We think of climate change as a gargantuan global crisis that will transform our environment in ways we can barely begin to imagine.

Yet few of us appreciate how it could hurt us individually.

An expert report released today by New Zealand's leading body for science, Royal Society Te Aparangi, warns of a warmer world bringing heatwaves, diseases, water contamination and air pollution.

Several degrees of warming this century could also have alarming implications for our mental health and social inequality - hitting Maori particularly hard.

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It followed a global report in 2015 that bleakly warned how the past 50 years of health gains could be undone by the "medical emergency" that was the threat of climate change.

"If we think of the basic building blocks of health, such as our shelter, the air we breathe, water we drink and the food we eat, all will be affected by climate change," said the society's president, Professor Richard Bedford.

But if we knew what the risks were, he added, we could prepare for them and lower their impact.

Here are five big factors in the report.

Turning up the heat

If greenhouse gas emissions continue to climb close to current levels, many parts of our country will see more than 80 days a year above 25C by 2100.

By contrast, most places today typically see only between 20 and 40 days above that.

Cities, with their large, impermeable surfaces, would act as heat islands, pushing up the temperature of hot days and retaining the heat at night.

Already in Auckland and Christchurch temperatures above 20C each year resulted in around 14 heat-related deaths among those over 65.

Seasonal fires - and major events like the fire that raged across Christchurch's Port Hills in February - will become a bigger threat. Photo / Supplied
Seasonal fires - and major events like the fire that raged across Christchurch's Port Hills in February - will become a bigger threat. Photo / Supplied

Yet if global temperatures rise just one, two or three degrees above current levels, that death rate could rise to 28, 51 and 88 respectively.

Elderly populations are especially vulnerable to heatwaves.

With about one in four Kiwis projected to be 65 and over by 2043 - that's including many people in their 40s today - the problem would be amplified.

Heat poses big risks to occupational health and labour productivity in areas where people work outdoors for many hours in susceptible regions.

Otago University public health expert Professor Michael Baker said that, in his own area of infectious diseases research, rises in temperature had been shown to contribute directly to an increased risk of foodborne diseases such as salmonellosis.

Hotter days could also lead to higher rates of aggression - and potentially heart attacks and strokes.

Flooding in the Bay of Plenty town on Edgecumbe in April. These one-off extreme events could become more common. Photo / File
Flooding in the Bay of Plenty town on Edgecumbe in April. These one-off extreme events could become more common. Photo / File

Data showed that when temperatures climbed above 18-20C, hospital and emergency room admissions increased for those with mental health or psychiatric conditions.

The problem could be eased or worsened by the magnitude and duration of the high temperatures - and the speed of temperature rise.

The report did not ignore the danger of one-off extreme events.

"Extreme events, such as the June 2015 flooding in South Dunedin and the 2017 Edgecumbe floods and Christchurch fires, pose immediate risks associated with being burned by fire, or being swept away when driving or walking through floodwaters or landslides.

Volunteers clean up damage in the wake of flooding that hit Edgecumbe in April. Photo / File
Volunteers clean up damage in the wake of flooding that hit Edgecumbe in April. Photo / File

"These extreme events may also have negative effects on wellbeing through disease outbreaks, toxic chemical contamination, effects of damp buildings, mental health issues, disruption to healthcare access and damage to homes, which can last from weeks to months after the initial event."

Fouling the water

Implications for our marine and freshwater environments were as concerning as they were broad.

Climate change would increase our exposure to waterborne diseases caused by bacteria, viruses and micro-organisms such as giardia and cryptosporidium.

Changing weather patterns, bringing extreme rainfall and flooding, would combine with agricultural run-off to heighten the risk of contamination to the water we drink or swim in.

"Many of the short- to medium-term health effects will come via climate change's impacts on freshwater," said Otago University environmental health senior lecturer Dr Alex Macmillan, who served on the advisory group for the report.

Blue-algae produced toxins which could cause liver damage, skin disorders, and gastrointestinal, respiratory and neurological symptoms. Photo / File
Blue-algae produced toxins which could cause liver damage, skin disorders, and gastrointestinal, respiratory and neurological symptoms. Photo / File

"We already have declining freshwater quality in New Zealand as a result of increasing agricultural and urban pressures on rivers, lakes and drinking-water sources.

"When these existing pressures are put together with warmer waters and heavier but more infrequent rainfall, then we are setting ourselves up for more outbreaks of waterborne illness like the one experienced by Havelock North in 2016.

"Protecting health from climate change will therefore require greater action on freshwater quality."

With between 18,000 and 34,000 cases of gastroenteritis per year, New Zealand already had relatively high rates of waterborne illness compared with other high-income countries.

By 2050, the World Health Organisation had suggested New Zealand could see about one to three more deaths of children due to all causes of diarrhoeal disease as a result of climate change.

