Winters are a month shorter than a century ago , Niwa research shows.

Photo: RNZ / Alexander Robertson

Principal climate scientist Brett Mullan has been looking at New Zealand temperature records going back to 1909.

Dr Mullan used data from seven geographically representative locations around the country.

Looking at two 30-year periods, 1909 to 1938 and 1987 to 2016, he calculated that winters have contracted by about one month.

If a threshold of 9°C was chosen there was an average of 100 days per year between 1909 and 1938 when the temperature was less than 9°C, compared to only 70 days per year between 1987 and 2016. Winter contracted about equally from both ends.

Winter is conventionally regarded as occurring between 1 June and 31 August.

Dr Mullan said with climate change causing rising temperatures around the world, the results showing winters were getting shorter didn't come as a surprise.

"What I didn't know was how large it was - one month out of three has basically gone."

A shorter winter period affected the environment, plant growth and what you can grow and where.

"Plants can ... mature more quickly, you might be able to grow two crops when you could only grow one before or grow different things that require more heating."

But there were downsides too, as higher temperatures allowed more pests and diseases.

And there were disadvantages for growers of crops like kiwifruit that require chilling in the winter for the fruit to set properly.

"That is why they have moved out of Northland, as it is not cold enough, and as time goes by they will have to move out of Bay of Plenty as well."

It was too late to return to having what was once regarded as a normal winter of three months.

"It is a matter of if we can get it to level off and stop at a level not too much different from today."

He said virtually all of the world is warming and only a few places are not.

"That is more in the oceans and around Antarctica and the North Atlantic is where conditions will take a long time to change."

There were fewer frosty days and the first frost of the year was a month later in many locations than 100 years ago.