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      Humans may not be able to handle as much heat as scientists thought

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      More than 2,000 people dead from extreme heat and wildfires raging in Portugal and Spain. High temperature records shattered from England to Japan. Overnights that fail to cool.

      Brutal heat waves are quickly becoming the hallmark of the summer of 2022.

      And even as climate change continues to crank up the temperature, scientists are working fast to understand the limits of humans’ resilience to heat extremes. Recent research suggests that heat stress tolerance in people may be lower than previously thought. If true, millions more people could be at risk of succumbing to dangerous temperatures sooner than expected.

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      “Bodies are capable of acclimating over a period of time” to temperature changes, says Vivek Shandas, an environmental planning and climate adaptation researcher at Portland State University in Oregon. Over geologic time, there have been many climate shifts that humans have weathered, Shandas says. “[But] we’re in a time when these shifts are happening much more quickly.”

      Just halfway through 2022, heat waves have already ravaged many countries. The heat arrived early in southern Asia: In April, Wardha, India, saw a high of 45° Celsius (113° Fahrenheit); in Nawabshah, Pakistan, in May recorded temperatures rose to 49.5° C (121.1° F).

      Extreme heat alerts blared across Europe beginning in June and continuing through July, the rising temperatures exacerbating drought and sparking wildfires. The United Kingdom shattered its hottest-ever record July 19 when temperatures reached 40.3° C in the English village of Coningsby. The heat fueled fires in France, forcing thousands to evacuate from their homes. 

      And the litany goes on: Japan experienced its worst June heat wave since record-keeping began in 1875, leading to the country’s highest-ever recorded June temperature of 40.2° C.  China’s coastal megacities, from Shanghai to Chengdu, were hammered by heat waves in July as temperatures in the region also rose above 40° C. And in the United States, a series of heat waves gripped the Midwest, the South and the West in June and July. Temperatures soared to 42° C in North Platte, Neb., and to 45.6° C in Phoenix.

      The current global rate of warming on Earth is unprecedented (SN: 7/24/19). And scientists have long predicted that human-caused climate change will increase the occurrence of heat waves. Globally, humans’ exposure to extreme heat tripled from 1983 to 2016, particularly in South Asia.

      The heat already is taking an increasing toll on human health. It can cause heat cramps, heat exhaustion and heat stroke, which is often fatal. Dehydration can lead to kidney and heart disease. Extreme heat can even change how we behave, increasing aggression and decreasing our ability to focus (SN: 8/18/21).

      Staying cool

      The human body has various ways to shed excess heat and keep the core of the body at an optimal temperature of about 37° C (98.6° F). The heart pumps faster, speeding up blood flow that carries heat to the skin (SN: 4/3/18). Air passing over the skin can wick away some of that heat. Evaporative cooling — sweating — also helps.

      But there’s a limit to how much heat humans can endure. In 2010, scientists estimated that theoretical heat stress limit to be at a “wet bulb” temperature of 35° C. Wet bulb temperatures depend on a combination of humidity and “dry bulb” air temperature measured by a thermometer. Those variables mean a place could hit a wet bulb temperature of 35° C in different ways — for instance, if the air is that temperature and there’s 100 percent humidity, or if the air temperature is 45° C and there’s 50 percent humidity. The difference is due to evaporative cooling.

      When water evaporates from the skin or another surface, it steals away energy in the form of heat, briefly cooling that surface. That means that in drier regions, the wet bulb temperature — where that ephemeral cooling effect happens readily — will be lower than the actual air temperature. In humid regions, however, wet and dry bulb temperatures are similar, because the air is so moist it’s difficult for sweat to evaporate quickly.

      So when thinking about heat stress on the body, scientists use wet bulb temperatures because they are a measure of how much cooling through evaporation is possible in a given climate, says Daniel Vecellio, a climate scientist at Penn State.

      “Both hot/dry and warm/humid environments can be equally dangerous,” Vecellio says — and this is where the body’s different cooling strategies come into play. In hot, dry areas, where the outside temperature may be much hotter than skin temperature, human bodies rely entirely on sweating to cool down, he says. In warm, humid areas, where the air temperature may actually be cooler than skin temperatures (but the humidity makes it seem warmer than it is), the body can’t sweat as efficiently. Instead, the cooler air passing over the skin can draw away the heat.

      How hot is too hot?

