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      With Theta, 2020 sets the record for most named Atlantic storms

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      It’s official: 2020 now has the most named storms ever recorded in the Atlantic in a single year.
      On November 9, a tropical disturbance brewing in the northeastern Atlantic Ocean gained enough strength to become a subtropical storm. With that, Theta became the year’s 29th named storm, topping the 28 that formed in 2005.
      With maximum sustained winds near 110 kilometers per hour as of November 10, Theta is expected to churn over the open ocean for several days. It’s too early to predict Theta’s ultimate strength and trajectory, but forecasters with the National Oceanic and Atmospheric Administration say they expect the storm to weaken later in the week.
      If so, like most of the storms this year, Theta likely won’t become a major hurricane. That track record might be the most surprising thing about this season — there’s been a record-breaking number of storms, but overall they’ve been relatively weak. Only five — Laura, Teddy, Delta, Epsilon and Eta — have become major hurricanes with winds topping 178 kilometers per hour, although only Laura and Eta made landfall near the peak of their strength as Category 4 storms.

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      Even so, the 2020 hurricane season started fast, with the first nine storms arriving earlier than ever before (SN: 9/7/20). And the season has turned out to be the most active since naming began in 1953, thanks to warmer-than-usual water in the Atlantic and the arrival of La Niña, a regularly-occurring period of cooling in the Pacific, which affects winds in the Atlantic and helps hurricanes form (SN: 9/21/19). If a swirling storm reaches wind speeds of 63 kilometers per hour, it gets a name from a list of 21 predetermined names. When that list runs out, the storm gets a Greek letter.
      While the wind patterns and warm Atlantic water temperatures set the stage for the string of storms, it’s unclear if climate change is playing a role in the number of storms. As the climate warms, though, you would expect to see more of the destructive, high-category storms, says Kerry Emanuel, an atmospheric scientist at MIT. “And this year is not a poster child for that.” So far, no storm in 2020 has been stronger than a Category 4. The 2005 season had multiple Category 5 storms, including Hurricane Katrina (SN: 12/20/05).
      There’s a lot amount of energy in the ocean and atmosphere this year, including the unusually warm water, says Emanuel. “The fuel supply could make a much stronger storm than we’ve seen,” says Emanuel, “so the question is: What prevents a lot of storms from living up to their potential?”
      On September 14, five named storms (from left to right, Sally, Paulette, Rene, Teddy and Vicky) swirled in the Atlantic simultaneously. The last time the Atlantic held five at once was 1971.NOAA
      A major factor is wind shear, a change in the speed or direction of wind at different altitudes. Wind shear “doesn’t seem to have stopped a lot of storms from forming this year,” Emanuel says, “but it inhibits them from getting too intense.” Hurricanes can also create their own wind shear, so when multiple hurricanes form in close proximity, they can weaken each other, Emanuel says. And at times this year, several storms did occupy the Atlantic simultaneously — on September 14, five storms swirled at once.
      It’s not clear if seeing hurricane season run into the Greek alphabet is a “new normal,” says Emanuel. The historical record, especially before the 1950s is spotty, he says, so it’s hard to put this year’s record-setting season into context. It’s possible that there were just as many storms before naming began in the ‘50s, but that only the big, destructive ones were recorded or noticed. Now, of course, forecasters have the technology to detect all of them, “so I wouldn’t get too bent out of shape about this season,” Emanuel says.
      Some experts are hesitant to even use the term “new normal.”
      “People talk about the ‘new normal,’ and I don’t think that is a good phrase,” says James Done, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colo. “It implies some new stable state. We’re certainly not in a stable state — things are always changing.” More

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      How frigid lizards falling from trees revealed the reptiles’ growing cold tolerance

