The Honga Tonga eruption put more than 50 billion kilograms of water into the stratosphere

Zoom / The Hongja Tonga eruption began underwater, yet it continued to erupt straight through much of the atmosphere.

In January this year, an undersea volcano in Tonga caused a massive eruption, the largest so far this century. Mixing hot volcanic material with cold ocean water triggered an explosion that sent an atmospheric shockwave across the planet and triggered a tsunami that devastated local communities and reached Japan. The only part of the crater rim extending above the water has been reduced in size and separated into two islands. A plume of material was blasted directly through the stratosphere and into the middle atmosphere, more than 50 km above the Earth’s surface.

We’ve taken a closer look at a number of past volcanic eruptions and studied how they affected the climate. But those eruptions (most notably the eruption of Mount Pinatubo) all came from volcanoes on Earth. Honga Tonga’s eruption may be the largest underwater eruption ever documented, and the eruption plume contained unusual amounts of water vapor – so much so that it was actually in the way of satellite observations at some wavelengths. Now, researchers have used weather balloon data to reconstruct the plume and follow its progress through two circles around the world.

boom meets balloon

Words of the day are your vocabulary Wireless probe, which is a small package and transmitter that can be carried into the atmosphere by a weather balloon. There are networks of sites where radiosondes are launched as part of weather forecasting services; The most closely related to the Hunga Tonga are found in Fiji and eastern Australia. An airship from Fiji was the first to use tools in the eruption plume, and did so less than 24 hours after the Honga Tonga erupted.

This radio probe saw increasing levels of water as it climbed through the stratosphere from an altitude of 19 to 28 kilometers. Water levels reached the highest level ever measured at the top of this range when the balloon burst, ending the measurements. But soon after, a plume began to appear along the east coast of Australia, which again recorded very high levels of water vapor. Again, the height of the water reached 28 km, but gradually settled at lower altitudes over the next 24 hours.

The amazing thing is how many there were. Compared to normal background levels of stratospheric water vapor, these radiosondes were recording 580 times as much water even two days after the eruption, after the plume had stretched for some time.

There were a lot of things still notable as the shaft drifted over South America. Researchers were able to track it for six weeks, following it as it spread out as it orbited the Earth twice. Using some of these readings, the researchers estimated the total volume of the water vapor column and then used the existing water levels to come up with the total amount of water that was put into the stratosphere by the explosion.

They came with 50 billion kilograms. This is a low estimate, because, as mentioned earlier, there was still water above the altitudes where some of the measurements left off.

Not like the others

Eruptions like the one at Mount Pinatubo put much of the reflective sulfur dioxide mist into the stratosphere, reflecting sunlight back into space. This had the net effect of cooling surface temperatures over the years immediately following the eruption, although the material gradually recedes through the atmosphere, causing the effect to fade over several years. At least in its immediate aftermath, Hunga Tonga does not appear to have made a similar effect.

Instead, the water vapor was acting as a greenhouse gas, as you’d expect. This means that the lower region of the eruption plume absorbs energy, leaving the upper parts about 2 K cooler.

The researchers suspect that the huge amount of water in the correct eruption prevented much of the sulfur dioxide from reaching the stratosphere. And items that have reached height may have been washed out faster. The researchers also suspect that changes in stratospheric chemistry may affect the amount of ozone there, but that it may take long-term observation to resolve.

Overall, the conclusion seems to be that it makes a really big difference when an underwater eruption occurs. Volcanic eruptions like Hunga Tonga would be rare compared to terrestrial eruptions, because the eruption would have to occur in relatively shallow water in order to blast material all the way into the stratosphere. But when they do occur, it seems that everything from the chemistry of the atmosphere to the effects of the climate is likely to be different.

Sciences2022. DOI: 10.1126 / Science. abq2299 (About DOIs).