      Given the complexity of the body’s cooling system, and the diversity of human bodies, there isn’t really a one-size-fits-all threshold temperature for heat stress for everybody. “No one’s body runs at 100 percent efficiency,” Vecellio says. Different body sizes, the ability to sweat, age and acclimation to a regional climate all have a role.

      Still, for the last decade, that theoretical wet bulb 35° C number has been considered to be the point beyond which humans can no longer regulate their bodies’ temperatures. But recent laboratory-based research by Vecellio and his colleagues suggests that a general, real-world threshold for human heat stress is much lower, even for young and healthy adults.

      The researchers tracked heat stress in two dozen subjects ranging in age from 18 to 34, under a variety of controlled climates. In the series of experiments, the team varied humidity and temperature conditions within an environmental chamber, sometimes holding temperature constant while varying the humidity, and sometimes vice versa.

      The subjects exerted themselves within the chamber just enough to simulate minimal outdoor activity, walking on a treadmill or pedaling slowly on a bike with no resistance. During these experiments, which lasted for 1.5 to two hours, the researchers measured the subjects’ skin temperatures using wireless probes and assessed their core temperatures using a small telemetry pill that the subjects swallowed.

      In warm and humid conditions, the subjects in the study were unable to tolerate heat stress at wet bulb temperatures closer to 30° or 31° C, the team estimates. In hot and dry conditions, that wet bulb temperature was even lower, ranging from 25° to 28° C, the researchers reported in the February Journal of Applied Physiology. For context, in a very dry environment at about 10 percent humidity, a wet bulb temperature of 25° C would correspond to an air temperature of about 50° C (122° F).

      These results suggest that there is much more work to be done to understand what humans can endure under real-world heat and humidity conditions, but that the threshold may be much lower than thought, Vecellio says. The 2010 study’s theoretical finding of 35° C may still be “the upper limit,” he adds. “We’re showing the floor.”

      And that’s for young, healthy adults doing minimal activity. Thresholds for heat stress are expected to be lower for outdoor workers required to exert themselves, or for the elderly or children. Assessing laboratory limits for more at-risk people is the subject of ongoing work for Vecellio and his colleagues.

      A worker wipes away sweat in Toulouse, France, on July 13. An intense heat wave swept across Europe in mid-July, engulfing Spain, Portugal, France, England and other countries.VALENTINE CHAPUIS/AFP via Getty Images

      If the human body’s tolerance for heat stress is generally lower than scientists have realized, that could mean millions more people will be at risk from the deadliest heat sooner than scientists have realized. As of 2020, there were few reports of wet bulb temperatures around the world reaching 35° C, but climate simulations project that limit could be regularly exceeded in parts of South Asia and the Middle East by the middle of the century.

      Some of the deadliest heat waves in the last two decades were at lower wet bulb temperatures: Neither the 2003 European heat wave, which caused an estimated 30,000 deaths, nor the 2010 Russian heat wave, which killed over 55,000 people, exceeded wet bulb temperatures of 28° C.

      Protecting people

      How best to inform the public about heat risk is “the part that I find to be tricky,” says Shandas, who wasn’t involved in Vecellio’s research. Shandas developed the scientific protocol for the National Integrated Heat Health Information System’s Urban Heat Island mapping campaign in the United States.

      It’s very useful to have this physiological data from a controlled, precise study, Shandas says, because it allows us to better understand the science behind humans’ heat stress tolerance. But physiological and environmental variability still make it difficult to know how best to apply these findings to public health messaging, such as extreme heat warnings, he says. “There are so many microconsiderations that show up when we’re talking about a body’s ability to manage [its] internal temperature.”

      One of those considerations is the ability of the body to quickly acclimate to a temperature extreme. Regions that aren’t used to extreme heat may experience greater mortality, even at lower temperatures, simply because people there aren’t used to the heat. The 2021 heat wave in the Pacific Northwest wasn’t just extremely hot — it was extremely hot for that part of the world at that time of year, which makes it more difficult for the body to adapt, Shandas says (SN: 6/29/21).

      Heat that arrives unusually early and right on the heels of a cool period can also be more deadly, says Larry Kalkstein, a climatologist at the University of Miami and the chief heat science advisor for the Washington, D.C.–based nonprofit Adrienne Arsht-Rockefeller Foundation Resilience Center. “Often early season heat waves in May and June are more dangerous than those in August and September.”

