The finding could have implications far beyond a single eruption. Scientists are increasingly interested in ways to reduce methane in the atmosphere because the gas is a major contributor to global warming and disappears much faster than carbon dioxide. Understanding how nature can accelerate that process could eventually help researchers develop new strategies for slowing near-term warming.

A Strange Signal in the Volcano Cloud

Researchers made the discovery using satellite observations of the huge plume created by the eruption. They detected exceptionally high levels of formaldehyde, a chemical that provided an important clue about what was happening inside the cloud.

Formaldehyde is produced briefly during the chemical reactions that break methane apart in the atmosphere. Because it survives for only a short time, unusually large amounts of formaldehyde can act as a chemical fingerprint showing that methane destruction is actively taking place.

“When we analyzed the satellite images, we were surprised to see a cloud with a record-high concentration of formaldehyde. We were able to track the cloud for 10 days, all the way to South America. Because formaldehyde only exists for a few hours, this showed that the cloud must have been destroying methane continuously for more than a week,” explains Dr. Maarten van Herpen from Acacia Impact Innovation BV, first author of the study, which was published in Nature Communications.

“It is known that volcanoes emit methane during eruptions, but until now it was not known that volcanic ash is also capable of partially cleaning up this pollution,” he adds.

Daily Emissions From 2 Million Cows

According to the researchers’ calculations, the volcano released around 300 gigagrams (Gg) of methane during the eruption. That is roughly equivalent to the annual methane emissions of more than two million cows.

At the same time, the plume removed approximately 900 megagrams (Mg) of methane per day, an amount comparable to the daily emissions of two million cows.

Salt, Sunlight and Unexpected Chemistry

The researchers believe the explanation involves an unusual combination of volcanic ash, seawater and sunlight. The basic chemistry was first identified by scientists in 2023, although in a very different environment.

In that earlier research, scientists found that Saharan dust carried across the Atlantic Ocean can mix with sea salt released by breaking waves. Together, these materials form tiny airborne particles called iron salt aerosols. Aerosols are microscopic particles suspended in the atmosphere.

When sunlight strikes those particles, chemical reactions can release chlorine atoms. Chlorine is highly reactive, allowing it to attack methane molecules and help break them apart. The discovery added a previously underappreciated process to scientists’ understanding of chemistry in the troposphere, the lowest major layer of Earth’s atmosphere where most weather occurs.

“What is new — and completely surprising — is that the same mechanism appears to occur in a volcanic plume high up in the stratosphere, where the physical conditions are entirely different,” says Professor Matthew Johnson from the Department of Chemistry at the University of Copenhagen, one of the researchers behind both discoveries.

The Hunga Tonga eruption created unusually favorable conditions for this chemistry. Because the volcano erupted beneath the ocean, it blasted enormous quantities of salty seawater upward along with volcanic ash. Some of that material reached the stratosphere, the atmospheric layer above the troposphere.

Researchers propose that sunlight striking this mixture produced highly reactive chlorine. Those chlorine atoms then reacted with methane in the plume, helping destroy some of the methane released during the eruption. The extraordinary amounts of formaldehyde detected from space provided evidence that this process was taking place.

Why Destroying Methane Matters

Methane currently accounts for about one third of global warming. Although it is far less abundant than carbon dioxide, methane traps heat extremely efficiently. Over a 20-year period, methane is about 80 times as potent as CO2.

There is also an important difference between the two greenhouse gases. Methane typically remains in the atmosphere for about 10 years before chemical reactions remove it, while a portion of carbon dioxide can influence the climate for far longer.

That relatively short atmospheric lifetime makes methane an especially attractive target for rapid climate action. Cutting methane emissions today could begin producing a noticeable climate benefit within about a decade.

For that reason, researchers sometimes describe methane reduction as an “emergency brake” on climate change, because rapidly lowering methane levels could help reduce warming during the coming decades and potentially lower the risk of crossing dangerous climate tipping points.

Reducing methane, however, is not a substitute for reducing carbon dioxide. Long-term temperature stabilization still requires major reductions in CO2 emissions.