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      After the coldest night in south Florida in a decade, lizards were dropping out of palm trees, landing legs up. The scientists who raced to investigate the fallen reptiles have now found that, despite such graceless falls, some of these tropical, cold-blooded creatures are actually more resilient to cold than previously thought.
      The finding sheds light on how some species might respond to extreme weather events caused by human-caused climate change (SN: 12/10/19). Although climate change is expected to include gradual warming globally, scientists think that extreme events such as heat waves, cold snaps, droughts and torrential downpours could also grow in number and strength over time.
      The idea for the new study was born after evolutionary ecologist James Stroud received a photo of a roughly 60-centimeter-long iguana prone on its back on a sidewalk from a friend in Key Biscayne, an island town south of Miami. The previous night, temperatures dropped to just under 4.4° Celsius (40° Fahrenheit).
      “When air temperatures drop below a critical limit, lizards lose the ability to move,” says Stroud, of Washington University in St. Louis. Lizards that sleep in trees “may lose their grip.” Stunned lizards on the ground are likely easy prey for predators, he notes.

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      Realizing that the cold snap could be used to study how future instances of extreme weather might affect such animals in the wild, Stroud and colleagues rushed to collect live specimens of as many different kinds of lizards as they could in the Miami area (SN: 8/27/20). The researchers then tested how well the six reptile species they captured tolerated cold by sticking thermometers on the animals, placing them in a large cooler of ice and observing how cold they got before becoming too stunned to right themselves after getting flipped on their backs.
      Stroud and colleagues had previously run similar tests on these lizard species as part of research on invasive species. That work in 2016 suggested that the reptiles might not easily withstand cold snaps like the recent one — cold tolerances ranged from as low as about 7.7° C for the Puerto Rican crested anole (Anolis cristatellus) to roughly 11.1° C for the brown basilisk (Basiliscus vittatus).
      Some tropical, cold-blooded lizards, such as this brown basilisk (Basiliscus vittatus), are more resilient to cold than previously thought, a new study finds.John Sullivan/iNaturalist (CC BY-NC 4.0)
      The new study, however, revealed that the reptiles now could withstand temperatures roughly 1 to 4 degrees C colder. Oddly, the lizards, on average, could all endure cold down to the same lowest temperature, about 5.5° C, the researchers report in the October Biology Letters. Given the great variation in size, ecology and physiology between these species, “this was a really unexpected result,” and one that the researchers don’t have an explanation for, Stroud says.
      Natural selection may be behind the change, meaning that abnormally cold temperatures are killing off those individuals that could not survive and leaving behind those that happen to be better able to tolerate cold. Alternatively, the reptiles’ bodies could have changed in some way to acclimate to the colder temperatures. Stroud hopes in the future to measure the cold tolerance of lizards immediately before a forecasted cold snap and then examine the same reptiles immediately afterward to look for signs of acclimation.
      Scientists have long thought that tropical species, which have typically evolved in thermally stable environments, might prove especially vulnerable to major shifts in temperature (SN: 5/20/15). This new study reveals a way in which species can either rapidly evolve or acclimate, which “may provide ecosystems with some resilience to extreme climate events,” says Alex Pigot, an ecologist at University College London who did not take part in the research.
      One remaining question “is whether this resilience also applies to extreme heating events,” Pigot adds. “Previous evidence has suggested that species’ upper thermal limits may be less flexible than their lower thermal limits.” More

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      Even the deepest, coldest parts of the ocean are getting warmer

