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    Extreme ocean heat off Florida has ebbed. But for marine life, the danger remains

    In late July, a fierce ocean heat wave ratcheted up temperatures in Florida’s coastal waters to unprecedented highs. One buoy bobbing in shallow, turbid Manatee Bay logged a measurement of 38.3˚ Celsius (101˚ Fahrenheit). That may be the highest temperature ever recorded in the ocean. A week later, that surge in ocean heat had ebbed. But South Florida’s denizens are still in hot water.

    The concern is not just that the Manatee Bay buoy recorded shockingly high, hot tub–level temperatures — actually, “close to the limit of hot tub temperatures” — for several days in a row, says Benjamin Kirtman, a climate scientist at the University of Miami’s Rosenstiel School of Marine, Atmospheric and Earth Science.

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    And it’s not just that June and July’s brutally hot water temperatures in the North Atlantic Ocean are linked to shockingly hot temperatures on land. This summer, Miami’s heat index, a measure of air temperature and humidity, soared to a record-breaking streak of nearly two months, reaching a daily heat index of 38° C (100° F).

    It’s not even that such ocean heat waves are becoming the new normal, as swells of heat more and more frequently crest atop the baseline warming of the global ocean due to climate change (SN: 2/1/22). Florida’s waters may have hit a record high, but July saw widespread ocean heat waves around the world, from the North Atlantic Ocean to the eastern equatorial Pacific to the Southern Indian Ocean.

    “The global oceans have warmed up so much … we’re seeing a ratcheting up that’s unprecedented in the modern instrument record, and maybe in the last 125,000 years,” Kirtman says. “It’s really quite remarkable.”

    ‘Way outside of the bounds of anything these corals have experienced’

    In Florida, the temperatures of the coastal waters have returned to a normal summertime range for now. But the danger remains acute for many ocean dwellers, from corals to fish, says Andrew Baker, a coral biologist also at the University of Miami’s Rosenstiel School.

    Murky Manatee Bay, swirling with sediment, isn’t home to corals — but the water temperatures in the reefs around the Florida Keys were still “incredibly hot,” perhaps reaching up to 36° C (96° F), Baker says.

    As the sweltering sea temperatures peaked in July, the Coral Restoration Foundation, a nonprofit marine conservation organization based in Key Largo, Fla., found 100 percent coral mortality at one site, Sombrero Reef off Key West. There, heat had caused the corals to bleach.

    Corals may survive a few days of very high temperatures but too long spent in extremely hot water can kill them. That accumulated heat stress is measured by coral scientists in “degree heating weeks,” or DHW, a measure of both the intensity and duration of the heat the corals experienced over the previous 12 weeks. Significant coral bleaching begins to occur at 4˚ C–weeks; at 8˚ C–weeks, corals experience severe, widespread bleaching and significant mortality is expected. The accumulated heat stress on the corals at Sombrero Reef in the Florida Keys (shown) generally peaks in August and September (gray lines). But the mounting heat stress on the corals in 2023 (red line) is unprecedented in records going back to 1985.

    Bleaching occurs when corals’ symbiotic algae, the main source of their food, flee, leaving the corals colorless and essentially starving. Corals can recover from bleaching, but if the events are too severe or too frequent, they can kill entire reefs. U.S. National Oceanic and Atmospheric Administration records show that the heat burden on corals globally has been rising since the 1980s (SN: 1/4/18).

    Even with the return to typical summertime water temperatures off Florida’s coasts, the impacts of July’s heat wave on the region’s corals will linger. That’s because corals have a limit to how much accumulated heat they can tolerate before bleaching. And with this heat wave, the corals have already received far too much heat far too early in the summer, researchers say.

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    NOAA records from sites across the Florida Keys each tell the same worrisome story — that what’s happened in 2023 so far is “way outside of the bounds of anything these corals have experienced,” Baker says. And the corals still must contend with two more months of expected, but still very hot, water in August and September.

    Meanwhile, scientists are racing to save corals growing at nurseries in the Keys, bringing them to onshore laboratories away from the overheated coastal waters. The cultivated corals are part of a decade-long effort to protect the two most important reef species in the region, staghorn and elkhorn coral, from the ever-looming threat of bleaching.

