Major new study: Massive heatwave behind NZ’s hottest summer could become ‘almost annual’ by 2040s

• Combined land and sea heatwaves, like that behind our warmest summer, have become more common in New Zealand and may soon be almost annual events, scientists report.
• A series of these “compound” events within the last decade have been driving scorching summer days and nights, while melting glaciers, disrupting growing seasons and harming marine life.
• It comes as the National Institute of Water and Atmospheric Research (Niwa) is predicting more above-average temperatures this summer – while nations are trying to strike new climate deals at this year’s UN summit.

Regional heatwaves like that which fuelled our hottest summer on record have become 12 times more likely under climate change – and could become almost annual events within the next few decades.

That’s the stark message of a major new study that comes as delegates from nearly 200 nations meet to thrash out new climate deals, with another Kiwi summer of above-average warmth lying beyond.

Marine heatwave conditions – like this, observed in late 2017 – helped fuel what remains New Zealand's hottest summer on record. Similar combined land and sea heatwave events have been observed over four summers since. Image / Ben Noll, Niwa

Summers in our region have been warming up at an unprecedented rate, on the back of global climate change and combined ocean and land heatwaves.

Climate scientist Professor Jim Salinger said he and colleagues were prompted to look closer at these “compound” extreme events after four of them were observed here in less than a decade.

Women and children paddle in a pool on Rangitoto Island in 1935. A combined land and sea heatwave behind that year's record-hot summer was considered a one-in-300-year event at the time – but it could become an almost annual one by the 2040s. Photo / NZ Herald

Further back in our past, there’d been just one significant compound heatwave – the historic drought summer of 1934/35 – which set a record for summer warmth unbroken until 2017/18.

In that season, and also 2018/19, 2021/22 and 2022/23, scientists observed unusually high sea surface temperatures in the region around New Zealand.

“The spatial patterns for 2017/18 and 2018/19 are very similar, with the largest temperature anomalies over a west-to-east across the southern Tasman Sea and to the Chatham Islands,” study co-author and Niwa ocean modeller Dr Erik Behrens said.

“Most of this warming has been attributed to blocking high-pressure conditions to the east [of] New Zealand, which reduced the wind-driven ocean mixing and enhanced incoming solar radiation – causing the near-surface waters to warm rapidly.”

At the peak of the summer of 2022-23, seas around Fiordland warmed to a striking 5C above normal temperatures, heaping more pressure on sea sponges that had already just suffered an extreme mass-bleaching event.

A bleached sea sponge in Fiordland, where sea temperatures soared to 5C above normal over the summer of 2022-23. Image / Victoria University

When Salinger and his colleagues put those recent compound heatwaves into historical context, the figures were sobering.

What was a one-in-320-year event back in 1934/35 had become a roughly one-in-25-year one today – while the average annual number of heatwave days had leapt from five over the 1932-2010 period to 12 in the period since.

More troublingly, the projections indicated with 1.5C or 2C of warming, such compound events could become almost annual by the 2040s.

The researchers pointed to impacts that could already be seen.

Between 2017 and 2021, the Southern Alps lost 20% of their ice volume – the most significant decline in a five-year period since records began – with seasonal snow melting earlier and faster.

The nearly-vanished Fog Glacier in the Southern Alps, as it appeared after summer 2023. Photo / Rebekah Parsons-King, Niwa

That meant cascading impacts for ecosystems and water supply. Agriculture had also been affected in other ways: for Marlborough’s famed sauvignon blanc grapes, warmer flowering seasons had contributed to dramatic shifts in yield.

The study, published in the International Journal of Climatology, described how the shifts under global warming had been influencing and amplifying effects of natural drivers.

They included more positive phases of the Southern Annular Mode – meaning more settled, anti-cyclonic conditions around New Zealand – and La Nina patterns that had pushed those anticyclones southeast.

The result was more balmy northerly winds flowing down over our region and the Tasman Sea, said co-author Dr Kevin Trenberth, of the US National Centre for Atmospheric Research and the University of Auckland.

“While this can indeed promote warmer conditions, it can also be favourable to atmospheric rivers from the tropics,” he said.

“This can bring copious rains and even flooding into New Zealand, as happened with the Auckland Anniversary weekend event in 2023.”

Last year's Auckland Anniversary weekend deluge caused hundreds of landslides around the region. Photo / Dean Purcell

At a time when improved climate information and predictions are much-needed to understand what is going on and why, Trenberth questioned Government funding cuts that had led to the recent loss of some of Niwa’s top climate modellers.

“None of the global climate models do a very good job around New Zealand.”

Moreover, Salinger said the findings underscored the urgency of tackling global climate change as we approach and pass local environmental tipping points.

“With 2024 looming as the hottest year on record globally – and next to no progress on commitments to reduce emissions in line with agreements from the Paris accord in 2015 by almost all countries – it’s vital to better plan, adapt and prepare for climate change-related extremes in New Zealand.”

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.

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