The Tonga Eruption injected water vapour into the stratosphere, temporarily altering global atmospheric chemistry

On January 15, 2022, a powerful underwater volcanic eruption near Tonga sent shock waves around the planet and propelled material higher into the atmosphere than any eruption in the satellite era. The event, known as the 2022 Hunga Tonga-Hunga Ha’apai eruption, did more than create dramatic pressure waves and towering ash plumes. It injected an extraordinary amount of water vapour directly into the stratosphere, temporarily altering global atmospheric chemistry in ways that scientists are still analysing.

An Unusual Eruption With an Unusual Signature

Most large volcanic eruptions influence climate by releasing sulfur dioxide, which forms reflective sulfate aerosols in the stratosphere and cools the planet by blocking incoming sunlight. However, satellite observations from instruments operated by NASA and the National Oceanic and Atmospheric Administration revealed that this eruption exhibited distinct behaviour.

Instead of being dominated by sulfur emissions, the Tonga eruption injected an estimated 146 teragrams of water vapour into the stratosphere, according to a 2022 study published in Geophysical Research Letters. This amount represented roughly a 10 per cent increase in the total stratospheric water vapour burden at the time. The injection reached altitudes of about 53 kilometres, which is unusually high and allowed the water vapour to persist for years rather than months. Atmospheric scientist Luis Millán, lead author of the study, explained that this was the first time scientists had observed such a large direct injection of water vapour into the stratosphere from a volcanic eruption. He noted that the scale of the injection was unprecedented in the modern observational record.

Why Water Vapour in the Stratosphere Matters

Water vapour is the most abundant greenhouse gas in the lower atmosphere, but it is normally scarce in the stratosphere because cold temperatures near the tropopause limit the amount of moisture that can rise from below. When large quantities are added directly at high altitude, the chemical and radiative consequences can be significant. A 2023 analysis published in Nature Climate Change showed that the additional water vapour slightly increased radiative forcing, indicating a small warming effect rather than the cooling commonly associated with volcanic eruptions. Climate model simulations estimated that the added stratospheric moisture could produce a transient global surface warming of several hundredths of a degree Celsius over the following years.

Susan Solomon, an atmospheric chemist known for her work on ozone depletion, commented in media interviews that the eruption was unique because it demonstrated how submarine volcanism can influence climate differently from land-based eruptions. She emphasised that water vapour at stratospheric levels alters both radiation balance and chemical reaction rates.

Impacts on Ozone Chemistry

Beyond its radiative effects, stratospheric water vapour participates in chemical cycles that influence ozone concentrations. Water vapour can generate hydroxyl radicals, which interact with ozone and other trace gases. Researchers analysing satellite data from the European Space Agency found regional variations in ozone concentrations following the eruption.

A study published in Atmospheric Chemistry and Physics reported that enhanced moisture contributed to increased formation of polar stratospheric clouds in certain regions, thereby accelerating ozone-depletion reactions under appropriate temperature conditions. However, scientists caution that the overall impact on global ozone appears modest and temporary, with recovery mechanisms already underway. Lead researchers have stressed that, although the chemical perturbation was measurable, it does not reverse the long-term recovery of the ozone layer, which is driven by international agreements such as the Montreal Protocol. Instead, it provides a real-world experiment for testing atmospheric chemistry models.

A Natural Experiment for Climate Science

The Tonga eruption has become a case study in how rare geophysical events can test scientific understanding of atmospheric processes. By combining satellite retrievals, balloon observations, and climate simulations, researchers are tracking how long the excess water vapour remains in the stratosphere and how it redistributes across latitudes.

According to NOAA atmospheric scientist Karen Rosenlof, who has studied stratospheric moisture trends for decades, the eruption offers an opportunity to observe how the stratosphere responds to a sudden moisture pulse. She explained that understanding these responses is essential for refining climate projections, especially because stratospheric water vapour variations can amplify or dampen warming trends.

A Temporary but Measurable Shift

Although the injected water vapour is expected to gradually decline over several years as it mixes and descends into lower atmospheric layers, its presence represents one of the most significant short-term perturbations of the stratosphere in recent history. Unlike eruptions that cool the planet through sulfate aerosols, this event highlighted the complexity of volcanic impacts on climate and chemistry.

The Tonga eruption demonstrated that interactions among the ocean, volcano, and atmosphere can produce outcomes that challenge conventional expectations. By documenting how water vapour altered the radiative balance and chemical cycles at high altitudes, scientists have obtained valuable data that improves understanding of both natural variability and long-term climate dynamics. In the aftermath of this eruption, the atmosphere itself became a laboratory, offering a rare glimpse into processes that usually unfold slowly and invisibly. Through careful measurement and analysis, researchers continue to unravel how a single explosive event temporarily reshaped global atmospheric chemistry.