Climate change likely increased extreme rain and flooding in New Zealand

Flood damage in Te Matau-a-Māui - Hawke’s Bay. Image by Rebekah Parsons-King, NIWA

A study has found that the rainfall from ex-tropical cyclone Gabrielle in New Zealand was about 30% heavier— most likely because of climate change.

The study was published on Tuesday by the World Weather Attribution (WWA) – an international initiative of climate scientists who study extreme weather events, led by Dr Friederike Otto at the Grantham Institute. 

In mid-February, severe flooding from Cyclone Gabrielle destroyed homes, roads, and bridges, leading to billions of dollars in damages and the tragic loss of 11 lives.   

Even before flood waters began to recede, many in New Zealand were quick to ask: is climate change to blame? 

A perfect storm 

New Zealand is no stranger to tropical cyclones. 

Every summer, about ten tropical cyclones form in the South Pacific tropics and, on average, at least one travels south and impacts the island nation.  

While most cyclones are fleeting, passing in a day or so, Gabrielle hung around.  

A “blocking” high-pressure system caused the cyclone to stall and “run aground” to the east of New Zealand.  

Between 13 and 14 February, a prodigious amount of rain fell on the Hawke’s Bay and Gisborne regions, on the east coast of the North Island. Rivers rose rapidly, leading to devastating floods in both rural and urban communities.  

With roads blocked, and power and telecommunications outages, thousands of people in the region were both uncontactable and unreachable for several days.  

Initial estimates of the economic damage linked to Cyclone Gabrielle is on par with the 2011 Christchurch earthquake, the country’s costliest disaster on record, at roughly NZ$13 billion. 

The World Weather Attribution investigates 

In late February, a week after Gabrielle, WWA decided to study Cyclone Gabrielle. 

Working with several climate scientists from Aotearoa New Zealand, the group focused the study on the rainfall that fell in the Hawke’s Bay and Gisborne regions over a 48-hour period. 

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. 

The researchers found that heavy rainfall events of a similar intensity to Cyclone Gabrielle occur in the region about four times more often than they did previously.  The group then analysed computer modelling simulations of the climate which compare the likelihood of the low rainfall event occurring in today’s world – with about 1.2°C of human induced global warming since the late 1800s – to the likelihood of it occurring in a hypothetical world without human induced global warming.  

The rarity of the event and the small size of the region limited the number of computer weather models that could be used, meaning the scientists could not confidently quantify the exact role of climate change.

Dr Otto says climate change undoubtedly influenced the rainfall from Gabrielle. 

“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.” 

Sam Dean, a Principal Scientist at the New Zealand National Institute of Water and Atmospheric Research and a co-author of the study, agrees.   

“I have no doubt whatsoever in my mind with my experience of my life as a climate scientist that climate change has influenced the event. 

“Climate change is a serious concern for flooding in New Zealand and you've got to understand these are gigantic amounts of rainfall.  

“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.” 

No country is too small 

According to data from the United Nations, New Zealand is one of just seven major countries that has not reduced its greenhouse gas emissions since 1990. 

In the same period, the UK has decreased its emissions by 49%. 

After Cyclone Gabrielle, James Shaw, the Minister for Climate Change, delivered an impassioned speech in parliament, decrying years of inaction. 

“I don’t think I’ve ever felt as sad or as angry about the lost decades that we spent bickering and arguing about whether climate change was real or not, whether it was caused by humans or not, whether it was bad or not, whether we should do something about it or not, because it is clearly here now, and if we do not act, it will get worse,” said Shaw.  

New Zealand makes up just 0.17% of the world’s total emissions and in the aftermath of Gabrielle, some commentators argued that the island nation is too small to make a difference.  

However, countries that emit less than 1% of the world’s emissions cumulatively make up about one quarter of the world’s total emissions – if all these countries decided to deprioritise actions to reduce emissions, global efforts to reach net zero emissions by 2050 would be hugely compromised.

Dr Otto said that every country must play their part to avoid more intense extreme weather events.  

“Every additional bit of warming will make these kind of events worse. 

“Climate change is not something that happens sometime in the future or to someone else… it affects people everywhere around the world today.”