New Zealand's climate is warming at a quickening pace - making severe events like drought more frequent. Photo / George Heard
They called it the "bummer summer", with January 2017 its lowest ebb.
Sun-starved Wellingtonians grumbled about the fewest monthly "beach days" in three decades, sparking a petition to shift the season forward.
In the small township of Otira, near Arthur's Pass, a wintry storm fueled by a "bomb low" caused a slip that moved a shed, destroyed a car and diverted a creek into a house.
Amid persistent cold southwesterly winds, a string of storms and miserable lows - and even the odd snowfall - the month's nationwide average temperature finished up at a paltry 16.4C.
Niwa forecaster Ben Noll, who was holidaying in Matapouri, still recalls how oddly cool seas were around the country that month.
But there's a bigger reason why he remembers it so clearly.
It was the last time New Zealand recorded a month with an overall temperature short of the 30-year average - the hot tailwind of climate change has pushed that trend overwhelmingly in the other direction.
That's partly why Niwa recently decided to stop mentioning the running absence of below-average temperatures since January 2017 – a count soon to hit 60 months - in its regular summaries.
A warmed world has simply made it unlikely to see months that are colder, be it by previous or current long-term measures.
That was reflected by the dismal record New Zealand just set – a winter declared our hottest for the second year in a row.
There's no shortage of other grim statistics to tell the story.
Across 30 different locations, the annual average temperature has climbed at 28 of them over summer - and across all in winter - while six of the eight warmest years on Niwa's books have occurred since only 2013.
With more widespread heat about the country this month, it's near-certain 2021 will finish up among the top five hottest years.
In just 111 years, the average has risen by more than 1C. The starkest changes have come within the last three decades, when the pace of warming has tripled.
Those who've long lived north would have noticed an increase in sweltering days over 25C - and a fading of frost mornings.
Other signs are clearer still.
The late-May deluge that put swathes of Canterbury underwater, for instance, was just calculated as being 10 to 15 per cent more intense as a direct result of our meddling with the climate.
It was part of a slew of bouts of extreme rainfall that's delivered insurers one of its costliest years for disaster claims.
For meteorologists like Noll, who demystifies weather to the public every day, climate change has fast become the elephant in the forecasting room.
It wasn't long ago that they were reluctant to acknowledge its influence, and for good reason.
Climate is, after all, different from weather – and the effects that we've caused ourselves can be notoriously difficult to disentangle from the natural variability that's always been a part of the planet's climate system.
Noll singles out one particular indicator called the Southern Annular Mode, or SAM, which describes how a vast band of westerly wind moves either north toward the mid-latitudes where New Zealand is, or south toward Antarctica.
When these winds are pushing south – or in a "positive" phase – systems that bring colder, stormier weather are kept bottled up over the Southern Ocean.
Up above in New Zealand, meanwhile, conditions are likelier to be tranquil, with higher air pressure and relatively light winds.
"We've seen a trend toward positive values, particularly in the most recent decade, and have had a strong run of positive values over the last few years — including being positive 71 per cent of the time from January to November this year," Noll said.
"If you compare the average pressure pattern from 2010-19 with 2000-09, there has been a trend toward a more expansive sub-tropical high-pressure belt into the North Island over the last 10 years, which has been associated with several droughts."
While this had been consistent with climate change expectations, Noll stressed that a bigger sample size would be needed to come to any grand conclusion.
Positive SAM conditions have also been implicated in a series of marine heatwaves that have enveloped New Zealand – including one unfolding in our coastal waters right now – as have other effects linked to global warming.
These dramatic events - which are predicted to become longer, stronger and more frequent as our planet continues to heat - have been shown to fuel heatwaves on land, melt glaciers, shift harvests and trigger a cascade of damaging effects in marine ecosystems.
Ultimately, though, the hand of climate change was most explicit in the abnormal statistics Noll routinely pulled from the record book.
The tricky question he now faced was, what happens when "abnormal" becomes normal?
Climate scientists regularly refer to the "boiling frog" to illustrate the danger of becoming too easily adjusted to what should be alarming changes.
If a frog jumps into a pot of boiling-hot water, it immediately hops out.
If, however, the water in the pot is slowly warmed to a boiling temperature while the frog is in it, it doesn't hop out, and is eventually cooked.
