In January this year, an undersea 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 shock wave across the planet and caused a tsunami that devastated local communities and reached as far as Japan. The only portion of the crater rim that extended above water shrank in size and split into two islands. A plume of material was shot straight through the stratosphere and into the mesosphere, more than 50 kilometers above the Earth’s surface. We’ve taken a good look at a number of past volcanic eruptions and studied how they affect the climate. But these eruptions (mainly that of Mount Pinatubo) all came from volcanoes on land. Hunga Tonga may be the largest underwater eruption we’ve ever documented, and the plume from the eruption contained unusual amounts of water vapor—so much of it that it actually blocked satellite observations at certain wavelengths. Now, researchers have used weather balloon data to reconstruct the plume and track its progress during two circuits around the world.

Boom meets balloon

Your vocabulary of the day is radiosonde, which is a small package of instruments and transmitter that can be carried in the atmosphere by a weather balloon. There are networks of sites where radio recordings are broadcast as part of weather forecasting services. The most relevant for Hunga Tonga are in Fiji and Eastern Australia. A balloon from Fiji was the first to instrument the eruption plume, doing so less than 24 hours after the Hunga Tonga eruption. This radiosonde saw rising water levels as it climbed through the stratosphere from 19 to 28 km altitude. Water levels had reached the highest yet measured at the top of this range when the balloon burst, ending the measurements. But shortly afterwards, the plume began to appear along the east coast of Australia, which again recorded very high levels of water vapour. Again, the water reached 28 km in elevation, but gradually settled to lower elevations over the next 24 hours. Advertising
The amazing thing was how much of it there was. Compared to normal background levels of stratospheric water vapor, these radio tubes record 580 times more water even two days after the explosion, after the plume has had enough time to spread. There was so much there that it still stood out as the plume drifted over South America. Researchers were able to track it for a total of six weeks, following it as it spread out while making two orbits around Earth. Using some of these measurements, the researchers calculated the total volume of the water vapor cloud and then used the water levels to come up with a total amount of water that entered the stratosphere from the eruption. They found 50 billion kilograms. And this is 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 that of Mount Pinatubo bring lots of reflective sulfur dioxide aerosols into the stratosphere and these reflect sunlight back into space. This had the net effect of cooling surface temperatures during the years immediately following the explosion, although the material gradually fell back into the atmosphere, causing the impact to fade for several years. At least in the immediate aftermath, Hunga Tonga doesn’t seem to have a similar effect. Instead, water vapor acted as a greenhouse gas, as one would expect. This meant that the energy was absorbed by the lower region of the explosion cloud, leaving the upper parts cooler by about 2 Kelvin. Researchers suspect that the sheer amount of water in the eruption greatly prevented the sulfur dioxide from ever reaching the stratosphere. And the material that reached the altitude was probably washed away faster. The researchers also suspect that changes in stratospheric chemistry may affect the amount of ozone there, but long-term monitoring may be needed to resolve. Overall, the bottom line seems to be that it really does make a big difference when an explosion takes place underwater. Eruptions like Hunga Tonga will be rare compared to land-based eruptions because the eruption must take place in relatively shallow water in order to eject material into the stratosphere. But when they do, it seems that everything from atmospheric chemistry to climate effects are likely to be distinct. Science, 2022. DOI: 10.1126/science.abq2299 (About DOIs).