Aucklanders could be suffering an equivalent of three months of extra-hot days within only a century's time - with residents in southern and western suburbs likely to be hit the hardest.
A sweeping, first-of-its-kind assessment, released during a three-day city symposium this week, has laid bare Auckland's possible future under climate change.
It warns of hotter days, wilder storms, higher seas, ecological devastation and a wave of pests and disease.
One report examined the city's vulnerability to extreme heat, finding that, if the planet continued to warm on its present trajectory, there could be at least 10 to 15 extra-hot days in Auckland each year by 2040.
This climbed to 60 to 70 by 2090, and 70 to 80 by 2110.
The heat would be felt even more intensely in the north and south of the region: these areas could be experiencing up to 25 of these days by 2040 – and more than 90 by 2110.
Extra-hot days were those in which the temperature reached over 25C, and which Auckland got a taste of during January's week-long heatwave.
A vulnerability index developed by Auckland Council scientists combined a range of socio-economic, health and environmental factors.
Heat-related deaths and health risks were more common among women, the elderly and very young; ethnic minorities and people with language barriers; those who rented or came from low-income households; and those who were socially isolated and had mental and chronic health conditions.
Tens of thousands of Aucklanders already suffered from heat-sensitive conditions like heart disease, diabetes and distress.
This would only get worse under climate change - and nowhere more so than in southern and western suburbs.
The reasons why ranged from deprivation and high numbers of children and elderly, to a lack of access to transport and green spaces.
Assets like community centres, along with green "cool spaces", could help, the experts said.
Another report found many of those same groups – Māori and Pacific people, the elderly, children and low-income homes – to be most at risk from the added threat of air pollution.
The biggest trouble-spots were densely populated urban street canyons surrounded by tall buildings and along heavily-trafficked roads, and smog wasn't the only danger.
Higher temperatures led to longer growing season for plants, increasing pollen in the atmosphere.
Once mixed with airborne pollutants, the allergenic properties can be enhanced – if inhaled, it could trigger asthma attacks and other acute respiratory disease symptoms.
FLOODING, HIGHER SEAS
Scientists have also examined the potential impacts of flooding, which was already the city's biggest natural risk, potentially affecting nearly a quarter of its buildings.
But a metre of sea level rise this century, along with more intense storms, would bring yet more headaches for hazard planners.
More flooding meant a higher danger of diseases spreading through water, or contamination outbreaks stemming from overflowing sewers.
One of the reports estimated that up to 7.5 per cent of council-owned green space was exposed to sea level rise, including sports fields, parks and cemeteries.
Another set out the peril to the region's unique natural ecosystems: in worst-case
scenarios, some could be completely wiped out.
Nearly half of Auckland's 48 types of native ecosystems were identified as having at least one risk factor, making them more susceptible.
While plant species like taraire and rimu were prone to drought stress, prospects for the region's seabird populations - already in delicate state - were just as grim.
Most of Auckland's 24 species of seabirds were considered either at risk of or threatened with extinction – and these species faced higher seas and storms destroying their habitats and nesting colonies.
But even subtle changes to Auckland's subtropical climate could spell trouble for bats, reptiles and amphibians, invertebrates, birds, plants and a host of aquatic and marine species.
Ocean acidification, as well, threatened the survival of shellfish, urchins, sea snails, and plankton – all of which were crucial blocks of the region's marine ecosystems.
In contrast with the danger to our cherished native species was the possible entry of unwanted ones: a more tropical climate would ripen conditions for new mosquitoes and other disease-carrying pests.
Although there has never been a confirmed case of a human acquiring a mosquito-borne disease in New Zealand, there are species here already capable of spreading diseases like the feared West Nile Virus.
Among the other potential nasties on the horizon are Murray Valley Encephalitis, Japanese Encephalitis and dengue fever.
'WE CANNOT WAIT'
Auckland, which contributed around 20 per cent of New Zealand's carbon emissions, was the first region in the country to research climate change risks on a local level.
Penny Hulse, chair of Auckland Council's environment and community committee, said the findings should come as no surprise.
"We are all experiencing the impacts of our changing climate; including more frequent and more severe weather events," she told the Herald.
"While issues like sea-level rise, increased flooding risk and worsening health impacts on our most vulnerable are daunting, we can't ignore the evidence in these new reports.
"They highlight the impacts to our community if we don't do anything. They show we cannot wait."
Auckland Mayor Phil Goff said the climate was changing – and so must the city.
Helping meet the Paris Agreement's ambitious goals to limit climate change demanded new approaches to transport, development, infrastructure and agriculture, he said.
"Auckland is making progress, but along with communities around the world there is much more that we need to do to meet the goals set in Paris."
The council's chief sustainability officer, John Mauro, said Auckland's Climate Action Plan was being developed in line with the UN's ambitious aspiration of limiting temperature rise to 1.5C.
Mauro said the biggest areas where Auckland could make a difference was offering transport choices, making urban centres more walkable and liveable, and growing a healthy urban forest.
"A more compact Auckland with greater reliance on electric-powered public transport will play a critical role in reducing carbon emissions from transport and traffic, which make up 40 per cent of our city's emissions," Mauro said.
"This will also reduce the high levels of black carbon pollution in the city centre."
Auckland Council was the first council in country to issue green bonds, and the $200 million raised so far would help fund Auckland's electric trains, along with a zero-emission bus fleet scheduled to be in place by 2025.
Mauro also highlighted the City Rail Link, currently being built; $900m worth of cycling infrastructure, including the SkyPath; and the Ports of Auckland's new hydrogen pilot plant, which could provide clean energy for buses and harbour ferries.
"We all need to work together. We all benefit when our society and people become more resilient, more equitable, more prosperous and healthier," Mauro said.
"Taking decisive action won't always be easy – it'll require some tough conversations and difficult decisions."
By the numbers
Northern and southern parts of Auckland could be experiencing 90 extra hot days, where it's warmer than 25C, by 2110.
Also by 2110, yearly maximum (daytime) temperatures are projected to increase between 1.5C and 3.75C for much of the Auckland region, and annual minimum (night-time) temperatures are projected to increase between 1.25C between 3.5C.
It is estimated that 16,000 buildings in Auckland are at risk of floor flooding in a 100-year flood event.
Over this century, approximately 1.5-2.5% of Auckland's land area could be exposed to sea level rise. This encompasses 0.3% of buildings, 80% of coastal ecosystems and 6% of dairy land.
22 of Auckland's 48 types of native ecosystems were identified as having at least one risk factor making them more susceptible to climate change.