The last 50 years of gains in development and global health could be undone by the "medical emergency" that is the threat of climate change to human health, scientists said today.
However, the report by a new global commission, published in major medical journal The Lancet, showed comprehensive evidence that tackling climate change through ways like reducing air pollution and improving diet could be one of the greatest chances to improve global health this century.
According to present projections, the mean temperature in New Zealand could be 2C higher by the end of the century - and even between 3C and 4C higher if no action is taken to curb the world's carbon emissions.
Within the same period, sea level is expected to rise between 50cm and 120cm, leaving populations to adapt by either abandoning coasts and islands, changing infrastructure and coastal zones, or protecting areas with barriers or dykes.
But Professor Anthony Costello, co-chair of the commission and director of the University College London (UCL) Institute for Global Health, said climate change also had the potential to un-do the health gains from economic development that have been made in recent decades.
"Not just through the direct effects on health from a changing and more unstable climate, but through indirect means such as increased migration and reduced social stability," he said.
"However, our analysis clearly shows that by tackling climate change, we can also benefit health, and tackling climate change in fact represents one of the greatest opportunities to benefit human health for generations to come."
The report showed the direct health impacts of climate change came from the increasing frequency and intensity of extreme weather events, especially heat waves, floods, droughts and storms.
Indirect impacts came from changes in infectious disease patterns, air pollution, food insecurity and malnutrition, involuntary migration, displacement and conflicts.
Another co-chair of the commission, Professor Hugh Montgomery, described climate change as a "medical emergency".
"It thus demands an emergency response, using the technologies available right now," he said.
"Under such circumstances, no doctor would consider a series of annual case discussions and aspirations adequate, yet this is exactly how the global response to climate change is proceeding."
Health gains which could stem from action on climate change included a reduction in respiratory diseases from burning fewer fossil fuels, and the cuts in rates of obesity, diabetes, coronary heart disease and stroke that came with choosing to walk or cycle instead of drive.
There were also health benefits from changes to diet which might arise from a concerted effort to tackle climate change, such as eating less red meat.
The commission concludes that a strong international consensus is essential to move the world to a global low-carbon economy, harnessing a crucial opportunity to protect human health, particularly of the poorest and most vulnerable populations, who stand to be hardest hit by the effects of climate change.
Dr Alexandra Macmillan, a senior lecturer in environmental health at Otago University, said climate change was most significant health threat facing the world globally, but well-designed action provided "exciting opportunities" to save lives, reduce illness and create a fairer society.
"New Zealand will not be immune from the negative impacts, which are already being felt in the form of greater frequency and severity of extreme weather events, ocean warming and acidification," said Dr Macmillan, who is presently co-convenor of OraTaiao: The New Zealand Climate and Health Council.
"These are already having direct impacts on people's health and wellbeing, as well as threatening the building blocks for good health - people's houses, neighbourhoods, food sources and livelihoods."
On the other hand, well-designed climate action in areas such as transport, housing, energy and food production could result in good health and fairness gains.
"New Zealand specific research has already demonstrated that health benefits from good climate policy can be accounted for in assessing the overall costs and benefits of climate action."
Dr Rhys Jones, a public health physician and senior lecturer at Auckland University, said the commission's new report was important because it brought together evidence from a range of disciplines - among them biodiversity, economics, energy policy, engineering and political science - but with a primary focus on human health.
"The key message is that tackling climate change is likely to be the greatest global health opportunity of the 21st century," he said.
"That means that urgent action to move to a low - and, very soon, zero - carbon society is not just necessary to avoid the worst effects of climate change, but if done correctly it will be enormously beneficial for our health and wellbeing right now.
"In other words, action to rapidly reduce greenhouse gas emissions is a no-brainer - both globally and for New Zealand."
The Government is presently considering what its new emissions reduction target beyond 2020 will be, following a consultation period that attracted more than 10,000 submissions from the public and organisations.
New Zealand presently has a current unconditional emissions target of five per cent below 1990 levels by 2020, and 50 per cent below 1990 levels by 2050.