    Members of the Coral Restoration Foundation retrieved these young staghorn corals from an ocean nursery to guarantee their survival after water temperatures rose to as high as 36° Celsius (96° Fahrenheit).Carolyn Cole / Los Angeles Times via Getty Images

    The fledgling finger- to hand-sized corals are cultivated in coastal waters atop bits of PVC tubing, and are ultimately destined to be planted in reefs. As the water temperatures rose, researchers hurried to collect the cultivated corals ahead of their expected spawning in early August. Scientists feared the “heat stress is just too much for these baby corals,” and that they might not spawn at all, Baker says. Happily, some of the rescued staghorn corals, now ensconced in the laboratory, did manage to spawn on August 3, releasing clouds of eggs and sperm into the water. Whether the sperm will fertilize the eggs remains uncertain, but Baker and colleagues are cautiously optimistic.

    It’s not just corals in trouble

    The overheated water is also bad news for everything from sponges to sea grasses to fish. “There are a lot of studies that show that species experiencing ocean heat waves are migrating [to cooler waters],” says Regina Rodrigues, a physical oceanographer at the Federal University of Santa Catarina in Brazil (SN: 8/10/20). But in tropical regions like the Caribbean and the Gulf of Mexico, where cooler waters are prohibitively far away, “that community doesn’t have anywhere to go.”

    That lack of access to an escape route to cooler waters is why the region’s cold-blooded ocean species, including fish, may be even more vulnerable to warming than their counterparts on land. On average, ocean ectotherms spend more time near the upper limits for body temperature than land ectotherms, as marine ecologist Malin Pinsky of Rutgers University in New Brunswick, N.J., and colleagues reported in 2019.

    Then there’s the anoxia. As water heats up, it releases oxygen, like bubbles escaping a pot boiling on the stove, leaving less oxygen available for sea life. Such heat-amped anoxic waters have been linked to increased sea grass die-offs as well as fish kills. In June, for example, thousands of fish killed by a low-oxygen event washed up on the Texas Gulf Coast just south of Houston.

    Florida’s sea grasses have been in free fall for years, with thousands of hectares of marine sea grass beds wiped out by anoxia as well as nutrient pollution, which can lead to harmful algal blooms that block out the light for underwater plants. The loss of those sea grass ecosystems has been deadly for manatees and other creatures that rely on the grasses for food.

    ‘It’s just bonkers hot’

    What’s driving the brutal ocean temperatures is still uncertain — but human-caused climate change is undeniably at its core, researchers say. “Ninety-three percent of the excess heat in the atmosphere is being absorbed by the ocean,” Rodrigues says. That’s raised the average temperature of ocean waters, “and once the mean temperature is raised, the extremes are easier to achieve.”

    Other factors are also likely playing a role, including this year’s onset of the global climate pattern known as the El Niño-Southern Oscillation (SN: 7/13/23). The El Niño phase of that climate pattern tends to increase the global average temperature, and this year’s El Niño is bidding to be “a strong one,” Kirtman says.

    “Certainly, one of the questions that’s come up is how much [of the heat] is internal natural variability, and how much a ratcheting up of climate change,” he says.

    Local extremes — such as the temporary hot tub in Manatee Bay — may also be influenced by factors such as the shallowness of the water and murkier, less-reflective waters absorbing more heat. 

    But, Kirtman says, the global oceans have warmed up so much that El Niño or sediment-laden waters alone can’t possibly explain what’s going on. “This is so crazy, so bonkers. It’s just bonkers hot.” More

  • in

    Extreme heat taxes the body in many ways. Here’s how

    Luis Melecio-Zambrano is the summer 2023 science writing intern at Science News. They are finishing their master’s degree in science communication from the University of California, Santa Cruz, where they have reported on issues of environmental justice and agriculture. More

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    Cow poop emits climate-warming methane. Adding red algae may help

    Earth has a cow problem. Cow agriculture is one of the largest emitters of climate-warming methane to the atmosphere.

