In January this year, an underwater volcano in Tonga caused a massive eruption, the largest so far this century. The mixing of hot volcanic material and cool ocean water created an explosion that sent an atmospheric shockwave across the planet and triggered a tsunami that devastated local communities and swept as far as Japan. The only part of the crater rim that extended above water was reduced and separated into two islands. A cloud of material was ejected straight through the stratosphere and into the mesosphere over 50 km above the Earth’s surface.
We looked at some past volcanic eruptions and how they affect climate. But these eruptions (Mount Pinatubo in particular) all came from volcanoes on land. Hunga Tonga is possibly the largest underwater eruption we’ve ever documented, and the plume contained unusual amounts of water vapor — so much that it actually got in the way of satellite observations at some wavelengths. Now researchers have used weather balloon data to reconstruct the cloud and track its progress during two laps around the globe.
Boom meets balloon
Your vocabulary 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 became the first to bring instruments into the plume less than 24 hours after Hunga Tonga blew up.
This radiosonde saw rising water levels as it climbed through the stratosphere from 12 to 17 miles. The water level had reached the highest level recorded at the top of this area when the balloon burst and measurements stopped. But shortly thereafter, the cloud appeared along the east coast of Australia, where very high levels of water vapor were again recorded. Again the water reached a height of 17 miles (28 km) but gradually dropped to lower levels over the next 24 hours.
The striking thing was how much of it was there. Compared to normal background levels of stratospheric water vapor, these radiosondes detected 580 times as much water even two days after the eruption, after the cloud had had some time to spread.
There was so much there that it was still noticeable as the cloud drifted across South America. Researchers were able to track it for a total of six weeks as it spread as it orbited the Earth twice. Using some of these readings, the researchers estimated the total volume of the water vapor plume, and then used existing water levels to calculate the total amount of water that the eruption pushed into the stratosphere.
They came up with 50 billion kilograms. And that’s a low estimate because, as mentioned above, there was still water above the elevations where some of the measurements stopped.
Not like the others
Eruptions like Mount Pinatubo release many reflective sulfur dioxide aerosols into the stratosphere, and these reflect sunlight back into space. This had the net effect of cooling surface temperatures in the years immediately following the eruption, although material gradually fell back through the atmosphere, easing the effects over several years. At least immediately afterwards, Hunga Tonga does not seem to have produced a similar effect.
Instead, as expected, the water vapor acted as a greenhouse gas. This meant that energy was absorbed by the lower region of the plume, leaving the upper parts about 2 Kelvin cooler.
The researchers suspect that the huge amount of water in the actual eruption prevented much of the sulfur dioxide from ever reaching the stratosphere. And material that made it to the top was likely to be washed out faster. The researchers also suspect that changes in stratospheric chemistry could affect the amount of ozone present there, but that could require longer-term monitoring to resolve.
Overall, the conclusion seems to be that when an underwater eruption occurs, it really makes a big difference. Eruptions like Hunga Tonga will be rare compared to land-based eruptions because the eruption must occur in relatively shallow water to blast material up into the stratosphere. But when they do occur, everything from atmospheric chemistry to climate impacts seems different.