Research reveals potentially dangerous 'urban heat island’ effect in central Auckland

Auckland's central city can get up to three degrees hotter at night than other parts of the region due to an urban heat island effect, new data shows.

Research commissioned by Auckland Council also found big differences in temperature between locations just a few blocks from each other, with areas with lots of hard surfaces tending to record the highest average temperatures.

While New Zealand does not currently experience the same extremes of heat as places like Australia or parts of Europe, climate change researchers say there are still health risks, which will worsen as global temperatures rise.

"With a warming climate we know that heat is going to become more prevalent, with more prevalent hot days and heat waves, when we haven't really experienced them much before in Auckland, " the council's lead climate resilience advisor Petra Pearce said.

At the moment, the temperature reaches 25°C in Auckland about 20 days each year. By the end of the century, that is expected to increase to 90 days a year.

The heat research, done by engineering consultancy Arup, divided Auckland into hexagons measuring about 300 metres across - the equivalent of one or two city blocks - and took into account wind patterns, surface cover such as trees, grass and concrete, and building density and materials.

The temperatures were modelled across a five-month summer period, using global climate data from 2021-2022 (Auckland's hottest summer on record). Researchers then checked them against actual temperatures collected from NIWA weather stations across the region during that time.

The hottest average temperature, 19.8°C, was recorded in Auckland's CBD, on a block of Nelson Street that has the highest proportion of concrete and other impervious surfaces in the central city.

Even overnight, this area remained much hotter, recording an average minimum temperature of 17.2°C, compared to 15.6°C in Albert Park, a central city park just a few hundred metres east.

"Some of the results found that the city centre was up to three degrees warmer than the surrounding rural areas at nighttime over this period of assessment, and also found that there were differences within the city as well, of up to three degrees during a peak temperature day."

Parts of South Auckland also recorded uniformly high average and maximum temperatures, while the coolest parts of the region were the Waitākere and Hunua Ranges, and coastal peninsulas like Whangaparāoa and the North Shore.

"We're a coastal city and have a lot of water bodies surrounding us, so we get the sea breeze happening during the afternoon on those hot summer days and that actually moderates some of our urban heat," Pearce said.

"You can see that the coastal boundary is often cooler than further inland."

Maximum temperatures during the day tended to be hottest in grassy, rural areas outside urban Auckland, which the researchers suggested was because of low moisture levels during summer.

Great Barrier Island also showed warm average and overnight temperatures.

Pearce said that was because the island has a warmer climate than the rest of the region, and warmer sea surface temperatures, which affect land temperatures.

The urban heat island effect is a common phenomenon in built-up areas around the world.

"It's more pronounced at night-time than daytime because the hard surfaces in the city like concrete roads, buildings, for example, hold that heat during the day and release it at night," Pearce said.

The amount of green cover, which provides shade and cooling through a process called transpiration, was markedly different across Auckland, she said.

"Some parts of the city are very green and then parts, particularly South Auckland, have very low tree canopy cover… So what you're seeing when you see a lot of warmer areas in the southern part of the city is the lack of green space."

Heat was an "underestimated" risk in New Zealand, Waikato University climate change lecturer and researcher Luke Harrington said.

"We've got to remember that extreme heat is the deadliest type of climate hazard worldwide. Even if you look at places which are considered cooler climates, you still have periods of relatively extreme heat that can kill people - and that's also the case in New Zealand."

A study published last year estimated that 500 children under five are hospitalised in New Zealand each year for heat-related reasons, Harrington said.

"If you think about expanding that to all age groups, and particularly older age groups, that number would rise again."

People with pre-existing cardiovascular issues were also at risk, because prolonged extreme heat placed extra stress on the heart, he said.

Other New Zealand research has estimated that heat-related deaths among older people could increase from 14 per year to 88, with a three-degree increase in global temperatures.

The urban heat island effect could be particularly dangerous during a heatwave.

"If you don't cool down then your body doesn't have that chance to have that respite before you go into the next day," Harrington said.

"So when you have this urban heat island effect exacerbating those overnight temperatures, that's further amplifying the potential risks from a health perspective."

Extreme heat was the climate extreme changing the most rapidly worldwide, but Auckland and elsewhere in New Zealand were "pretty poorly equipped" to deal with it, he said.

"Right now we haven't really thought about this at all, because I think there's still this prior view that we're not actually that vulnerable in New Zealand."

Creating a formal heat alert system, similar to the heavy rain warning system Metservice already operates, would be an important step to take, combined with clear health advice, Harrington said.

"A lot of it just comes from behavioural messages… Often it's quite simple things, like don't do exercise at the peak of the day, make sure you stay hydrated."

It was also important to identify which groups in a city or community were most at risk and come up with a plan to protect them during periods of extreme heat, he said.

Auckland already has a heat vulnerability index, developed in 2019, that shows parts of south and west Auckland are most vulnerable because of their demographics.

Petra Pearce said other options the council was considering or implementing included more blue-green infrastructure, such as urban wetlands and green rooftops, planting more shade trees - especially in South Auckland - and using more permeable materials to construct footpaths and other hard surfaces.

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