New analysis reveals New Zealand's 150-year climate transformation

Scientists have just pieced together 150 years of New Zealand climate history, revealing cool spells driven by volcanic eruptions - and the growing handprint of climate change. Photo / Leon Menzies

Scientists have just pieced together 150 years of New Zealand climate history, revealing cool spells driven by volcanic eruptions - and the growing handprint of climate change.

A team of researchers have compared land temperatures recorded around the country between 1871 and 2019 with surrounding ocean surface data, finding our wider region has warmed by about 0.66C over that time.

Much of the heat has come over recent years, on the back of global warming.

"While there were some very warm years in the early 1970s, all of the warmest years have occurred since 1998," said the study's lead author, Professor Jim Salinger.

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The new study offered a deeper picture than that of our official record to track national temperature trends - "seven station" series - comprised of land-based measurements that have been continuously taken since 1908.

Salinger - the seven-station series' original architect - said New Zealand had one of the best and oldest temperature records in the world, spanning back to the foundational work of Sir James Hector and colleagues in the 1860s.

"What we've done here is pushed the record back, while also bringing it up to date to 2019," he said.

"We wanted to cover the larger New Zealand area, not just land, because we are dealing with four million square kilometres that's home to our fisheries and other parts of our economy."

Source / Niwa

Drawing on a range of datasets, Salinger and colleagues took a sweeping look at what drove our coldest and warmest periods.

They found two big natural drivers that dominated trends were interdecadal Pacific oscillation (IPO) and El Nino-southern oscillation (ENSO).

Both indicated major cyclical changes in the Pacific ocean-atmosphere system, which in turn influenced our own climate.

When the IPO was in negative phases, for instance, New Zealand tended to get more easterlies and northeasterlies, and warmer temperatures.

ENSO - which see-sawed between states of El Nino, neutral, and La Nina - similarly brought a mix of effects.

While La Nina years spelt above-average temperatures with more northeasterlies, El Nino years typically proved cooler than average, because of more southwesterly winds.

Another natural influencer was something called the Southern Annular Mode (SAM) - a ring of climate variability that encircles the South Pole, but stretches far out to our own latitudes.

Some warm years had been put down to the SAM being locked in a mostly positive phase, which came with westerly winds farther south over the southern oceans but lighter winds and sunnier skies over New Zealand.

Salinger said it had been increasingly trending this way over recent times.

Fascinatingly, major eruptions had left their own fleeting footprint, by spewing massive amounts of dust and sulphate aerosols into the atmosphere.

The record showed how local temperatures slightly dipped after six big blows, including Krakatoa in 1883, Tarawera in 1886 and Mt Pinatubo in 1991.

Major eruptions like Tarawera in 1886 have been shown to cause temporary dips in New Zealand's average temperature. Image / Charles Blomfield

"We found how these can mean temperatures drop by about 0.3C to 0.5C less than they were for several seasons before - and there also seems to be more southwesterlies."

More striking, Salinger said, were the clear trends that have emerged with climate change.

The paper found our coolest years had all played out before 1933.

Between 1870 and 1895, temperatures were running at about 0.4C below the 1981-2010 average, then, in the early 1900s, dropped even further to 0.8C below that baseline.

Temperatures were also noticeably colder in the early 1930s, and then 0.4C below the average in the 1940s, and generally close to normal over the 1950s, 1970s and 1980s.

While they dropped again to around 0.5C below normal in early 1990s - partly because of the Pinatubo eruption - they increased rapidly to 0.1C above normal, before averaging 0.4C above normal throughout the 2010s.

When this trend came in tandem with natural drivers that had historically made for balmier conditions, New Zealand saw extremely warm years.

A case in point was our hottest year on record - 2016 - which co-incided with a pressure set-up that drove more northerlies and northwesterlies across the country.

But study co-author and prominent climate scientist Professor James Renwick, of Victoria University, pointed out other human-driven effects have also changed our climate.

Victoria University climate scientist Professor James Renwick. Photo / Mark Mitchell

"The fact that we've had a trend toward a positive southern annular mode - which is related to the ozone hole, mostly - has also added to warming over the past 50-odd years," he said.

"That effect is probably going to weaken because the ozone hole is starting to fill in, helping stop that trend in the SAM.

"New Zealand will keep getting warmer in future, but the rate of warming we'll see for the rest of the century might actually be a bit slower than we saw over the last 50 years."

Globally, emissions are expected to keep rising - meaning rising, acidifying oceans and more weather events like heavy storms, drought and heatwaves.

At the current rate, the global average temperature is likely to cross the threshold of 1.5C above the pre-industrial level within the next 10 to 20 years.

And a rise of 3C by the end of this century is projected, even if all the current emissions reduction commitments and goals are met by the international community.

Deep cuts to net emissions would be needed to hold warming to 1.5C - about 45 per cent below 2010 levels by 2030, and net zero by about 2050.

The study team - which also included Dr Howard Diamond, of the US National Oceanic and Atmospheric Administration - dedicated the paper to late Niwa climate scientist Dr Brett Mullan, whose decades of work has been instrumental.