Further back in our past, there had been only one significant compound heat wave – the Historic drought of summer 1934/35 – which set an uninterrupted summer heat record until 2017/18.

During this season, as well as in 2018/19, 2021/22 and 2022/23, scientists observed unusually high sea surface temperatures in the region around New Zealand.

“Spatial patterns for 2017/18 and 2018/19 are very similar, with the largest temperature anomalies from west to east across the southern Tasman Sea and down to the Chatham Islands,” co-author L Niwa’s study and ocean modeler, Dr. Erik. » said Behrens.

“Most of this warming was attributed to blocking high pressure conditions east of New Zealand, which reduced wind-driven ocean mixing and increased incoming solar radiation, causing rapid warming of the waters close to the surface.”

At the height of summer 2022-23, the seas around Fiordland warmed to a striking temperature of 5°C above normal temperatures, thereby increasing pressure on sea sponges that had already just experienced an extreme mass bleaching event.

When Salinger and his colleagues put these recent heat waves into historical context, the numbers are sobering.

What was an event occurring every 320 years in 1934/35 has now become an event occurring approximately every 25 years – while the average annual number of heatwave days has fallen from five in the period 1932-2010 to 12 . in the period since.

Even more troubling, projections indicate that with warming of 1.5°C or 2°C, such compound events could become almost annual by the 2040s.

The researchers highlighted already visible impacts.

Between 2017 and 2021, the Southern Alps have lost 20% of their ice volume – the largest decline over a five-year period since records began – with earlier and faster seasonal snowmelt.

This meant cascading impacts on ecosystems and water supplies. Agriculture was also affected in other ways: for Marlborough’s famous sauvignon blanc grapes, warmer flowering seasons contributed to dramatic changes in yields.

The study, published in the International Journal of Climatologydescribed how changes linked to global warming had influenced and amplified the effects of natural factors.

They included more positive phases of the southern annular mode – meaning more stable anticyclonic conditions around New Zealand – and La Nina patterns which had pushed these anticyclones southeast.

The result was milder northerly winds blowing across our region and the Tasman Sea, said co-author Dr Kevin Trenberth, of the US National Center for Atmospheric Research and the University of Auckland.

“While this may indeed favor warmer conditions, it may also be favorable to atmospheric rivers in the tropics,” he said.

“This can bring heavy rain and even flooding to New Zealand, as has happened with Auckland Anniversary Weekend Event in 2023.”

At a time when better climate information and forecasts are critical to understanding what is happening and why, Trenberth questioned government budget cuts that led to the recent loss of some of Niwa’s best climate modelers.

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

Additionally, Salinger said the findings underscore the urgency of addressing global climate change as we approach and pass local environmental tipping points.

“As 2024 promises to be the hottest year on record globally – and almost all countries make virtually no progress on emissions reduction commitments under the Paris Agreement in 2015 – it is essential to better plan, adapt and prepare for climate change. associated extremes in New Zealand.

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

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