ANALYSIS:
A gigantic volcanic eruption belches the equivalent of 58,000 Olympic swimming pools of water into the atmosphere, then New Zealand records its wettest year and summer on record.
It seems a simple enough explanation, and one that some on social media have reached for when attempting to downplay the reality of climate change.
Only, scientists remind us, what influences our climate patterns – and causes extreme rainfall events like those that’ve swamped the northeast this year – is never so straightforward.
Rather, the dramatic impacts we’ve all seen this summer likely owe much more to what’s been happening in and on the tropical waters well to the north of New Zealand – and in our planet’s warming atmosphere – than any one-off effects of last year’s big blow at Hunga Tonga-Hunga Ha’apai.
It’s worth noting that, in the Earth’s past, large volcanic eruptions have indeed influenced major climate shifts.
“Generally, they result in cooling, from the sunlight-blocking nature of the sulphate clouds they emit into the stratosphere,” Victoria University climate scientist Professor James Renwick explained.
“These effects are well modelled by global climate models, and it can be a big deal, for a year or two.”
Over New Zealand’s last 150 years, several huge eruptions had left their own fleeting footprints.
One 2020 analysis, led by climate scientist Professor Jim Salinger and co-authored by Renwick, showed how local temperatures slightly dipped after six big blows, including Krakatoa in 1883, Tarawera in 1886 and Mt Pinatubo in 1991.
“Pinatubo was able to block substantial amounts of solar radiation from reaching Earth’s surface, resulting in a heating of the stratosphere by 3.5C, and a cooling of the troposphere of the northern hemisphere by 0.2 to 0.7C,” Salinger said.
“This was owing to massive amounts of sulphur dioxide injected into the stratosphere, hydrolysing into sulphuric acid mist in the stratosphere, and blocking out some sunlight.”
These historic episodes happened to accompany mean temperature drops of about 0.3C to 0.5C, compared with previous seasons, along with more southwesterlies over New Zealand.
Niwa climate scientist Dr Olaf Morgenstern noted the 1783 eruption of Iceland’s Laki volcano was powerful enough to cause both a massive heatwave during summer, and an extremely cold winter afterwards, leading to a spike in mortality in England.
When Indonesia’s Mt Tambora erupted in 1815, it was followed by the Northern Hemisphere’s famous “year without a summer”, along with widespread crop failures and famines.
“So, volcanoes are important,” Morgenstern said, “but only ever perturb climate for limited periods of time”.
In today’s world, any climate-forcing effect of volcanism was generally considered negligible.
In 2021, the UN’s Intergovernmental Panel on Climate Change reported that the best estimate for contributions of “natural forcings” - such as volcanic or solar influences – to global warming was zero degrees, or effectively no contribution.
The 2020 study observed that, while eruptions had sometimes caused short-term effects, most of New Zealand’s coolest years had all played out before 1933.
Since then, of course, we’ve observed the mounting impact of human-driven global warming.
Our country’s average temperature has risen by 0.10C per decade since 1909, but that rate has quickened to 0.31C per decade in the past 30 years, with the last decade bringing seven of the eight warmest years on Niwa’s books.
While Hunga Tonga’s eruption was one of the most violent in modern times, interestingly, climate scientists were suggesting its net effect mightn’t be global cooling, but more warming.
Renwick noted it had sent unusually vast amounts of water vapour into our stratosphere – as much as 100 teragrams (Tg), or 100 million tonnes, by current estimates - against a background of 1500 Tg that’s usually there.
“Estimates from JPL and Nasa say it was about a 10 per cent increase in the total amount of water vapour in the stratosphere,” Renwick said.
“Most of it went into the stratosphere, where it can affect the atmospheric circulation and the climate for a few years.”
What water vapour that was released into the troposphere, the lowest layer of the atmosphere, would’ve been rained out within weeks of the eruption, he said.
“So, the short-term effect would have been increased rainfall under the tropospheric water vapour plume, early last year.”
Yet, there were some important medium-term effects to consider.
One was that potential contribution to global warming.
Water vapour was, after all, a greenhouse gas.
Renwick expected the sheer volume of H2O ejected had far outweighed the usual volcanic cooling effect of sulphate, meaning a short-term contribution to global heating was the likeliest outcome.
“The other medium-term effect is that, while the H2O is acting to warm the surface of the earth, it’s also cooling the stratosphere.”
This in turn encouraged the positive phase of what’s called the Southern Annular Mode (SAM), a ring of climate variability that encircles the South Pole, but stretches far out to the latitudes of New Zealand.
In its positive phase, the SAM was associated with relatively light winds and more settled weather over New Zealand latitudes, together with enhanced westerly winds over the southern oceans.
“But,” Renwick added, “there’s a tendency in summer in the positive SAM to see increased rainfall in the east and north of the North Island, as subtropical storms can penetrate more easily down the east coast”.
