Richard Musgrove
Dr Richard Musgrove is a zoologist and science journalist. He writes for Cosmos and Double Helix.
The Hunga Tonga volcanic eruption of January 2022 was expected to warm the planet, as hundreds of millions of tonnes of water vapour was injected into the atmosphere. But it didn’t according to US researchers.
Hunga Tonga-Hunga Ha’apai is a submarine volcano, with a vent 200m below the ocean’s surface. It sits 20o south of the Equator (Sydney is 33oS), on the boundary of the Pacific and Indo-Australian tectonic plates, about 65km north of Tongatapu, Tonga’s main island. The small islands projecting above sea level were uninhabited prior to the eruption but are known for their guano deposits.
Being underwater complicates the effect of eruptions on climate. “Large volcanic eruptions near the equator that eject a lot of sulphate aerosols generally have a cooling effect on the atmosphere” CSIRO climate scientist, Dr Michael Grose told Cosmos.
Sulphur dioxide gas ejected by volcanoes combines with water vapour in the atmosphere to form sulphates, tiny particles, which physically block sunlight, reducing temperatures. It’s referred to as a shading effect, which is the basis of some contested geoengineering theories.
Famous examples include the ‘year without a summer’ in Europe and North America in 1816, following the Tambora eruption in 1815 in Indonesia. That year, global temperatures cooled by around 2oC, contributing to famines in Europe, India and China. The massive 1991 Mt Pinatubo eruption in the Philippines cooled the atmosphere by 0.4-0.5oC, making 1993 the coolest in the past 35 years.
Hunga-Tonga,below sea level, was expected to do the opposite, because of the huge amounts of water vapour ejected, says atmospheric scientist Dr Ashok Gupta of the University of California at Los Angeles
Water vapour traps surface heat, preventing its radiation into space. The researchers thought that this might push global temperatures past the 1.5oC threshold set by the Paris Climate Accords.
Indeed 2022 would turn out to be the fifth warmest year on record to that date, tied with 2015. NASA says that heating in that year was 0.89oC above the average for their baseline 1951-1980. (Since then, 2024 has been confirmed as the hottest year on record, at 1.5oC above pre-industrial levels.)
But researchers found a net cooling effect of about 0.1oC in the Southern Hemisphere, by the end of 2022 and 2023, despite the eruption of “unprecedented amounts” of water vapor into the stratosphere, says lead author, Gupta.
Gupta concluded that because sulphate particles in this eruption were 50% smaller than those of Pinatubo, they may have been better at blocking sunlight, despite the heavy water vapour load. “Smaller particles move more erratically and therefore have more chances to reflect sunlight”, he says.
“The bottom line is that sulphate aerosols did indeed contribute to temporary cooling in the Southern Hemisphere, although the overall magnitude was relatively small. Part of this cooling effect can be attributed to sulphate aerosols being in a ‘sweet spot’ in terms of particle size, an outcome influenced by complex chemical interactions and stratospheric mixing processes still not fully understood.”
These results have a much broader context. Geoengineering, altering the global climate to prevent catastrophic climate change, has included discussion of approaches such as injecting sulphate aerosols into the atmosphere to counteract global heating.
“If we plan to use approaches that involve releasing sulphate aerosols into the stratosphere to reflect sunlight, we have to consider how other factors — such as water vapor and atmospheric mixing — could change the outcome,” says Gupta.
“The overall impact of such measures depends on understanding the complex interactions among atmospheric components that affect the formation and properties of stratospheric sulphate aerosols.”
Gupta adds that geoengineering efforts can have multiple, potentially unforeseen consequences, and working out whether a given approach would lead to warming or cooling must be based on a thorough understanding of an atmospheric system.
Atmospheric scientist Dr Martin Jucker of the Climate Research Centre at the University of New South Wales, was not involved in the research, but “certainly agrees that geoengineering can have multiple, potentially unforeseen consequences.” He also urges caution with the interpretation of the results.
Hunga Tonga eruption and the weather
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