As the world swelters through record temperatures, scientists say an unusual culprit may be partly to blame: an underwater volcanic eruption off Tonga in the South Pacific last year.
While most big blasts cool the planet with a sun-dimming haze, the eruption of Hunga Tonga-Hunga Ha’apai in January 2022 blew the equivalent of 60,000 Olympic swimming pools of water into the stratosphere, high above the planet.
Water vapour is a natural greenhouse gas, trapping heat as it swirls around the globe. By contrast, major land eruptions — such as Pinatubo in the Philippines in 1991 — temporarily dim sunshine with an ashen sunshade before falling back to Earth.
“The majority of volcanoes will have a cooling effect,” said Peter Thorne, a professor of climate science at Maynooth University in Ireland. The Tongan volcano “is an exception to the rule and a significant wild card we haven’t seen before”.
The June-August period this year was the warmest on record worldwide by a puzzlingly wide margin, with heatwaves occurring from Japan to the US.
Many scientists say more research into volcanoes is vital to gauge how far eruptions can briefly affect the long-term trend of global warming.
The eruption in the Polynesian archipelago ejected 150mn-odd tonnes of water vapour into the stratosphere, about 10% of the 1.4bn tonnes typically swirling there, said Margot Clyne, an atmospheric scientist at the University of Colorado, Boulder, in the US.
“We can say with pretty good confidence that a volcanic eruption like this didn’t happen all the way back... to the 1880s, when Krakatoa erupted in 1883,” she said.
The eruption also blew about 500,000 tonnes of sulphur dioxide into the stratosphere, which tends to cool the planet. That mix of water and sulphur complicate the volcano’s impact.
A study in the journal Nature in January said the eruption slightly increased the risk that global temperatures would temporarily breach 1.5C in at least one of the next five years.
Holger Voemel, a senior scientist at the US National Center for Atmospheric Research (NCAR), said it was possible that the eruption would have some effect on global warming.
“But I think the verdict is still out,” Voemel said.
Before it erupted, the Hunga Tonga-Hunga Ha’apai was about 150m below sea level. It is unclear how many volcanoes are in water shallow enough to blow material into the atmosphere if they erupt.
The IPCC says at least one Pinatubo-style eruption is likely this century but that volcanoes have had a negligible effect on the overall trend of global warming driven by human greenhouse gas emissions since the Industrial Revolution.
“Volcanic activity is irregular, unpredictable and uncontrollable,” said Ingo Bethke, of the Bjerknes Centre for Climate Research at the University of Bergen in Norway.
Bethke and Thorne argue that the IPCC should do more to examine the risks of a string of eruptions.
“We can deal with one Pinatubo, but several would be a major stress test for society on top of climate change,” Thorne said.
Amid the unpredictability, however, some scientists say climate change might make eruptions more frequent in certain icy areas where the weight of thick glaciers keeps a lid on some volcanoes. A thaw could unleash eruptions.
In Iceland, for instance, the ending of the last Ice Age about 12,000 years ago coincided with eruption rates that were about 100 times higher than recent times.
And downpours linked to climate change could erode the sides of volcanoes. In Hawaii in 2018, unusually heavy rain may have weakened the flanks of Kilauea volcano.
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