Security camera captures moment family flee their Twyford home and the Ngaruroro River breaches stop bank after Cyclone Gabrielle. Video / Iain Trotter
Big deluges have become up to four times more common – and drop up to 30 per cent more rain – in the East Coast regions that Gabrielle hit hardest.
That’s according to an early study that’s explored climate change’s potential role in our worst weather event this century, but which stops short of carving out its precise contribution.
The destructive power of the ex-tropical cyclone, which claimed 11 lives and wrought billions of dollars of damage across the north and east of the North Island, owed to a freak mix of factors.
They ranged from extreme aspects of the system itself – and how they combined in a catastrophic way for the East Coast – to the vulnerability of those communities that bore its brunt.
While some have gone as far as labelling Gabrielle a “climate disaster”, the role of human-driven warming wasn’t so straightforward for scientists to untangle.
The new analysis, released overnight by the World Weather Attribution group, marks the formal effort to do so, and indicates Gabrielle’s torrential rainfall likely carried a sizeable climate-change handprint.
The study, authored by 23 local and international scientists, drew on computer model simulations comparing today’s warmed climate state with that of the past.
These focused on that 48-hour window in which the most rain dropped on the East Coast: a period in which some exposed sites above Gisborne received more than 550mm in just a day and a half.
Around 200mm fell in another high country station north of Napier in a single night, while the city itself recorded its second wettest February day ever.
Based on historical weather station data, the team found such deluges in those regions could now involve around a third more rainfall – and occur up to four times more frequently – than before humans made the planet some 1.2C warmer.
Yet, extreme rainfall totals like those of Gabrielle’s remained rare at any given location in the studied regions, with a roughly three per cent chance – or less – of this occurring each year.
The team however stressed such estimates were constrained by large mathematical uncertainties, due to limited weather station data and the fact that rainfall was highly variable across the regions.
Next, the scientists combined the weather station data with climate models to find that, while the influence of climate change couldn’t be quantified, its signal was there.
Of those models that could be applied to what was a rare event, in a relatively small focus area, most couldn’t match those increases seen over time in weather station data.
Because well-established global models indicated that climate change made rainfall events heavier and more frequent – and with no other plausible explanation – the scientists had only human-caused warming to point to.
That finding was in line with increasing international evidence showing that rainfall from tropical cyclones was intensifying as our planet heated up.
Gabrielle itself powered up to category 3 strength over tropical waters that were running abnormally hotter against a background of global warming, but also a long-lingering La Niña climate pattern.
After moving out of the Coral Sea, it underwent an “extra-tropical transition” in the colder waters of the Tasman Sea, where strong upper atmosphere winds took its driving wheel.
As it was rolling close to New Zealand, the system deepened and strengthened once more as it ingested a piece of “spin” in the upper atmosphere, while also feeding off the influence of a vigorous subtropical jetstream.
During this process, Gabrielle also tugged closer to the North Island and slowed down to unload torrents of rain on the East Coast, with catastrophic consequences.
“Cyclone Gabrielle was a rare event even in today’s climate, with some places [in the study region] experiencing rain that would’ve had a 1.5 per cent chance of occurring in any year, others with a 0.4 per cent chance,” study co-author and Waikato University climate scientist Dr Luke Harrington said.
Long-running observations clearly showed extreme rainfall was occurring more often over the Hawke’s Bay and Gisborne regions, even after accounting for the role of La Niña.
“However, many of our models found no such trend,” Harrington said.
“We have several explanations as to why the models show no clear change in the frequency of extreme rainfall but only one explanation for the clarity of the observed trends, and that is climate change.”
Niwa climate scientist Dr Sam Dean said that, while Gabrielle still would’ve been a destructive system even without the climate change component, rainfall potential was now “in a different place”.
The exercise wasn’t the first time scientists had tried to tease out the effect of climate change – known as attribution – in local extreme weather events.
In the aftermath of 2017′s Debbie, which swamped the Bay of Plenty town of Edgecumbe, researchers estimated that around 5 to 10 per cent more rain fell as a result of it.
Later, scientists found it had made Canterbury’s flooding in 2021 perhaps 10 to 15 per cent more intense, with a similar-sized influence also observed in that year’s Westport disaster.
But the speed at which this assessment was carried out marked a first for New Zealand, Dean said, and tried to “address one of the questions being regularly raised about whether this summer is the country’s new reality”.
The study also underscored the urgent need to make New Zealand infrastructure and communities more resilient to climate impacts, the team said.
Models suggested that a climate 2C warmer than pre-industrial times – which could be reality by century’s end – would mean slightly increased rainfall intensity, although there were wide uncertainties.
“Climate change extremes like cyclone Gabrielle threaten not only our lives and livelihoods but also our social and cultural connections between people and place,” said Dr Shaun Awatere, research impact leader at Manaaki Whenua-Landcare Research.
“It’s difficult enough already as Māori landowners navigating the various institutional and economic challenges, let alone trying to protect the whenua for future generations from the impacts of erosion and flooding.
“My hope is that politicians listen to those voices, who are often overlooked but who so often are the ones that are most vulnerable to the impacts of climate change.”