Hunga Tonga's Eruption Destroyed 900 Metric Tons of Methane Daily Through Rare Chemistry

When the Hunga Tonga volcano erupted in January 2022, it sent an enormous plume of ash, water, and gas rocketing 55 kilometers into the stratosphere. What scientists discovered months later, by studying satellite data, was that something unexpected had happened inside that plume: methane was vanishing at a rate never before observed in nature.

Satellite instruments detected the disappearance indirectly, through a chemical fingerprint. The Sentinel-5P satellite, carrying an instrument called TROPOMI, spotted formaldehyde concentrations reaching 12 parts per billion in the volcanic plume roughly 30 kilometers above Earth. Formaldehyde is a short-lived molecule that normally survives only a few hours in the stratosphere. Yet it persisted in the plume for at least 10 days as the cloud drifted across the Pacific. That persistence was the puzzle. If the volcano had simply ejected formaldehyde during the eruption, about 95 percent of it should have disappeared before the satellite's first observation, roughly 20 hours after the main blast. Instead, large amounts remained on subsequent days, suggesting that new formaldehyde was being continuously produced.

The explanation lay in methane oxidation. When a methane molecule breaks down, it produces roughly one formaldehyde molecule during the reaction sequence. Because formaldehyde disappears quickly, its presence reveals where active methane destruction is occurring. Researchers, led by Dr. Maarten van Herpen of Acacia Impact Innovation BV, calculated that the plume was producing about 4.7 million moles of formaldehyde per hour around midday on January 16. That corresponded to approximately 75 metric tons of methane oxidation per hour at midday. Accounting for changing sunlight over the full day, the team estimated the plume was destroying roughly 900 metric tons of methane daily, with an uncertainty range of plus or minus 220 metric tons.

The next question was what could drive such rapid methane destruction. Hydroxyl radicals, the molecules normally responsible for breaking down atmospheric methane, could not explain the observations without requiring implausibly high methane concentrations. Reactive chlorine fit the data far better. The researchers estimated that sustaining the methane destruction required the production of roughly 2 to 5 gigagrams of chlorine each day. But known chlorine-producing chemistry could not easily account for such a large amount. The team proposed an unusual mechanism: volcanic ash, seawater, and sunlight working together. Hunga Tonga was distinctive because it erupted underwater, propelling enormous quantities of seawater into the stratosphere alongside volcanic material. Iron in the fine volcanic ash could interact with chloride supplied by seawater. When sulfate-coated ash particles were exposed to sunlight, they could support reactions that release highly reactive chlorine atoms. A similar mechanism had been identified previously in the lower atmosphere, where Saharan mineral dust mixes with sea-salt aerosols over the North Atlantic. The Hunga Tonga findings suggested that iron-chloride photochemistry might operate under very different conditions high in the stratosphere.

The research, published in Nature Communications, revealed something unexpected about one of the most powerful volcanic eruptions of the modern era: it had become an accidental experiment in atmospheric chemistry. Yet the explanation remains a proposed mechanism. Laboratory experiments and atmospheric modeling will be needed to establish exactly what happened. The findings also carry uncertainty. Aerosols complicated the formaldehyde measurements, introducing roughly 20 percent additional uncertainty. Other processes, including biomass burning, can generate formaldehyde and complicate its interpretation elsewhere. For Hunga Tonga, however, researchers found strong relationships between formaldehyde, sulfur dioxide, and volcanic aerosols, helping identify the signal as part of the eruption plume.

The discovery has implications beyond volcanology. 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. Scientists are 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. 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. Researchers are not proposing that the volcanic process should be copied. Any deliberate attempt to alter atmospheric chemistry would require extensive study of effectiveness and unintended consequences. 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 and whether humans might someday accelerate that process.