    But adding a type of red algae known for its methane-inhibiting properties to cow feces might help. Doing so reduces the production of methane within feces by about 44 percent, researchers report July 13 in Frontiers in Sustainable Food Systems. That offers a promising new avenue to reduce overall methane emissions from cattle, the scientists say.

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    Cow agriculture is responsible for nearly a quarter of the world’s emissions of methane, a potent greenhouse gas (SN: 11/18/15; SN: 5/5/22). The cows make methane in their guts during digestion that is then released to the world, mostly via burps. A smaller — but not insignificant — amount of methane is also emitted directly from the cows’ feces during decomposition.

    Researchers have been actively seeking solutions to the gut-produced methane. Adding just a pinch — 0.5 percent of the dry feed — of the red algae Asparagopsis taxiformis to the cows’ food can prevent about 65 percent of that methane production.

    Ubiquitous in tropical ocean waters, A. taxiformis contains an organic compound called bromoform, which inactivates an enzyme that normally helps the methane reaction along. This research has raised concerns that the milk of dairy cows fed the algae may contain toxic levels of bromoform as well as iodine in their milk and meat. The U.S. Environmental Protection Agency has assessed bromoform as a probable human carcinogen, and too much iodine can cause thyroid malfunction.

    The red algae species Asparagopsis taxiformis (pictured) produces an organic compound that interferes with bacterial methane production, including in cows’ guts and feces.Jean-Pascal Quod/Wikimedia Commons (CC BY-SA 3.0)

    Mohammad Ramin, an animal scientist at the Swedish University of Agricultural Sciences in Umeå, and colleagues wondered whether it might be possible to cut out the middleman — by adding the algae directly to the cows’ poop. That wouldn’t reduce the gut-produced methane, but it might reduce overall cattle emissions without impacting meat or milk.

    Methane emitted from feces is primarily a problem when it comes to dairy cows, says Sara Place, an animal scientist at Colorado State University in Fort Collins who was not involved in the study. Dairy cows tend to be raised in environments with more oxygen-poor soils, and methane-producing bacteria thrive in such anaerobic environments. Cows raised for beef tend to live out their lives and defecate in open pasturelands or in an enclosed, but dry, feedlot, which is less conducive ground for methane production.

    In the new study, Ramin and colleagues added algae to four dairy cows’ feces. Two had been fed the algae, and two hadn’t. Each fecal sample was divided further, with one subsample given additional algae and the other left alone. Then, all the fecal samples were allowed to incubate, slowly decomposing in the laboratory. After nine weeks, the team analyzed the subsamples to see how much methane they contained.

    As expected, adding algae to the cows’ food did initially reduce methane in their poop. But once the poop began to decompose, the production of new methane wasn’t affected by whether the cows had eaten the algae or not. The team also examined the microbial communities living in the different types of poop, and they found that there wasn’t much difference between the algae-fed cows and the control cows. That suggests that algae food supplements aren’t that effective at inhibiting methane production outside the stomach.

    But adding the algae directly to the feces did make a noticeable difference to methane coming from decomposition. That, the team says, suggests that this would be an effective part of the solution to the larger cow-methane problem.

    The major strength of this new work is that it focuses on providing a solution to an understudied part of the cow-methane problem, says Christopher Glasson, a chemist at the University of Waikato in Tauranga, New Zealand, who studies agrichemicals derived from seaweed. But ultimately, he says, it may just not be cost-effective to produce A. taxiformis for this particular purpose. “I think [this strategy] is likely to be nonviable due to the cost of the production of the seaweed.”

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    A. taxiformis may still be most effective at suppressing fermentation in a cow’s guts rather than in its manure. The good news, Glasson says, is that state-of-the-art feed additive technologies that use specific extracts from the algae rather than the whole biomass greatly mitigate the risk of iodine or bromoform toxicity.

