Environment

Some forests take one-two punches surprisingly well.

Researchers have shown that certain California forests exposed to two successive droughts weathered the second one much better than forests only hit by the later dry period. Given that the frequency and severity of droughts is increasing with climate change (SN: 3/10/22), the findings suggest that forested regions might fare better than predicted in the future, the research team proposes in the May 17 AGU Advances. That’s important because of the many resources that forests provide, including their ability to sequester about a quarter of the carbon dioxide that humans put into the atmosphere every year.

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Carl Norlen and Mike Goulden, ecologists at the University of California, Irvine, studied roughly 520,000 hectares of California forest (about 4 percent of the state’s forested areas). The researchers focused on conifers, trees such as pines and firs that have needles rather than flat leaves. Using archival data gleaned from airplane observations, Norlen and Goulden estimated the extent of conifer die-off during two droughts that struck from 1999–2002 and 2012–2015. “Both of them when they happened were considered some of the most severe droughts ever in California,” Norlen says.

When the researchers looked at forests that just experienced the 2012–2015 drought, they found that over 50 percent of forested areas showed evidence of die-off. But when Norlen and Goulden analyzed forests that had weathered both droughts, they were surprised to find that only 13 percent of those forested areas showed evidence of die-off after the second drought. The researchers also analyzed satellite data that revealed the severity of tree die-off, and they again found that forests hurt by one drought were better prepared to weather another.

The underlying cause of those differences probably has to do with the tree mortality that occurred during the first drought, Norlen and Goulden propose. Roughly 31 percent of the forested areas that experienced both droughts showed signs of die-off during the 1999–2002 drought. “We think that it has to do with experiencing die-off the first time,” Norlen says.

That makes sense, says William Anderegg, an ecologist at the University of Utah in Salt Lake City who was not involved in the research. “You might have culled out the weak trees,” he says.

Norlen and Goulden speculate that the first drought eliminated trees weakened by pests like bark beetles (SN: 2/21/23), or perhaps that the dry conditions prompted trees to protect themselves by growing deeper roots. Those changes would have helped protect the remaining trees from future droughts, Goulden says. “You effectively have a stronger population.”

Getting at the root of why exactly some forests are better prepared to weather successive droughts will require more investigation, the researchers say. One way forward is to monitor individual trees to better understand their physiology, which other teams have done by outfitting trees with sensors (SN: 4/25/23).

However, the new findings already provide some hope for the forests of the future. Modeling work has suggested that upwards of 50 percent of present-day conifer tree coverage could be lost in the Northern Hemisphere by the end of the century. But this new work shows that that prediction might well turn out to be an overestimate. “There’s an element of it that’s actually hopeful,” Goulden says. More

Meet the house that diapers built.

Researchers have designed and erected a house that has shredded, disposable diapers mixed into its concrete and mortar. A single-story home of about 36 square meters can pack nearly 2 cubic meters of used diapers into its floors, columns and walls, the team reports May 18 in Scientific Reports.

Using recycled diapers as composite building materials would not only shrink landfill waste but also could make such homes more affordable, the team says, a particular need in developing countries like Indonesia where the demand for low-cost housing far outstrips the supply.

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Indonesia’s urban population has increased by about 4 percent per year in the last three decades, and more of its people are moving to urban centers. Over two-thirds of Indonesians are expected to live in urban areas by 2025, says environmental engineer Siswanti Zuraida of the University of Kitakyushu in Japan. That population boom is putting a heavy strain on both housing demand and waste management, says Zuraida, who is originally from Indonesia. Used disposable diapers mostly pile up in landfills or get incinerated, adding to a growing waste problem.

The materials used to build a house, meanwhile, particularly those needed to shore up its structural integrity, are often the biggest barrier to making homes affordable. So researchers have previously examined the possibility of using a wide variety of unconventional materials that could also save costs. These materials included many that would otherwise pile up as waste, such as the husks of rice grains or fly ash, the fine residue left over from the combustion of pulverized coal. Disposable diapers, as it happens, contain a lot of potentially useful building material, such as wood pulp, cotton, rayon and plastic.

