Heavy rain that flooded northeast Aotearoa New Zealand is now more common on warmer planet

Climate change a likely cause for the increase, but scientists cannot yet quantify the precise influence of human-caused warming on last month’s rainfall.

Very heavy rain, like the event that flooded parts of Aotearoa New Zealand in February during Cyclone Gabrielle, has become about four times more common in the region, and extreme rainfall events now drop 30% more rain, according to rapid analysis by an international team of climate scientists from the World Weather Attribution group.

While climate change is the likely cause of the increased rainfall, a lack of
long-term weather data and limits in climate models meant that the scientists could not quantify the extent to which human-caused warming is responsible.

Extreme rainfall brought by Cyclone Gabrielle from 11-14 February led to extensive damages in Aotearoa New Zealand’s North Island, Te Ika-a-Māui. More than 200,000 houses were left without electricity and at least 11 people died, with billions of dollars of estimated economic losses, making it one of the deadliest and costliest cyclones to hit the region on record. Some of these regions had also been hit by widespread heavy rainfall and flooding only two weeks earlier.

To quantify the effect of climate change on the heavy rain in the region, scientists analysed weather data and computer model simulations to compare the climate as it is today, after about 1.2°C of global warming since the late 1800s, with the climate of the past, following peer-reviewed methods. The analysis focused on the 48-hour period in which the most rain fell over Te Matau-a-Māui (Hawke’s Bay)/Te Tairāwhiti (Gisborne) region, on the east coast of Te Ika-a-Māui (North Island).

Based on historical weather station data, the scientists found that heavy rainfall events, like those seen during Cyclone Gabrielle, now produce around 30% more rain than before humans warmed the planet. Such heavy rainfall events also now happen about four times more often than they did previously; even so, rainfall this heavy is still rare, with roughly a 3% chance, or less, of happening each year in any location. These estimates have large mathematical uncertainties due to limited weather station data and the fact that rainfall is highly variable in the region.

The scientists then combined the weather station data with climate models, but were unable to quantify the influence of climate change. The rarity of the event and the small size of the analysed region limited the number of models that could be used, and most of those that could be used did not match the increase in rainfall seen in weather station data.

While this meant the scientists could not quantify the influence of climate change on the increased rainfall seen in this region of Aotearoa New Zealand, they considered 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.

Evidence from elsewhere on the planet also shows that climate change is increasing rainfall from tropical cyclones.

Luke Harrington, Senior Lecturer in Climate Change at the University of Waikato, said:

“Cyclone Gabrielle was a rare event even in today’s climate, with some places experiencing rain that would’ve had a 1.5% chance of occurring in any year, others with a 0.4% chance. 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. 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.”

Sam Dean, Principal Scientist at the National Institute of Water and Atmospheric Research, said:

“A rapid assessment of the role of climate change in this flood event is a first for Aotearoa New Zealand, and tries to address one of the questions being regularly raised about whether this summer is the country’s new reality. While there is a mixed bag of results from this particular rapid analysis, the study contributes to a wealth of evidence that here in Aotearoa New Zealand, adapting to a changing flood risk now and for the foreseeable future is one of the greatest challenges we face.”

Shaun Awatere, Research Impact Leader at Manaaki Whenua Landcare Research, said:

“Climate change extremes like cyclone Gabrielle threaten not only our lives and livelihoods but also our social and cultural connections between people and place. 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.”

Friederike Otto, Senior Lecturer in Climate Science at Grantham Institute – Climate Change and the Environment, Imperial College London, said:

“Weather observations in the region show exactly what we expect from physics, which is that a warmer atmosphere accumulates more water and increases the frequency and intensity of downpours. And with the world getting even warmer we will see more and more of events like this. Reducing exposure and vulnerability of populations in flood-prone areas is thus an urgent priority.”