their efforts to maximize the return on that investment. For instance, the US National Research Council conducts a decadal survey of NASA earth science and applications to prioritize observations of the atmosphere, ocean, land, and cryosphere. The most recent NASA Decadal survey, published in 2017, prioritized observations of surface biology and geology using a visible to shortwave infrared (VSWIR) imaging spectrometer and a multi-spectral thermal infrared (TIR) imager to meet a range of needs. As announced by NASA in May 2021, SBG will become integrated within a larger NASA Earth System Observatory (ESO)  that will include observations of aerosols, clouds, convection, and precipitation, mass change, and surface-deformation and change.

The SBG science, applications and technology build on over a decade of experience and planning for such a mission based on the previous Hyperspectral Infrared Imager (HyspIRI) mission study. During the course of a three-year study (2018-2021), the SBG team analyzed needed instrument characteristics (spatial, temporal and spectral resolution, measurement uncertainty) and assessed the cost, mass, power, volume, and risk of different architectures. The SBG Research and Applications team examined available algorithms, calibration and validation, and societal applications, and used end-to-end modeling to assess uncertainty.  The team also identified valuable opportunities for international collaboration to increase the frequency of revisit through data sharing, adding value for all partners. Analysis of the science, applications, architecture, and partnerships led to a clear measurement strategy and a well-defined observing system architecture.

SBG addresses global vegetation, aquatic, and geologic processes that quantify critical aspects of the land surface, responding to NASA’s Decadal Survey priorities, which then interact with the Earth’s climate system. The SBG observing system has a defined set of critical observables that equally inform science and environmental management and policy for a host of societal benefit areas. Click image for larger version. Credit: NASA JPL

First, and perhaps, foremost, SBG will be a premier integrated observatory for observing the emerging impacts of climate change. It will characterize the diversity of plant life by resolving chemical and physiological signatures. It will address wildfire, observing pre-fire risk, fire behavior and post-fire recovery. It will provide information for the coastal zone on phytoplankton abundance, water quality, and aquatic ecosystem classification. It will inform responses to natural and anthropogenic hazards and disasters guiding responds to a wide range of events, including oil spills, toxic minerals, harmful algal blooms, landslides and other geological hazards, including volcanic activity.

The NASA Earth System Observatory initiates a new era of scientific monitoring, with SBG providing an unprecedented perspective of the Earth surface through new spatial, temporal, and spectral information with high signal-to-noise. The Earth in Living Color special collection will showcase the latest advances in remote sensing that are providing vital insights into changes in planet Earth.

—David Schimel (david.schimel@jpl.nasa.gov,  0000-0003-3473-8065), NASA Jet Propulsion Laboratory, USA; and Benjamin Poulter ( 0000-0002-9493-8600), NASA Goddard Space Flight Center, USA

When it opened in 1963, Bhakra Dam was called a “new temple of resurgent India” by Jawaharlal Nehru, India’s first prime minister. Today the dam is threatened as its reservoir rapidly fills with silt.

Much to the worry of hydrologists monitoring the situation, the reservoir—Gobind Sagar Lake—has a rapidly growing sediment delta that, once it reaches the dam, will adversely affect power generation and water deliveries.

Bhakra Dam stands 226 meters tall and stretches 518 meters long, making it one of the largest dams in India. Electricity generated by the dam supports the states of Himachal Pradesh (where the dam is located), Punjab, Haryana, and Rajasthan, and the union territories of Chandigarh and Delhi. The reservoir supplies these areas with water for drinking, hygiene, industry, and irrigation. Loss of reservoir capacity as a result of sedimentation could thus have severe consequences for the region’s water management system and power grid.

A Leopard’s Leap to a Green Revolution

In 1908, British civil services officer Sir Louis Dane claimed to have witnessed a leopard leaping from one end of a gorge on the Sutlej River to the other. “Here’s a site made by God for storage,” he wrote. Little happened, however, until 40 years later, when Nehru took up the proposal as one of the first large infrastructure projects in India after independence.

“Before the canal brought water to our area, we were poor [and] used to live [lives] of nomads, in the sand dunes. Now we grow a variety of crops…and we are referred [to] as affluent farmers.”Bhakra Dam’s waters quickly catalyzed the nation’s green revolution of increased agricultural production. In the early 1960s, for instance, 220,000 hectares of rice were under paddy cultivation in Punjab. Within 10 years, that number increased to 1.18 million, which doubled by 1990. Today Punjab contributes up to 50% of India’s rice supply.