Zuraida and colleagues assessed how much of the sand, gravel and other traditional building materials used in mortar and concrete could be replaced by diapers — washed, dried, sterilized and shredded — without reducing the strength of the structures. They created six different samples of concrete and mortar by mixing different proportions of diaper material with cement, sand, gravel and water. Crushing the samples in a machine let the researchers test how much weight each could bear.

The team then went on to design — and then build — a small, single-story, two-bedroom, one-bathroom home based on the maximum amount of diaper waste they calculated they could use. Recycled diapers could replace up to 27 percent of the traditional materials used in load-bearing structural components like columns and beams without losing significant strength, the team found. For buildings with more floors, that fraction is somewhat less: A three-story home could use up to 10 percent disposable diapers in its load-bearing structures, the team calculated. As for nonstructural components like wall partitions or garden paving blocks, shredded diapers could replace up to 40 percent of the sand.

Despite the need for more affordable housing, there are significant hitches that stand in the way of adopting diapers or other low-impact nonconventional materials, Zuraida says.

Diapers’ plastic components would have to be separated from the organic fibers, a complicated recycling process currently available only in developed nations. And Indonesia’s building regulations restrict construction materials to concrete, bricks, wood and ceramics — materials that also bear a high cost in terms of carbon emissions.

“Thinking about how to use waste for other purposes is an excellent idea,” says chemist Christof Schröfl of Technische Universität Dresden in Germany. But there may be limits on the ultimate environmental friendliness of repurposing used diapers in buildings, he says, due to the existing challenges of separating and sanitizing diapers in waste. “It’s maybe worthwhile to start thinking about ways to replace single-use diapers” with something less frequently disposed of. More

When a strong tornado roars through a city, it often leaves behind demolished buildings, broken tree limbs and trails of debris. But a similarly powerful storm touching down over barren, unvegetated land is much harder to spot in the rearview mirror.

Now, satellite imagery has revealed a 60-kilometer-long track of moist earth in Arkansas that was invisible to human eyes. The feature was presumably excavated by a tornado when it stripped away the uppermost layer of the soil, researchers report in the March 28 Geophysical Research Letters. This method of looking for “hidden” tornado tracks is particularly valuable for better understanding storms that strike in the winter, when there’s less vegetation, the researchers suggest. And recent research has shown that wintertime storms are likely to increase in intensity as the climate warms (SN: 12/16/21).

Over 1,000 tornadoes strike the United States each year, according to the National Weather Service. But not all are equally likely to be studied, says Darrel Kingfield, a meteorologist at the National Oceanic and Atmospheric Administration in Boulder, Colo., who was not involved in the research.

For starters, storms that pass over populated areas are more apt to be analyzed. “There’s historically been a pretty big population bias,” Kingfield says. Storms that occur over vegetated regions also tend to be well studied, simply because they leave obvious scars on the landscape. Ripped-up grasses or downed trees function like beacons to indicate the path of a storm, says Kingfield, who has studied forests damaged by tornadoes.

Spring and summer are peak storm seasons in the United States — more than 70 percent of tornadoes strike from March through September, according to NOAA. But on December 10, 2021, a cluster of storms started racing across the central and southern United States. Those tornadoes, which claimed more than 80 lives, swept across cities and also farmland, much of which had already been harvested for the season.

Jingyu Wang, a physical geographer at Nanyang Technological University in Singapore, and his colleagues set out to detect the signatures of those deadly storms in unpopulated, barren landscapes.

Swirling winds, even relatively weak ones, can suction up several centimeters of soil. And since deeper layers of the ground tend to be wetter, a tornado ought to leave behind a telltale signature: a long swath of moister-than-usual soil. Two properties linked with soil moisture level — its texture and temperature — in turn impact how much near-infrared light the soil reflects.

Wang and his collaborators analyzed near-infrared data collected by NASA’s Terra and Aqua satellites and looked for changes in soil moisture consistent with a passing tornado.

