Here’s what you’ll learn when you read this story:

• The 2022 Hunga Tonga–Hunga Haʻapai volcanic eruption was the largest eruption in the 21st century, sending up the highest plume ever recorded.
• A new study finds that salt water lifted into the stratosphere by the explosion mixed with volcanic ash and reacted with sunlight, creating highly reactive chlorine that destroyed methane.
• Scientists found that the volcanic plume contained formaldehyde, a short-lived intermediary that forms when methane is destroyed in the atmosphere.

On January 15, 2022, after weeks of rumblings, the submarine volcano Hunga Tonga–Hunga Haʻapai in the South Pacific blew its top. The massive eruption—the largest and highest underwater eruption ever recorded by modern instruments, and the first to ever reach the mesosphere—could be heard 1,200 miles away in New Zealand and sent ash a stunning 40 miles (65 kilometers) into the air. In many, many ways, the eruption of Hunga Tonga–Hunga Haʻapai was extraordinary.

Now, more than four years after that impressive volcanic display, a new study in the journal Nature Communications details a completely surprising new fact about Hunga Tonga–Hunga Haʻapai eruption: It cleaned up after itself. Well, at least a little bit.

Typically, volcanoes spill tons of methane during an eruption. The Permian-Triassic extinction, for example—which decimated most life on Earth—was likely the result of elevated levels of sulfur dioxide, carbon dioxide, and methane burped up from the Siberian Traps. Methane is 80 times more potent than carbon dioxide as a greenhouse gas (over a 20-year period), so you really don’t want this stuff reaching the atmosphere.

But it turns out that Hunga Tonga–Hunga Haʻapai did us a solid by cleaning up at least some of its methane emissions itself. Scientists discovered this rare example of volcanic eitquette using data gathered from the TROPOMI instrument aboard the European Space Agency’s Sentinel-5P (a satellite designed to study air pollution and greenhouse gas emissions). What they discovered is that the record-breaking plume contained incredibly high concentrations of the volatile organic compound formaldehyde,—a short-lived intermediary that forms when methane is destroyed in the atmosphere.

“Because formaldehyde only exists for a few hours, this showed that the cloud must have been destroying methane continuously for more than a week,” Maarten van Herpen, lead author of the study, said in a press statement. “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.”

The engine behind this natural clean-up process is sea water, which Hunga Tonga–Hunga Haʻapai hurled into the sky in excess during the eruption. Once in the stratosphere, this salt—along with plentiful volcanic ash—interacted with sunlight, turning the mixture into highly reactive chlorine that effectively broke down methane. Scientists discovered a similar process in 2023 that showed how dust from the Sahara (once blown over the Atlantic) can, via sunlight, also create chlorine atoms that remove methane from the atmosphere.

“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,” Matthew Johnson, a co-author of the study from the University of Copenhagen, said in a press statement.

During the eruption, Hunga Tonga–Hunga Haʻapai emitted roughly 300 gigagrams (Gg) of methane, which the scientists estimate to be equal to the annual emissions of roughly two million cows. Unlike cows, however, the researchers say the volcano cleaned up 900 megagrams per day. While that’s roughly the equivalent of cleaning one fork after a giant Thanksgiving dinner, the natural process shows that humans might attempt to remove methane from the atmosphere using a similar method.

“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,” Johnson said. “Our satellite method could offer a way to help figure out how humans might slow global warming.”