Scientists say climate change likely behind the intensity of rainfall from Cyclone Gabrielle

Scientists have discovered the severity of rainfall during New Zealand's most significant weather event this century, Cyclone Gabrielle, is likely caused by climate change.

A new study released on Wednesday has found global warming could cause extreme rainfall events, such as the cyclone, to produce almost one-third more rain than before - and concerningly heavy rainfall is now more common on a warmer planet.

Cyclone Gabrielle tore through the North Island last month. It brought widespread destruction to the East Coast and took the lives of 11 people.

Rain fell at a significant intensity, causing flash floods for parts of the country, with one area recording enough rainfall to fill 72 Olympic swimming pools every minute, for six hours. 

But now scientists claim global warming could be behind the intensity of the rainfall. 

The rapid study was conducted by 23 researchers as part of the World Weather Attribution group, including scientists from universities and meteorological agencies in Aotearoa New Zealand, France, Germany, the Netherlands, the United Kingdom and the US.

It found very heavy rain, similar to that of Cyclone Gabrielle, although still rare, has become around four times more common in the region. It also found extreme rainfall events produce around 30 percent more rain than before humans warmed the planet.

However, these estimates have large mathematical uncertainties due to limited weather station data and the fact that rainfall is highly variable in the region.

Cyclone Gabrielle caused flooding in Esk Valley.
Cyclone Gabrielle caused flooding in Esk Valley. Photo credit: Facebook/Valley D'Vine Restaurant

University of Waikato's climate change senior lecturer Luke Harrington said Cyclone Gabrielle was a rare event in today's climate, with some places experiencing rain that would've had a 1.5 percent chance of occurring in any year, others with a chance as low as 0.4 percent.

"Long-running observations clearly show extreme rainfall is occurring more often over Te Matau-a-Māui and Te Tairāwhiti regions, even after accounting for the role of La Niña. However, many of our models found no such trend," Dr 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."

The rarity of the event and the small size of the analysed regions (Hawke's Bay and Gisborne) limited the number of models used, meant scientists could not quantify the influence of climate change on the increased rainfall seen.

However, despite this, the researchers still consider it likely that climate change was the cause of the increase.

"They reached this conclusion because well-established weather science and climate models both indicate that further greenhouse gas emissions, leading to more warming, will make heavy rainfall more intense and more frequent. The scientists also could find no plausible explanation, other than human-caused warming, for the observed increase in heavy rainfall," the study said.

Flooding in Wairoa, Hawke's Bay.
Flooding in Wairoa, Hawke's Bay. Photo credit: Newshub.

NIWA principal scientist Sam Dean told a press conference while the findings are an uncomfortable result, there was no doubt in his mind that climate change is increasing the intensity of this event.

Senior lecturer at the Grantham Institute for Climate Change and the Environment Dr Friederike Otto said the weather observations in the area are what we expect from basic physics, which is that a warmer atmosphere accumulates more water and increases the frequency and intensity of downpours.

"With the world getting even warmer we will see more and more events like this. Reducing exposure and vulnerability of populations in flood-prone areas is thus an urgent priority," Dr Otto said.

Dr Dean said while the results were a mixed bag the study contributes to the wealth of evidence that adapting to a changing flood risk now and for the foreseeable future is one of the greatest challenges we face.