So, one might say that the eruption put loads of extra vapour into the atmosphere, which influenced the SAM, or caused more warming, which in turn directly caused all of these East Coast storm events?
Not so fast: we also had to consider those much more obvious contributors to our stormy summer, and two in particular.
One was La Niña, the ocean-driven system that’s been meddling with our weather and climate since the start of the decade, and which has helped pile up warmer water in the tropical Southwest Pacific – a potent source region for big rain-makers.
As Salinger explained: “La Niña causes more summer easterlies and northeasterlies with lower pressures over the north and northeast of New Zealand, and higher pressures to the south.”
That effect’s been dramatically shown in local climate conditions across the country, with parts of the South Island entering meteorological drought, while soils in the north sit water-logged to the point of saturation.
Coupled with local marine heatwave conditions, La Niña’s remarkable three-year run – now finally coming to an end – has coincided with New Zealand’s two latest warmest years on record (2022, 2021); three back-to-back record warm winters (2020, 2021, 2022); and our wettest year (2022).
The other big factor was background climate change itself.
“Global warming puts more water vapour into the air, as evaporation works better in warmer conditions, and the water-holding capacity of the air increases exponentially with warming,” Renwick explained.
As per one oft-cited measure, one degree of temperature rise was equivalent, on average, to a seven per cent increase in the amount of water vapour in the atmosphere.
Warming sea temperatures – as have been observed across the Southwest Pacific and Tasman Sea over the past seven decades – also translated to more water vapour.
“So, a warmer climate is associated with heavier rainfalls, more often,” Renwick said.
“And water vapour is fuel to thunderstorms, because as water vapour condenses into liquid water – or cloud droplets and rain - that releases heat into the air, making it more buoyant and more able to rise, making the storm cloud more violent.”
Well before the Tonga eruption, climate scientists were teasing out the precise contribution of climate change on local extreme weather events, finding it’d made Canterbury’s flooding in 2021 perhaps 10 to 15 per cent more intense - with a similar-sized influence also observed in that year’s Westport disaster.
Following Auckland’s historic anniversary weekend floods, Niwa climate scientist Dr Sam Dean suggested the likely difference similarly would’ve been around 10 to 20 per cent.
That sounded a reasonable estimate to Renwick.
“For one degree of warming, the increase is around 20 per cent over much of the North Island, for a one-hour rainfall extreme,” he said.
“For a six-hour extreme it’s more like 10 to 15 per cent, while for 24-hour totals, it’s more like 5 per cent - close to the average increase in atmospheric water vapour.
“We’ll need to wait and see what the climate modelling tells us about these events, but a 20 per cent increase for the short-duration thunderstorm events is plausible.”
That weather event also owed to a freakish alignment of drivers, right down to tightly-squeezed, localised thunderstorms and a low-lying jet of air that helped ferry rain and wind right into the city.
In the case of Gabrielle, the ex-tropical cyclone system’s most devastating impacts came as it slowed and intensified right over the northeast of the North Island, largely because it was merging with a nearby piece of “spin” in the upper atmosphere.
It’s a virtual certainty that climate change would’ve contributed more moisture to both systems.
As for La Niña, Renwick noted this tended to boost the strength of trade winds, while restricting tropical cyclones to the western part of the tropical south Pacific.
“That doesn’t necessarily mean more ex-tropical cyclones heading to New Zealand, but when we have a positive SAM, and a La Niña, there is evidence that we get more of these kinds of storms affecting New Zealand.”
Just where the effects of the Hunga-Tonga eruption fitted in amid this very complex picture wasn’t yet clear, but we’d learn more as international studies were published.
To Renwick, the key questions were how long its extra water vapour was likely to linger in the stratosphere; its overall effect on warming, versus sulphate-related cooling; and how that all interacted with natural variability in the atmosphere.
Morgenstern expected the water vapour to remain elevated for between three and five years.
“This is the time it takes for stratospheric air to return to the troposphere: I don’t think this will have long-term effects beyond this timescale.”
Niwa meteorologist Ben Noll listed off many more contributors in the mix.
“What I would say, is that this summer, we had a perfect storm of climate drivers that are much more readily attributable to extreme rainfall,” he said.
“We had a triple dip La Niña; a very warm local and regional seas across the board; and a precursor condition of what’s called the negative Indian Ocean Dipole, which primed the source region for more moisture-laden atmospheric rivers,” Noll said.
“And we’ve also had a positive SAM, which has kept those highs sitting near the South Island and acting as an ‘atmospheric stop sign’ blocking those pesky lows but allowing them to linger over the North Island.
“If the eruption was causing everything, we’d be seeing abundant rain everywhere in the hemisphere, not just the North Island, right?”
The decision was influenced by Hawke’s Bay hydrologists, who opposed the warning.