      One way to improve communities’ resilience to the heat may be to treat heat waves like other natural disasters — including give them names and severity rankings (SN: 8/14/20). As developed by an international coalition known as the Extreme Heat Resilience Alliance, those rankings form the basis for a new type of heat wave warning that explicitly considers the factors that impact heat stress, such as wet bulb temperature and acclimation, rather than just temperature extremes.

      The rankings also consider factors such as cloud cover, wind and how hot the temperatures are overnight. “If it’s relatively cool overnight, there’s not as much negative health outcome,” says Kalkstein, who created the system. But overnight temperatures aren’t getting as low as they used to in many places. In the United States, for example, the average minimum temperatures at nighttime are now about 0.8° C warmer than they were during the first half of the 20th century, according to the country’s Fourth National Climate Assessment, released in 2018 (SN: 11/28/18).

      By naming heat waves like hurricanes, officials hope to increase citizens’ awareness of the dangers of extreme heat. Heat wave rankings could also help cities tailor their interventions to the severity of the event. Six cities are currently testing the system’s effectiveness: four in the United States and in Athens, Greece, and Seville, Spain. On July 24, with temperatures heading toward 42° C, Seville became the first city in the world to officially name a heat wave, sounding the alarm for Heat Wave Zoe.

      As 2022 continues to smash temperature records around the globe, such warnings may come not a moment too soon. More

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      How to make jet fuel from sunlight, air and water vapor

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      Jet fuel can now be siphoned from the air.

      Or at least that’s the case in Móstoles, Spain, where researchers demonstrated that an outdoor system could produce kerosene, used as jet fuel, with three simple ingredients: sunlight, carbon dioxide and water vapor. Solar kerosene could replace petroleum-derived jet fuel in aviation and help stabilize greenhouse gas emissions, the researchers report in the July 20 Joule.

      Burning solar-derived kerosene releases carbon dioxide, but only as much as is used to make it, says Aldo Steinfeld, an engineer at ETH Zurich. “That makes the fuel carbon neutral, especially if we use carbon dioxide captured directly from the air.”

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      Kerosene is the fuel of choice for aviation, a sector responsible for around 5 percent of human-caused greenhouse gas emissions. Finding sustainable alternatives has proven difficult, especially for long-distance aviation, because kerosene is packed with so much energy, says chemical physicist Ellen Stechel of Arizona State University in Tempe who was not involved in the study.

      In 2015, Steinfeld and his colleagues synthesized solar kerosene in the laboratory, but no one had produced the fuel entirely in a single system in the field. So Steinfeld and his team positioned 169 sun-tracking mirrors to reflect and focus radiation equivalent to about 2,500 suns into a solar reactor atop a 15-meter-tall tower. The reactor has a window to let the light in, ports that supply carbon dioxide and water vapor as well as a material used to catalyze chemical reactions called porous ceria.

      Within the solar reactor, porous ceria (shown) gets heated by sunlight and reacts with carbon dioxide and water vapor to produce syngas, a mixture of hydrogen gas and carbon monoxide.ETH Zurich

      When heated with solar radiation, the ceria reacts with carbon dioxide and water vapor in the reactor to produce syngas — a mixture of hydrogen gas and carbon monoxide. The syngas is then piped to the tower’s base where a machine converts it into kerosene and other hydrocarbons.

      Over nine days of operation, the researchers found that the tower converted about 4 percent of the used solar energy into roughly 5,191 liters of syngas, which was used to synthesize both kerosene and diesel. This proof-of-principle setup produced about a liter of kerosene a day, Steinfeld says.

      “It’s a major milestone,” Stechel says, though the efficiency needs to be improved for the technology to be useful to industry. For context, a Boeing 747 passenger jet burns around 19,000 liters of fuel during takeoff and the ascent to cruising altitude. Recovering heat unused by the system and improving the ceria’s heat absorption could boost the tower’s efficiency to more than 20 percent, making it economically practical, the researchers say. More

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      Underground heat pollution could be tapped to mitigate climate change

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      The secret to efficiently heating some buildings might lurk beneath our feet, in the heat that humans have inadvertently stored underground. 

      Just as cities warm the surrounding air, giving rise to urban heat islands, so too does human infrastructure warm the underlying earth (SN: 3/27/09). Now, an analysis of groundwater well sites across Europe and parts of North America and Australia reveals that roughly a couple thousand of those locations possess excess underground heat that could be recycled to warm buildings for a year, researchers report July 8 in Nature Communications.

