Huge volcanic eruption offers clues to combat climate change

A gigantic underwater volcano has become a massive chemical experiment that could help researchers quantify the success of tactics designed to combat climate change.

In January 2022, the Hunga Tonga-Hunga Ha’apai volcano in the South Pacific Ocean exploded with the power of several atomic bombs, launching a towering plume of ash, gas and seawater 55 kilometers into the atmosphere. Researchers now report that chemical reactions inside the plume may have partially cleaned up some of the pollution from the eruption by degrading methane, a powerful greenhouse gas, as revealed by satellite data that tracked the destruction of methane. The results, published on May 7 in Natural communicationscould help researchers evaluate proposals to speed up the removal of methane from the atmosphere, thereby slowing global warming.

Methane is responsible for around a third of current global warming. Although methane traps more heat than carbon dioxide, it is easier to break down and only persists in the atmosphere for about a decade, compared to the centuries that CO₂ lingers. This relatively short lifespan has made methane an attractive target for geoengineering projects aimed at further accelerating its degradation.

Having a reliable way to measure success is a prerequisite for any attempt to methane removal strategysays Maarten van Herpen, a physicist at Acacia Impact Innovation, a consulting company in Heesch, the Netherlands. As it turns out, the eruption provided van Herpen and his colleagues with a rare opportunity to test their ability to quantify the destruction of methane from space. “If we can see it in the volcano, we would also see it in a hypothetical intervention,” says van Herpen.

One way to separate methane molecules is to use highly reactive chlorine atoms. Previous work by van Herpen and colleagues suggested that chlorine atoms could form when iron-rich dust blown from the Sahara Desert mixes with salt-rich sea spray — which contains chlorine in a different form — over the Atlantic Ocean. Sunlight causes chemical reactions between iron and salt, releasing chlorine in a highly reactive atomic form. The team suspected that volcanic ash could cause similar reactions, and the 2022 eruption created the perfect setting to test them.

The researchers turned to the European Space Agency’s Tropospheric Monitoring Instrument, a satellite tool that monitors air pollution and greenhouse gases on a global scale. Because methane is difficult to measure over the ocean due to the similar wavelengths at which water absorbs light, the team looked for formaldehyde as evidence of the presence of reactive chlorine. Formaldehyde is not emitted by volcanoes, but is formed during the breakdown of methane. Formaldehyde remained detectable in the volcanic plume for several days, although it normally decomposes within a few hours, suggesting that it was constantly being produced by ongoing chemical reactions.

“It is quite surprising that these levels of formaldehyde were observed,” says Folkert Boersma, an atmospheric scientist at Wageningen University and Research in the Netherlands who was not involved in the study. “It reveals something I didn’t know myself.”

The 2022 eruption provided exceptionally favorable conditions for this chemistry. Chlorine is not usually a major component of volcanic eruptions, but in this case the explosion occurred 150 meters below sea level, releasing more than a hundred million tons of salt water into the atmosphere. Researchers estimate that chlorine-driven reactions destroyed about 900 tons of methane per day after the eruption. This is a modest amount compared to the explosion’s total methane emissions, estimated at 300,000 tonnes.

However, some researchers believe that using chlorine to break down methane would likely create a bigger problem than the methane itself. “I don’t think we should be injecting chlorine into the stratosphere. We’ve done it before, and it didn’t go well,” says Pete Edwards, an atmospheric chemist at the University of York, England, referring to chlorofluorocarbons, the chlorinated chemicals that spread into the atmosphere from sources such as refrigerants and aerosols, which cause serious depletion of the ozone layer and Ozone hole in Antarctica. Chlorine is much more likely to react with the more abundant molecules in the atmosphere, such as ozone, than with methane, which is relatively rare. This is especially true in the cold stratosphere, where chlorine reacts with ozone about 380 times faster than with methane, Edwards says. “Chlorine in the stratosphere is a bad thing.”

Boersma says that before moving forward with such projects, the priority should be to emit less methane and CO₂. “We all know what to do,” he said. “It’s not about releasing chlorine into the stratosphere, it’s just about making sure we reduce emissions.”