When the team looked at data obtained shortly after the 2021 storm outbreak, they noticed a signal in northeastern Arkansas. The feature was consistent with a roughly 60-kilometer-long track of wet soil. Tornadoes had been previously reported in that area — outside the city of Osceola — so it’s likely that this feature was created by a powerful storm, the team concluded.

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That makes sense, Kingfield says, and observations like these can reveal tornado signatures that might otherwise be missed. However, it’s important to acknowledge that this new technique works best in places where soils are capable of retaining water, he says. “You need to have clay-rich soils.”

Even so, these results hold promise for analyzing other tornadoes, Kingfield says. It’s always useful to have a new tool for estimating the strength, path and structure of a storm, but many storms go relatively unexamined simply because of where and when they occur, he says. “Now we have this new ground truth.” More

The 19th century landscape paintings hanging in London’s Tate Britain museum looked awfully familiar to climate physicist Anna Lea Albright. Artist Joseph Mallord William Turner’s signature way of shrouding his vistas in fog and smoke reminded Albright of her own research tracking air pollution.“I started wondering if there was a connection,” says Albright, who had been visiting the museum on a day off from the Laboratory for Dynamical Meteorology in Paris. After all, Turner — a forerunner of the impressionist movement — was painting as Britain’s industrial revolution gathered steam, and a growing number of belching manufacturing plants earned London the nickname “The Big Smoke.”

Turner’s early works, such as his 1814 painting “Apullia in Search of Appullus,” were rendered in sharp details. Later works, like his celebrated 1844 painting “Rain, Steam and Speed — the Great Western Railway,” embraced a dreamier, fuzzier aesthetic.

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Perhaps, Albright thought, this burgeoning painting style wasn’t a purely artistic phenomenon. Perhaps Turner and his successors painted exactly what they saw: their environs becoming more and more obscured by smokestack haze.

To find out how much realism there is in impressionism, Albright teamed up with Harvard University climatologist Peter Huybers, who’s an expert in reconstructing pollution before instruments existed to closely track air quality. Their analysis of nearly 130 paintings by Turner, Paris-based impressionist Claude Monet and several others tells a tale of two modernizing cities.

Low contrast and whiter hues are hallmarks of the impressionist style. They are also hallmarks of air pollution, which can affect how a distant scene looks to the naked eye. Tiny airborne particles, or aerosols, can absorb or scatter light. That makes the bright parts of objects appear dimmer while also shifting the entire scene’s color toward neutral white.

The artworks that Albright and Huybers investigated, which span from the late 1700s to the early 1900s, decrease in contrast as the 19th century progresses. That trend tracks with an increase in air pollution, estimated from historical records of coal sales, Albright and Huybers report in the Feb. 7 Proceedings of the National Academy of Sciences.

“Our results indicate that [19th century] paintings capture changes in the optical environment associated with increasingly polluted atmospheres during the industrial revolution,” the researchers write.

Albright and Huybers distinguished art from aerosol by first using a mathematical model to analyze the contrast and color of 60 paintings that Turner made between 1796 and 1850 as well as 38 Monet works from 1864 to 1901. They then compared the findings to sulfur dioxide emissions over the century, estimated from the trend in the annual amount of coal sold and burned in London and Paris. When sulfur dioxide reacts with molecules in the atmosphere, aerosols form.

The early works of British painter Joseph Mallord William Turner, such as “Apullia in Search of Appullus,” left, painted in 1814, were rendered in sharp details. His later works, like “Rain, Steam and Speed — the Great Western Railway,” right, painted in 1844, embraced a dreamier aesthetic. The decrease in contrast between the paintings tracks with increasing air pollution from the industrial revolution, researchers say.From left: Apullia in Search of Appullus vide Ovid, Joseph Mallord William Turner/The Tate Collection (CC BY-NC-ND 3.0); World History Archive/Alamy Stock Photo

As sulfur dioxide emissions increased over time, the amount of contrast in both Turner’s and Monet’s paintings decreased. However, paintings of Paris that Monet made from 1864 to 1872 have much higher contrast than Turner’s last paintings of London made two decades earlier.