Today's report further highlighted a "significant impact" of higher temperatures on increasing diarrhoeal disease transmission, and increasing risk of illness which could range from days off work to hospitalisation.

Here and overseas, extreme rainfall events had been linked to increased levels of harmful micro-organisms like norovirus, and those causing cryptosporidiosis and giardiasis diarrhoea in treated drinking-water supplies.

Havelock North's gastro disaster - which affected more than 5,500 people last August - was New Zealand's single biggest outbreak of waterborne illness. Photo / File
Havelock North's gastro disaster - which affected more than 5,500 people last August - was New Zealand's single biggest outbreak of waterborne illness. Photo / File

In our streams, concentrations of salmonella and E. coli could rise significantly over summer months, and after heavy rain.

The bacteria Leptospira, introduced into water from the urine of infected animals, could further bring illness, ranging from nausea to renal failure.

Small community or private groundwater wells, and other drinking water supplies where water was untreated or minimally treated, were especially susceptible to contamination after heavy deluges.

Flooding in South Dunedin in June 2015. Photo / File
Flooding in South Dunedin in June 2015. Photo / File

In ocean waters, the marine bacteria vibrio - whose growth rates were highly responsive to rising sea surface temperatures, particularly in coastal waters - could cause infected wounds, or diarrhoea or septicaemia if it contaminated sea food.

Algae, also fuelled by a warming climate, could cause problems in our waterways and shores.

In rivers and lakes, blue-algae produced toxins could cause liver damage, skin disorders, and gastrointestinal, respiratory and neurological symptoms.

In our marine environment, climate change could mean toxic algae spreading and becoming more abundant and toxic.

Polluting the air

Rising air pollution is a well-known risk of climate change - but in New Zealand, it isn't just smog we would have to worry about.

Higher concentrations of CO2, together with higher temperatures and changes in precipitation, might extend the start or duration of the growing season - increasing the quantity and allergic potential of pollen.

Historical trends had shown climate change has forced shifts in the length of the growing season for certain plant species that were sources of allergenic pollens.

Studies had also found that increases in CO2 levels resulted in greater pollen production, and increased allergic potential of grass and pine trees.

Studies have also found that increases in CO2 levels resulted in greater pollen production - and increased allergic potential of grass and pine trees. Photo / File
Studies have also found that increases in CO2 levels resulted in greater pollen production - and increased allergic potential of grass and pine trees. Photo / File

While grasses formed the main source of atmospheric pollen in spring and summer here, annual birch pollen production in 2020 and 2100 was projected to be 1.3 and eight times higher respectively - with pollen season also arriving several weeks earlier.

Meanwhile, exposure to PM2.5 and PM10, two major man-made air pollutants, could rise with heightened seasonal fire severity.

Patients with underlying diseases, the elderly, and children were particularly sensitive.

Even in 2012, exposure to PM10 was estimated to have caused 1000 premature deaths and more than 500 hospital admissions in New Zealand.

PM2.5 was linked with serious chronic and acute health effects, among them lung cancer, chronic obstructive pulmonary disease, cardiovascular disease, and asthma.

The amount of soil-derived PM10 dust in the air could also increase in areas more frequently affected by drought.

In Masterton, for example, soil had been found to contribute up to 14 per cent of the PM10 particulate matter in the air.

A new tide of pests and disease

We know there are many organisms - notably mosquitoes, ticks, and fleas - capable of spreading infectious diseases between us, or from animals.

The seasonality, range, and occurrence of diseases spread by these carriers are largely influenced by climatic factors - especially high and low temperature extremes, and precipitation patterns.

These factors can affect disease outbreaks by changing the population size, population density, and survival rates of the disease carriers.

Further, climate change could affect the relative abundance of other animals that are part of the disease cycle - and higher temperatures can increase the infectious agent's own reproduction rates.

Collectively, these changes could contribute to an increase in the risk of the infectious diseases being spread to humans in some areas.

There are many organisms - notably mosquitoes, ticks, and fleas - capable of spreading infectious diseases between us, or from animals. Photo / File
There are many organisms - notably mosquitoes, ticks, and fleas - capable of spreading infectious diseases between us, or from animals. Photo / File

We would face an assortment of nasty bug-borne diseases we don't have here already.

Worst among them are West Nile virus, dengue fever, Murray Valley encephalitis, Ross River virus and Barmah Forest virus.

Similarly, there were emerging pathogens that had recently spread across the globe - such as mosquito-borne Zika - which were present in the Pacific Islands today.

These could become more of a risk in New Zealand if climate change allowed important disease-spreading mosquitos to become established here.

Added to that were a host of parasitic diseases, also characteristic of warmer climates, that might arrive here, via flies whose larvae infested skin, or lung fluke-carrying snails.

Harmful species such as sea snakes or toxic jellyfish might similarly extend their range, while the invasive Australian redback spider, presently found only in Central Otago and Taranaki, could cross into other regions as they warmed.