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      What’s more, even if humans managed to remove all this accumulated thermal pollution, existing infrastructure at about a quarter of the locations would continue to warm the ground enough that heat could be harvested for many years to come. That could reduce reliance on fossil fuels, and help mitigate climate change.

      This work showcases the impact that underground heat recycling could have if harnessed on a large scale, says hydrogeologist Grant Ferguson of the University of Saskatchewan in Saskatoon, Canada, who was not involved in the study. “There’s a lot of untapped potential out there.”

      Heat leaks into the subsurface from the warm roots of structures such as buildings, parking garages and tunnels, and from artificial surfaces such as asphalt, which absorb solar radiation. In Lyon, France, for example, researchers in 2016 found that human infrastructure warmed groundwater by more than 4 degrees Celsius.

      Scientists don’t fully understand how heat pollution alters underground environments. But warming of the subsurface can cause contaminants, such as arsenic, to move through groundwater more readily.

      Extracting the thermal pollution could be accomplished by piping groundwater to heat pumps at the surface. The water, warmed underground by all that trapped heat, could then warm buildings as it releases heat into their cooler interiors, says Susanne Benz, an environmental scientist at Dalhousie University in Halifax, Canada.

      Harnessing underground heat in this way could provide some communities with a reliable and low-energy means to warm their homes, Benz says. “And if we don’t use it, it will just continue to accumulate,” she says.

      Benz and her colleagues analyzed the population size, heating demand and groundwater temperature at more than 6,000 locations, most of which were in Europe. The researchers found that at about 43 percent of the locations — mostly those near highly populated areas — enough heat had accumulated in the top 20 meters of earth to satisfy a year’s worth of the local heating demand.

      Curious about sustainability, the researchers also identified places where the continuous flow of heat into the underground — and not just the stockpiled thermal pollution — was high. Their calculations show that if all of the accumulated heat was first extracted, the heat that continued leaking from existing infrastructure could be harvested at about 25 percent of the 6,000 locations. At 18 percent of locations, this recycled heat could satisfy at least a quarter of the heating demand of the local population.

      Constructing systems to take advantage of human heat pollution today could one day help residents harvest heat from climate change, the researchers say.

      Using climate projections for the end of the century, the team probed the feasibility of extracting underground heat in a warmer world. In the most optimistic warming scenario considered, which assumes greenhouse gas emissions peak about the year 2040, the researchers found that climate change would warm the ground enough by the end of the century that underground heat recycling at 81 percent of the studied locations could meet more than a quarter of locals’ heating demands. If there are no efforts to curb emissions, that number rises to 99 percent of locations.

      Though the researchers focused mostly on Europe, Benz says that other continents probably also possess abundant underground heat that could be harnessed. In Europe and elsewhere, heat recycling might be most feasible in suburban areas, she says, where there is sufficient accumulated underground heat to help meet local heating demands, and space to install heat recycling systems.

      Looking ahead, Benz plans to investigate whether cooling the subsurface can help reduce aboveground temperatures in urban environments. “This might actually be a little additional tool to control [aboveground] urban heat.” More

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      In the battle of human vs. water, ‘Water Always Wins’

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      Water Always WinsErica Gies Univ. of Chicago, $26

      Humans have long tried to wrangle water. We’ve straightened once-meandering rivers for shipping purposes. We’ve constructed levees along rivers and lakes to protect people from flooding. We’ve erected entire cities on drained and filled-in wetlands. We’ve built dams on rivers to hoard water for later use.

      “Water seems malleable, cooperative, willing to flow where we direct it,” environmental journalist Erica Gies writes in Water Always Wins. But it’s not, she argues.

      Levees, which narrow channels causing water to flow higher and faster, nearly always break. Cities on former wetlands flood regularly — often catastrophically. Dams starve downstream environs of sediment needed to protect coastal areas against rising seas. Straightened streams flow faster than meandering ones, scouring away riverbed ecosystems and giving water less time to seep downward and replenish groundwater supplies.

      In addition to laying out this damage done by supposed water control, Gies takes readers on a hopeful global tour of solutions to these woes. Along the way, she introduces “water detectives”— scientists, engineers, urban planners and many others who, instead of trying to control water, ask: What does water want?