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      Things are heating up at the seafloor.
      Thermometers moored at the bottom of the Atlantic Ocean recorded an average temperature increase of about 0.02 degrees Celsius over the last decade, researchers report in the Sept. 28 Geophysical Research Letters. That warming may be a consequence of human-driven climate change, which has boosted ocean temperatures near the surface (SN: 9/25/19), but it’s unclear since so little is known about the deepest, darkest parts of the ocean.
      “The deep ocean, below about 2,000 meters, is not very well observed,” says Chris Meinen, an oceanographer at the U.S. National Oceanic and Atmospheric Administration in Miami. The deep sea is so hard to reach that the temperature at any given research site is typically taken only once per decade. But Meinen’s team measured temperatures hourly from 2009 to 2019 using seafloor sensors at four spots in the Argentine Basin, off the coast of Uruguay.
      Temperature records for the two deepest spots revealed a clear trend of warming over that decade. Waters 4,540 meters below the surface warmed from an average 0.209° C to 0.234° C, while waters 4,757 meters down went from about 0.232°C to 0.248°C. This warming is much weaker than in the upper ocean, Meinen says, but he also notes that since warm water rises, it would take a lot of heat to generate even this little bit of warming so deep.
      It’s too soon to judge whether human activity or natural variation is the cause, Meinen says. Continuing to monitor these sites and comparing the records with data from devices in other ocean basins may help to clarify matters. More

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      How planting 70 million eelgrass seeds led to an ecosystem’s rapid recovery

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      In the world’s largest seagrass restoration project, scientists have observed an ecosystem from birth to full flowering.
      As part of a 20-plus-years project, researchers and volunteers spread more than 70 million eelgrass seeds over plots covering more than 200 hectares, just beyond the wide expanses of salt marsh off the southern end of Virginia’s Eastern Shore. Long-term monitoring of the restored seagrass beds reveals a remarkably hardy ecosystem that is trapping carbon and nitrogen that would otherwise contribute to global warming and pollution, the team reports October 7 in Science Advances. That success provides a glimmer of hope for the climate and for ecosystems, the researchers say.
      The project, led by the Virginia Institute of Marine Science and The Nature Conservancy, has now grown to cover 3,612 hectares — and counting — in new seagrass beds. By comparison, the largest such project in Australia aims to restore 10 hectares of seagrass.
      The results are “a game changer,” says Carlos Duarte. “It’s an exemplar of how nature-based solutions can help mitigate climate change,” he says. The marine ecologist at King Abdullah University of Science and Technology in Thuwal, Saudi Arabia is a leader in recognizing the carbon-storing capacity of mangroves, tidal marshes and seagrasses.
      The team in Virginia started with a blank slate, says Robert Orth, a marine biologist at the Virginia Institute of Marine Science in Gloucester Point. The seagrass in these inshore lagoons had been wiped out by disease and a hurricane in the early 1930s, but the water was still clear enough to transmit the sunlight plants require.
      A researcher collects seeds from a restored seagrass meadow in a coastal Virginia bay.Jay Fleming
      Within the first 10 years of restoration, Orth and colleagues witnessed an ecosystem rebounding rapidly across almost every indicator of ecosystem health — seagrass coverage, water quality, carbon and nitrogen storage, and invertebrate and fish biomass (SN: 2/16/17).
      For instance, the team monitored how much carbon and nitrogen the meadows were capturing from the environment and storing in the sediment as seagrass coverage expanded. It found that meadows in place for nine or more years stored, on average, 1.3 times more carbon and 2.2 times more nitrogen than younger plots, suggesting that storage capacity increases as meadows mature. Within 20 years, the restored plots were accumulating carbon and nitrogen at rates similar to what natural, undisturbed seagrass beds in the same location would have stored. The restored seagrass beds are now sequestering on average about 3,000 metric tons of carbon per year and more than 600 metric tons of nitrogen, the researchers report.

