Kiwi scientists have contributed to a study that's found evidence climate change fuelled the impacts of Cyclone Gabrielle in Hawke's Bay and Tairāwhiti last month.
Emissions of greenhouse gases from human activity caused a 20 to 30% increase in rainfall in the area, and extreme rainfall events like this have become three to four times more likely, while still rare, the study reports.
NIWA principal climate scientist Sam Dean said it's the first time New Zealand has participated in a rapid attribution study run by World Weather Attribution.
"It's absolutely front and centre of people's minds right now: 'What role does climate change play, and what will it mean for the future?'" he said.
"We want to try and give them certainty that we have confidence that climate change has contributed to this event," Dean told 1News.
"Like everyone else, I watched the pictures of what's happened, and it really was a gargantuan amount of rainfall that came down these valleys.
"It's terrifying and destructive and distressing, and we're really just trying to help in any way we can for people to understand what's happened and what it all means."
The rapid study period — as well as climate data having only been recorded in New Zealand from 1979 — have led to limitations with the study.
Further rainfall data from locals in the affected regions are yet to arrive.
"We're confident that climate change is unquestionably affecting... has contributed to making this event worse than it would have otherwise been," Dean said.
Observational data led to the study findings, but the analysis of climate models didn't align with this.
Dean said reviewing climate models over a longer time period was needed.
"We always like to believe our observations because they're our truth, and the models are a useful tool," he said.
"We're looking at a very short record here — we got a result with these observations, that isn't always going to be the case, but I think when we look at those models... over a much longer time, then we will see a stronger signal that will support that argument that climate change has contributed to this event," Dean said.
World Weather Attribution co-lead and Imperial College London climate lecturer Friederike Otto said the result is frustrating but not surprising as climate models have "difficulties to disentangle".
"There is no doubt climate change played an important role in making this rainfall more intense," she said.