    And the study’s conclusion that algae in the cows’ feed doesn’t affect methane production in their feces might also be good news, in a way, Place says. One proposed avenue for mitigating emissions from cow feces is to harness the methane to make biogas. “If you feed [algae] to cattle for methane mitigation [and] if you don’t see any results [in the manure], that could be good for biogas production,” she adds — a possible two-fer for the industry.  More

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    Here’s how much climate change increases the odds of brutally hot summers

    The recent, record-breaking heat waves that have scorched the southwestern United States and northern Mexico, China and southern Europe were made dramatically more likely due to human-caused climate change, researchers report July 25 in a study from the World Weather Attribution network.

    “This is absolutely not a surprise,” climate scientist Friederike Otto of Imperial College London said at a July 24 news briefing. But “while the weather is changing as expected, how much it hurts us is larger than expected.”

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    These intense and at times deadly heat waves are occurring as high-pressure systems stall across the Northern Hemisphere, creating barely budging heat domes (SN: 7/19/23).  Phoenix, for example, has reached at least 43.3° C (110° Fahrenheit) every day for more than three weeks. 

    Otto and her colleagues used computers to simulate Earth’s climate, with and without human-caused climate change, to assess how likely the recent heat waves would have been under different climate conditions.

    In a world without climate change, they found, the recent extreme heat in China would be expected roughly once every 250 years. Now, it’s a once-in-five-years phenomenon, or 50 times more likely to occur. Meanwhile the extreme heat waves in southern Europe and North America, which would have been virtually impossible without climate change, are now likely to occur once every 10 years and 15 years, respectively.

    Should climate warming reach 2 degrees Celsius above preindustrial levels, these events could occur every two to five years, the simulations showed. We’ve already warmed by at least 1.1 degrees C since then (SN: 12/22/22).

    Though it’s still too early to pin down the human toll of these extreme events, hundreds of deaths have already been reported from regions around the world, and power shortage concerns grow as the demand for cooling surges.

    “The risks are rising faster than we are adapting,” Otto said. “We are much more vulnerable than we might have liked to believe in the past.” More

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    What’s causing this summer’s extreme heat waves?

    The dog days of summer are upon us. Brutal heat waves are roasting regions around the globe, smashing records with unrelenting severity.

    In the southwest United States and northern Mexico, devastating heat has been scorching the region for weeks. For 19 straight days and counting, temperatures in Phoenix have reached above 43.3° Celsius (110° Fahrenheit), surpassing a record streak from 1974. The Texas city of El Paso has endured an unprecedented 33 consecutive days of temperatures reaching over 37.8° C (100° F), and that streak is only expected to continue. And just after midnight on July 17, Death Valley, Calif., may have sweltered under the highest temperature ever recorded anywhere for that time: 48.9° C (120° F).

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    China has also been enduring extreme heat for weeks. On July 16, the township of Sanbao broke not only the national record with a temperature of 52.2° C (126° F), but also the record for highest temperature above 40˚ N latitude. Meanwhile, southern Europe is in its second heat wave in a week, with Rome recording a new all-time high of 42.9° C (109.2° F) on July 18 while a town in Catalonia, Spain set a new record for the region, 45.3° C (113.5° F).

    What’s concocting these bouts of extreme heat? It’s partly because the world has been exceptionally warm this year, thanks to the compounding of human-caused climate change with a natural climate phenomenon called El Niño, whose influence is known to temporarily warm our planet (SN: 7/13/23).

    But it’s not just that Earth’s a hotter stovetop; the cooks have been busy. The jet streams, powerful ribbons of wind that control much of the planet’s weather, have been meandering and getting stuck, holding bulges of hot air over many parts of the Earth. While that’s not unusual, some scientists have suggested that climate change may be altering the dynamics of the consequential winds.

    Here’s what we know about how climate change is impacting extreme heat and how these potentially dangerous events occur.

    Extreme heat waves are becoming more likely

    Let’s start with that hot stovetop. Humans have been warming the planet for decades by emitting climate-warming greenhouse gases into the atmosphere. That’s made extreme heat waves more common, many researchers say.

    Since 2004, scientists have conducted attribution studies to estimate how much climate change may have influenced the probability and severity of a specific bout of extreme weather. These studies essentially simulate the world with and without climate change to compare how often certain types of extreme weather events occur.

