In January 2022, a submarine volcano near Tonga exploded in what became the most powerful volcanic eruption in over 30 years. Unlike previously known eruptions, the Hunga event hurled not only ash and sulfur into the atmosphere, but also an astonishing amount of water. A new international report, The Hunga Volcanic Eruption Atmospheric Impacts Report, shows that the eruption had remarkable and long-lasting effects on the stratosphere, fundamentally changing our understanding of how volcanoes influence Earth’s climate system.

Associate Professor Freja Chabert Østerstrøm from the Department of Environmental Science at Aarhus University participated in the international research collaboration, in which 159 scientists from 21 countries analyzed the unusual eruption and its implications for climate and atmospheric chemistry.

“We have never before observed such large quantities of water vapor from a volcanic eruption. The amount corresponds to about 20 percent of the water content in the stratosphere across the entire Southern Hemisphere,” says Freja Chabert Østerstrøm.

An eruption of unprecedented power

The eruption began 150 meters below sea level and was so intense that the plume reached a height of 58 kilometers, the highest ever recorded in the satellite era. Parts of the plume even reached the mesosphere. Within the first hour, it had spread to a width of 400 km and ultimately reached a diameter of about 600 km.

The explosion injected about 150 million tons of water vapor into the stratosphere, roughly 10 percent of the stratosphere’s total water vapor. Most of the water came from seawater and magma, and because Hunga sat at an ideal depth beneath the sea, conditions were perfect for a so-called "wet explosive" eruption.

Under normal circumstances, the boundary between the troposphere and stratosphere, the tropopause, acts as a cold trap, preventing water vapor from rising. But this explosion was powerful enough to breach that barrier.

“What makes Hunga so special is the combination of water and sulfate particles that reached unprecedented altitudes. It has completely changed our view of what happens after a volcanic eruption. We simply didn’t think this was possible,” says Freja Chabert Østerstrøm.

The stratosphere is usually a dry and stable layer, where sunlight and low temperatures keep chemical reactions in check. But with the injection of water, significant changes occurred. Water affects temperature and accelerates chemical processes that deplete ozone, in part by forming hydroxyl radicals (OH), which help turn sulfur dioxide (about 0.5 million tons released) into sulfuric acid particles (sulfate aerosols).

“Water vapor has a dual effect. It cools the stratosphere but also shifts the chemical balance and particle formation. This allows reactions that typically occur only over the poles to happen elsewhere, and we’ve seen that here,” Østerstrøm explains.

In the months following the eruption, large ozone losses were recorded outside polar regions. Locally over the Southern Hemisphere, researchers observed up to 14% ozone loss and 22% hydrogen chloride (HCl) reduction in the mid-stratosphere, up to 5% ozone loss occurred within the first two weeks after the eruption, with up to 40% on a single day.

“It’s highly unusual. And it happened because the water vapor and volcanic particles accelerated reactions that break down ozone,” explains Freja Chabert Østerstrøm.

A volcano with limited climate effect

Large volcanic eruptions are normally associated with global cooling. Sulfuric acid particles formed from sulfur dioxide spread through the stratosphere and reflect sunlight. After Mount Pinatubo erupted in 1991, for example, global temperatures dropped by about 0.5 degrees for two years.

But the Hunga eruption defied that pattern. The water vapor counteracted the cooling from sulfur particles because water behaves very differently in the atmosphere. Water vapor absorbs heat and acts as a greenhouse gas, warming Earth’s surface.

“This is the first time we've seen this type of response from a volcanic eruption,” says Østerstrøm.

Climate models estimate the overall effect to be around -0.05°C during 2022–2023, making it difficult to detect. But it highlights the substantial impact water vapor can have in the stratosphere, forcing researchers to reconsider their assumptions in volcanic climate models.

New insights into supervolcanoes

One of the biggest surprises was how long the water remained in the atmosphere. It only began to dissipate in early 2024, nearly a year and a half after the eruption. In 2022, 78% of the water stayed in the Southern Hemisphere, and remnants of the plume remain measurable in 2025.

“The stratosphere simply isn’t designed to handle that much water. It changes how we understand atmospheric chemistry, and also how we interpret earlier supervolcano eruptions,” explains Østerstrøm.

The Hunga eruption injected such a massive amount of water that it could indirectly inform us about what might have happened during even larger, prehistoric eruptions, so-called supervolcanoes. Since we’ve never before had the technology to measure such water quantities, Hunga becomes a key example in future climate models.

“The Hunga eruption was unlike anything observed before,” says Yunqian Zhu, research associate at the University of Colorado Boulder and one of the main authors of the report. “It taught us how profoundly water-rich eruptions can affect the stratosphere and how essential global cooperation is in capturing and understanding such rare events.”

How water vapor affects the ozone layer

• Normally absent in the stratosphere, water vapor disrupts its stability.
• Rising water vapor changes temperature and triggers new chemical reactions.
• Water reacts with sulfur dioxide (SO₂) to form sulfuric acid (H₂SO₄), which condenses into aerosols.
• These aerosols serve as reaction surfaces where harmful halogen compounds (like HCl) are transformed into free radicals.
• These radicals destroy ozone (O₃), not only over the poles but also in tropical and mid-latitudes.
• The result: lower ozone levels, increased surface UV radiation, and changes in atmospheric energy balance.

Associate Professor Freja Chabert Østerstrøm 
Department of Environmental Science, Aarhus University 
Mail: freja@envs.au.dk
Tel.: +4587150623