The mental and social toll

Higher temperatures, extreme weather events and displacement of people from homes and communities would all have a major toll on our mental health and wellbeing.

Effects could range from minimal stress and distress symptoms to clinical disorders such as anxiety, depression, post-traumatic stress and suicidal thoughts, the report authors said.

Research in Australia during the decade-long drought which officially ended in 2012 revealed an increase in anxiety, depression, and possibly suicide in rural populations.

A projected increase in drought frequency will put pressure on our rural economy, researchers say. Photo / File
A projected increase in drought frequency will put pressure on our rural economy, researchers say. Photo / File

In these communities, concerns about financial and work-related issues were compounded by loss of hope for the future and by a sense of powerlessness or lack of control.

For Kiwis, our natural environment is at the heart of our nation's identity - profoundly so for Maori - and shapes our economy, lifestyles and culture.

"There's going to be a reduction in rain, particularly on the eastern side of the country - and more rain on the west - but the increase in drought frequency is going to put a lot of pressure on our rural economy," University of Auckland epidemiologist and biostatistician Professor Alistair Woodward said.

"We know that there is a relationship between the rural economy, the welfare of the people working in the rural economy, and the frequency of mental health problems."

Disruption of cherished bonds between individuals and their environment, such as during the managed retreat of threatened coastal communities, could cause grief, loss, and anxiety.

Even routine exposure to news articles like this one can add stress to an individual's everyday environment.

University of Auckland epidemiologist and biostatistician Professor Alistair Woodward. Photo / File
University of Auckland epidemiologist and biostatistician Professor Alistair Woodward. Photo / File

Between 2005 and 2016 an average of 422 articles were published each month mentioning climate change or global warming in print and online media in the New Zealand region, according to the global media database.

In the United States, psychological responses to such stress have been shown to include heightened risk perceptions, general anxiety, pessimism, helplessness, eroded sense of self and collective control, stress, distress, sadness, loss, and guilt.

For Maori, climate change threatened to further entrench patterns of social disadvantage and unacceptable health inequities, said Dr Rhys Jones, an Auckland University senior lecturer and co-convenor of OraTaiao: NZ Climate and Health Council.

"On the flip side, the report highlights the significant health benefits that could be realised through well-planned climate action."

A wake-up call

Mitigation measures could actually lower air pollution and its associated health problems - and dozens of lives could saved each year if we moved from driving cars to cycling.

One assessment of the effect of retrofitting houses in New Zealand with insulation suggested there could be savings of 217kg of CO2 per household per year through improved energy efficiency.

At the same time there would be fewer inpatient hospital respiratory admissions for the elderly, days off school for school-age children, and days off work for adults.

"A clear implication is that New Zealand must act urgently as part of global efforts to address climate change, and must do so in ways that centralise and prioritise the most vulnerable groups in society," Jones said.

Auckland public health expert Dr Rhys Jones. Photo / Supplied
Auckland public health expert Dr Rhys Jones. Photo / Supplied

"A business-as-usual approach would predictably see the opportunities and benefits accrue disproportionately to those who are already privileged, leading to widening social and health disparities.

"This makes it critical that health and equity are at the core of decision making as we transition to a zero-carbon society."

Macmillan echoed Jones' call for urgent action.

"By bringing together such a comprehensive body of evidence, the report should be a wake-up call for us to shift from thinking about climate change as an environmental problem which will be expensive to address, to it being fundamentally an issue of health and quality of life for all New Zealanders," she said.

Indeed, many of the health effects described in the report were already beginning to occur.

"The report adds further weight to arguments that New Zealand can no longer afford to delay urgent action as part of the global effort under the Paris Agreement.

By switching from cars to bikes, we could see short-term gains in air pollution reduction and health benefits. Photo / File
By switching from cars to bikes, we could see short-term gains in air pollution reduction and health benefits. Photo / File

"The overwhelmingly negative impacts of climate change on health require a co-ordinated health sector response - to adapt health services to expected changes, to reduce the health sector's own climate pollution, and to ensure health is at the heart of climate decision-making."

New Zealand and climate change

• Under present projections, the sea level around New Zealand is expected to rise between 30cm and 100cm this century. Temperatures could also increase by several degrees by 2100.
• Climate change would bring more floods; worsen freshwater problems and put more pressure on rivers and lakes; acidify our oceans; put even more species at risk and bring problems from the rest of the world.
• Climate change is also expected to result in more large storms compounding the effects of sea-level rise.
• New Zealand, which reported a 23 per cent increase in greenhouse gas emissions between 1990 and 2014, has pledged to slash its greenhouse gas emissions by 30 per cent from 2005 levels and 11 per cent from 1990 levels by 2030.
• The new coalition Government has promised greater action, with a proposed new Climate Commisson and Zero Carbon Act and goals for a carbon-neutral economy by 2050 and 100 per cent renewable energy by 2035.