In January this year, an underwater volcano in Tonga produced a massive eruption, the largest so far this century. The mixture of hot volcanic material and cold ocean water created an explosion that sent an atmospheric shock wave around the planet and triggered a tsunami that devastated local communities and reached as far as Japan. The only part of the crater rim that protruded from the water shrank in size and separated into two islands. A plume of material was launched straight through the stratosphere and into the mesosphere, more than 50 km above the Earth’s surface.
We have carefully analyzed several past volcanic eruptions and studied how they influence the climate. But all those eruptions (particularly the one on Mount Pinatubo) came from volcanoes on land. Hunga Tonga may be the largest eruption we’ve ever documented to have taken place underwater, and the eruption column contained unusual amounts of water vapor, so much so that it actually interfered with satellite observations at some wavelengths. Now, researchers have used data from weather balloons to reconstruct the plume and track its progress during two circuits around the world.
The boom meets the balloon
Your vocabulary word of the day is radiosonde, which is a small package of instruments and a transmitter that a weather balloon can carry into the atmosphere. 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 carry instruments to the eruption column, less than 24 hours after Hunga Tonga exploded.
That radiosonde saw increasing levels of water as it rose through the stratosphere from 19 to 28 kilometers in altitude. Water levels had reached the highest level ever measured at the top of that range when the balloon burst ended the measurements. But soon after, the plume began to appear along Australia’s east coast, which again recorded very high levels of water vapour. Once again, the water reached 28 km in altitude, but gradually settled to lower heights over the next 24 hours.
What was surprising was how many there were. Compared to normal background levels of stratospheric water vapor, these radiosondes recorded 580 times more water even two days after the eruption, after the plume had time to spread.
There was so much there that it still stood out when the plume of smoke drifted over South America. The researchers were able to track it for a total of six weeks, following it as it spread as it circled the Earth twice. Using some of these readings, the researchers estimated the total volume of the water vapor column and then used the water levels present to arrive at the total amount of water put into the stratosphere by the eruption.
They reached 50 billion kilograms. And that’s a low estimate, because, as mentioned above, there was still water above the altitudes where some of the measurements stopped.
not like the others
Eruptions like Mount Pinatubo spew many reflective aerosols of sulfur dioxide into the stratosphere, and these reflect sunlight back into space. This had the net effect of cooling surface temperatures for years immediately following the eruption, although the material gradually receded through the atmosphere, causing the impact to fade over several years. At least immediately afterward, Hunga Tonga does not appear to have produced a similar effect.
Instead, the water vapor acted as a greenhouse gas, as expected. This meant that energy was absorbed by the lower region of the eruption column, leaving the upper parts cooler by about 2 Kelvin.
The researchers suspect that the enormous amount of water in the eruption prevented much of the sulfur dioxide from reaching the stratosphere. And the material that reached the height was probably washed faster. The researchers also suspect that changes in stratospheric chemistry may influence the amount of ozone present there, but that may require longer-term monitoring to resolve.
Overall, the bottom line seems to be that it really does make a big difference when an underwater eruption occurs. Eruptions like Hunga Tonga are going to be rare compared to terrestrial eruptions, because the eruption has to take place in relatively shallow water to blow material up into the stratosphere. But when they do occur, it seems that everything from atmospheric chemistry to climate impacts is likely to be different.
Sciences2022. DOI: 10.1126/science.abq2299 (About DOIs).