The potential hand of climate change in one of the worst storms to hit Northland, causing tens of millions of dollars worth of damage, has been revealed in a new study.

Over five days in July 2014, Northland was swamped by a low pressure system that brought extreme rainfall, flooding and high winds, knocking out power to 17,000 homes in the Far North alone.

Damage to Top Energy's power network was estimated at $1.5 million, the worst since Cyclone Bola in 1988.

Slips and flooding cut the Far North off from the rest of the country, leading to shortages of fuel, bread and other basics.


The storm even had political repercussions. Perceptions that the Government had backed away from its promise to cover more than 90 per cent of the roading repair bill helped NZ First leader Winston Peters unseat National in the Northland byelection earlier this year.

In a report published last week by the Bulletin of the American Meteorological Society, Niwa climate scientist Dr Suzanne Rosier picked apart the potential influence of human-driven climate change on the event.

She drew upon data collected with help from the weather@home project, in which people donated their computers' spare processing power to crunch a vast amount of weather data from climate models. With that extra computing power she used the state-of-the-art Australia New Zealand regional climate model to run two sets of experiments.

The first looked at the five-day weather data under present climate conditions, while the second analysed the weather data under climate conditions as they might have been had there been no human influence.

While the rainfall that led to the 2014 Northland floods could have occurred in a world without climate change, Dr Rosier found that the chances of such heavy rain had increased as a result of human influence.

The study's best estimate was that the risk of such an event had approximately doubled because of human interference with the climate system.

"Since weather patterns are chaotic, we need such a large number of simulations to help us estimate the range of possible weather outcomes under any particular set of driving conditions," she said.

It followed similar Niwa studies analysing the effects of climate change on two previous events - the extreme rainfall in Golden Bay and Nelson in 2011 that flooded more than 300 homes and properties, and drought in the North Island in 2013.


The Golden Bay study concluded that an increase in greenhouse gases helped increase atmospheric moisture levels, contributing to extreme rainfall in the area.

The Northland storm of July 8-11, 2014

* Kaikohe recorded a July rainfall total more than 300 per cent of normal.
* Total insurance claims topped $18 million.
* 17,000 homes were without power at the height of the storm in the Far North, another 10,000 in Whangarei and Kaipara.
* About 5000ha of dairy land was under water for up to 10 days.
* The Fire Service carried out 15 rescues, mostly of motorists trapped in floodwaters.
* The Far North was all but cut off from the rest of the country by flooding and slips on State Highway 1, Mangakahia Rd, Pipiwai Rd and Ruapekapeka Rd. The only route south was a badly damaged State Highway 14.