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      Seagrasses can take a hit. When a sudden marine heat wave killed off a portion of the seagrass, it took just three years for the meadow to fully recover its plant density. “It surprised us how resilient these seagrass meadows were,” says Karen McGlathery, a coastal ecologist at the University of Virginia in Charlottesville.
      She believes the team’s work is more than just a great case study in restoration. It “offers a blueprint for restoring and maintaining healthy seagrass ecosystems” that others can adapt elsewhere in the world, she says.
      Reestablished eelgrass beds off Virginia not only store carbon efficiently, they also support rich biodiversity, such as the seahorse seen here.VIMS
      Seagrasses are among the world’s most valuable and most threatened ecosystems, and are important globally as reservoirs of what’s known as blue carbon, the carbon stored in ocean and coastal ecosystems. Seagrasses store more carbon, for far longer, than any other land or ocean habitat, preventing it from escaping to the atmosphere as heat-trapping carbon dioxide. These underwater prairies also support near-shore and offshore fisheries, and protect coastlines as well as other marine habitats. Despite their importance, seagrasses have declined globally by some 30 percent since 1879, according to an Aug. 14 study in Frontiers in Marine Science.
      “The study helps fill some large gaps in our understanding of how blue carbon can contribute to climate restoration,” says McGlathery. “It’s the first to put a number on how much carbon restored meadows take out of the atmosphere and store,” for decades and potentially for centuries.
      The restoration is far from finished. But already, it may point the way for struggling ecosystems such as Florida’s Biscayne Bay, once rich in seagrass but now suffering from water quality degradation and widespread fish kills.  Once the water is cleaned up, says Orth, “our work suggests that seagrasses can recover rapidly” (SN: 3/5/18).
      McGlathery also believes the scale of the team’s success should be uplifting for coastal communities. “In my first years here, there was no seagrass and there hadn’t been for decades. Today, as far as I can swim, I see lush meadows, rays, the occasional seahorse. It’s beautiful.” More

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      Large-scale changes in Earth’s climate may originate in the Pacific

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      The retreat of North America’s ice sheets in the latter years of the last ice age may have begun with “catastrophic” losses of ice into the North Pacific Ocean along the coast of modern-day British Columbia and Alaska, scientists say. 
      In a new study published October 1 in Science, researchers find that these pulses of rapid ice loss from what’s known as the western Cordilleran ice sheet contributed to, and perhaps triggered, the massive calving of the Laurentide ice sheet into the North Atlantic Ocean thousands of years ago. That collapse of the Laurentide ice sheet, which at one point covered large swaths of Canada and parts of the United States, ultimately led to major disturbances in the global climate (SN: 11/5/12).
      The new findings cast doubt on the long-held assumption that hemispheric-scale changes in Earth’s climate originate in the North Atlantic (SN: 1/31/19). The study suggests that the melting of Alaska’s remaining glaciers into the North Pacific, though less extreme than purges of the past, could have far-ranging effects on global ocean circulation and the climate in coming centuries.
      “People typically think that the Atlantic is where all the action is, and everything else follows,” says Alan Mix, a paleoclimatologist at Oregon State University in Corvallis. “We’re saying it’s the other way around.” The Cordilleran ice sheet fails earlier in the chain of reaction, “and then that signal is transmitted [from the Pacific] around the world like falling dominoes.”