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      These water detectives have found ways to give the slippery substance the time and space it needs to trickle underground. Around Seattle’s Thornton Creek, for instance, reclaimed land now allows for regular flooding, which has rejuvenated depleted riverbed habitat and created an urban oasis. In California’s Central Valley, scientists want to find ways to shunt unpolluted stormwater into ancient, sediment-filled subsurface canyons that make ideal aquifers. Feeding groundwater supplies will in turn nourish rivers from below, helping to maintain water levels and ecosystems.

      While some people are exploring new ways to manage water, others are leaning on ancestral knowledge. Without the use of hydrologic mapping tools, Indigenous peoples of the Andes have a detailed understanding of the plumbing that links surface waters with underground storage. Researchers in Peru are now studying Indigenous methods of water storage, which don’t require dams, in hopes of ensuring a steady flow of water to Lima — Peru’s populous capital that’s periodically afflicted by water scarcity. These studies may help convince those steeped in concrete-centric solutions to try something new. “Decision makers come from a culture of concrete,” Gies writes, in which dams, pipes and desalination plants are standard.Understanding how to work with, not against, water will help humankind weather this age of drought and deluge that’s being exacerbated by climate change. Controlling water, Gies convincingly argues, is an illusion. Instead, we must learn to live within our water means because water will undoubtedly win.

      Buy Water Always Wins from Bookshop.org. Science News is a Bookshop.org affiliate and will earn a commission on purchases made from links in this article. More

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      Flower shape and size impact bees’ chances of catching gut parasites

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      Bees that land on short, wide flowers can fly away with an upset stomach.  

      Common eastern bumblebees (Bombus impatiens) are more likely to catch a diarrhea-inducing gut parasite from purple coneflowers, black-eyed Susans and other similarly shaped flora than other flowers, researchers report in the July Ecology. Because parasites and diseases contribute to bee decline, the finding could help researchers create seed mixes that are more bee-friendly and inform gardeners’ and land managers’ decisions about which flower types to plant.

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      The parasite (Crithidia bombi) is transmitted when the insects accidentally ingest contaminated bee feces, which “tends to make the bees dopey and lethargic,” says Rebecca Irwin, a community and evolutionary ecologist at North Carolina State University in Raleigh. “It isn’t the number one bee killer out there,” but bees sickened with it can struggle with foraging.    

      In laboratory experiments involving caged bees and 16 plant species, Irwin and her colleagues studied how different floral attributes affected transmission of the gut parasite. They focused on three factors of transmission: the amount of poop landing on flowers when bees fly and forage, how long the parasite survives on the plants and how easily the parasite is transmitted to new bees. Multiplied together, these three factors show the overall transmission rate.

      Compared with plants with long, narrow flowers like phlox and bluebeards, short, wide flowers had more feces land on them and transmitted the parasite more easily to the pollinators, increasing the overall parasite transmission rate for these flowers. However, parasite survival times were reduced on these blooms. This is probably due to the open floral shapes increased exposure to ultraviolet light, speeding the drying out of parasite-laden “fecal droplets,” Irwin says.

      The findings confirm a new theory suggesting that traits, such as flower shape, are better predictors of disease transmission than individual species of plants, says Scott McArt, an entomologist focusing on pollinator health at Cornell University who wasn’t involved with the study. Therefore, “you don’t need to know everything about every plant species when designing your pollinator-friendly garden or habitat restoration project.”

      Instead, to limit disease transmission among bees, it’s best to choose plants that have narrower, longer flowers, he says. “Wider and shorter flowers are analogous to the small, poorly ventilated rooms where COVID is efficiently transmitted among humans.”  

      If ripping out coneflowers or black-eyed Susans isn’t palatable, don’t fret. Irwin recommends continuing to plant a diversity of flower types. This helps if one type of flower is “a high transmitting species,” she notes. In the future, she plans to conduct field experiments examining other factors that could influence parasite transmission, such as whether bees are driven to visit certain types of flowers more often in nature.   More

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      How to build better ice towers for drinking water and irrigation

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      There’s a better way to build a glacier.

      During winter in India’s mountainous Ladakh region, some farmers use pipes and sprinklers to construct building-sized cones of ice. These towering, humanmade glaciers, called ice stupas, slowly release water as they melt during the dry spring months for communities to drink or irrigate crops. But the pipes often freeze when conditions get too cold, stifling construction.

