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Luis Melecio-Zambrano was 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

On the morning of September 5, a loosely swirling system of thunderstorms formed off the western coast of Africa. By September 6, the system had become a Category 1 storm, with maximum winds at least 130 kilometers per hour (80 miles per hour).

Just 24 hours later, fueled by the record-warm waters of the North Atlantic Ocean, Hurricane Lee whipped itself into a Category 5 monster. In that short time span, its wind speeds doubled, to 260 kilometers per hour (160 miles per hour).

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As the world’s oceans continue to stockpile heat from global warming, stories of such rapid intensification of tropical cyclones are becoming more commonplace, and not just in the Atlantic.

“While all eyes are on [Hurricane Lee], [Hurricane Jova] is bombing out in the eastern Pacific,” wrote Miami-based U.S. National Hurricane Center meteorologist Eric Blake on X, formerly called Twitter, on September 6. “This was just named 36 hours ago and has exploded into a Category 4 hurricane.”

These storms formed just weeks after Hurricane Idalia, which also rapidly intensified. Its wind speeds cranked up from about 120 kph to 209 kph (or 75 mph to 130 mph) in 24 hours. Shortly afterward, Idalia slammed into Florida’s Gulf Coast.

All of these hurricanes easily met — and some greatly exceeded — the National Hurricane Center’s definition of rapid intensification, in which a storm’s maximum sustained winds jump by at least 56 kph (35 mph) in less than a day. Such storms can leave people little time to prepare, making the hurricanes particularly dangerous to lives and property.

Here ‘s what to know about such rapidly intensifying storms.

A warmer ocean and atmosphere can supercharge storms

The key ingredients to boost a storm’s power quickly are very warm ocean waters, a lot of moisture in the atmosphere and low vertical wind shear, says atmospheric scientist Philip Klotzbach of Colorado State University in Fort Collins (SN: 9/28/18; SN: 9/13/18).

Vertical wind shear is what happens when winds at different heights in the atmosphere are moving at different speeds and in different directions. Those winds can chip away at a storm as it tries to organize into a tight swirl by pulling heat and moisture away from the storm’s center and sweeping away the upper structure of the storm.

This year saw the onset of an El Niño phase of the El Niño-Southern Oscillation ocean-climate pattern, a phase that tends to bring more vertical wind shear conditions to the North Atlantic. That means that El Niño years tend to feature fewer Atlantic tropical storms (SN: 5/26/23).

But so far in 2023, El Niño hasn’t done much to minimize hurricane formation, or dampen the storms’ power. “The first half of the season has not seen the unfavorable upper-level wind conditions in the western Atlantic that are typically observed in an El Niño year,” says Ryan Truchelut, president and chief meteorologist of WeatherTiger, a weather consulting firm based in Tallahassee, Fla. That’s true even in the Caribbean Sea, where El Niño’s shearing power tends to be strongest, Truchelut adds.

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Generally, there’s more of a contrast in temperature between the Atlantic and Pacific ocean basins during an El Niño year — the weather pattern heats up the eastern tropical Pacific while the Atlantic stays relatively cool.  But 2023 has seen record-breaking ocean temperatures in the North Atlantic Ocean and in the Gulf of Mexico (SN: 6/15/23). While the El Niño-warmed Pacific is about 1.5 degrees Celsius above normal, parts of the Atlantic are 1 to 3 degrees C above normal.

“Temperature contrasts drive jets,” Trechelut says, “and the lack of this contrast is likely responsible for the missing shear.”

The extremely warm waters of the Atlantic and the Gulf of Mexico set the stage — all the storms needed was a window of time with favorable wind conditions, says John Kaplan, a hurricane modeler with the U.S. National Oceanographic and Atmospheric Administration’s Atlantic Oceanographic and Meteorological Laboratory, based in Miami.  “What it really comes down to is whether the conditions for rapid intensification are favorable for even a short period of time,” he says. “If there’s a window — even if not a very long one — the system can take advantage of it. That was the case for both Lee and Idalia.”

More storms are rapidly intensifying as Earth warms

It certainly feels like most of the hurricanes in recent years are rapidly intensifying (SN: 8/27/20). But is that a true trend? And if it is, is it linked to climate change?

Studies suggest that it’s not just anecdotal. In August, researchers reported that the yearly number of tropical storms around the globe that rapidly intensified just offshore (within 400 kilometers of land) increased by about three a decade over the last 40 years, from fewer than five per year in the 1980s to about 15 per year by 2020.

Open-ocean storms, spinning far out in the big blue, showed no discernible trend, the team found. But that’s perhaps not wholly reassuring, as it’s the storms closer to shore that are most threatening to coastal populations. And a 2021 study reported that tropical cyclones have been migrating closer to the coasts since 1982.

In 2019, another team focused on observational records of wind-speed changes over 24-hour increments. Based on those data, the researchers found that episodes of rapid intensification in tropical storms tripled from 1982 to 2009. Using climate simulations, the team researchers determined that the rapid intensification trend was strongly linked to anthropogenic human-caused climate change.

Truchelut notes that the same team authored a 2022 study that further supported an upward trend over the last few decades, determining that a larger proportion of tropical cyclones are now undergoing rapid intensification at some point in their life cycle.

Regional and local weather patterns — such as more frequent La Niña events (the flipside of the El Niño-Southern Oscillation pattern) — “could dampen rapid intensification trends locally,” Truchelut says. “But there is strong objective evidence that anthropogenic global warming is driving increased proportions of tropical cyclones to undergo rapid intensification worldwide.”

Klotzbach and colleagues, meanwhile, found another fingerprint of climate change in the rapid intensification of cyclones. The researchers reported in 2022 that the increase in global sea surface temperatures correlate well with an increase in the potential intensity of tropical cyclones around the world — essentially, just how strong the winds are capable of becoming — over the last 30 years. That observed increase was particularly apparent for the most monstrous storms — those whose wind speeds increased by a whopping 93 kph (57 mph) in a single day.

Despite the fury of Lee and Jova, their threat has largely fizzled. Hurricane Lee downgraded to a Category 3 by September 12 and curved northward; however, it’s also tripled in size, threatening Bermuda with tropical storm force winds. Hurricane Jova, meanwhile, ultimately spun harmlessly in the Pacific before weakening.

But with the hurricane season only half over, there’s still time for all that hot water to fuel the next big ones. 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

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