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      In 2013, Mix and colleagues pulled sediment cores from the seafloor of the Gulf of Alaska in the hope of figuring out how exactly the Cordilleran ice sheet had changed prior to the end of the last ice age. These cores contained distinct layers of sand and silt deposited by the ice sheet’s calved icebergs during four separate occasions over the last 42,000 years. The team then used radiocarbon dating to determine the chronology of events, finding that the Cordilleran’s ice purges “surprisingly” preceded the Laurentide’s periods of abrupt ice loss, known as “Heinrich events,” by 1,000 to 1,500 years every single time.
      “We’ve long known that these Heinrich events are a big deal,” says coauthor Maureen Walczak, a paleoceanographer also at Oregon State University. “They have global climate consequences associated with increases in atmospheric CO2, warming in Antarctica … and the weakening of the Asian monsoon in the Pacific. But we’ve not known why they happened.”  
      Though scientists can now point the finger at the North Pacific, the exact mechanism remains unclear. Mix proposes several theories for how Cordilleran ice loss ultimately translated to mass calving of ice along North America’s east coast. It’s possible, he says, that the freshwater deposited in the North Pacific traveled northward through the Bering Strait, across the Arctic and down into the North Atlantic. There, the buoyant freshwater served as a “cap” on the ocean’s denser saltwater, preventing it from overturning. This process could have led to the water getting warmer, destabilizing the adjacent ice sheet.
      Another theory posits that the lower elevation of the diminished Cordilleran ice sheet altered how surface winds entered North America. Normally, the ice sheet would act like a fence, diverting winds and their water vapor southward as they entered North America. Without this barrier, the transport of heat and freshwater between the Pacific and Atlantic Ocean basins is disrupted, changing the salinity of the Atlantic waters and ultimately delivering more heat to the ice there.
      Today, Alaska’s glaciers serve as the last remnants of the Cordilleran ice sheet. Many are in a state of rapid retreat due to climate change. This melting ice, too, drains into the Pacific and Arctic oceans, raising sea levels and interfering with normal ocean mixing processes. “Knowing the failure of ice in the North Pacific seemed to presage really rapid ice loss in the North Atlantic, that’s kind of concerning,” Walczak says.
      If the ice melt into the North Pacific follows similar patterns to the past, it could yield significant global climate events, the researchers suggest. But Mix cautions that the amount of freshwater runoff needed to trigger changes elsewhere in the global ocean, and climate, is unknown. “We know enough to say that such things happened in the past, ergo, they are real and could happen again.”
      It’s not clear, though, what the timing of such global changes would be. If the ice losses in the Atlantic occurred in the past due to a change in deep ocean dynamics triggered by Pacific melting, that signal would likely take hundreds of years to reach the other remaining ice sheets. If, however, those losses were triggered by a change in sea levels or winds, other ice sheets could be affected a bit faster, though still not this century.
      The Laurentide ice sheet is, of course, long gone. But two others remain, in Greenland and Antarctica (SN: 9/30/20, 9/23/20). Both have numerous glaciers that terminate in the ocean and drain the interior of the ice sheets. This makes the ice sheets susceptible to both warmer ocean water and sea level rise.
      Alaska’s melting glaciers have already fueled about 30 percent of global sea level rise. “One of the hypotheses we have is that sea level rise is going to destabilize the ice shelves at the mouths of those glaciers, which will break off like champagne corks,” Walczak explains. When that happens, the idea goes, the ice sheets will start collapsing faster and faster.
      Records of climate change in the Pacific, like the one Walczak and colleagues have compiled, have been hard to come by, says Richard Alley, a glaciologist at Pennsylvania State University who wasn’t involved with the study. “These new data may raise more questions than they answer,” he says. “But by linking North Pacific Ocean circulation … to the global template of climate oscillations, the new paper gives us a real advance in understanding all of this.” More

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      By 2100, Greenland will be losing ice at its fastest rate in 12,000 years

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      By 2100, Greenland will be shedding ice faster than at any time in the past 12,000 years, scientists report October 1 in Nature.
      Since the 1990s, Greenland has shed its ice at an increasing rate (SN: 8/2/19). Meltwater from the island’s ice sheet now contributes about 0.7 millimeters per year to global sea level rise (SN: 9/25/19). But how does this rapid loss stack up against the ice sheet’s recent history, including during a 3,000-year-long warm period?
      Glacial geologist Jason Briner of the University at Buffalo in New York and colleagues created a master timeline of ice sheet changes spanning nearly 12,000 years, from the dawn of the Holocene Epoch 11,700 years ago and projected out to 2100.
      The researchers combined climate and ice physics simulations with observations of the extent of past ice sheets, marked by moraines. Those rocky deposits denote the edges of ancient, bulldozing glaciers. New fine-tuned climate simulations that include spatial variations in temperature and precipitation across the island also improved on past temperature reconstructions.
      During the past warm episode from about 10,000 to 7,000 years ago, Greenland lost ice at a rate of about 6,000 billion metric tons each century, the team estimates. That rate remained unmatched until the past two decades: From 2000 to 2018, the average rate of ice loss was similar, at about 6,100 billion tons per century.
      Over the next century, that pace will accelerate, the team says. How much depends on future greenhouse gas emissions: Under a lower-emissions scenario, ice loss is projected to average around 8,800 billion tons per century by 2100. With higher emissions, the rate of loss could ramp up to 35,900 billion tons per century.
      Lower emissions could slow the loss, but “no matter what humanity does, the ice will melt this century at a faster clip than it did during that warm period,” Briner says. More