Could Scientists Copy What the Volcano Did?

The discovery could also provide inspiration for researchers and companies investigating ways to accelerate methane destruction deliberately.

One emerging area of climate research focuses on atmospheric methane removal. Instead of only preventing new methane emissions, scientists are exploring whether chemical processes could safely increase the rate at which methane already in the atmosphere is broken down.

Hunga Tonga may have provided a dramatic natural demonstration of one possible mechanism.

A major challenge, however, is proving that any proposed technology actually removes methane. Atmospheric methane is spread across enormous areas, making relatively small changes difficult to measure with confidence.

“How do you prove that methane has been removed from the atmosphere? How do you know your method works? It’s very difficult. But here we address that problem by showing that methane breakdown can in fact be observed using satellites,” says Dr. Jos de Laat from the Royal Netherlands Meteorological Institute, senior author of the study.

Changing the Methane Budget

The findings may also force scientists to reconsider calculations of the global methane budget.

The methane budget is essentially an accounting system for the gas. Scientists estimate how much methane enters the atmosphere from sources such as wetlands, agriculture, fossil fuels and geological activity, then compare that with the amount removed through atmospheric chemistry and other processes.

According to the researchers, atmospheric dust has not previously been fully incorporated into those calculations. If volcanic ash and other forms of mineral dust can accelerate methane destruction, estimates of how methane moves through the atmosphere may need to be adjusted.

“We now know that atmospheric dust — for example from a volcanic eruption — impacts the methane budget, meaning the budget of how much methane is added to the atmosphere and how much is removed. Because dust has not previously been taken into account, it is important that we correct the data on which these estimates are based,” says Matthew Johnson.

Satellites Watched the Chemistry Unfold

The researchers used measurements from TROPOMI, an advanced instrument aboard the European Space Agency’s Sentinel-5P satellite. TROPOMI scans Earth’s atmosphere every day, monitoring gases associated with air pollution and climate change.

Detecting formaldehyde inside a volcanic plume in the stratosphere, however, pushed the instrument well beyond the conditions for which its standard measurements are normally designed.

“Retrieving formaldehyde from TROPOMI in a stratospheric volcanic plume is far outside the instrument’s standard operating conditions — we had to carefully correct the satellite’s sensitivity for the unusual altitude of the signal and account for interference from the high sulfur dioxide concentrations. Getting these corrections right was essential to confirm that what we were seeing was real,” said Dr. Isabelle De Smedt, Royal Belgian Institute for Space Aeronomy.

Those corrections allowed the researchers to conclude that the unusually strong formaldehyde signal was genuine and could be used to track methane destruction within the volcanic cloud.

A Possible Blueprint From Nature

The team believes the discovery could encourage engineers to investigate whether the natural chemistry observed after Hunga Tonga can be reproduced safely and effectively.

“It’s an obvious idea for industry to try to replicate this natural phenomenon ­ — but only if it can be proven to be safe and effective. Our satellite method could offer a way to help figure out how humans might slow global warming,” concludes Matthew Johnson.

Any attempt to manipulate atmospheric chemistry would require careful study to understand potential unintended effects. But Hunga Tonga provides researchers with an unusual real-world example of methane destruction occurring on a huge scale, along with a possible way to monitor the process from space.

About the Study

The scientific article was published in Nature Communications.

The researchers behind the study are Maarten van Herpen (Acacia Impact Innovation BV, Netherlands); Isabelle De Smedt (Royal Belgian Institute for Space Aeronomy, Belgium); Daphne Meidan and Alfonso Saiz-Lopez (CSIC, Spain); Matthew Johnson (University of Copenhagen, Denmark); Thomas Röckmann (Utrecht University, Netherlands); and Jos de Laat (Royal Netherlands Meteorological Institute, Netherlands).

The research was supported by Spark Climate Solutions.

“Exposing moderately trained men to chocolate odors right before and between sets of resistance exercise significantly increased their overall training volume without increasing their perceived exertion,” said senior author Dr. Mohamed Nashrudin bin Naharudin, an assistant professor at the Faculty of Sports and Exercise Science at the University of Malaya. “Seeing a substantial increase in repetitions without the athletes feeling like they were exerting themselves any harder is a fascinating psychobiological outcome.”

