A 12-month run that’s delivered record-breaking deluges and dramatic marine heatwaves has proven New Zealand’s warmest period since observations began more than 150 years ago.

That’s according to a prominent climate scientist’s analysis, as global agencies report that 2023 looks likely to go down as the planet’s hottest in recorded history.

In assessing the local picture, Professor Jim Salinger compared October-to-November temperature data from 22 land sites against a 1951-80 average.

The result came in at 1.55C above that three-decade baseline – the highest for any such period in records stretching back to 1870.

Regional sea temperatures, too, had been running unusually warm over the period, at 1.06C above average.

When these were combined with land observations, the total 4.2 million sq km area around New Zealand showed a record-high average of 1.07C for the 12 months.

Salinger – who pioneered Niwa’s long-running “seven stations” temperature series – said the record run owed to several major drivers that have been influencing our climate.

These included marine heatwaves, the warm and wet signal of a La Nina climate pattern, fewer southerly systems reaching New Zealand and background climate change.

“The reason for using a 1951-80 average to compare against is it allows us to put temperatures in the current context – and means we can see the real components of global warming.”

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This graph shows more than 150 years of average New Zealand temperatures as anomalies against a 1951-80 baseline. The blue line shows the average from 22 land sites, the red shows the average optimum interpolation sea surface temperature (OISST) in our surrounding oceans, and the green shows the combined average. The last 12 months have been the warmest on the record. Source / Jim Salinger

Marine heatwaves had been responsible for driving up coastal sea surface temperatures by an average 1.6C over the past two years, while causing a cascade of effects above and below water.

“It’s been an incredibly warm period in our oceans; a bleaching event in Fiordland earlier this year was just one of the impacts.”

Salinger said local seas had only recently begun to cool with the end of a three-year La Nina run, and its replacement with an El Nino pattern that’s helped to churn up the ocean and haul up colder flows from deep below New Zealand.

Just how 2023 closed out in Niwa’s own records remained to be seen – but meteorologist Ben Noll expected a “photo finish”.

Relative to the 1991-2020 baseline, temperatures from January to October had tracked about 0.9C above average - and the final figure for 2023 was likely to put the year at least within New Zealand’s five hottest, Noll said.

While November’s temperatures had run close to average, a warm end to the year could come with a pulse of hot air expected from Australia in early December.

Even if 2023 didn’t prove another record-breaker, there’s been no shortage of others set over recent months and years - including our warmest September (2023), warmest year (2022) and consecutive record-warm winters (2020, 2021, 2022).

That might be unsurprising if we consider that seven of the eight warmest years on Niwa’s books have all occurred since 2013, with annual average temperatures having climbed by 1.26C in less than 120 years.

Globally, the January-to-October mean temperature was also the highest yet measured, as just reported by the EU’s Copernicus Climate Change Service.

That came to 1.43C above the pre-industrial average, and some 0.10C higher than the 10-month average for 2016 - still the planet’s warmest calendar year on record.