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      Invasive jumping worms damage U.S. soil and threaten forests

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      What could be more 2020 than an ongoing invasion of jumping worms?
      These earthworms are wriggling their way across the United States, voraciously devouring protective forest leaf litter and leaving behind bare, denuded soil. They displace other earthworms, centipedes, salamanders and ground-nesting birds, and disrupt forest food chains. They can invade more than five hectares in a single year, changing soil chemistry and microbial communities as they go, new research shows. And they don’t even need mates to reproduce.
      Endemic to Japan and the Korean Peninsula, three invasive species of these worms — Amynthas agrestis, A. tokioensis and Metaphire hilgendorfi — have been in the United States for over a century. But just in the past 15 years, they’ve begun to spread widely (SNS: 10/7/16). Collectively known as Asian jumping worms, crazy worms, snake worms or Alabama jumpers, they’ve become well established across the South and Mid-Atlantic and have reached parts of the Northeast, Upper Midwest and West.
      Jumping worms are often sold as compost worms or fishing bait. And that, says soil ecologist Nick Henshue of the University at Buffalo in New York, is partially how they’re spreading (SN: 11/5/17). Fishers like them because the worms wriggle and thrash like angry snakes, which lures fish, says Henshue. They’re also marketed as compost worms because they gobble up food scraps far faster than other earthworms, such as nightcrawlers and other Lumbricus species.

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      But when it comes to ecology, the worms have more worrisome traits. Their egg cases, or cocoons, are so small that they can easily hitch a ride on a hiker’s or gardener’s shoe, or can be transported in mulch, compost or shared plants. Hundreds can exist within a square meter of ground.  
      Compared with Lumbricus worms, jumping worms grow faster and reproduce faster — and without a mate, so one worm can create a whole invasion. Jumping worms also consume more nutrients than other earthworms, turning soil into dry granular pellets that resemble coffee grounds or ground beef — Henshue calls it “taco meat.” This can make the soil inhospitable to native plants and tree seedlings and far more likely to erode.
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      Asian jumping worm species thrash furiously, unlike the more placid movements of other earthworm species. The jumping worms can also slime and shed their tails as defense mechanisms.
      To date, scientists have worried most about the worms’ effects on ground cover. Prior to a jumping worm invasion, the soft layer of decomposing leaves, bark and sticks covering the forest floor might be more than a dozen centimeters thick. What’s left afterward is bare soil with a different structure and mineral content, says Sam Chan, an invasive species specialist with Oregon Sea Grant at Oregon State University in Corvallis. Worms can reduce leaf litter by 95 percent in a single season, he says.
      That in turn can reduce or remove the forest understory, providing less nutrients or protection for the creatures that live there or for seedlings to grow. Eventually, different plants come in, usually invasive, nonnative species, says Bradley Herrick, an ecologist and research program manager at the University of Wisconsin–Madison Arboretum. And now, new research shows the worms are also changing the soil chemistry and the fungi, bacteria and microbes that live in the soils.
      Invasive jumping worms can clear a forest of leaf litter in just a couple of months, as these pictures taken in Jacobsburg State Park near Nazareth, Pa., in June 2016 (left) and August 2016 (right) show.Nick Henshue
      In a study in the October Soil Biology and Biochemistry, Herrick, soil scientist Gabriel Price-Christenson and colleagues tested samples from soils impacted by jumping worms. They were looking for changes in carbon and nitrogen levels and in soils’ release of carbon dioxide, which is produced by the metabolism of microbes and animals living in the soil. Results showed that the longer the worms had lived in the soils, the more the soils’ basal metabolic rate increased — meaning soils invaded by jumping worms could release more carbon dioxide into the atmosphere, says Price-Christenson, who is at the University of Illinois at Urbana-Champaign.
      Relative amounts of carbon and nitrogen in soils with jumping worms also shifted, the team found. That can affect plant communities, Herrick says. For example, although nitrogen is a necessary nutrient, if there’s too much, or it’s available at the wrong time of year, plants or other soil organisms won’t be able to use it. 
      The team also extracted DNA from worm poop and guts to examine differences in microbes among the jumping worm species, and tested the soils for bacterial and fungal changes. Each jumping worm species harbors a different collection of microbes in its gut, the results showed. That’s “a really important find,” Herrick says, “because for a long time, we were talking about jumping worms as a large group … but now we’re learning that [these different species] have different impacts on the soil, which will likely cascade down to having different effects on other worms, soil biota, pH and chemistry.”  
      The finding suggests each species might have a unique niche in the environment, with gut microbes breaking down particular food sources. This allows multiple species to invade and thrive together, Herrick says. This makes sense, given findings of multiple species together, but it’s still a surprise that such similar worms would have different niches, he says.    
      Scientists have been working hard to get a good handle on the biology of these worms, Henshue says. So the newly discovered soil chemistry and microbiology changes are “thoughtful” and important lines of research. But there’s still a lot that’s unknown, making it hard to predict how much farther the worms might spread and into what kinds of environments. One important question is how weather conditions affect the worms. For example, a prolonged drought this year in Wisconsin seems to have killed off many of the worms, Herrick says. Soils teeming with wriggling worms just a few weeks ago now hold far fewer.
      Perhaps that’s a hopeful sign that even these hardy worms have their limits, but in the meantime, the onslaught of worms continues its march — with help from the humans who spread them. More