    The work of the World Weather Attribution initiative has repeatedly indicated that climate change has made extreme weather events like heat waves more likely and more severe (SN: 4/11/22; SN: 7/7/21).

    A May report concluded that an April heat wave in South Asia — during which locations in Thailand and Laos set new national temperature records of 45.4° C and 42.9° C, respectively — was made at least 30 times more likely due to climate change.

    Another study suggested that a different heat wave in northern Africa and southwestern Europe, which subjected some areas to temperatures 20 degrees C higher than is normal in April, was at least 100 times more likely due to climate change.

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    Climate change is pushing heat waves in general to have higher temperatures, but it’s having a particularly strong impact on the frequency of the most extreme events, says atmospheric scientist Noboru Nakamura of the University of Chicago.

    “What used to be once every 1,000 years might now occur every 20 years,” he says. “It’s still a rare event, but … you can actually feel that in our daily lives.”

    How heat waves form

    What’s actually whipping up these summer scorchers, and why are only certain regions getting roasted?

    The answer lies roughly 8 to 14 kilometers high in the sky. There, the jet streams flow at about 177 kilometers per hour on average, though they can reach speeds of more than 400 kilometers per hour — faster than a Shinkansen bullet train.

    These powerful winds control much of Earth’s weather by transporting high- and low-pressure systems around the world.

    Jet streams develop where large masses of air with different temperatures meet, flowing faster where the temperature contrast is stronger. When jet streams are blowing strong, they tend to orient themselves more parallel to the equator, says atmospheric scientist Jennifer Francis of the Woodwell Climate Research Center in Falmouth, Mass. “But when those winds get weaker … then we tend to see the jet stream take these bigger meanders.”

    When the jet stream meanders, it forms broad waves, with crests and troughs that reach north and south for hundreds of kilometers. Jet streams in the northern and southern hemispheres typically undulate more during their respective summers. Due to Earth’s axial tilt, polar regions receive more warming sunlight during their summers, weakening their temperature contrast with the tropics. As the waves become amplified, high- and low-pressure systems in the crests and troughs encroach farther north and south. Sometimes these pressure systems become stuck over one spot for days to weeks, causing weather to persist over a region.

    When a high-pressure system gets stuck over an area, it pushes air down toward the surface, compressing and warming the air. The high pressure also pushes clouds away, clearing the sky for the hot sun to beat down unabated. These factors compound to produce a heat dome, a phenomenon that scorches and often dries landscapes.

    Jet streams are mostly high-velocity wind currents (shown in red and purple) that often flow at the altitudes passenger planes frequent. When the currents form broad crests and troughs (one shown over the eastern United States), the winds can weaken (slow winds are shown in green). Swirling high-pressure systems (one shown over the U.S. Southwest and northern Mexico) can get stuck over areas, generating persistent heat. The city of Phoenix is indicated by the green dot. This graphic shows jet stream activity on July 18.N. Ogasa, C. Beccario/earth.nullschool.net

    Source: GFS/NCEP/NOAA

    An exception is when heat domes form by coastlines — such as the one that has formed by the U.S. Gulf Coast. Since warmer air can carry more moisture, heat domes near the ocean can make for weather that is both hot and humid, a potentially lethal combination for humans (SN: 7/27/22).

    It’s a bit of a mystery why pressure systems become stuck, Nakamura says, making the phenomenon difficult to predict. It may occur when jet streams become especially wavy, he and a colleague reported in 2018 in Science. The waves may get stuck like cars in a traffic jam, causing weather to idle in place.

    But this explanation is a theoretical one, and more evidence is needed to validate it, Nakamura says. Until then, he says, the underlying mechanics of those jams will remain elusive.

    The jet streams’ uncertain future

    A related, but similarly unresolved, problem is how climate change may affect the dance of the jet streams in the future. In 2012, Francis and a colleague proposed climate change could make the powerful winds more wobbly.

    “The Arctic is warming about four times faster than the globe as a whole,” Francis says. “That means that the north-south temperature difference has been getting weaker and weaker.” As a result, jet streams may be becoming more unstable, she says, and more prone to meandering.

