Hunga Tonga eruption sent more than 50 billion kilograms of water into the stratosphere

Enlarge / Hunga Tonga eruption started underwater, but still went through much of the atmosphere. Nasa

In January this year, an underwater volcano in Tonga produced a massive eruption, the largest this century. The mixture of hot volcanic material and cold ocean water created an explosion that sent an atmospheric shock wave across the planet and triggered a tsunami that devastated local communities and reached Japan. The only part of the crater rim that extended above the water was reduced in size and separated into two islands. A plume of material was propelled directly through the stratosphere and into the mesosphere, more than 50 km above the Earth's surface.

We have carefully examined a number of past volcanic eruptions and studied their influence on the climate. But these eruptions (notably that of Mount Pinatubo) all came from terrestrial volcanoes. Hunga Tonga is possibly the largest eruption we have ever documented that took place underwater, and the eruption plume contained unusual amounts of water vapor, so much so that it actually interfered with satellite observations at certain wavelengths. Now, researchers have used weather balloon data to reconstruct the plume and track its progress on two circuits around the globe.

The boom meets the ball

Your vocabulary word for the day is radiosonde, which is a small set of instruments and a transmitter that can be carried through the atmosphere by a weather balloon. There are networks of sites where radiosondes are launched as part of weather forecasting services; those most relevant to Hunga Tonga are in Fiji and Eastern Australia. A balloon from Fiji was the first to carry instruments into the eruption plume, less than 24 hours after the Hunga Tonga explosion.

This radiosonde saw rising water levels as it rose through the stratosphere from 19 to 28 kilometers above sea level. Water levels had reached the highest ever measured at the top of this beach when the balloon burst, ending measurements. But soon after, the plume began to appear along the east coast of Australia, which again recorded very high levels of water vapour. Again the water rose to 28 km high but gradually leveled off at lower heights over the next 24 hours.

What is striking is how many there were. Compared to normal background levels of stratospheric water vapour, these radiosondes recorded 580 times more water even two days after the eruption, after the plume had had time to spread out.

There were so many 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, tracking it as it spread while circling the Earth twice. Using some of these readings, the researchers estimated the total volume of the water vapor plume, then used the water levels present to arrive at a total amount of water introduced into the stratosphere by the eruption. .

They found 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 send 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 the material gradually fell back into the atmosphere, causing the impact to fade over several years. At least in its immediate aftermath, Hunga Tonga does not appear to have produced a similar effect.

Instead, the water vapor acted as a greenhouse gas, as you might expect. This meant that the energy was absorbed by the lower region of the eruption plume, leaving the upper parts colder by about 2 Kelvin.

Researchers suspect that the huge amount of water in the eruption itself prevented much of the sulfur dioxide from reaching the stratosphere. And the materials that reached the altitude were probably washed out faster. The researchers also suspect that changes in stratospheric chemistry may influence the amount of ozone present, but this may require longer-term monitoring to resolve.