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      Global warming may lead to practically irreversible Antarctic melting

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      How is melting a continent-sized ice sheet like stirring milk into coffee? Both are, for all practical purposes, irreversible.
      In a new study published in the Sept. 24 Nature, researchers outline a series of temperature-related tipping points for the Antarctic Ice Sheet. Once each tipping point is reached, changes to the ice sheet and subsequent melting can’t be truly reversed, even if temperatures drop back down to current levels, the scientists say.
      The full mass of ice sitting on top of Antarctica holds enough water to create about 58 meters of sea level rise. Although the ice sheet won’t fully collapse tomorrow or even in the next century, Antarctic ice loss is accelerating (SN: 6/13/18). So scientists are keen to understand the processes by which such a collapse might occur.
      “What we’re really interested in is the long-term stability” of the ice, says Ricarda Winkelmann, a climate scientist at Potsdam Institute for Climate Impact Research in Germany. In the new study, Winkelmann and her colleagues simulated how future temperature increases can lead to changes across Antarctica in the interplay between ice, oceans, atmosphere and land.

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      In addition to direct melting due to warming, numerous processes linked to climate change can speed up overall melting, called positive feedbacks, or slow it down, known as negative feedbacks.
      For example, as the tops of the ice sheets slowly melt down to lower elevations, the air around them becomes progressively warmer, speeding up melting. Warming temperatures also soften the ice itself, so that it slides more quickly toward the sea. And ocean waters that have absorbed heat from the atmosphere can transfer that heat to the vulnerable underbellies of Antarctic glaciers jutting into the sea, eating away at the buttresses of ice that keep the glaciers from sliding into the sea (SN: 9/11/20). The West Antarctic Ice Sheet is particularly vulnerable to such ocean interactions — but warm waters are also threatening sections of the East Antarctic Ice Sheet, such as Totten Glacier (SN: 11/1/17).
      In addition to these positive feedbacks, climate change can produce some negative feedbacks that delay the loss of ice. For example, warmer atmospheric temperatures also evaporate more ocean water, adding moisture to the air and producing increased snowfall (SN: 4/30/20).
      The new study suggests that below 1 degree Celsius of warming relative to preindustrial times, increased snowfall slightly increases the mass of ice on the continent, briefly outpacing overall losses. But that’s where the good news ends. Simulations suggest that after about 2 degrees Celsius of warming, the West Antarctic Ice Sheet will become unstable and collapse, primarily due to its interactions with warm ocean waters, increasing sea levels by more than 2 meters. That’s a warming target that the signatories to the 2015 Paris Agreement pledged not to exceed, but which the world is on track to surpass by 2100 (SN: 11/26/2019).
      As the planet continues to warm, some East Antarctic glaciers will follow suit. At 6 degrees Celsius of warming, “we reach a point where surface processes become dominant,” Winkelmann says. In other words, the ice surface is now at low enough elevation to accelerate melting. Between 6 and 9 degrees of warming, more than 70 percent of the total ice mass in Antarctica is loss, corresponding to an eventual sea level rise of more than 40 meters, the team found.
