Climate change added 2-4cm of rain to Auckland’s storm, researcher says

Climate change is likely responsible for 10 to 20% of the rain that flooded Auckland and Northland, according to Niwa climate scientist Sam Dean​.

At Albert Park, 211mm of rain fell in under 6 hours. Global heating contributed 21 to 42mm of water – at max, the depth of five iPhone 11s.

The extra rain created a disproportionate amount of damage to homes, cars and possessions, Dean said. “The peak of the storm causes the peak of the flooding.”

Dean and his Niwa colleagues can estimate the role of climate change in natural disasters, such as the Westport floods in 2021.

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Climate change’s responsibility is estimated with a computer that simulates a world where humans never produced climate-heating greenhouse gases, and by adding the same weather conditions that created Auckland’s storm.

“We look at what those events are like in that natural world and compare them to what they’re like in the world we have,” Dean said.

In the Pacific, the La Niña climate pattern – the opposite of the El Niño phenomenon – is bringing warmer, wetter weather to northern and eastern parts of New Zealand.

The Indian Ocean is brewing up pulses of wet air and wind and sending them towards the country. The seas surrounding the motu are in the middle of a heatwave, and these warmer waters effectively power up storms as they pass.

On January 27, there was also a “blocking high” sitting to the east of Aotearoa. This can act as an atmospheric wall, so the storm stayed over Auckland rather than blowing through.

These events could all line up in a hypothetical climate-change-free world – though emissions may be influencing these conditions, including blocking highs, Dean said.

In the simulated natural world, the line-up would create downpours. However, Dean’s initial work suggested climate change added up to 20% more rainfall that evening.

Dean and other “climate attribution” scientists would complete a full study on the record-breaking rain, but it could take 18 months to complete.

“I would certainly expect this to break the 10% barrier and our previous work would suggest it could be as high as 20%.”

Climate change’s extra few centimetres of rain produces more damage than the first few centimetres, Dean said.

“It’s the flood peak that does the damage – how high the flood gets when it reaches its peak is what causes the trouble,” he added.

Hotter air is capable of holding more water vapour, Dean said.

“We have an increase in the amount of moisture in the atmosphere that’s been observed by satellites. All other things being equal, that means more rainfall.”

Auckland has just experienced its wettest month since records began in 1853, according to Niwa. In January, 539mm of rain fell at Albert Park’s weather station – 28%​ more than the previous record, set in 1869.

While climate change was a factor in February’s storm, bad luck also played a role. A band of rain hit the most-populated city like a bullseye.

Within these storms, patches of moist air rise and become unstable – releasing clouds, rain and heat, with the new warmth triggering even more rain.

A storm band undergoing this vicious cycle “ran a line north to south through the middle of Auckland”, Dean said.

If that had unfolded over a rural area, some properties might have been very wet. But fewer people would have been affected compared to the “bullseye hit on Auckland”, he added.

Areas able to produce this type of intense rain can be difficult to identify with weather forecasting models. Yet these short but powerful downpours are “the most affected by climate change,” Dean said.

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