Here’s what we can and can’t say about what might be causing this extreme spring weather

Wednesday, 29 October 2025

A Canterbury farmer describes waking to find rows of trees blown down after a violent nor’wester ripped through the district. Locals worked to clear roads and restore power, while grateful no one was hurt, they say it was the worst gale they’ve seen.

Perhaps you’ve found yourself looking out of the window over the past week and wondering.

Perhaps the rain was pouring, the snow falling or the wind howling.

Perhaps you wondered whether this weather is the new spring normal, just a symptom of a changing climate.

If you did, you weren’t alone.

With property damage up and down the motu, a state of emergency down south and centimetres of snow settling in late October, Kiwis around the country have been looking for answers.

And many of them emailed Stuff.

As Stuff’s explainer reporter, I would love to provide an answer.

But the reality is, there are too many factors and too much natural variability in the weather to say what caused this and whether it will happen again.

But here’s what we do know.

Large elms, planted on Anzac Ave in Dunedin in the 1920s, were no match for the wind.

Climate change makes this kind of weather more likely

It is not possible to say whether climate change caused the spring storms that have lashed New Zealand over the past week, but it likely did contribute.

That’s according to Earth Sciences NZ principal scientist Sam Dean, who was speaking with the utmost caution.

“There are so many other things that might be going on to make this particular spring so windy over New Zealand,” he said. “But, in general, what climate change does is drive heating in the tropics.

“At the same time, the atmosphere over Antarctica in the winter is very dark and isolated and cold. This means you get a very strong temperature gradient between the heat in the tropics and the cold in Antarctica.”

Last week, we explained how high highs and low lows were creating a steep pressure gradient - causing strong winds across the country. Because temperature changes affect pressure, temperature gradients can’t be divorced from this.

So an inference could be drawn that the hotter the tropics, the steeper the gradient and the worse the weather in the middle.

And just to be clear: New Zealand is the cheese in the middle of this sandwich.

But without time, evidence and resources, there are no clear conclusions.

“We haven't done a study to say what the contribution of climate change is to this particular spring, but a feature of climate change is increasing spring Westerlies,” Dean said.

Snow settled on the ground - and the wool - across the South Island this week.

“I guess you could say climate change increases the chances of the weather we just had.”

Last year temperature increases topped 1.5 degrees

Aside from the warming tropics, there’s also the fact that the earth in general is hotter than it should be right now.

“Last year, the global average temperature was 1.55 degrees above pre-industrial levels,” Dean said. “But remember, we’re trying to stay below 1.5 degrees.”

Before anyone panics, this doesn’t mean that we have failed the Paris Agreement’s target already (although things aren’t looking great in that space). That looks at average warming over a number of years.

But it does mean the earth is unusually warm, which comes with consequences.

“This sort of thing goes up and down, and we’re probably in a bit of an up at the moment,” Dean said. “It will likely come back down. But that fact that it is so warm right now - and warmer than it should be at this point in time - means it is not unreasonable to expect that global warming has made some contribution to the unsettled spring we have seen.”

There’s something funny going on in Antarctica

Have you ever heard of sudden stratospheric warming? I hadn’t.

A quick science lesson: in the stratosphere above Antarctica - about 30km above ground - is a polar vortex. Basically, a large area of cold, rotating air which - in winter - becomes particularly cold with very strong winds.

Normally, the lack of sun and strong winds over winter keep the air temperature around –55°C.

But this September, temperatures rose to –20°C. That’s 35ºC higher than normal.

So, did this cause our recent storms? As above, it probably contributed.

Basically, the warming means the band of Antarctic winds have expanded north, bringing cold fronts and low pressure systems over the Southern Ocean.

Thunderstorms and downpours hit Auckland on Tuesday.

This has the effect that I explained last week. Deeper valleys at the bottom of the pressure gradient cliff means a steeper pressure gradient, and means worse weather as the warmer air rushes south.

“For New Zealand, the implications are usually wetter, colder and windier,” University of New South Wales climate change researcher Martin Jucker said.

If you’re wondering whether this will happen in years to come - and bring more bad spring weather with it - I can’t be much help I’m afraid.

In the Northern Hemisphere, sudden stratospheric warming events happen about every two years. But thanks to the geography of the area, their impacts are a lot less significant.

Over Antarctica, satellite observations recorded the first event in 2002. This may not have been the first, but it was the first since observations began in 1979.

In 2019, it happened again.

“At this point we did a study that used 10,000 years of weather and climate data to try to find out how often we should expect these events,” Jucker said.

“We found that, on the conditions of the 1990s, it should happen about once every 22 years. And that worked well with the first one happening in 2002 - more than 22 years after 1979 - and the second one in 2019.”

The researchers then re-did the study, but included increasing carbon dioxide levels in their calculations. According to this modelling, the events would only come once every 300 years.

“Our conclusion was that we probably wouldn’t see another one, because climate change would make them extremely rare.”

That conclusion has been proven wrong. In addition to September’s event, Antarctica also saw a (weaker) sudden stratospheric warming event last year.

Plenty of scientists will already be in the depths of trying to find out why, Jucker said, but he has his theories.

“My guess is that it is due to very warm sea surface temperatures and very low Antarctic sea ice,” he said.

Both things that have been attributed to climate change.