    But that’s still “a very hotly contested hypothesis,” Nakamura says, pointing out that some climate simulations have suggested that in the Northern Hemisphere, the jet stream may actually become less wavy. “There is not a widely accepted consensus on this,” Nakamura says.

    Even if the fate of the jet streams remains up in the air, one thing seems clear: Extreme heat waves aren’t going anywhere. More

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    Antarctic sea ice has been hitting record lows for most of this year

    Something strange is happening to the Antarctic’s sea ice. The areal expanse of floating ice fringing the continent is not only at a record low for this time of year — surpassing a record just set in 2022 — but ice extent has been hitting record lows throughout the year.

    “What’s happened here is unlike the Arctic sea ice expanse,” says Mark Serreze, a climate scientist and the director of the U.S. National Snow and Ice Data Center, or NSIDC, in Boulder, Colo. We’ve come to expect a dramatic decline in sea ice at Earth’s other pole, he says (SN: 9/25/19). “Not much has happened to Antarctica’s sea ice until the last few years. But it’s just plummeted.”

    NSIDC uses satellite-gleaned data, collected daily, to keep an eye on the spread of sea ice at both poles. Throughout most of 2023, the ring of sea ice around Antarctica has repeatedly set new record lows, staying well below the average extent from 1981 to 2010. On February 21 — the height of the Southern Hemisphere’s summer — the sea ice expanse hit an all-time low since record-keeping began in 1978, of 1.79 million square kilometers. That’s 130,000 square kilometers — about the size of the state of New York — smaller than the previous recorded minimum, reached on February 25, 2022.

    Subpar sea ice

    The amount of ocean around Antarctica covered in sea ice in 2023 (red) has stayed well below the average from 1981 to 2010 (black). Sea ice expanse hit a record low in late February — surpassing a record set just in 2022 (blue). The sea ice expanse for every year from 1981 to 2021 is shown in gray.

    Even as the Southern Hemisphere shifted into winter, Antarctic sea ice remained at record low levels. On June 27, the ice was dotted across about 11.7 million square kilometers of ocean. That’s about 2.6 million square kilometers below the 1981–2010 average, and about 1.2 million square kilometers below the previous lowest extent on record for June 27, set in 2022.

    Unlike Arctic ice, whose dwindling is known to be closely tied to global warming, it’s been harder to parse the reasons for changes in Antarctic sea ice extent. That difficulty has made it unclear whether changes are the result of natural variability or whether “something big has changed,” Serreze says.

    As of June 28, the sea ice surrounding Antarctica, as measured by satellite, covered a smaller area of ocean than the average extent from 1981 to 2010 for this time of year. Yellow lines and dots represent missing satellite data.U.S. National Snow and Ice Data Center

    The last few years have given scientists pause (SN: 6/27/17). “We’re kind of dropping off an edge,” Serreze says. It’s not yet clear whether this year’s extent is part of a larger trend, he notes. But “the longer that persists, the more likely it is that something big is happening.”

    The Arctic and the Antarctic regions are polar opposites, so to speak, in their geographic setting. Ice in the Arctic Ocean is confined to a relatively small body of water ringed by land. The Antarctic, by contrast, is a landmass surrounded by ocean, which means the sea ice around the continent is much more mobile than up north, with a larger seasonal range as it expands in the Southern Hemisphere’s winter and shrinks in summer. Climate simulations have, accordingly, consistently predicted that the Arctic would show bigger sea ice losses as the planet warms, at least at first, while Antarctica would be slower to respond.

    As to why the Antarctic ice has tracked so low this year there are a few possible culprits. Regional climate patterns — particularly an air pressure pattern known as the Southern Annular Mode that shifts the direction of winds blowing around the continent — can pack or diffuse the sea ice cover around Antarctica. And other regional patterns, such as the El Niño Southern Oscillation, can affect both ocean and air circulation in the southern high latitudes.

