How climate change is driving bigger NZ deluges

The Waiho River bridge between between Fox Glacier and Franz Josef Glacier was washed out amid heavy rains this year. Photo / File

Scientists have confirmed what many people in rain-soaked regions have long suspected: climate change has been driving a widespread rise in big deluges.

And they expect the patterns that have been pieced together in a just-published study to only intensify as the world warms.

Big rainfall events are one of the most damaging consequences of climate change, packing the potential to damage homes and trigger landslides and flash floods.

One dramatic example was a deluge that washed out a closed landfill in South Westland and left much of the Fox River strewn with rubbish.

Typically, we measure rainfall intensity by what's called the annual maximum one-day rainfall, which measures the highest amount of rain recorded in a single day.

Over the past five decades, this has shown no clear evidence that intense events had changed since 1960, besides some drops and increases at certain places.

But, when looking at the picture a different way - recording all of the rain that fell across an extreme event, rather than just a day of it - the trends appeared much more dramatic.

Climate scientist Professor Jim Salinger found that, between between 1961 and 2010, annual extreme rainfall when measured by event had increased noticeably in the west and south of the South Island - and had fallen in the drier north and east of both islands.

When compared with the one-day measure, the "extreme event annual maximum" ranged from 25 per cent higher in the east, to 60 per cent higher in the west and south of the South Island.

Salinger said climate change would boost the intensity of these events even more.

With each degree of warming, he estimated the event annual maximum climbed by 5 per cent in the west of the South Island, and north-west of the North Island.

"Essentially with warming, heavy precipitation maximum annual events get wetter."

Explaining the logic behind measuring extreme rainfall by event, Salinger said these big dumps typically lasted longer than a day - and usually two or three days.

"It is essentially the period of continuous rainfall - rather than rain showers with breaks," he said.

"We can think of it in terms of Cyclone Bola on Gisborne - this system would have produced rainfall over two to four days.

"So the 'event' rainfall will be higher than the one-day maximum rainfall. For example, the one-day rainfall maximum was 415mm during Bola, but the entire three-day event brought 900mm."

Building the picture was a matter of looking at 25 long-term weather stations with climate data stretching over at least the last 50 years.

"The station data had to be high quality. Once these stations had been identified, the data had to undergo quality control and checks for missing precipitation data."

Whatever the indicator used, scientists expect extreme rainfall events to become more common over most of the country, with the exception of Northland and Hawke's Bay.

A rise of up to 20 per cent is possible in the south of the South Island by the end of the century.

Scientists are now working on a new million-dollar model that will ultimately be able to tell us the precise difference that climate change made to an extreme event – and within just a day or two of it happening.