      Those losses in ice can’t be regained, even if temperatures return to preindustrial levels, the study suggests. The simulations indicate that for the West Antarctic Ice Sheet to regrow to its modern extent, temperatures would need to drop to at least 1 degree Celsius below preindustrial times.
      “What we lose might be lost forever,” Winkelmann says.
      There are other possible feedback mechanisms, both positive and negative, that weren’t included in these simulations, Winklemann adds — either because the mechanisms are negligible or because their impacts aren’t yet well understood. These include interactions with ocean-climate patterns such as the El Niño Southern Oscillation and with ocean circulation patterns, including the Atlantic Meridional Overturning Circulation.
      Previous research suggested that meltwater from the Greenland and Antarctic ice sheets might also play complicated feedback roles. Nicholas Golledge, a climate scientist with Victoria University of Wellington in New Zealand, reported in Nature in 2019 that flows of Greenland meltwater can slow ocean circulation in the Atlantic, while cold, fresh Antarctic meltwater can act like a seal on the surface ocean around the continent, trapping warmer, saltier waters below, where they can continue to eat away at the underbelly of glaciers.
      In a separate study published Sept. 23 in Science Advances, Shaina Sadai, a climate scientist at the University of Massachusetts Amherst, and her colleagues also examined the impact of Antarctic meltwater. In simulations that look out to the year 2250, the researchers found that in addition to a cool meltwater layer trapping warm water below it, that surface layer of freshwater would exert a strong cooling effect that could boost the volume of sea ice around Antarctica, which would in turn also keep the air there colder.
      A large plug of such meltwater, such as due to the West Antarctic Ice Sheet’s sudden collapse, could even briefly slow global warming, the researchers found. But that boon would come at a terrible price: rapid sea level rise, Sadai says. “This is not good news,” she adds. “We do not want a delayed surface temperature rise at the cost of coastal communities.”
      Because the volume and impact of meltwater is still uncertain, Winkelmann’s team didn’t include this factor. Robert DeConto, an atmospheric scientist also at the University of Massachusetts Amherst and a coauthor on the Science Advances study, notes that the effect depends on how scientists choose to simulate how the ice breaks apart. The study’s large meltwater volumes are the result of a controversial idea known as the marine ice-cliff hypothesis, which suggests that in a few centuries, tall ice cliffs in Antarctica might become brittle enough to suddenly crumble into the ocean like dominoes, raising sea levels catastrophically (SN: 2/6/19).
      Despite lingering uncertainties over the magnitude of feedbacks, one emerging theme — highlighted by the Nature paper — is consistent, DeConto says: Once the ice is lost, we can’t go back.
      “Even if we get our act together and reduce emissions dramatically, we will have already put a lot of heat into the ocean,” he adds. For ice to begin to grow back, “we’ll have to go back to a climate that’s colder than at the beginning of the Industrial Revolution, sort of like the next ice age. And that’s sobering.” More