Parminder Singh Dhanju, a rural resident of Rajasthan whose village is about 565 kilometers from Bhakra Dam, has a farm fed by canals originating from the reservoir. “The water availability has changed the lives of us villagers,” he said. “Before the canal brought water to our area, we were poor [and] used to live [lives] of nomads, in the sand dunes. Now we grow a variety of crops such as wheat, rice, cotton, and citrus fruits (oranges and kinnows), and we are referred [to] as affluent farmers.”

The Saga of Silt

According to investigations led by D. K. Sharma, former chairman of the Bhakra Beas Management Board (BBMB, the power company responsible for the dam), nearly a quarter of Gobind Sagar Lake has filled with silt. The sedimentation flows from the lake’s catchment areas, which are spread over 36,000 square kilometers in the Himalayas.

“The storage of the reservoir is 9.27 billion cubic meters, out of which 2.13 billion cubic meters are filled with silt, which is an alarming situation,” explained Sharma. He said the studies related to silt pileup are carried out every 2 years.

Sharma and other BBMB engineers submitted a report last year on siltation at Bhakra Dam. In it, Sharma said the dam was projected to be an effective reservoir for at least 100 years. However, he explained, the silt buildup will likely shorten that time frame. “It depends on the amount of silt in the reservoir,” he said. “The increase in siltation will hasten the process of turning the dam into a dead project, making the canal system downstream vulnerable to deposition of silt and floods.”

The Way Out

To combat siltation, Sharma suggested extensive reforestation in the reservoir’s catchment area. “The partner states of BBMB—Punjab, Haryana, Rajasthan, and Himachal Pradesh—need to plan forestation to bind the loose soil,” he said.

“If we can reduce silt inflows by 10%, the dam’s life can be extended by 15–20 years,” he added.

“We need to act fast and engage local population and NGOs to carry out plantation, before it’s too late.”BBMB joint secretary Anurag Goyal heads the reforestation project around the dam. He said that in 2019, 600,000 saplings were planted over the reservoir’s catchment area. “We have resumed plantation that was temporarily halted in 2020 due to COVID-19 pandemic.”

Other suggestions to prevent or mitigate siltation include dredging the reservoir, although Goyal dismisses that idea as cost prohibitive. Goyal agreed with Sharma that reforestation or other mitigation projects must include local governments. “Reforestation over [such a] vast area needs a road map and the involvement of the north Indian states…. We need to act fast and engage local population and NGOs to carry out plantation, before it’s too late.”

—Gurpreet Singh (@JournoGurpreet), Science Writer

The Gulf Stream, which has reliably channeled warm water from the tropics northward along the East Coast of North America for thousands of years, is changing. Recent research shows that it may be slowing down, and more and more often, the current is meandering into the Mid-Atlantic Bight—a region on the continental shelf stretching from North Carolina to Massachusetts and one of the most productive marine ecosystems in the world.

Previous studies have suggested that this intrusion of Gulf Stream water, which is comparatively low in nutrients at the surface, could hamper productivity. But in a new study, Oliver et al. found that intrusions of deeper, nutrient-rich Gulf Stream water can also feed hot spots of primary productivity.

By analyzing data collected by R/V Thomas G. Thompson in July of 2019, the team spotted a series of hot spots about 50 meters below the surface, just east of a large eddy known as a warm-core ring. This ring had formed off the side of the Gulf Stream current and was pushing westward toward the continental shelf, drawing cool water into the slope region off the edge of the shelf.

The hot spots had chlorophyll levels higher than those typically seen in the slope region and were packed with a diverse load of diatoms, a class of single-celled algae. Studying images of the hot spots, the team found that the colony-forming diatom Thalassiosira diporocyclus was an abundant type in the hot spots.

The researchers used a model that combined upper ocean and biogeochemical dynamics to support the idea that the upwelling of Gulf Stream water moving northward into the Mid-Atlantic Bight could cause the hot spots to form. The study demonstrates how Gulf Stream nutrients could influence subsurface summer productivity in the region and that such hot spots should be taken into account when researchers investigate how climate change will reshape circulation patterns in the North Atlantic. (Geophysical Research Letters, https://doi.org/10.1029/2020GL091943, 2021)

—Kate Wheeling, Science Writer