The difference, Albright and Huybers say, can be attributed to the much slower start of the industrial revolution in France. Paris’ air pollution level around 1870 was about what London’s was when Turner started painting in the early 1800s. It confirms that the similar progression in their painting styles can’t be chalked up to coincidence, but is guided by air pollution, the pair conclude.

The researchers also analyzed the paintings’ visibility, or the distance at which an object can be clearly seen. Before 1830, the visibility in Turner’s paintings averaged about 25 kilometers, the team found. Paintings made after 1830 had an average visibility of about 10 kilometers. Paintings made by Monet in London around 1900, such as “Charing Cross Bridge,” have a visibility of less than five kilometers. That’s similar to estimates for modern-day megacities such as Delhi and Beijing, Albright and Huybers say.

To strengthen their argument, the researchers also analyzed 18 paintings from four other London- and Paris-based impressionists. Again, as outdoor air pollution increased over time, the contrast and visibility in the paintings decreased, the team found. What’s more, the decrease seen in French paintings lagged behind the decrease seen in British ones.

Overall, air pollution can explain about 61 percent of contrast differences between the paintings, the researchers calculate. In that respect, “different painters will paint in a similar way when the environment is similar,” Albright says. “But I don’t want to overstep and say: Oh, we can explain all of impressionism.” More

In Appalachia’s coal country, researchers envision turning toxic waste into treasure. The pollution left behind by abandoned mines is an untapped source of rare earth elements.

Rare earths are a valuable set of 17 elements needed to make everything from smartphones and electric vehicles to fluorescent bulbs and lasers. With global demand skyrocketing and China having a near-monopoly on rare earth production — the United States has only one active mine — there’s a lot of interest in finding alternative sources, such as ramping up recycling.

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Pulling rare earths from coal waste offers a two-for-one deal: By retrieving the metals, you also help clean up the pollution.

Long after a coal mine closes, it can leave a dirty legacy. When some of the rock left over from mining is exposed to air and water, sulfuric acid forms and pulls heavy metals from the rock. This acidic soup can pollute waterways and harm wildlife.

Recovering rare earths from what’s called acid mine drainage won’t single-handedly satisfy rising demand for the metals, acknowledges Paul Ziemkiewicz, director of the West Virginia Water Research Institute in Morgantown. But he points to several benefits.

Unlike ore dug from typical rare earth mines, the drainage is rich with the most-needed rare earth elements. Plus, extraction from acid mine drainage also doesn’t generate the radioactive waste that’s typically a by-product of rare earth mines, which often contain uranium and thorium alongside the rare earths. And from a practical standpoint, existing facilities to treat acid mine drainage could be used to collect the rare earths for processing. “Theoretically, you could start producing tomorrow,” Ziemkiewicz says.

From a few hundred sites already treating acid mine drainage, nearly 600 metric tons of rare earth elements and cobalt — another in-demand metal — could be produced annually, Ziemkiewicz and colleagues estimate.

Currently, a pilot project in West Virginia is taking material recovered from an acid mine drainage treatment site and extracting and concentrating the rare earths.

If such a scheme proves feasible, Ziemkiewicz envisions a future in which cleanup sites send their rare earth hauls to a central facility to be processed, and the elements separated. Economic analyses suggest this wouldn’t be a get-rich scheme. But, he says, it could be enough to cover the costs of treating the acid mine drainage. More

Our modern lives depend on rare earth elements, and someday soon we may not have enough to meet growing demand.

Because of their special properties, these 17 metallic elements are crucial ingredients in computer screens, cell phones and other electronics, compact fluorescent lamps, medical imaging machines, lasers, fiber optics, pigments, polishing powders, industrial catalysts – the list goes on and on (SN Online: 1/16/23). Notably rare earths are an essential part of the high-powered magnets and rechargeable batteries in the electric vehicles and renewable energy technologies needed to get the world to a low- or zero-carbon future.

