Hunga Tonga-Hunga Ha’apai (Hunga Tonga for short) erupted on January 15, 2022 in the Pacific Kingdom of Tonga. It created a tsunami that triggered warnings across the Pacific basin and sent sound waves around the globe multiple times.
A new study published in the Journal of Climate explores the climate impacts of this eruption.
Our findings suggest that the volcano could explain last year’s unusually large ozone hole, as well as the much wetter-than-expected summer of 2024.
The eruption could have lasting effects on our winter weather for years to come.
A cloud of cooling smoke
Usually, the smoke from a volcano – and in particular the sulfur dioxide contained within the smoke cloud – eventually leads to a cooling of the Earth’s surface for a short period.
That’s because sulfur dioxide turns into sulfate aerosols, which send sunlight back into space before it reaches the surface. This shadow effect means that the surface cools for a while, until the sulfate falls back to the surface or it rains.
This is not what happened with Hunga Tonga.
Because it was an underwater volcano, Hunga Tonga produced little smoke but a lot of water vapor: 100-150 million tons, or the equivalent of 60,000 Olympic swimming pools. The intense heat of the explosion turned large amounts of seawater into steam, which was then ejected into the atmosphere by the force of the explosion.
Japan Meteorological Agency, CC BY
All that water ended up in the stratosphere: a layer of the atmosphere between about 15 and 40 kilometers above the surface, which produces neither clouds nor rain because it is so dry.
Water vapor in the stratosphere has two main effects. One, it aids in the chemical reactions that destroy the ozone layer, and two, it is a very powerful greenhouse gas.
There is no precedent in our observations of volcanic eruptions for knowing what all that water would do to our climate and for how long. This is because the only way to measure water vapor throughout the stratosphere is via satellites. These have only existed since 1979, and there hasn’t been a Hunga Tonga-like eruption in that time.
Follow the steam
Experts in stratospheric science around the world began reviewing satellite observations from the first day of the eruption. Some studies focused on the more traditional effects of volcanic eruptions, such as the amount of sulfate aerosols and their evolution after the eruption, some focused on the possible effects of water vapor, and some included both.
But no one really knew how water vapor would behave in the stratosphere. How long will it stay in the stratosphere? Where will it go? And, more importantly, what does this mean for the climate while the water vapor is still there?
These were exactly the questions we set out to answer.
We wanted to find out about the future, and unfortunately it is impossible to measure this. That’s why we turned to climate models, which are designed specifically to see the future.
We ran two simulations with the same climate model. In one, we assumed no volcanoes erupted, while in the other we manually added 60,000 Olympic pools of water vapor to the stratosphere. Next, we compared the two simulations, knowing that any difference must be due to added water vapor.
NASA
What did we discover?
The large ozone hole from August to December 2023 was at least partly due to Hunga Tonga. Our simulations predicted that ozone hole almost two years ago.
Notably, this was the only year we would expect any impact of the volcanic eruption on the ozone hole. By then, water vapor has had enough time to reach the polar stratosphere above Antarctica, and in later years there will not be enough water vapor to expand the ozone hole.
As the ozone hole lasted until the end of December, it brought with it a positive phase of the southern annular mode during the summer of 2024. For Australia this meant a higher chance of a wet summer, which was the exact opposite of that most people expected with El Niño declared. Again, our model predicted this two years ahead.
In terms of global average temperatures, which are a measure of how much climate change we are experiencing, Hunga Tonga’s impact is very small, only about 0.015 degrees Celsius. (This was independently confirmed by another study.) This means that the extremely high temperatures we have been measuring for about a year now cannot be attributed to the Hunga Tonga eruption.
Disruption for the rest of the decade
But there are some surprising and lasting impacts in some regions of the planet.
For the northern half of Australia, our model predicts colder and wetter-than-normal winters through 2029. For North America, it predicts warmer-than-normal winters, while for Scandinavia, it again predicts colder-than-normal winters .
The volcano appears to change the way certain waves travel through the atmosphere. And atmospheric waves are responsible for highs and lows, which directly affect our weather.
It is important to clarify here that this is only one study and one particular way of investigating what impact the Hunga Tonga eruption may have on our weather and climate. Like any other climate model, ours is not perfect.
We also did not include any other effects such as the El Niño–La Niña cycle. But we hope our study will spur scientific interest to try and understand what such a large amount of water vapor in the stratosphere might mean for our climate.
Whether it confirms or contradicts our findings remains to be seen – we welcome either result.