Testing Chocolate Scents During Exercise

The study included 23 healthy men in their early to mid-20s who were moderately trained. Researchers separated them into three groups and exposed each group to one of three samples: liquified dark chocolate containing 90% cocoa, liquified milk chocolate containing 60% cocoa, or water as a control.

“We know olfaction is powerfully wired into the brain’s appetite and emotion networks, but surprisingly, no study has systematically looked at the three-way interaction between smell, appetite, and actual resistance exercise capacity,” said Nashrudin Naharudin.

Before exercising, the participants had gone without food for at least 10 hours. They then performed leg extensions, an exercise in which a person sits and straightens the lower legs against resistance to lift a weight.

Researchers measured leg extension performance both before and during the training session. Before exercising, participants also rated their hunger, fullness, desire to eat, and intention to eat in the near future. While completing their sets, researchers measured hunger and desire to eat after each 30-second exposure to one of the scent samples.

Dark and Milk Chocolate Produced Different Effects

The two chocolate aromas influenced appetite in notably different ways.

Compared with both the water control and milk chocolate conditions, smelling dark chocolate was consistently associated with lower hunger, less desire and intention to eat, and greater feelings of fullness before exercise. The dark chocolate aroma therefore appeared to reduce appetite mainly by decreasing hunger and increasing perceived fullness.

Milk chocolate had a different effect. Participants rated its smell as more pleasant than either dark chocolate or water, but the aroma did not significantly change their hunger or appetite.

The effects were not limited to appetite. Chocolate aromas also appeared to influence how much exercise the participants could complete.

“Sniffing a 90% dark chocolate odor added about 18 more repetitions to participants’ leg extensions, while a 60% milk chocolate odor added about nine repetitions compared to the water control,” said Nashrudin Naharudin.

How Smell Could Influence the Brain

The researchers think learned associations with food may help explain the results. From an early age, people repeatedly experience certain smells alongside eating. Over time, those aromas can become signals that lead the brain and body to anticipate what normally happens after food is consumed.

Those expectations may even shift a person’s perception from hunger toward fullness before any food has actually been eaten.

“The dark chocolate scent serves as a learned cue for a rich, bitter, and highly satiating food, which essentially tricks the system into an anticipatory state of fullness,” said Nashrudin Naharudin. “Conversely, the sweeter milk chocolate scent acts more like a hedonic reward cue, enhancing training volume by creating a highly pleasant sensory environment rather than by shifting basic metabolic hunger signals.”

The findings raise the possibility that simply anticipating food could trigger some responses resembling those caused by actually eating it. That possibility is especially intriguing because the participants had been fasting when the effects were observed.

Food aromas may prepare the digestive system for an upcoming meal or produce psychological and physiological changes as the body anticipates eating. Some of these responses could resemble changes that normally happen after food consumption.

Important Limitations of the Study

The researchers caution that the biological explanation remains speculative. They did not measure blood hormones or neural activity, so the study cannot show exactly which biological pathways were responsible for the changes in appetite and exercise performance.

There were other limitations as well. The intensity of the dark and milk chocolate aromas may not have been perfectly identical. Because the control sample was odorless water, participants also may have realized when they were in the control condition.

The study was also small and included only healthy, moderately trained young men. Research involving larger and more diverse groups will be needed to determine how broadly the findings apply.

Could Other Food Smells Work Too?

Another unanswered question is whether chocolate has something special about it or whether other familiar foods could produce similar effects.

“We don’t think chocolate is entirely unique, though it is a food cue with incredibly strong, universally recognized reward associations,” Nashrudin Naharudin concluded.

Other foods strongly associated with feeling satisfied after eating might potentially produce comparable responses, although researchers have not yet tested that idea.

“A person likely needs to find the odor familiar and appealing – or at least not repulsive – to trigger the psychological shift in appetite that’s needed to see a performance boost.”

Archaeological evidence now shows that one of these sites, Cloggs Cave, was used for such activities over an extraordinary span of roughly 25,000 years.

