Climate change is projected to increase fire danger in almost all parts of the country - and push the average fire season length out by about 70 per cent within just two decades. Photo / NZ Herald
Heatwaves, deluges, rising seas: climate change isn't a future scenario but one that we're living in now.
How much worse might it get? That all depends on what happens over the next few decades – or even in the next one.
The best case scenario is that the world succeeds in ramping down man-made emissions of carbon dioxide, within this century, to net zero.
Emissions might even have to be forced down to negative toward the end of the century, something that would likely require CO2 to be actively sucked back out of the atmosphere.
Mean global warming would likely flatten off a little under 2C above pre-industrial temperatures – the target number of the Paris Agreement that New Zealand and around 200 other nations are signed up to.
But even under this optimistic, and perhaps unlikely, trajectory, our world would still be transformed, with a big change in extreme events like floods and droughts.
Across the globe, rainfall levels could rise or drop by 10 per cent, and the occurrence of heatwaves and the risk of forest fires would soar.
Warming could reach up to 4C near the Arctic, while global sea level rise could climb anywhere between 0.28m and 0.61m by the close of the century, with more to come – maybe even 7m over 1000 years – as greenhouse gases gradually fell.
A high-carbon world – as the world is on track to experience if it continued to pump out CO2 at its current rate – would be much bleaker.
"By just another human lifetime away, people could be living in a climate outside anything that any of us have ever experienced," Victoria University climate scientist Professor James Renwick said.
"Where we live, where we grow food, how we live our lives – all of these things will change completely. The potential for mayhem is astronomical and it really could be catastrophic."
With continued strong emissions, global mean temperature rises were likely to rise by somewhere between 2.6C to 4.8C, plunging the planet into a climate not seen for at least several hundred thousand years.
The Arctic could lose of all its summer time sea ice and oceans could be 1.5m higher, with another 10m locked in over coming centuries.
Major food-producing areas in existence today would be badly affected, much of the tropics would become uninhabitable, and extreme high temperatures would make many mid-latitude areas hard to bear in summer.
Added to that could be quadrupling or more of heavy rainfall and flooding events - with some events well beyond anything observed to date.
In New Zealand, only another metre of sea level rise would add 116,000 people to the 72,000 – not to mention $12.5b worth of buildings – already exposed to extreme coastal flooding.
Hundreds of thousands more lay in the path of potential flooding from storm-swollen rivers.
"Here, temperatures might be 3C or 4C higher than today, and extreme high temperatures would be completely off the scale – warm summer days in Canterbury and Hawke's Bay might be reaching into the high 40s," Renwick said.
"Most of New Zealand could be in what I'd call a climate that's subtropical, at least, with heavy rainfall events possibly double the amounts that we're getting today.
"In drier, eastern parts of the country, there would be very long, very severe droughts. The risk of forest fires would have gone up by a factor of probably five, at least. In parts of New Zealand, you'd get conditions not unlike those in the south-east of Australia today, with constant bushfires."
By the end of the century, assuming greenhouse gas emissions continued to climb close to current levels, many parts of our country will record more than 80 days a year above 25C.
Most places typically only have between 20 and 40 days above that now, yet, already, about 14 elderly people in Auckland and Christchurch die each year when the mercury climbs above 20C.
If global temperatures climb just one, two or three degrees above current levels, that same death rate could rise to 28, 51 and 88 respectively.
With about one in four New Zealanders projected to be 65 and over by 2043 - that's including many people in their 40s today - the problem will be amplified.
Despite that, New Zealand might be seen by the rest of the world as a comparatively nice place to live.
"We could be looking at large numbers of migrants wanting to come here, as is happening with people trying to sail across the across the Mediterranean from North Africa into Greece and Italy."
In the Pacific, where about half the population live within 1.5km of the ocean, even a further 2C of warming could see small island countries inundated by sea level rise.
The 180,000 people living in low-lying Kiribati, Tuvalu and the Marshall Islands would be among the most threatened.
Another estimate predicted that some 75 million people from the wider Asia-Pacific region would be forced to shift by 2050.
"We are also talking about resource scarcity, disruption, loss of land, conflict over food and water – all of that could lead to a complete breakdown of the rule of law in many countries."
Already, crop yields have declined in 30 countries because of global warming, raising concerns about malnutrition in some regions, while the risks of dengue fever and malaria have markedly increased around the planet.
The most recent figures show that, if warming continues at the current rate, the Paris Agreement's aspirational threshold of 1.5C – and that of New Zealand's Zero Carbon Bill - will be crossed at some point between 2030 and 2052.
To keep within that mark, carbon dioxide emissions will need to be halved over the next decade, and other greenhouse gases like methane and nitrous oxide will also need to be forced down.
If the 1.5C threshold can be held, the world could escape an extra 10cm of sea level rise, over and above what's already been locked in for this century.
If the world can't hold the line, then the 2C threshold - the ultimate limit the Paris Agreement was built around - could be over-shot only about a decade later.
Given the past trajectory of global emissions of greenhouse gases - nearly a doubling in the past 30 years – climate scientists say turning this picture around requires "far reaching and unprecedented changes" to every aspect of society.
Renwick believed our attention shouldn't be fixed on worst case scenarios – psychological studies have suggested too much focus on doomsday narratives can be counter-productive to taking action – but these should at least motivate us.
"We should all be thinking about how good our future can be, and how do we get there," he said.
"We should be imagining a sustainable future, where our power is renewable and we're not digging up coal.
"Because this warmer future hasn't happened yet, it's hard for people to take seriously. But I can guarantee that climate models do not lie. If we don't take action, it's going to be a very difficult future."
This story is part of the Herald's contribution to Covering Climate Now, an international campaign by more than 170 media organisations to draw attention to the issue of climate change ahead of a United Nations summit on September 23. To read more of our coverage go to nzherald.co.nz/climate
Today's briefing is set to spell out what commuters can expect in coming months.