"We've adapted to a new normal with Covid, and we've done the exact same thing with climate change, even though it's changing right in front of our eyes."
For that reason, Noll keeps the boiling frog metaphor in the back of his mind when he's discussing the weather with journalists – and the often - stark contrast of today's temperatures to the 30-year baseline Niwa uses in its climate reporting.
Yet that baseline is just about to shift.
When Niwa transitioned from the former 1971-2000 baseline a decade ago, the switch in warmer averages was striking enough.
How to separately recognise the unprecedented amount of recent change alongside the next reference baseline, spanning from 1991-2020, is something scientists are grappling with now.
Niwa's Dr Andrew Tait explained these baselines – known as "climate normals" - covered more than just temperature.
"There is the normal daytime temperature in summer, for example. Or the normal number of rainy days in January."
These normals, which are also ascribed to specific locations around the country, have been calculated in 30-year blocks since the mid-20th century.
"Keeping track of the climate normals over the decades is useful for determining whether our climate is changing, but fundamentally the normals are used for determining how 'abnormal' the climate of a recent month, season or year is when compared against the 30-year values," Tait said.
"Thus, they provide a baseline for comparison which helps to contextualise recent climate conditions.
"For example, we can say the total rain in Gisborne that fell in November 2021 was over four times as much as 'normal'."
To help construct normals, scientists draw on around 200 climate stations dotted around the country, all of which automatically gather weather data every day, and every hour.
That information is collected up in Niwa's national climate database, where the values were quality checked.
"If there has been a continuous stream of good quality data from a climate station over the climate normal period, then the calculation of the normals is really simple," Tait said.
"Where it gets complicated is when there are gaps in the data due to instrument failure, or when a decision is made to stop measurements at a climate station site all together."
Under these conditions, scientists had to follow strict international guidelines on how to build their baselines.
The basic approach was to fill in gaps with data from nearby climate station locations, but only after carefully comparing the data during any overlapping period from the two sites, and making adjustments if required.
"It would be incorrect to fill in any data gaps from another site which is consistently colder than the target site, for example," Tait said.
"So, adjustments must be made before data from other locations can be stitched together with the climate station data being used for the calculation of the climate normals."
This internationally recognised process is called "data homogenisation", and can involve an enormous amount of work, depending how wide the data gaps are.
"Sometimes, we have to accept that climate normals for a particular location can't be calculated if the source data is just not good enough."
Niwa was looking into newer methods for data homogenisation, particularly using data science methods, but Tait said Niwa still had to ensure it was meeting global standards.
"We had hoped to be introducing the 1991-2020 normals in early 2022, but we're running behind schedule due to a number of things," he said.
"It is still very high priority for us, so we hope to make the switch as soon as possible."
When it came to the risk of normalising climate change, Tait acknowledged the move to a 1991-2020 baseline could pose a "communication challenge".
That was why Niwa was looking to present a historical baseline, such as 1961-1990, alongside its next one, to illustrate the impact of climate change – an approach its US counterpart has already taken.
"The fascinating thing about this exercise is that we're warming at such a fast pace that we might find every month is above average, even with the new 1991-2020 normal," Noll said.
"We've seen that happen in the US where the baseline transition has happened, yet, because December has been such an incredibly warm month there, even the new normal has been blown out of the water.
"So, I think, certainly the way to overcome this communication challenge is to use a couple of different baselines just to tell how much warmer things have got."
General awareness of climate change is growing – and Horizon Research's latest poll indicated that concern about the crisis was at its highest for nearly a decade.
Noll expected the experience of the last 10 years would've brought many climate sceptics around to our inconvenient new reality.
Still, he worried that people weren't relating what was arguably the planet's most pressing problem to their everyday experiences.
Temperature records were being "smashed", as he put it, all too regularly. On the day he spoke with the Herald, Westport chalked up its balmiest December day – 26C – in 70 years.
"What we don't want to happen is to see this all just become white noise to people. We don't want these things to become normal, because, frankly, they're not.
"Unfortunately, this is going to be something more and more frequent as we go through the next few decades and beyond," he said.
"We should be buckling the seatbelt here, because this won't be ending any time soon."
Emergency services were called to the scene about 8.30pm.
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