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Like people, leaves have their limits when it comes to heat.

Scientists first reported in 1864 that the leaves of some plants could survive up to 50° Celsius, only to perish beyond that threshold. More than 150 years later, researchers are making similar findings. In 2021, a study of 147 tropical tree species reported that the average temperature beyond which photosynthesis failed was 46.7° C.

Now, in the upper canopies of Earth’s tropical forests, roughly 1 in every 10,000 leaves experiences temperatures at least once a year that may be too high for photosynthesis, researchers report August 23 in Nature.

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That might seem a paltry sum, but a photosynthetic breakdown could harm entire forests if climate change is not halted, the scientists warn. A rise of about 4 degrees C above current temperatures in tropical forests could potentially cause wide swaths of leaves to die en masse, simulations suggest. Still, the researchers acknowledge that the prediction comes with uncertainties.   

“One small possibility that we’re suggesting … is an incredibly dire tipping point” beyond which tropical forests perish, Christopher Doughty, an ecologist at Northern Arizona University in Flagstaff, said at an August 21 news briefing. But “there’s a lot we don’t know.”

When leaves get too hot, their photosynthetic machinery — proteins that convert light energy into sugars — breaks down. Keen to figure out whether tropical forests were approaching such a threshold, Doughty and colleagues obtained data collected by ECOSTRESS, a thermal sensor aboard the International Space Station, which captures vegetation temperatures on Earth’s surface in 70-meter pixels. That’s about the area that two large tropical trees could fill.

The team compared the data with measurements from devices on the planet’s surface. These included an instrument in the Amazon, mounted 64 meters high on a tower, as well as swarms of sensors taped to the bottoms of leaves in Brazil, Puerto Rico, Panama and Australia.

The analysis revealed a mosaic of temperatures in forest canopies. During periods when forests were hot and their soils were dry, temperatures across the canopy could reach an average peak of 34° C. But there was variability; some tracts exceeded 40° C.

The comparison also revealed a detail unseen by ECOSTRESS — a scatter within the mosaic. Individual leaf temperatures varied in single forest tracts, with some leaves reaching temperatures that far exceeded the tract average. About 0.01 percent of the time, upper canopy leaves sweltered at temperatures above the 46.7° C threshold, the team found.

The researchers also analyzed data from leaf-warming experiments in Brazil, Puerto Rico and Australia. These experiments showed that each degree of ambient warming had a disproportionate impact on leaf temperatures. For example, when Amazon leaves were subjected to an additional 2 degrees C of ambient warming, maximum leaf temperatures rose from 42.8° to 50.9° C.

The team used the experimental data, along with the satellite and ground-based data, to simulate the future of tropical forests under climate change. Most forests could endure about 4 degrees C of warming above current levels before trees lose all their leaves, and potentially die, the simulations suggest. That amount of warming might be possible by 2100 in a worst-case scenario in which greenhouse gas emissions continue rising through the century, the researchers say.

Still, there’s a lot of uncertainty. That’s in part because the adaptive capabilities of different tree species and how the deaths of individual leaves impact a tree’s mortality aren’t well understood.  

The study may even overestimate vulnerability by “assuming that when leaves hit this critical temperature, they die,” says ecologist Christopher Still of Oregon State University in Corvallis. That’s possible, he says, but we don’t fully understand how long it takes various temperatures to kill different species’ leaves.

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Predicting the future of these forests will also require more insights into what’s unfolding beneath the canopy, says ecologist Marielle Smith of Bangor University in Wales. “There is still a question mark over the role of small trees and understory leaves, which aren’t going to be as hot.”

Among tropical forests, the Amazon may be most vulnerable to the type of reckoning predicted by the researchers. “There’s more trees dying [there] now than there were 10 years ago or 20 years ago. We don’t see that in Africa,” Doughty said. That could be because “temperatures are a bit hotter … in the Amazon than in Africa.”

Some researchers have been warning for years that climate change and deforestation could trigger large parts of the Amazon to transform into savanna and shrubland (SN: 6/16/23).

“This is a glimpse into a potential tipping point. It’s not saying that the tropical forests are now going to be savannas tomorrow,” study coauthor and ecologist Joshua Fisher of Chapman University in Orange, Calif., said at the briefing. “We can now see this insight … and because we can see that, it means we can act.” More

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

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

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