In January of this year, a submarine volcano in Tonga produced the largest eruption this century has ever had. The mix of hot volcanic material and cold ocean water created an eruption that sent an atmospheric shock wave across the planet, decimating local communities and triggering a tsunami that reached as far as Japan. The only portion of the crater’s rim that lies above the water has been scaled down and divided into two islands. A cloud of material was ejected through the stratosphere and into the mesosphere, 50 km above the Earth’s surface.
We took a good look at several past volcanic eruptions and how they affected the climate. But these eruptions (especially those on Mount Pinatubo) came from volcanoes on land. Hunga Tonga may be the largest underwater eruption we’ve ever documented, and the eruption cloud contained unusual amounts of water vapor – so much that it actually outstripped satellite observations at some wavelengths. Now, the researchers have used weather balloon data to reconstruct the cloud and track its progress across two circuits around the world.
Boom meets balloon
Your word of the day radiosondeIt is a small instrument package and transmitter that can be transported into the atmosphere by a weather balloon. There are networks of sites where radiosondes are launched as part of weather forecasting services; The best suited for Hunga Tonga are in Fiji and eastern Australia. Less than 24 hours after the eruption of Hunga Tonga, a balloon from Fiji became the first balloon to take instruments into the eruption cloud.
This radiosonde saw rising water levels as it climbed from 19 kilometers to 28 kilometers in the stratosphere. When the balloon burst, the water levels reached the highest measured at the top of this range, ending measurements. Soon, however, plume began to appear along Australia’s east coast, where again very high water vapor levels were recorded. Again, the water reached a height of 28 km, but gradually settled to lower elevations over the next 24 hours.
What was striking was how many there were. Compared to normal background levels of stratospheric water vapor, these radiosondes were recording 580 times more water even two days after the eruption had spread.
There was so much there that the feathers still stood out as they drifted over South America. The researchers were able to track it for a total of six weeks as it spread out as it orbited the Earth twice. Using some of these readings, the researchers estimated the total volume of the water vapor cloud and then using current water levels to find the total amount of water put into the stratosphere by the eruption.
They came with 50 billion kilograms. And that’s an underestimate because, as mentioned above, there was still water above the altitudes at which some measurements stopped.
not like others
Eruptions like Mount Pinatubo put large numbers of reflective sulfur dioxide aerosols into the stratosphere, which reflect sunlight back into space. This had the net effect of cooling surface temperatures over the years immediately following the eruption, although this caused the material to gradually fall back into the atmosphere and the effect faded within a few years. At least not immediately after that, Hunga Tonga doesn’t seem to have had a similar effect.
Instead, water vapor was acting as a greenhouse gas, as you might expect. This meant that energy was absorbed by the lower region of the blast cloud, and the upper parts were about 2 Kelvin cooler.
The researchers suspect that the large amount of water in the eruption prevented too much sulfur dioxide from reaching the stratosphere. And the material that made it to the height probably washed out faster. The researchers also suspect that changes in stratospheric chemistry may affect the amount of ozone there, but solving this may require longer-term monitoring.
Overall, the result seems like it really makes a big difference when an explosion occurs underwater. Eruptions like the Hunga Tonga will be rarer than land-based eruptions, because the eruption must take place in relatively shallow water to blast material into the stratosphere. But when they do occur, everything from atmospheric chemistry to climate effects seems likely to be different.
Science2022. DOI: 10.1126/science.abq2299 (About DOIs).
