December 2019 will go down as New Zealand's 35th straight month of above-average temperatures – a record run that's been put down to climate change.

And while we won't know until the final figures are revealed next month how 2019 itself measured up in the record books, the year will fall somewhere among the five hottest.

If that doesn't sound too dramatic, it might be because three of the past few years – 2013, 2016 and 2018 – already rank in the top five, and because New Zealand has been seeing consistently higher temperatures since the start of 2017.

"It's now been 35 months since we've had a month with below average temperatures," Niwa forecaster Nava Fedaeff said.

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"To find a cooler month, we'd have to go right back to January 2017."

The stretch had been pushed to the warmer side by a handful of factors – among them, a scorching marine heatwave that took hold over the record-hot summer of 2017/18 and left seas warm enough to create another one the following summer.

"Over the first few months of this year, getting a below-average month was out of the question, just because the seas around New Zealand were so warm."

Even a cold turn in September, when a rare polar phenomenon played out high above Antarctica, couldn't send the temperature statistics below average.

That was partly because of high pressure systems keeping things settled, and a general lack of snow.

But the wider reason was a changing planet.

An overall warming trend made it increasingly less likely to get a month south of average against the 1981 to 2010 baseline Niwa used to draw comparisons.

"We've been getting a warm average, but only because of climate change – if we didn't have that background warming trend, there wouldn't really be an issue."

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The baseline itself was built off Niwa's Seven Station Series - taking in Auckland, Masterton, Wellington, Hokitika, Nelson, Lincoln and Dunedin.

For each location, temperature records from a number of local sites were merged together to form a long time series.

Source / Niwa
Source / Niwa

Overall, the series showed New Zealand's average annual temperature had risen by about 1C over the past 100 years.

Before January 2017, the last below average month was August 2016. Before that, it was September 2015.

"The year 2013 had no below average months. The last time we had more than one month of below average temperatures was 2012, when there were six months. So that would have been our last properly cold year."

She said we might soon hear forecasters again describing months as "average" again – but only as a result of the 30-year baseline shifting to 1991 to 2020 in a year's time.

"That's going to become an interesting communication challenge, but, having said that, it's not going to affect things like rankings.

"If we are talking about a year being the warmest on record, it won't matter what average baseline we use. And that record will probably be broken again, probably not in a very long time."

The best estimates of New Zealand temperatures are for an expected increase of about 1C by 2040, and 2C by 2090.

However, owing to the different emission scenarios and model climate sensitivities, the projections of future warming cover a wide range, spanning from 0.C to 2C by 2040 and 0.7C to 5.1C by 2090.

NZ's hottest years

• The warmest year since the start of Niwa's Seven Station Series record began in 1909 is the year 2016, with an annual mean temperature of 13.44C, or +0.84C above the 1981-2010 average. The year 2018 is ranked 2nd-equal warmest in the record, tied with 1998.

• The five warmest years on the basis of monthly mean temperatures are: 2016 (+0.84C), 2018 and 1998 (tied on +0.80C), 1999 (+0.74C), and 2013 (+0.72C).

• Considering monthly minimum temperatures instead, the five warmest years are: 2018 (+0.94C), 2016 (+0.80C), 1998 (+0.72C), 2013 (+0.67C), and 1999 (+0.62C).

• Considering monthly maximum temperatures, the five warmest years are: 1998 (+0.88C), 2016 (+0.87C), 1999 (+0.85C), 2013 (+0.79C) and 2018 (+0.68C).