Climate change worsened Westport deluge - study

Scientists have teased out the hidden hand of climate change in one of our worst flood events - finding it made last year's West Coast deluge about 10 per cent more intense. Photo / George Heard

Scientists have teased out the hidden hand of climate change in one of our worst flood events in recent times – finding the planet's warming made last year's West Coast deluge about 10 per cent more intense.

A slew of bouts of extreme rainfall made 2021 a record year for weather damage claims – and a moisture-packed system that wheeled into the country in mid-July cost private insurers more than $130m alone.

Fuelled by an "atmospheric river" stretching back to the tropics, the system dumped more than 150mm of rain over Westport in just three days.

As the Buller River swelled to its highest flows in 95 years and burst its banks, waist-high floodwaters forced many residents to evacuate.

While the same system brought heavy flooding across the lower North Island and upper South Island, Westport was hardest hit, leaving around 450 homes damaged or uninhabitable.

The West Coast isn't unused to big downpours – a predominant westerly airflow over the country and the Southern Alps' orographic effect on rainfall make it New Zealand's wettest region.

Still, scientists are increasingly trying to understand how climate change is packing extra energy into its traditional rain-makers, with another event last month bringing Westport record-breaking levels.

Last year, a team of researchers working under the MBIE-funded Extreme Weather Event Risk Attribution Machine (EWERAM) project confirmed last May's Canterbury floods – which the Ministry for Primary Industries spent $4.5m responding to – was made more intense by climate change.

Their latest study, just published in the journal Weather and Climate Extremes, estimated the difference at Westport last July to have been around 10 per cent.

In the past, scientists trying to calculate the role of climate change have had to draw heavily on global climate models which, while useful for determining how climate pollution influenced systems arriving here, couldn't give them a more granular picture of what played out at local scales.

About 450 homes were left damaged or unihabitable following the July flood. Photo / George Heard

"So, another approach we looked at was to take forecasts of the event from the MetService and Niwa weather forecast models, and then re-run the forecasts with reduced greenhouse gas concentrations, a cooler ocean, and a cooler and drier atmosphere," study leader and Niwa climate scientist Dr Daithi Stone explained.

Again, that wasn't so straight-forward.

"These forecasts models are great at describing what happens when a weather system hits, but they are assuming the same weather system would have been hitting New Zealand in the absence of our emissions."

In any case, no matter which of these modelling approaches were used, the study – also supported by the Whakahura project, and involving researchers from Niwa, MetService, Bodeker Scientific and Victoria University – found the results to be generally similar.

Stone said the Canterbury event had been much harder to study, given global models did a poor job of representing how the Southern Alps blocked rainfall to the region from weather systems sweeping in from the Tasman Sea.

"This meant we were restricted to using a smaller set of global climate models and weather forecast models."

Still, he said, the work showed the overall influence of climate change on our weather was clear.