      Now, preliminary results show that an automated system can erect an ice stupa while avoiding frozen pipes, using local weather data to control when and how much water is spouted. What’s more, the new system uses roughly a tenth the amount of water that the conventional method uses, researchers reported June 23 at the Frontiers in Hydrology meeting in San Juan, Puerto Rico.

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      “This is one of the technological steps forward that we need to get this innovative idea to the point where it’s realistic as a solution,” says glaciologist Duncan Quincey of the University of Leeds in England who was not involved in the research. Automation could help communities build larger, longer-lasting ice stupas that provide more water during dry periods, he says.

      Ice stupas emerged in 2014 as a means for communities to cope with shrinking alpine glaciers due to human-caused climate change (SN: 5/29/19). Typically, high-mountain communities in India, Kyrgyzstan and Chile pipe glacial meltwater into gravity-driven fountains that sprinkle continuously in the winter. Cold air freezes the drizzle, creating frozen cones that can store millions of liters of water.

      The process is simple, though inefficient. More than 70 percent of the spouted water may flow away instead of freezing, says glaciologist Suryanarayanan Balasubramanian of the University of Fribourg in Switzerland.

      So Balasubramanian and his team outfitted an ice stupa’s fountain with a computer that automatically adjusted the spout’s flow rate based on local temperatures, humidity and wind speed. Then the scientists tested the system by building two ice stupas in Guttannen, Switzerland — one using a continuously spraying fountain and one using the automated system.

      After four months, the team found that the continuously sprinkling fountain had spouted about 1,100 cubic meters of water and amassed 53 cubic meters of ice, with pipes freezing once. The automated system sprayed only around 150 cubic meters of water but formed 61 cubic meters of ice, without any frozen pipes.

      The researchers are now trying to simplify their prototype to make it more affordable for high-mountain communities around the world. “We eventually want to reduce the cost so that it is within two months of salary of the farmers in Ladakh,” Balasubramanian says. “Around $200 to $400.” More

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      Megatooth sharks may have been higher on the food chain than any ocean animal ever

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      Whenever paleontologist Dana Ehret gives talks about the 15-meter-long prehistoric sharks known as megalodons, he likes to make a joke: “What did megalodon eat?” asks Ehret, Assistant Curator of Natural History at the New Jersey State Museum in Trenton. “Well,” he says, “whatever it wanted.”

      Now, there might be evidence that’s literally true. Some megalodons (Otodus megalodon) may have been “hyper apex predators,” higher up the food chain than any ocean animal ever known, researchers report in the June 22 Science Advances. Using chemical measurements of fossilized teeth, scientists compared the diets of marine animals — from polar bears to ancient great white sharks — and found that megalodons and their direct ancestors were often predators on a level never seen before.

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      The finding contradicts another recent study, which found megalodons were at a similar level in the food chain as great white sharks (SN: 5/31/22). If true, the new results might change how researchers think about what drove megalodons to extinction around 3.5 million years ago.

      In the latest study, researchers examined dozens of fossilized teeth for varieties of nitrogen, called isotopes, that have different numbers of neutrons. In animals, one specific nitrogen isotope tends to be more common than another. A predator absorbs both when it eats prey, so the imbalance between the isotopes grows further up the food chain. 

      For years, scientists have used this trend to learn about modern creatures’ diets. But researchers were almost never able to apply it to fossils millions of years old because the nitrogen levels were too low. In the new study, scientists get around this by feeding their samples to bacteria that digest the nitrogen into a chemical the team can more easily measure.

      The result: Megalodon and its direct ancestors, known collectively as megatooth sharks, showed nitrogen isotope excesses sometimes greater than any known marine animal. They were on average probably two levels higher on the food chain than today’s great white sharks, which is like saying that some megalodons would have eaten a beast that ate great whites.

      “I definitely thought that I’d just messed up in the lab,” says Emma Kast, a biogeochemist at the University of Cambridge. Yet on closer inspection, the data held up.

      The result is “eyebrow-raising,” says Robert Boessenecker, a paleontologist at the College of Charleston in South Carolina who was not involved in the study. “Even if megalodon was eating nothing but killer whales, it would still need to be getting some of this excess nitrogen from something else,” he says, “and there’s just nothing else in the ocean today that has nitrogen isotopes that are that concentrated.”

      “I don’t know how to explain it,” he says.

      There are possibilities. Megalodons may have eaten predatory sperm whales, though those went extinct before the megatooth sharks. Or megalodons could have been cannibals (SN: 10/5/20).  

