How the Hunga Tonga Eruption Cooled the Stratosphere and Rewrote Volcanic Rules

The 2022 Hunga Tonga eruption uniquely cooled the stratosphere, a departure from typical warming effects of major volcanoes, driven by an unprecedented injection of water vapor from its submarine location. This fundamentally altered our understanding of volcanic climate impacts.

David Grossman

May 20, 2026 · Updated October 4, 2026 · 4 min read

The colossal eruption of the Hunga Tonga-Hunga Ha’apai volcano on January 15, 2022, was an event of planetary scale, sending a sonic boom twice around the globe and launching a plume of ash and gas 55 kilometers into the atmosphere. Yet its most profound legacy is a scientific revelation that has fundamentally altered our understanding of how volcanoes influence Earth's climate. A landmark international assessment in 2025, involving more than 100 scientists, delivered a definitive and surprising verdict: unlike other major eruptions that warm the upper atmosphere, Hunga Tonga did the opposite. It cooled the stratosphere, a unique effect driven by its submarine location, which powered an unprecedented injection of water vapor—not the typical sulfur gases—into the sky.

Submarine Genesis: The Root of Hunga Tonga's Uniqueness

The key to Hunga Tonga’s anomalous behavior lies deep beneath the South Pacific Ocean. The 2022 event was not a typical terrestrial eruption but a violent submarine explosion defined by a rapid caldera collapse. Research published in Nature Geoscience detailed how this underwater setting allowed the volcano's immense power to interact directly with a vast quantity of seawater. The eruption, which produced a tsunami larger than the one from the historic 1883 Krakatau event, effectively vaporized and propelled an enormous volume of the ocean straight into the upper atmosphere. This direct, high-velocity injection of seawater is the fundamental reason its atmospheric consequences diverged so dramatically from those of land-based volcanoes, which primarily vent gases from the Earth's interior.

Water Vapor's Role: A Cooling Paradox

While most large volcanic eruptions are known for spewing vast quantities of sulfur dioxide, which forms a sun-reflecting haze of sulfate aerosols, Hunga Tonga’s primary payload was water. An analysis led by scientists at NASA’s Jet Propulsion Laboratory, using data from the Microwave Limb Sounder instrument, quantified the staggering scale of the injection. The eruption blasted approximately 146 teragrams of water vapor into the stratosphere, an amount sufficient to fill over 58,000 Olympic-sized swimming pools. This single event increased the total amount of water in that atmospheric layer by 10% globally. “We’ve never seen anything like it,” said Luis Millán, a NASA atmospheric scientist who led the study.

This massive influx of water upended scientific expectations. The 2025 international assessment report noted that very large eruptions usually cause a warming effect in the stratosphere, as sulfur aerosols absorb radiation. But Hunga Tonga did the opposite, cooling the stratosphere. While the eruption did release a quantity of sulfur similar to that of Mount Pinatubo in 1991, its shallow underwater setting meant that about 95% of this sulfur was washed out and returned to the surface before it could reach the stratosphere. The dominant atmospheric agent was, therefore, water vapor.

A 2025 UCLA-led study further clarified that the smaller sulfate aerosols that did reach the stratosphere contributed to a cooling effect that, combined with the effects of the water vapor, resulted in a net temperature reduction of 0.1°C over the Southern Hemisphere, countering initial fears that the water vapor would act as a warming greenhouse gas.

Comparative Climate Impacts: Hunga Tonga vs. Mount Pinatubo

The distinction between an eruption dominated by water vapor and one dominated by sulfur aerosols is critical to understanding their profoundly different climate impacts. The 2022 Hunga Tonga event and the 1991 Mount Pinatubo eruption in the Philippines provide a stark contrast in volcanic mechanics and atmospheric consequences, with Pinatubo serving as the textbook example of a major terrestrial eruption.

A comparison of the atmospheric and climate effects of the submarine Hunga Tonga eruption and the terrestrial Mount Pinatubo eruption.
Feature	Hunga Tonga (2022)	Mount Pinatubo (1991)
Eruption Type	Submarine, involving a rapid caldera collapse in the ocean.	Terrestrial, occurring on land.
Primary Stratospheric Injection	An unprecedented amount of water vapor, increasing global stratospheric content by 10%.	Primarily sulfur dioxide, which formed a layer of sulfate aerosols.
Stratospheric Temperature Effect	Caused a cooling of the stratosphere, contrary to typical large eruptions.	Sulfur aerosols from large eruptions typically warm the stratosphere by absorbing radiation.
Global Surface Temperature Effect	Had minor effects on the global surface climate and did not cause warming.	Caused a significant, temporary cooling of Earth's surface by reflecting sunlight.