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    Right now, scientists are concerned most with what lies beneath the ice (SN: 12/13/21). “There’s growing evidence that there has been some kind of change in ocean circulation that is bringing more heat” to the region, which affects the ice cover, Serreze says. “There are a bunch of people looking into this; we’re really blitzing to get the data. We need to understand what the heck is going on in the ocean.” More

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    Why is the North Atlantic breaking heat records?

    In the past few weeks, sea-surface temperatures in some parts of the North Atlantic Ocean have soared to record heights.

    The anomalous warming is occurring in a large swath stretching almost one-third of the way across the Atlantic westward from the northwestern coast of Africa. Satellite data reveal that some surface waters in the area are almost 4 degrees Celsius (about 7 degrees Fahrenheit) above normal for this time of the year, says Brian McNoldy, a meteorologist at the University of Miami in Coral Gables, Fla.

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    “There’s been record-breaking warmth since March, but even more so now,” he says.

    On June 10, for instance, the average sea-surface temperature for the portion of the Atlantic that stretches from the equator to 60 degrees north — up to southern Norway, southern Greenland and the central portions of Canada’s Hudson Bay — was 22.7° C (nearly 73° F). That’s about 1 degree C higher than the average recorded from 1991 through 2020, McNoldy notes. The previous record for the same date, 22.1° C, occurred in 2010.

    Hot Atlantic

    Sea-surface temperatures in the North Atlantic in 2023 (red) have been breaking daily records since March. In this plot, the 1982–2022 daily mean temperatures have been subtracted from this year’s temperatures as well of those from 1982 to 2022 (gray).

    This year’s warmer-than-normal waters might help strengthen storms that form in the eastern Atlantic and eventually spawn hurricanes, scientists say.

    What’s causing the unusual warm-up isn’t clear. But here’s a rundown of several factors that might be at play.

    A dearth of Sahara dust

    Occasionally, vast swathes of desert dust from the Sahara waft across the ocean. They are carried by winds stirred up by a semipermanent high-pressure system dubbed the “Azores high” due to its proximity to those islands.

    But lately, the Azores high has weakened and shifted southwest away from Africa. So those winds that typically pick up and transport Saharan dust westward over the North Atlantic are calmer and largely dust free, says Michael Mann, a climate scientist at the University of Pennsylvania.

    As a result, solar radiation that normally would be scattered back into space by the dust reaches the ocean surface, warming the dark waters (SN: 9/25/01).

    If and when the trade winds strengthen, increased dust from Africa could help cool the area somewhat.

    In a typical year, vast plumes of dust from the Sahara Desert (left in this satellite image of northwest Africa from 2020) block sunlight, shading and cooling the underlying ocean. This spring, a lack of dusty air has contributed to warming of the North Atlantic.NOAA

    Decreased air pollution

    In 2020, new emissions rules kicked in for long-haul container ships that spew sulfate-rich exhaust plumes. There’s been some speculation that less pollution could lead to more heating. With fewer plumes scattering sunlight back into space, more radiation reaches the sea surface.

    But some studies suggest that the cooling effect of ship plumes may have been minor to begin with: Not only do the exhaust plumes have a short life span, the pollutants can cause natural clouds to evaporate more quickly and thus lead to warming, not cooling (SN: 2/1/21).

    Global warming trends

    This year marks the return of El Niño, a climate phenomenon whose hallmark is warmer-than-normal sea-surface temperatures along the equator west of South America. By winter, there’s a more than 4 in 5 chance that the El Niño will be either strong or moderate, according to scientists with the National Oceanic and Atmospheric Administration’s Climate Prediction Center.

    Each El Niño has its own personality (SN: 5/2/16). But in general, El Niño boosts average surface temperatures both on land and at sea worldwide, Mann says. Human-caused climate change has done the same, he notes.

    But there’s still a lot of uncertainty about how current conditions may affect the coming forecast.

    The unusually warm waters of the North Atlantic may tend to strengthen storm systems that later develop into tropical depressions and then hurricanes. But the El Niño that’s now developing in the equatorial Pacific may hamper their formation by strengthening winds in the upper atmosphere that can shear the tops off nascent hurricanes. How active this year’s hurricane season will be depends on which of these forces will prevail, scientists say (SN: 5/26/23).  More