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In 2021, the world mined 280,000 metric tons of rare earths — roughly 32 times as much as was mined in the mid-1950s. And demand is only going to increase. By 2040, experts estimate, we’ll need up to seven times as much rare earths as we do today.

Satisfying that appetite won’t be easy. Rare earth elements are not found in concentrated deposits. Miners must excavate huge amounts of ore, subject it to physical and chemical processes to concentrate the rare earths, and then separate them. The transformation is energy intensive and dirty, requiring toxic chemicals and often generating a small amount of radioactive waste that must be safely disposed of. Another concern is access: China has a near monopoly on both mining and processing; the United States has just one active mine (SN Online: 1/1/23).

For most of the jobs rare earths do, there are no good substitutes. So to help meet future demand and diversify who controls the supply — and perhaps even make rare earth recovery “greener” — researchers are looking for alternatives to conventional mining.   

Proposals include everything from extracting the metals from coal waste to really out-there ideas like mining the moon. But the approach most likely to make an immediate dent is recycling. “Recycling is going to play a very important and central role,” says Ikenna Nlebedim, a materials scientist at Ames National Laboratory in Iowa and the Department of Energy’s Critical Materials Institute. “That’s not to say we’re going to recycle our way out of the critical materials challenge.”

Still, in the rare earth magnets market, for instance, by about 10 years from now, recycling could satisfy as much as a quarter of the demand for rare earths, based on some estimates. “That’s huge,” he says.

But before the rare earths in an old laptop can be recycled as regularly as the aluminum in an empty soda can, there are technological, economic and logistical obstacles to overcome.

Why are rare earths so challenging to extract?

Recycling seems like an obvious way to get more rare earths. It’s standard practice in the United States and Europe to recycle from 15 to 70 percent of other metals, such as iron, copper, aluminum, nickel and tin. Yet today, only about 1 percent of rare earth elements in old products are recycled, says Simon Jowitt, an economic geologist at the University of Nevada, Las Vegas.

“Copper wiring can be recycled into more copper wiring. Steel can just be recycled into more steel,” he says. But a lot of rare earth products are “inherently not very recyclable.”

Rare earths are often blended with other metals in touch screens and similar products, making removal difficult. In some ways, recycling rare earths from tossed-out items resembles the challenge of extracting them from ore and separating them from each other. Traditional rare earth recycling methods also require hazardous chemicals such as hydrochloric acid and a lot of heat, and thus a lot of energy. On top of the environmental footprint, the cost of recovery may not be worth the effort given the small yield of rare earths. A hard disk drive, for instance, might contain just a few grams; some products offer just milligrams.

Chemists and materials scientists, though, are trying to develop smarter recycling approaches. Their techniques put microbes to work, ditch the acids of traditional methods or attempt to bypass extraction and separation.

Microbial partners can help recycle rare earths

One approach leans on microscopic partners. Gluconobacter bacteria naturally produce organic acids that can pull rare earths, such as lanthanum and cerium, from spent catalysts used in petroleum refining or from fluorescent phosphors used in lighting. The bacterial acids are less environmentally harmful than hydrochloric acid or other traditional metal-leaching acids, says Yoshiko Fujita, a biogeochemist at Idaho National Laboratory in Idaho Falls. Fujita leads research into reuse and recycling at the Critical Materials Institute. “They can also be degraded naturally,” she says.

In experiments, the bacterial acids can recover only about a quarter to half of the rare earths from spent catalysts and phosphors. Hydrochloric acid can do much better — in some cases extracting as much as 99 percent. But bio-based leaching might still be profitable, Fujita and colleagues reported in 2019 in ACS Sustainable Chemistry & Engineering.

In a hypothetical plant recycling 19,000 metric tons of used catalyst a year, the team estimated annual revenues to be roughly $1.75 million. But feeding the bacteria that produce the acid on-site is a big expense. In a scenario in which the bacteria are fed refined sugar, total costs for producing the rare earths are roughly $1.6 million a year, leaving around just $150,000 in profits. Switching from sugar to corn stalks, husks and other harvest leftovers, however, would slash costs by about $500,000, raising profits to about $650,000.

