On January 15, 2022, a volcano on the floor of the South Pacific exploded with enough force to punch a plume 35 miles into the sky. Scientists expected the usual aftermath: sulfur aerosols drifting upward, the stratosphere warming slightly, surface temperatures dipping for a year or two. Instead, the atmosphere cooled in a layer where it was supposed to warm, and three years of international research were needed to explain why.
A new scientific assessment confirmed that the Hunga Tonga-Hunga Haʻapai eruption chilled the stratosphere rather than heated it, left no measurable fingerprint on the record global temperatures of 2023 and 2024, and drove more water into the upper atmosphere than any eruption scientists have been able to measure.
The figure at the center of the story is 146 teragrams, the mass of water vapor the eruption delivered to the stratosphere. That equals 10 percent of all the moisture already sitting in that atmospheric layer. According to a study led by atmospheric scientist Luis Millán at NASA’s Jet Propulsion Laboratory, published inGeophysical Research Letters, the total was nearly four times the water contribution of the 1991 Mount Pinatubo eruption, which had been the modern benchmark for volcanic atmospheric disruption.
Why the Caldera’s Depth Changed Everything
Nearly every unusual feature of this eruption traces to one fact: the Hunga caldera sat 490 feet below the ocean surface. That depth placed it in a narrow physical window. Deep enough for erupting magma to superheat vast volumes of seawater into vapor. Shallow enough that ocean pressure could not muffle the explosion. A few hundred feet in either direction and the eruption likely unfolds as an ordinary event.
The seawater that fueled the plume also blocked something. In most large eruptions, sulfur dioxide climbs into the stratosphere, where it forms aerosol particles that absorb solar radiation and warm the layer from within. At Hunga Tonga, the water intercepted most of that sulfur before it could escape upward. The sulfate signal was negligible compared to Pinatubo’s. What reached the stratosphere instead was water, in quantities the atmosphere had not absorbed from a single eruption in the observational record.
A study in the Journal of Volcanology and Geothermal Research proposed the blast was driven by gas-forced hydraulic failure, where expanding gas shattered rock and vaporized seawater at an accelerating rate. Seafloor surveys by New Zealand’s National Institute of Water and Atmospheric Research estimated the explosion excavated roughly 2.3 cubic miles of rock.
Water Vapor Radiates Heat Out. It Does Not Hold It In.
This is the core of why the eruption defied expectations. Sulfate aerosols act like a lid, trapping solar energy inside the stratosphere and raising its temperature. Water vapor behaves the opposite way: at high altitude it emits heat back toward space, pulling energy out of the layer rather than concentrating it.
The result was stratospheric cooling of 0.5 to 1 degree Celsius across broad regions of the upper atmosphere, according to the assessment. Researchers at the University of Colorado Boulder described this as behavior with no clear parallel in the modern eruption record. At ground level, the effect was barely detectable, a surface temperature shift of around 0.05 degrees Celsius. Pinatubo, by comparison, cooled the surface by 0.25 to 0.5 degrees.
Professor Amanda Maycock of the University of Leeds confirmed the eruption had no measurable role in the record heat years of 2023 and 2024. That finding closed a question climate scientists had been working through since those temperatures were recorded.
The Column That Punched Through the Stratosphere
National Geographic confirmed the eruption column reached 35.4 miles, making it the tallest volcanic plume on record. It did not stop at the stratosphere. It pierced into the mesosphere, the layer above, where air is so thin that meteors burn out on entry.
The force of the blast also sent atmospheric pressure waves looping around the planet four times over six days. Those waves were strong enough to push ocean water upward in distant basins. In the Mediterranean, sea levels rose by about a foot, a phenomenon scientists call a meteo tsunami, driven not by a submarine earthquake but by the atmosphere itself. The last time something comparable was documented was the 1883 Krakatau eruption.
Nearer to Tonga, the collapse of the caldera floor generated a conventional tsunami. Waves hit surrounding shores at heights above 50 feet in some locations. Both events happening simultaneously from a single eruption had no precedent in the instrumental record.
Three Years Later, the Signal Is Still There
As of late 2025, stratospheric water vapor levels from the eruption remain above normal. Dr. Sandip Dhomse of the University of Leeds said the elevated moisture is expected to linger for several more years, a longer atmospheric footprint than sulfate-driven eruptions typically leave. Across parts of the Southern Hemisphere, short-term ozone losses were detected in the months after the eruption, linked to shifts in air circulation rather than direct chemical breakdown of ozone molecules. Antarctic ozone remained within seasonal ranges.
Dr. Yunqian Zhu, a senior research scientist at the University of Colorado Boulder, said the eruption exposed a gap in how scientists model water-rich volcanic events and their capacity to reshape stratospheric conditions. The moisture Hunga Tonga injected is projected to keep affecting atmospheric chemistry into the late 2020s.
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