The eruption of the Hunga Tonga-Hunga Haʻapai volcano in the island nation of Tonga on January 15 was the largest in decades. Photo / Japan Meteorology Agency via AP
The eruption of the Hunga Tonga-Hunga Haʻapai volcano in the island nation of Tonga on January 15 was the largest in decades. Photo / Japan Meteorology Agency via AP
The eruption of an underwater volcano in the Pacific Ocean in January that was so large it produced a global shock wave also spewed huge amounts of water vapour into the upper atmosphere, where it may cause a small, short-term spike in global warming, scientists say.
The injection of whatthe researchers estimated was at least 55 million tons of water vapour into the stratosphere may also temporarily cause more depletion of the protective ozone layer in the atmosphere, the scientists said on Thursday (Friday NZT).
The eruption of the Hunga Tonga-Hunga Haʻapai volcano in the island nation of Tonga on January 15 this year was the largest in decades. It spawned a tsunami that devastated parts of Tonga, as well as smaller tsunamis thousands of kilometres distant that were caused by changes in air pressure as the shock wave circled the world.
The eruption spawned a tsunami that devastated parts of Tonga. Photo / Dr Faka Taumoefolau, Twitter
Because it occurred about 500 feet (152 metres) underwater, the eruption of superheated molten rock also caused seawater to flash explosively into steam. A plume of water vapour, volcanic gases and ash reached an altitude of 56 kilometres. That increased the amount of water vapour in the stratosphere, which ends at an altitude of 50km, by at least 5 per cent.
"It's absolutely unique," said Holger Vömel, a senior scientist at the National Center for Atmospheric Research in Boulder, Colorado. "This has not happened since we've been capable of measuring stratospheric water vapour, which started something like 70 years ago."
Vömel is the lead author of a paper on the findings published in the journal Science.
Like carbon dioxide and other greenhouse gases, water vapour absorbs heat in the form of infrared radiation from the Earth's surface and re-emits it. So, adding a large amount of water vapour would be expected to add to warming for several years until the gas dissipated.
The underwater eruption sent a plume of water vapour, volcanic gases and ash 56 kilometres into the air. Photo / Tongan Navy
Large eruptions of land-based volcanoes don't release much water vapour, but they can inject huge amounts of sulphur dioxide gas into the stratosphere, which can have a short-term cooling effect. After the latest such eruption, of Mount Pinatubo in the Philippines in 1991, global average temperatures dropped by 1 degree Fahrenheit, or 0.6 degrees Celsius, for more than a year.
Vömel said any estimate of the amount of additional warming that the Tonga eruption will add was highly speculative at this point. "But I wouldn't be surprised if it was the same order of magnitude" as Pinatubo, he said, just in the opposite direction. The extra warming would likely continue for longer than the cooling after Pinatubo, he added.
Susan Solomon, an atmospheric scientist at the Massachusetts Institute of Technology who described the temperature impacts of changes in stratospheric water vapour in a 2010 study, said the Tonga eruption "could add something on the order of 0.05 degrees of warming to global average temperatures", probably for three to five years.
"That's less than what we expect from carbon dioxide, which is closer to 0.1 to 0.2 degrees per decade," she said. Solomon was not involved in the Tonga research.
All that water vapour will very likely also alter the atmospheric chemistry that destroys ozone, the oxygen molecule that protects life on Earth from harmful ultraviolet radiation from the Sun.
'Uiha Village before Hunga Tonga-Hunga Haʻapai's eruption, left, and after, right. Photos / Supplied
"By increasing the amount of water vapour drastically, that should decrease the amount of ozone," Vömel said. But that would be temporary, he said, because ozone formation and destruction "is a cycle that keeps going".
Solomon said that any loss of ozone near the boundary of the stratosphere and the lower atmosphere would also quite likely lead to some surface cooling, which would counteract the warming from the added water vapour.
A study published in July estimated the amount of water vapour injected by the Tonga eruption to be about three times higher, at about 160 million tons.
That study used data from a Nasa satellite, which provides water vapour measurements globally on a daily basis. Vömel and his colleagues took a different approach, using data from instruments in small packages called radiosondes that are carried aloft by balloons. Radiosondes are launched on a regular timetable, usually every 12 hours, at weather stations around the world.
This approach was only possible because there were regular balloon launches from Australia, Fiji and other locations that were close enough to the eruption that the instruments were carried into the volcanic plume. It also helped that the water vapour concentrations in the plume were extremely high.
"Any self-respecting scientist who knows stratospheric water vapour knows you cannot measure it with radiosondes," Vömel said. "Don't even think about it. However, this event was so unbelievably huge."
He said his team's estimate of 55 million tons was conservative, and their calculations showed the amount could be double that. While that is still lower than the earlier study, he said the difference in impact "probably would not be that huge".
"This is just scientific discourse," he said of the difference between the two studies' estimates. "At some point we'll get a better understanding, once all the dust has settled."
