Hunga volcano eruption cooled, rather than warmed, the Southern Hemisphere

Hunga volcano eruption cooled, rather than warmed, the Southern Hemisphere
by Holly Ober for UCLA News
Los Angeles CA (SPX) Apr 03, 2025

When Hunga Tonga-Hunga Ha'apai, an underwater volcano near Tonga in the South Pacific Ocean, erupted in 2022, scientists expected that it would spew enough water vapor into the stratosphere to push global temperatures past the 1.5 C threshold set by the Paris Accords. A new UCLA-led study shows that not only did the eruption not warm the planet, but it actually reduced temperatures over the Southern Hemisphere by 0.1 C.

The reason: The eruption formed smaller sulfate aerosols that had an efficient cooling effect that unexpectedly outweighed the warming effect of the water vapor. Meanwhile, the water vapor interacted with sulfur dioxide and other atmospheric components, including ozone, in ways that did not amplify warming.

While that's good news, the study also suggests that efforts to reverse climate change by loading the atmosphere with substances that react with solar radiation to send heat back out into space, an effort known as geoengineering, are potentially even riskier than previously thought and must take new complications into account.

"If we plan to use approaches that involve releasing sulfate 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," said UCLA atmospheric scientist Ashok Gupta, the first author of a paper describing the findings in Nature Communications Earth and Environment. "The overall impact of such measures depends on understanding the complex interactions among atmospheric components that affect the formation and properties of stratospheric sulfate aerosols."

The Hunga Tonga volcano erupted Jan. 15, 2022, from a vent just 200 meters below the ocean's surface, shooting an enormous amount of water vapor, along with a moderate amount of sulfur dioxide, into the stratosphere. The sulfur dioxide was quickly converted into tiny particles called sulfate aerosols that reflect sunlight back into space. Scientists were concerned because sulfate aerosols and water vapor have opposite climate effects. Sulfate aerosols lead to cooling in the atmosphere. Past volcanic eruptions, such as that of nearby Pinatubo in 1991, have had this kind of cooling effect on the climate.

On the other hand, water vapor, a greenhouse gas, cools the stratosphere but warms the Earth's surface. But this effect also has to do with the water vapor's altitude: The higher into the stratosphere it goes, the greater the warming effect on Earth. Given the quantity of high-altitude water vapor from the Hunga eruption and the relatively small amount of sulfur dioxide, an increase in global warming seemed the most likely result.

Methods for studying the volcanic emissions

Gupta and UCLA atmospheric sciences professor Jasper Kok worked with Ralf Bennartz and Kristen Fauria at Vanderbilt University and Tushar Mittal at Pennsylvania State University to study how volcanic emissions spread throughout the atmosphere over the two years following the eruption and how they affected the Earth's energy balance. They used satellite data to track the distribution of water vapor, sulfate aerosols and ozone over time and space.

Next, they analyzed how these satellite observations revealed the impact of altered stratospheric water vapor, sulfate aerosols and ozone on the interaction between solar radiation and the Earth's heat. This detailed analysis helped them determine how the volcanic eruption changed the movement of energy in the atmosphere and affected surface temperatures.

How researchers discovered the volcano caused cooling

The analysis showed that these components caused almost instantaneous net radiative energy losses, or cooling, at both the top of the atmosphere and near the tropopause, the boundary separating the troposphere (the lowest layer of Earth's atmosphere) from the stratosphere, resulting in a cooling of about 0.1 C in the Southern Hemisphere by the end of 2022 and 2023. The sulfate aerosols were about 50% smaller than those that occurred after the Pinatubo eruption, which made them better at blocking sunlight and cooling the atmosphere in spite of the heavy water vapor load. Smaller particles move more erratically and, therefore, have more chances to reflect sunlight. The surprising result came because the researchers included ozone and other components of the atmosphere in their analysis, while previous studies focused mostly on sulfate aerosol and water vapor.

This study shows that shallow undersea eruptions can trigger complex changes in the atmosphere. While the Southern Hemisphere experienced a cooling effect largely due to particles that bounce sunlight away, some signs hint at a very slight warming influence due to this eruption in the Northern Hemisphere because water vapor can linger for years in the stratosphere. But overall, the 2022 Hunga eruption induced a slight cooling effect on the planet from 2022.

"The bottom line is that sulfate aerosols did indeed contribute to temporary cooling in the Southern Hemisphere, although the overall magnitude was relatively small," said Gupta. "Part of this cooling effect can be attributed to sulfate 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. This work also highlights that geoengineering efforts can have multiple, potentially unforeseen consequences. It is crucial to thoroughly understand a given atmospheric system to determine whether a proposed geoengineering approach will ultimately lead to cooling or warming."

Research Report:The January 2022 Hunga eruption cooled the southern hemisphere in 2022 and 2023