One experimental recycling approach uses organic acids made by bacteria to extract rare earths from waste products. This reactor at the Idaho National Laboratory prepares an organic acid mixture for such recycling.Idaho National Lab

Other microbes can also help extract rare earths and take them even further. A few years ago, researchers discovered that some bacteria that metabolize rare earths produce a protein that preferentially grabs onto these metals. This protein, lanmodulin, can separate rare earths from each other, such as neodymium from dysprosium — two components of rare earth magnets. A lanmodulin-based system might eliminate the need for the many chemical solvents typically used in such separation. And the waste left behind — the protein — would be biodegradable. But whether the system will pan out on a commercial scale is unknown.

How to pull rare earths from discarded magnets

Another approach already being commercialized skips the acids and uses copper salts to pull the rare earths from discarded magnets, a valuable target. Neodymium-iron-boron magnets are about 30 percent rare earth by weight and the single largest application of the metals in the world. One projection suggests that recovering the neodymium in magnets from U.S. hard disk drives alone could meet up about 5 percent of the world’s demand outside of China before the end of the decade.

Nlebedim led a team that developed a technique that uses copper salts to leach rare earths out of shredded electronic waste that contains magnets. Dunking the e-waste in a copper salt solution at room temperature dissolves the rare earths in the magnets. Other can be scooped out for their own recycling, and the copper can be reused to make more salt solution. Next, the rare earths are solidified and, with the help of additional chemicals and heating, transformed into powdered minerals called rare earth oxides. The process, which has also been used on material left over from magnet manufacturing that typically goes to waste, can recover 90 to 98 percent of the rare earths, and the material is pure enough to make new magnets, Nlebedim’s team has demonstrated.

In a best-case scenario, using this method to recycle 100 tons of leftover magnet material might produce 32 tons of rare earth oxides and net more than $1 million in profits, an economic analysis of the method suggests.

That study also evaluated the approach’s environmental impacts. Compared with producing one kilogram of rare earth oxide via one of the main types of mining and processing currently used in China, the copper salt method has less than half the carbon footprint. It produces an average of about 50 kilograms of carbon dioxide equivalent per kilogram of rare earth oxide versus 110, Nlebedim’s team reported in 2021 in ACS Sustainable Chemistry & Engineering.

But it’s not necessarily greener than all forms of mining. One sticking point is that the process requires toxic ammonium hydroxide and roasting, which consumes a lot of energy, and it still releases some carbon dioxide. Nlebedim’s group is now tweaking the technique. “We want to decarbonize the process and make it safer,” he says.

Meanwhile, the technology seems promising enough that TdVib, an Iowa company that designs and manufactures magnetic materials and products, has licensed it and built a pilot plant. The initial aim is to produce two tons of rare earth oxides per month, says Daniel Bina, TdVib’s president and CEO. The plant will recycle rare earths from old hard disk drives from data centers.

Noveon Magnetics, a company in San Marcos, Texas, is already making recycled neodymium-iron-boron magnets. In typical magnet manufacturing, the rare earths are mined, transformed into metal alloys, milled into a fine powder, magnetized and formed into a magnet. Noveon knocks out those first two steps, says company CEO Scott Dunn.

After demagnetizing and cleaning discarded magnets, Noveon directly mills them into a powder before building them back up as new magnets. Unlike with other recycling methods, there’s no need to extract and separate the rare earths out first. The final product can be more than 99 percent recycled magnet, Dunn says, with a small addition of virgin rare earth elements — the “secret sauce,” as he puts it — that allows the company to fine-tune the magnets’ attributes.

Compared with traditional magnet mining and manufacturing, Noveon’s method cuts energy use by about 90 percent, Miha Zakotnik, Noveon’s chief technology officer, and other researchers reported in 2016 in Environmental Technology & Innovation. Another 2016 analysis estimated that for every kilogram of magnet produced via Noveon’s method, about 12 kilograms of carbon dioxide equivalent are emitted. That’s about half as much of the greenhouse gas as conventional magnets.

