Science

Hunga Tonga eruption put over 50B kilograms of water into the stratosphere

enlarge / The Hunga Tonga eruption started underwater, but still swept through much of the atmosphere.

In January this year, a submarine volcano in Tonga caused a massive eruption, the largest yet this century. The mixing of hot volcanic material and cool ocean water created an explosion that sent an atmospheric shock wave across the planet, triggering a tsunami that devastated local communities and even reached Japan. The only part of the crater rim that extended above water was reduced in size and divided into two islands. A plume of dust was blown across the stratosphere and into the mesosphere, more than 50 km above the Earth’s surface.

We’ve taken a closer look at some past volcanic eruptions and how they affect the climate. But those eruptions (especially those from Mount Pinatubo) all came from volcanoes on land. Hunga Tonga is arguably the largest eruption we’ve ever documented that took place underwater, and the eruption plume contained unusual amounts of water vapor — so much so that it got in the way of satellite observations at some wavelengths. Now, researchers have used data from weather balloons to reconstruct the plume and track its progress on two circuits around the world.

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Tree meets balloon

Your vocabulary word of the day is radiosonde, a small instrument 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 relevant to Hunga Tonga are in Fiji and Eastern Australia. A balloon from Fiji was the first to bring instruments into the eruption plume, less than 24 hours after the Hunga Tonga exploded.

That radiosonde saw increasing water levels as it climbed through the stratosphere from 19 to 28 kilometers high. Water levels were the highest yet measured at the top of that range when the balloon burst, ending the measurements. But soon after, the plume appeared along Australia’s eastern coast, again registering very high levels of water vapor. Again, the water reached an elevation of 28 km, but gradually subsided to lower elevations over the next 24 hours.

What was striking was how much of it there was. Compared to normal background levels of stratospheric water vapor, these radiosondes recorded 580 times as much water two days after the eruption, after the plume had some time to disperse.

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There was so much there that it still stood out as the plume floated over South America. The researchers were able to track it for a total of six weeks, as it spread out as it circled the Earth twice. Using some of these measurements, the researchers estimated the total volume of the water vapor plume and then used the water levels present to calculate a total amount of water released into the stratosphere by the eruption.

They came out at 50 billion kilograms. And that’s a low estimate because, as mentioned above, there was still water above the heights where some measurements stopped.

Not like the others

Eruptions like those at Mount Pinatubo release many reflective sulfur dioxide aerosols into the stratosphere, and these bounce sunlight back into space. This had the net effect of cooling surface temperatures in the years immediately following the eruption, although the material gradually receded through the atmosphere, fading the impact over several years. In any case, in its immediate aftermath, Hunga Tonga does not appear to have had a comparable effect.

Instead, the water vapor acted as a greenhouse gas, as you might expect. This meant that energy was absorbed by the lower part of the eruption plume, keeping the upper parts about 2 Kelvin cooler.

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The researchers suspect that the massive amount of water in the actual eruption prevented much of the sulfur dioxide from reaching the stratosphere. And material that did make it to the height is likely to have been washed away faster. The researchers also suspect that the changes in stratospheric chemistry may affect the amount of ozone present there, but that may take longer-term monitoring to resolve.

All in all, the conclusion seems to be that it really makes a big difference when an eruption occurs underwater. Eruptions like Hunga Tonga will be rare compared to eruptions on land, as the eruption must occur in relatively shallow water to blast material all the way into the stratosphere. But when they do occur, it seems that everything from atmospheric chemistry to climate effects is likely to be different.

Science2022. DOI: 10.1126/science.abq2299 (About DOIs).

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