      Another complication comes from the earlier, contradictory study. Those researchers examined the same food chain —  in some cases, even the same shark teeth — using a zinc isotope instead of nitrogen. They drew the opposite conclusion, finding megalodons were on a similar level as other apex predators.

      The zinc method is not as established as the nitrogen method, though nitrogen isotopes have also rarely been used this way before. “It could be that we don’t have a total understanding and grasp of this technique,” says Sora Kim, a paleoecologist at the University of California, Merced who was involved in both studies. “But if [the newer study] is right, that’s crazy.”

      Confirming the results would be a step toward understanding why megalodons died off. If great whites had a similar diet, it could mean that they outcompeted megalodons for food, says Ehret, who was not involved in the study. The new findings suggest that’s unlikely, but leave room for the possibility that great whites competed with — or simply ate — juvenile megalodons (SN: 1/12/21). 

      Measuring more shark teeth with both techniques could solve the mystery and reconcile the studies. At the same time, Kast says, there’s plenty to explore with their method for measuring nitrogen isotopes in fossils. “There’s so many animals and so many different ecosystems and time periods,” she says. 

      Boessenecker agrees. When it comes to the ancient oceans, he says, “I guarantee we’re going to find out some really weird stuff.” More

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      Earth’s oldest known wildfires raged 430 million years ago

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      Bits of charcoal entombed in ancient rocks unearthed in Wales and Poland push back the earliest evidence for wildfires to around 430 million years ago. Besides breaking the previous record by about 10 million years, the finds help pin down how much oxygen was in Earth’s atmosphere at the time.

      The ancient atmosphere must have contained at least 16 percent oxygen, researchers report June 13 in Geology. That conclusion is based on modern-day lab tests that show how much oxygen it takes for a wildfire to take hold and spread.

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      While oxygen makes up 21 percent of our air today, over the last 600 million years or so, oxygen levels in Earth’s atmosphere have fluctuated between 13 percent and 30 percent (SN: 12/13/05). Long-term models simulating past oxygen concentrations are based on processes such as the burial of coal swamps, mountain building, erosion and the chemical changes associated with them. But those models, some of which predict lower oxygen levels as low as 10 percent for this time period, provide broad-brush strokes of trends and may not capture brief spikes and dips, say Ian Glasspool and Robert Gastaldo, both paleobotanists at Colby College in Waterville, Maine.

      Charcoal, a remnant of wildfire, is physical evidence that provides, at the least, a minimum threshold for oxygen concentrations. That’s because oxygen is one of three ingredients needed to create a wildfire. The second, ignition, came from lightning in the ancient world, says Glasspool. The third, fuel, came from burgeoning plants and fungus 430 million years ago, during the Silurian Period. The predominant greenery were low-growing plants just a couple of centimeters tall. Scattered among this diminutive ground cover were occasional knee-high to waist-high plants and Prototaxites fungi that towered up to nine meters tall. Before this time, most plants were single-celled and lived in the seas.

      Once plants left the ocean and began to thrive, wildfire followed. “Almost as soon as we have evidence of plants on land, we have evidence of wildfire,” says Glasspool.

      That evidence includes tiny chunks of partially charred plants — including charcoal as identified by its microstructure — as well as conglomerations of charcoal and associated minerals embedded within fossilized hunks of Prototaxites fungi. Those samples came from rocks of known ages that formed from sediments dumped just offshore of ancient landmasses. This wildfire debris was carried offshore in streams or rivers before it settled, accumulated and was preserved, the researchers suggest.

      The microstructure of this fossilized and partially charred bit of plant unearthed in Poland from sediments that are almost 425 million years old reveals that it was burnt by some of Earth’s earliest known wildfires.Ian Glasspool/Colby College

      The discovery adds to previous evidence, including analyses of pockets of fluid trapped in halite minerals formed during the Silurian, that suggests that atmospheric oxygen during that time approached or even exceeded the 21 percent concentration seen today, the pair note.

      “The team has good evidence for charring,” says Lee Kump, a biogeochemist at Penn State who wasn’t involved in the new study. Although its evidence points to higher oxygen levels than some models suggest for that time, it’s possible that oxygen was a substantial component of the atmosphere even earlier than the Silurian, he says.

      “We can’t rule out that oxygen levels weren’t higher even further back,” says Kump. “It could be that plants from that era weren’t amenable to leaving a charcoal record.” More