Image of a circular explosion area surrounded by clouds.

Hunga Tonga eruption injects more than 50Bkg of water into the stratosphere

enlarge / The Hungatanga eruption started underwater but still passed directly through most of the atmosphere.

In January, an underwater volcano in Tonga erupted in the largest massive eruption so far this century. The mix of hot volcanic material and cool ocean water created an explosion that sent an atmospheric shock wave across Earth, setting off a tsunami that devastated local communities and traveled as far as Japan. The only part of the crater rim above the water shrunk in size and split into two islands. A plume of material travels directly through the stratosphere and into the mesosphere more than 50 kilometers from the Earth’s surface.

We took a closer look at some of the past volcanic eruptions and how they affected the climate. But these eruptions (most notably Mount Pinatubo) came from volcanoes on land. Hunga Tonga may have been the largest underwater eruption we’ve ever recorded, and the eruption plume contained an unusual amount of water vapor — which actually hampered satellite observations of certain wavelengths. Now, researchers have used weather balloon data to reconstruct the plume and track its progress on two global cycles.

prosperity meets balloons

Your word today is a radiosonde, which is a small instrument package and transmitter that can be carried into the atmosphere by a weather balloon. As part of the weather forecast service, there is a network of stations transmitting radiosondes; the most relevant to Hungatanga are Fiji and eastern Australia. A balloon from Fiji was the first to carry the instrument into the eruption plume, and did so less than 24 hours after it exploded in Hungatanga.

The radiosonde found increasing water levels as it climbed across the stratosphere from altitudes of 19 to 28 kilometers. When the balloon popped, the water level had reached its highest level at the top of the range, ending the measurement. But it wasn’t long before the plume began to appear off Australia’s east coast, again recording very high levels of water vapour. Likewise, the water reached an altitude of 28 km, but gradually dropped to a lower altitude over the next 24 hours.

What is striking is how much it has. Compared to the normal background level of stratospheric water vapor, even two days after the eruption, these radiosondes recorded 580 times the amount of water vapor in the stratosphere, after the plume had some time to spread out.

There’s a lot going on there, and the plume still stands out as it drifts across South America. The researchers were able to track it for a total of six laps, tracking it as it spread out as it circled the Earth twice. Using some of these readings, the researchers estimated the total volume of the water vapor plume and then used the amount of water present to calculate the total amount of water that erupted into the stratosphere.

They came up with 50 billion kilograms. This is a low estimate because, as mentioned above, there is still water above the height where some measurements stopped.

unlike others

Eruptions like Mount Pinatubo drop large amounts of reflective sulfur dioxide aerosols into the stratosphere, which reflect sunlight back into space. This had the net effect of cooling the surface temperature in the years following the eruption, although the material gradually fell back from the atmosphere, causing the effect to wear off within a few years. Hunga Tonga doesn’t seem to have a similar effect, at least in its immediate aftermath.

Instead, as you might expect, water vapor acts as a greenhouse gas. This means that energy is absorbed by the lower region of the eruption plume, lowering the upper temperature by about 2 Kelvin.

The researchers suspect that the large amount of water in the eruption prevented a large amount of sulfur dioxide from reaching the stratosphere. Material that does reach high altitudes may be washed away more quickly. The researchers also suspect that changes in stratospheric chemistry may affect the amount of ozone present there, but this may require longer-term monitoring to resolve.

Overall, the conclusion seems to be that eruptions do make a big difference when they occur underwater. Eruptions like Hunga Tonga will be rare compared to land-based eruptions, which must occur in relatively shallow water in order to eject material all the way to the stratosphere. But when they do occur, it seems likely that everything from atmospheric chemistry to climate impacts could be different.

science2022. DOI: 10.1126/science.abq2299 (on DOI).

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