Surprising find after volcanic eruption: Methane was broken down again directly

When the underwater volcano Hunga Tonga-Hunga Haʻapai erupted with immense force in mid-January 2022, it not only released large amounts of the potent greenhouse gas methane but also removed some of it from the atmosphere. This surprising discovery was made with the help of an ESA satellite and could point to a way to reduce methane emissions, according to the European research team presenting it now. They detected a cloud with a record amount of formaldehyde, which remained for a few hours after the methane was broken down. The cloud existed for ten days, during which about 3 percent of the emitted methane was directly removed from the atmosphere.

Underlying process was only recently discovered

According to the University of Copenhagen, the formaldehyde cloud was found using the TROPOMI instrument on ESA's Sentinel-5 Earth observation satellite. However, it was not designed for this purpose; instead, it had to be carefully recalibrated until the finding was confirmed. Then, they were able to track the cloud eastward to South America for days. “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 research leader Maarten van Herpen. While it was already known that methane can enter the atmosphere during volcanic eruptions, it was not known that it can also be broken down in the process.

According to the research group, everything points to a very specific process that was discovered only after the volcanic eruption. It involves the discovery that Saharan dust over the Atlantic can mix with seawater to form so-called iron salt aerosols. When sunlight hits them, chlorine atoms are formed, which in turn react with methane and break down the gas. Their discovery suggests that this process can also occur under completely different conditions in volcanic clouds high up in the stratosphere. There, salty seawater was mixed with volcanic ash, from which sunlight in turn formed chlorine. This then continuously broke down methane.

The eruptions of Hunga Tonga-Hunga Haʻapai on January 14 and 15, 2022, and the subsequent tsunami claimed at least six lives; the islands of Tonga were covered by a thick layer of ash. It was the most violent explosion on Earth since the eruption of the Krakatoa volcano in 1883, and the ash cloud was the highest in the satellite era. Many buildings in the island nation were destroyed; entire settlements were lost in some cases. Tonga was cut off from the outside world for weeks afterward due to the severed internet cable. Insufficiently secured aid deliveries then also led to an outbreak of COVID-19 on the island nation. The eruption may also have temporarily warmed the Earth's atmosphere.

The research group led by van Herpen has determined that approximately 300,000 tons of methane were released into the atmosphere during the eruption, and 900 tons of it were broken down daily in the cloud. This is roughly equivalent to what two million cows emit in a year or in one day. Methane is a much more effective greenhouse gas than carbon dioxide and contributes significantly to global warming. At the same time, however, it does not remain in the atmosphere as long, so its effects are much shorter-term than those of CO₂. Therefore, if emissions are reduced immediately or methane is specifically broken down, much faster measurable effects can be achieved in the fight against climate change. The discovery could point the way. It is now being presented in Nature Communications.

(mho)