Wetter wet days mean more chance of flooding. Photo: Guy Dowding/High Country Helicopters
The saying that it "never rains but it pours" will become truer in much of New Zealand as the planet heats, a new study has found.
Wet days will be even wetter and the driest group of days even drier across much of New Zealand - even in places where average rainfall over the course of a year is expected to stay about the same.
The trend may dry up some of those drizzly days at the edges of summer, which can be crucial for stopping drought, according to the study, published in the scientific journal Environmental Research Letters.
The lead author, climate scientist Luke Harrington of Waikato University, said the results showed even places where scientists did not expect much change to average annual rainfall would still see significant changes on a daily basis.
That includes much of the North Island.
The findings have implications for droughts and floods.
Wet days could get 10 percent wetter and the driest group of days 10 percent drier even in places where previous modelling has shown no change to average yearly rainfall.
Harrington said the results showed the importance of looking beyond averages to prepare for climate change.
The study adds to a growing body of climate projections, which are used by businesses, the government and town planners to plan corporate strategies, housing developments, infrastructure and flood resilience measures.
Those projections have already established that wet places such as the South Island's West Coast will get wetter on average with climate change, while some already dry places - like South Canterbury and Central Otago - will get drier on average. The same pattern holds in many places around the globe.
The latest research sheds light on those places in the middle, where changes to wet and dry days can cancel each other out in the course of a year.
That includes large parts of the middle of the North Island, down to Wairarapa, including many areas where rain is crucial for farming.
"For a large part of the North Island, you see no average change in annual rainfall, but we've come up with a way of separating out what's happening for the wettest days of the year and the driest days of the year around the country," Harrington said.
"When you do that, you find that the wettest days of the year are increasing the amount of rain falling within them and the driest days, which include the drizzle days around the edges of the summer... are decreasing in their amount of rain, and these two things are cancelling each other out."
He said although the changes might cancel each other out in average terms, they have important implications.
Wetter wet days mean more chance of flooding.
Separate research has found drizzly days can be crucial for preventing or lessening drought - so if days of light drizzle become drier, they will not be as effective at busting droughts.
The timing of those heavier rain days also becomes important.
In some places with no average annual changes, "we're seeing as much as a threefold increase of really extreme dry years, even relative to today's climate," Harrington said.
"There are some years where it's shaping up to look quite dry in the future but then you have these supercharged individual wet days, such that the amount of rain which falls in a year looks quite normal by the end of it.
"But then there are other years where you start out and it's shaping up to be quite dry, but these supercharged wet days don't arrive in that year, and then you do have a really extreme drought."
The results were calculated for a planet 3 degrees Celsius hotter than today, but the same pattern (only less severe) would apply at lower temperatures, he said.
There were already multiple lines of evidence showing wet days would get wetter in New Zealand, Harrington said.
This study reinforced that, and also found "unprecedented" dry years were in store.
The drier dry days affected some of country's most prominent farming areas, including the middle of the North Island down to Wairarapa, he said.
As a general rule, the authors concluded that even parts of the country where no average change was expected, they could expect around 10 percent more rain on wet days, and dry days to be around 10 percent drier, in a 3C hotter world.
Harrington said it took a lot of computing power to study changes at a daily scale, at the level of New Zealand regions.
The latest research used a network of volunteers who have offered their home computers to supply the computing power needed to run complex climate models, a project called Weather@Home.
The study was published in the scientific journal Environmental Research Letters, and includes authors from NIWA and the University of Canterbury.