“Here we show that the Old Ancestors selected whole grasses from the wider landscape and carried them into Cloggs Cave to spread out in thin layers and burn. Over time, sediments covered these burnt layers, preserving each layer on top of another over thousands of years,” said Dr Elle Grono, a postdoctoral researcher at the Australian National University and the corresponding author of a new study published in Frontiers in Environmental Archaeology.

Microscopic Clues From Ancient Plants

Grono and researchers from Australia, France, and New Zealand examined phytoliths recovered from buried sediments inside Cloggs Cave. Phytoliths are microscopic silica structures that form within plant tissues and can survive long after the plants themselves have disappeared.

The research grew out of archaeological excavations carried out in 2019 and 2020. Those excavations were designed to determine how people collected and used plants inside the cave in the distant past. The work was requested by the GunaiKurnai Land and Waters Corporation, and GunaiKurnai representatives were involved in the research design, fieldwork, interpretation, and communication of the findings.

“Unlike pollen, which is dispersed by wind or insects, phytoliths mostly accumulate at the spot where a plant decays. No plants grow inside the cave due to a lack of light, so the phytoliths must have been carried in by people or by animals such as possums on their fur or through their scats,” explained Grono. “We focused on half-burnt phytoliths from ashy layers, which can only have been brought in by people, as unburnt ones might have arrived through either means.”

Because vegetation cannot grow naturally inside the dark cave, the burned plant remains offered especially strong evidence that people had intentionally carried grasses into the site.

Layers of Ash and a Standing Stone

Using microscopy, the researchers classified thousands of phytoliths collected from two excavation pits measuring 1.5 m and 2.3 m deep.

The upper sections of those pits contained 73 thin layers of ash deposited by people between approximately 4,400 and 1,600 years ago. Researchers established those ages using optically stimulated luminescence and carbon dating via accelerator mass spectrometry.

Hidden among the layers was another striking discovery: a standing stone measuring 28 cm high. Later analysis showed that Old Ancestors had intentionally placed the stone inside the cave for ritual purposes around 2,000 years ago.

Grasses Were Carefully Chosen

The team identified 29 different kinds of phytoliths representing grasses, herbs, and woody plants. Many were exceptionally well preserved, including delicate silica structures such as hair cells.

Grass phytoliths dominated the samples, accounting for as much as 97% of the total. They included members of the Pooideae subfamily, which favor temperate or cool conditions, as well as Panicoideae grasses, which generally thrive in warmer and more humid environments.

Phytoliths from Chloridoideae grasses, which tolerate drought and are adapted to warm, dry arid conditions, appeared less frequently. Remains from herbs and woody plants were also uncommon.

The pattern suggests that Old Ancestors did not simply gather whatever vegetation was nearby. Instead, they deliberately selected particular grasses.

Before European settlement, the landscape around Cloggs Cave included riparian forest and open woodland containing many different plant species. Yet those plants were largely absent from the archaeological deposits. The people using the cave instead chose grasses growing relatively close to its entrance.

“The phytoliths found at Cloggs Cave were from different parts of grass plants such as stems, leaves, flowers and roots, indicating that whole grasses were brought in, including the inedible parts,” said Grono.

Evidence of Ritual Burning Across Millennia

As much as 18% of the phytoliths showed signs of burning or melting. Those altered plant remains occurred throughout the dated deposits, suggesting that people repeatedly carried vegetation into the cave and burned it there across the span of the past 25,000 years.

The finding fits with other archaeological evidence indicating that Cloggs Cave was a place of long-lasting ritual activity rather than an ordinary living site.

Professor Bruno David from Monash University, who co-led the excavations with GunaiKurnai Land and Waters Corporation, concludes that “their burning is consistent with GunaiKurnai traditional practices of using ash to perform magic and rituals, and also to track the footsteps of anyone coming in, including spirit-Beings. Other macro-botanical evidence, such as fat-smeared, trimmed wooden sticks from Casuarina trees, was also found in deeper and older deposits, indicating that other kinds of rituals using woody plants were also performed.”