Hunga Tonga’s record-breaking eruption removed 900 metric tons of methane per day

The enormous cloud blasted into the atmosphere by the 2022 Hunga Tonga-Hunga Ha'apai eruption carried an unexpected surprise. Inside it, methane was disappearing at an unusually rapid rate.

Satellite observations indicate that roughly 900 ± 220 metric tons of methane were oxidized each day in the volcanic plume. Researchers traced that destruction indirectly through formaldehyde, a short-lived chemical produced as methane breaks down.

The findings, published in Nature Communications by Dr. Maarten van Herpen of Acacia Impact Innovation BV and his team, turn one of the most powerful volcanic eruptions of the modern era into an unusual experiment in atmospheric chemistry.

The research also shows how satellites could measure enhanced methane removal, a capability that may become important if future efforts seek to deliberately accelerate methane destruction.

The key signal came from TROPOMI, an instrument aboard the Sentinel-5P satellite.

TROPOMI detected formaldehyde concentrations reaching 12 parts per billion, with uncertainty of about 10%, roughly 30 kilometers above Earth. Researchers could follow the enhancement for at least 10 days as the plume traveled across the Pacific.

That persistence immediately presented a puzzle. Formaldehyde survives only a few hours under those conditions.

When the satellite first observed the plume about 20 hours after the main eruption, the calculated formaldehyde lifetime was roughly 2.5 hours. If Hunga Tonga had simply emitted the formaldehyde during the eruption, about 95% should have disappeared before that first observation.

Instead, large amounts remained on subsequent days. The total enhancement on Jan. 17 was similar to the previous day's measurement, indicating that new formaldehyde was continually being produced.

Methane oxidation offered an explanation.

Each methane molecule that is oxidized produces roughly one formaldehyde molecule during the reaction sequence. Because formaldehyde disappears quickly, its presence can reveal where active methane destruction is occurring.

Researchers calculated that the plume was producing 4.7 ± 1.1 million moles of formaldehyde per hour around midday on Jan. 16. That corresponded to approximately 75 ± 18 metric tons of methane oxidation per hour at midday.

Accounting for changing sunlight over the day produced the estimate of 900 ± 220 metric tons of methane destroyed daily.

The plume itself appears to have contained far more methane than the surrounding stratosphere. Researchers estimated methane concentrations of at least 14 parts per million if chlorine caused 90% of the oxidation, compared with background levels of roughly 1 part per million.

That calculation implies the eruption injected at least 330 gigagrams, or 330,000 metric tons, of methane into the stratosphere.

The next question was what could destroy methane so quickly.

Hydroxyl radicals, or OH, normally play a major role in atmospheric methane loss. But explaining the Hunga Tonga observations with OH would require methane concentrations of at least 95 parts per million and an estimated methane injection exceeding 2,300 gigagrams.

Later satellite observations did not support a release that large.

Reactive chlorine fit the observations better. Researchers estimated that sustaining the methane destruction required primary production of roughly 2 to 5 gigagrams of chlorine each day.

Even that created another mystery because known chlorine-producing chemistry could not easily account for such a large amount.

The researchers instead point to volcanic ash, seawater and sunlight as a possible combination.

Hunga Tonga was especially unusual because the volcano erupted underwater.

The blast propelled volcanic material, enormous quantities of seawater and about 146 ± 5 teragrams of water into the stratosphere. Its plume reached approximately 55 kilometers high.

“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,” said Professor Matthew Johnson of the University of Copenhagen’s Department of Chemistry, who contributed to both discoveries.

The proposed mechanism involves iron in fine volcanic ash interacting with chloride supplied by seawater. Sulfate-coated ash particles exposed to sunlight could support reactions that release highly reactive chlorine atoms.

A related mechanism had previously been identified much lower in the atmosphere, where Saharan mineral dust mixes with sea-salt aerosols over the North Atlantic. The Hunga Tonga findings suggest similar iron-chloride photochemistry might operate under very different conditions in the stratosphere.

Calculations showed that the process could potentially produce chlorine at the necessary rate. The researchers estimated iron catalytic cycling of 1.1 to 4.5 times per hour in one part of the plume.

Still, the explanation remains a proposed mechanism. Dedicated laboratory experiments and atmospheric modeling will be needed to establish exactly what happened.

The result does not mean every volcanic eruption provides a significant methane sink.

Hunga Tonga combined several unusual conditions. It erupted beneath seawater, injected extraordinary amounts of water and salt high into the atmosphere, and produced a plume containing volcanic ash alongside comparatively modest sulfur dioxide emissions.

Those circumstances may have created a particularly favorable chemical environment.

The satellite measurement also carries uncertainty. Aerosols complicated the formaldehyde retrieval, and the team estimated uncertainty of about ±20% from those effects. Other processes, including biomass burning, can generate formaldehyde and complicate its interpretation elsewhere.

For Hunga Tonga, however, the researchers found strong relationships between formaldehyde, sulfur dioxide and volcanic aerosols, helping identify the signal as part of the eruption plume.

The finding has another potential use beyond understanding volcanoes.

Methane lasts roughly a decade in the atmosphere, considerably less time than carbon dioxide, making changes in its abundance capable of affecting warming relatively quickly. Researchers are therefore investigating whether atmospheric methane removal could someday supplement efforts to prevent emissions.

Proving that such a system actually destroys methane would be difficult, particularly over oceans where some methane-observing satellites struggle because the surface reflects little shortwave infrared light.

TROPOMI detects formaldehyde using ultraviolet wavelengths and can operate over oceans. The Hunga Tonga plume therefore offered a natural demonstration that enhanced methane oxidation could be measured through its short-lived chemical products.

The researchers are not proposing that the volcanic process should simply be copied. Any deliberate attempt to alter atmospheric chemistry would require extensive study of effectiveness and unintended effects.

But the eruption provided something difficult to create intentionally: a massive real-world experiment. By revealing both unexpected methane chemistry and a way to observe it from orbit, Hunga Tonga may help scientists better understand how methane disappears from Earth's atmosphere.

These resources explore the chlorine chemistry, atmospheric methane sinks and emerging methane-removal research that place the Hunga Tonga findings in a broader climate-science context.

Photocatalytic chlorine atom production on mineral dust–sea spray aerosols over the North Atlantic: This research identified sunlight-driven chlorine production when Saharan dust mixes with sea spray, providing the closely related mechanism used to interpret the Hunga Tonga plume. (Proceedings of the National Academy of Sciences, 2023)

Global environmental implications of atmospheric methane removal through chlorine-mediated chemistry-climate interactions: Modeling showed that deliberately increasing atmospheric chlorine could reduce methane but would also alter ozone and other climate-relevant chemistry, emphasizing the need to evaluate unintended consequences. (Nature Communications, 2023)

Enhanced atmospheric oxidation toward carbon neutrality reduces methane’s climate forcing: Climate-model simulations examined how changes in hydroxyl radicals can shorten methane's atmospheric lifetime and alter its future climate forcing. (Nature Communications, 2024)

Emergent methane mitigation and removal approaches: A review: This review assesses methane mitigation and proposed removal approaches, including enhancement of atmospheric chemical sinks, while emphasizing major feasibility and scaling challenges. (Atmospheric Environment: X, 2024)

A Research Agenda Toward Atmospheric Methane Removal: This consensus report identifies the scientific, technical and social research needed before atmospheric methane-removal approaches could be assessed for climate-scale use. (National Academies of Sciences, Engineering, and Medicine, 2024)

Research findings are available online in the journal Nature.

The original story "Hunga Tonga’s record-breaking eruption removed 900 metric tons of methane per day" is published in The Brighter Side of News.

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