Dunn declined to share what volume of magnets Noveon currently produces or how much its magnets cost. But the magnets are being used in some industrial applications, for pumps, fans and compressors, as well as some consumer power tools and other electronics.

To help with recycling, Apple developed the robot Daisy (shown), which can dismantle 23 models of iPhones. Other robots in the works — Taz and Dave — will specialize in recovering rare earth magnets.Apple

Rare earth recycling has logistical hurdles

Even as researchers clear technological hurdles, there are still logistical barriers to recycling. “We don’t have the systems for collecting end-of-life products that have rare earths in them,” Fujita says, “and there’s the cost of dismantling those products.” For a lot of e-waste, before rare earth recycling can begin, you have to get to the bits that contain those precious metals.

Noveon has a semiautomated process for removing magnets from hard disk drives and other electronics.

Apple is also trying to automate the recycling process. The company’s Daisy robot can dismantle iPhones. And in 2022, Apple announced a pair of robots called Taz and Dave that facilitate the recycling of rare earths. Taz can gather magnet-containing modules that are typically lost during the shredding of electronics. Dave can recover magnets from taptic engines, Apple’s technology for providing users with tactile feedback when, say, tapping an iPhone screen.

Even with robotic aids, it would still be a lot easier if companies just designed products in a way that made recycling easy, Fujita says.

No matter how good recycling gets, Jowitt sees no getting around the need to ramp up mining to feed our rare earth–hungry society. But he agrees recycling is necessary. “We’re dealing with intrinsically finite resources,” he says. “Better we try and extract what we can rather than just dumping it in the landfill.” More

U.S. rivers are getting into hot water. The frequency of river and stream heat waves is on the rise, a new analysis shows.

Like marine heat waves, riverine heat waves occur when water temperatures creep above their typical range for five or more days (SN: 2/1/22). Using 26 years of United States Geological Survey data, researchers compiled daily temperatures for 70 sites in rivers and streams across the United States, and then calculated how many days each site experienced a heat wave per year. From 1996 to 2021, the annual average number of heat wave days per river climbed from 11 to 25, the team reports October 3 in Limnology and Oceanography Letters.

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The study is the first assessment of heat waves in rivers across the country, says Spencer Tassone, an ecosystem ecologist at the University of Virginia in Charlottesville. He and his colleagues tallied nearly 4,000 heat wave events — jumping from 82 in 1996 to 198 in 2021 — and amounting to over 35,000 heat wave days. The researchers found that the frequency of extreme heat increased at sites above reservoirs and in free-flowing conditions but not below reservoirs — possibly because dams release cooler water downstream.

Most heat waves with temperatures the highest above typical ranges occurred outside of summer months between December and April, pointing to warmer wintertime conditions, Tassone says.

Human-caused global warming plays a role in riverine heat waves, with heat waves partially tracking air temperatures — but other factors are probably also driving the trend. For example, less precipitation and lower water volume in rivers mean waterways warm up easier, the study says.

“These very short, extreme changes in water temperature can quickly push organisms past their thermal tolerance,” Tassone says. Compared with a gradual increase in temperature, sudden heat waves can have a greater impact on river-dwelling plants and animals, he says. Fish like salmon and trout are particularly sensitive to heat waves because the animals rely on cold water to get enough oxygen, regulate their body temperature and spawn correctly.

There are chemical consequences to the heat as well, says hydrologist Sujay Kaushal of the University of Maryland in College Park who was not involved with the study. Higher temperatures can speed up chemical reactions that contaminate water, in some cases contributing to toxic algal blooms (SN: 2/7/18). 

The research can be used as a springboard to help mitigate heat waves in the future, Kaushal says, such as by increasing shade cover from trees or managing stormwater. In some rivers, beaver dams show promise for reducing water temperatures (SN: 8/9/22). “You can actually do something about this.” More