No climate upside in blast
Tonga eruption may affect temperatures regionally, but make no difference globally
I t produced an ash cloud spanning hundreds of square kilometres — but scientists say it is unlikely Hunga Tonga-Hunga Ha'apai’s big blow will put any dent in global warming.
The Tongan volcano’s Saturday eruption was so large it could be clearly observed from space, and created an enormous umbrella of ash that stretched to about 260km.
In our planet’s past, major eruptions have been able to cool the climate by spewing massive amounts of dust and sulphate aerosols into the atmosphere.
While larger ash particles fell from the air quickly, smaller ones rose to form a dark cloud in the troposphere — and the very smallest entered the stratosphere, where they could block sunlight and cause cooling over large areas of the Earth.
When Indonesia’s Mt Tambora erupted in 1815, belching an estimated 100 cubic km of ash into this atmosphere, the average global temperature fell by as much as 3C in 1816, influencing the famous “year without a summer” in North America and Europe.
A 2020 study found six major eruptions — including Krakatoa in 1883, Tarawera in 1886 and Mt Pinatubo in 1991 — had their own fleeting footprint on New Zealand’s climate over the last 150 years.
These events had led to mean temperatures about 0.3C to 0.5C lower compared with several seasons before — and often an increase in southwesterly winds.
Climate scientist Professor Jim Salinger, who co-authored the study, said Hunga Tonga-Hunga Ha'apai’s eruption was big enough to send 0.4 teragrams — equivalent to 0.4 megatons — of sulphur dioxide (SO2) into the stratosphere.
That likely wouldn’t have any global climate effect, he said, but it could have a regional one.
“What we’ll probably see in the next two months is rather magnificent sunsets as the sulfuric acid mist slowly descends from the strato
It’s certainly a very significant effect locally, but it’s not likely to have an effect on the global climate. Professor James Renwick
sphere, but I’d also expect cooling in our region to amount to a few tenths of a degree, maximum.”
Salinger said any climate effect would likely take months to play out, given it took time for these tiny particles to disperse over the southern hemisphere.
Victoria University climate scientist Professor James Renwick agreed.
“It’s certainly a very significant effect locally, but it’s not likely to have an effect on the global climate,” he said.
“Sure, it’s cutting a bit of sunlight reaching that particular part of the tropics, and what’s in the aerosol cloud will spread out in the stratosphere — but that’s not going to lead to significantly cooler temperatures.”
By contrast, Mt Pinatubo’s eruption — the second-largest terrestrial eruption of the 20th century after the 1912 eruption of Novarupta in Alaska — lowered global temperatures by 0.5C.
That owed to the sheer volume of SO2 it ejected into the stratosphere: about 15 million tonnes, and more than any eruption since Krakatoa.
In a media statement, Niwa also said the world's highest concentration of SO2 is currently over the Pacific.
Some of the SO2 had been wrapped into Cyclone Cody, east of New Zealand.
“While large and locally devastating for Tonga, the eruption looks unlikely to change the direction of the global temperature trend,” the statement said.
“SO2 is a toxic, invisible pollutant, so for those in Tonga, this could cause short-term hazards to human health, including acid rain and ‘ volcanic smog’.”