New Zealand is having five more hot days than it was 70 years ago – as the number of frost days slowly drops away.
The fresh insights have been captured in a global index of extreme weather trends, bringing together a century of data from more than 36,000 stations.
Kiwi climate scientist Professor Jim Salinger, who calculated New Zealand's data, said shifts in local extremes were consistent with global warming trends: extra-warm days are becoming even hotter and more common.
His analysis showed the frequency of those days, where temperatures reached 25C or more, climbed from 8 per cent to 12 per cent between 1950 and 2018, or from 19 to 24 average hot days in a year.
"Warm spells of three days or more have similarly increased."
At the other end of the scale, the picture was just as striking.
"In many parts of New Zealand cold extremes are changing faster than the hot extremes, which is entirely consistent with global trends."
New Zealand had an annual average of 30 frost days – below 0C - 70 years ago, but that had fallen to about 25 days now. Over the same period, the annual proportion of cool nights fell from around 15 per cent to about 5 per cent today.
"With anthropogenic global warming, the extra greenhouse gases in the air keep more heat in at night like putting an extra blanket around the planet to keep it warmer," Salinger explained.
"Thus, low temperatures aren't so low and there are fewer frosts."
The shifts were particularly dramatic in different parts of the country. Northern New Zealand had effectively become "frost free", he said, enabling subtropical crops and pastures to be grown.
At the same time, crops in the region that required winter frosts to set fruit were no longer successful, or could only be grown with chemical treatments.
Across the country, the amount of heat available for crop growth during growing season had been rising, which meant winegrowers had to shift some varieties further south.
Meanwhile, the annual rate of very wet days increased from 15 to 20 per cent over the 20th century in the east of the North Island, while a slight rise was also observed in the west and south of the South Island.
"The trends in consecutive dry days contrast across New Zealand."
Salinger said the general trends would continue under climate change: the dramatic heatwave that marked the sweltering summer of 2017-18 would be the norm by 2080.
Future warming – coming on top of an average 1C temperature rise over the past century – is also expected to lift snowlines, melt glaciers and raise flood risks in many parts of the country.
By 2040, the frequency of drought could double or triple in eastern and northern New Zealand, while the average fire season length could soar by about 70 per cent.
Scenarios calculated by Niwa found mean temperatures could rise by between 0.7C and 1C by 2040, and by between 0.7C and 3C by 2090.
The number of days reaching more than 25C could increase by between 40 and 100 per cent by 2040, and between 40 and 300 per cent by 2090, and the average number of frost days could fall between 30 and 50 per cent by 2040, and 30 and 90 per cent by 2090.
Globally, the new index, called HadEX3, suggested the intensity, frequency and duration of extreme weather such as heatwaves and heavy rain would increase as the planet warmed.
One of the clearest trends among 29 indices was an upward trend in daily maximum temperature, showing a clear increase in the number of warm days globally, when compared with the number of warm days between 1961 and 1990.
The greatest increase had been observed in tropical regions of South America, northern Africa and through Asia.
Similarly, the number of warm nights in these regions had risen by more than eight days per decade, doubling since the late 1970s.
"Looking at the changing climate by studying the frequency of extreme weather events provides a different perspective on climate change, complementing other metrics such as annual average temperature rise," said the index's lead author, Dr Robert Dunn of the UK Met Office.
"It is extreme weather events which make the impacts of climate change real for people, and these events also present the greatest shocks to human health and wellbeing, financial economies and of course, the biosphere."
The index has been published in a paper in the Journal of Geophysical Research – Atmospheres.