New Zealanders born today will live to see their country’s great glaciers shrink into extinction. No wonder there’s a boom in the number of people going to see them.

On the gentle, stony path that weaves through rain forest at the base of the Southern Alps, a red arrow on a large sign points to the ground and says: ‘Here is where it used to be’.

There are more signs along the path. Once, it was here, and then it was here. This is a historical timeline, beginning hundreds of years ago and moving into the recent past, through podocarp rainforest and Rātā trees that blaze in the summer, trees that get younger and younger until you reach the viewing platform where the timeline ends with a grey and distant glacier, recoiling up a valley.

There are hundreds of thousands of glaciers in the world, but Franz Josef is special, largely because of the path that leads to it. The glacier starts high in the Southern Alps with a heavy, inaccessible snowfield, but drops steeply into a valley a few hundred metres above sea-level, ending with a braided river flowing beside rain forest to the ocean.

Glaciers usually reside in the hostile places where people are not, like Greenland and Antarctica. Franz Josef is just a short walk from a sprawling car park, minutes from the comfort of a tourist village.

Like glaciers all over the world, the modern story of Franz Josef is one of decline. During the last ice age, it surged many kilometres further in a glistening wall of ice. By the time it was found and named by colonial settlers in the mid-19th century, it reached where the first viewing platform is today. Now, it is several kilometres further back, high up a valley, its tongue severed.

The irony is that although Franz Josef glacier is likely the smallest it has been for many thousands of years, it has never been in higher demand. Tourism has boomed to an unprecedented level, largely through a buoyant Chinese tourism industry - in the summer months, the entire town is booked out. The township must grow to meet that demand.

It risks, quite literally, building onto thin ice. The glacier will continue to shrink, likely at a rapid pace as the world gets warmer. Even if the world stopped polluting the climate today, the retreat would not stop, although it would probably slow. There will come a time where Franz Josef will no longer be spectacular, at least from ground level, and visitors will stop coming.

It's a long term threat to those who live and work in Glacier Country, the engine room of the West Coast economy which has spawned a multi-million dollar industry, employing hundreds of people. But for now, business is booming. The ice remains.

A few days earlier, ex-tropical cyclone Fehi had landed on the West Coast, destroying roads and flattening buildings in Glacier Country. But nevertheless, the path to Franz Josef glacier is packed. The car park is straining with rented campervans and tour buses, their colourful slogans popping in the grey valley.

After a 15-minute walk through the forest, the glacier first comes into view with a viewing platform hanging over the riverplain below. There are young families and tour groups and grandparents with their grandchildren, some from China, others from Germany, Australia, the USA. Two-hundred years ago, they would have been standing on the glacier but now it is a speck on the horizon, partly covered by a patch of low, dark cloud, unspooling down the ice.

An older German couple, Edeltraud and Wolfgang Mueller, have perched in a spot amongst the crowd.

Wolfgang was last here as a young man 48 years ago, he says, when the glacier roared down the valley, near where he is standing now.

It is much smaller now: “It looks completely different,” he says. “I expected that”.

Edeltraud, visiting for the first time, sees the withering glacier as a warning.

“Twenty or thirty years ago, we thought about climate change and everybody said ‘oh, you’re silly’,” she says.

“But now we have it, it’s right here.”

We know climate change influences massive storms and rising seas, acidifies the oceans and kills forests; we know it can increase the range of diseases, while reducing the range of rare species. But when we see those things, they can seem one step removed from the process of warming.

A glacier is useful because it is simple. The way it sheds ice in chunks and leaks meltwater through its tongue, how it slowly retracts into the mountains, taps into a basic truth that everyone knows: Ice melts in heat.

“I think they’re the most valuable measure of climate change,” says Dr Trevor Chinn, a glaciologist.

“Every single item to do with climate is fed into them.”

A glacier is effectively a frozen river, which starts when snow falls faster than it melts. As the snow piles up, it compresses into a thick slab of ice which becomes unsustainable, squeezing ice slowly through steep valleys. In person, a glacier looks static, but take a photo every hour and you see it is constantly moving, sometimes by a few metres every day.

If we did not have glaciers trapping the water in the skies, the oceans would be 60m higher, and many low-lying cities would not exist. During the last ice age, the glaciers held so much water that sea levels were more than 100m lower than they are today: Cook Strait would have been a land bridge, if anyone had been around to walk it, and glaciers completely covered the Southern Alps, effectively separating the west from the east.

The study of glaciers used to be obscure, but has entered a renaissance period. By looking at one glacier, a scientist can begin to understand the climate it exists within and make projections about the future, which has stark implications for monitoring climate change.

This is particularly apparent in New Zealand, which is one of the best places in the world to study glaciers and climate. Our best-known glaciers are steep, accessible, and warm, due to the maritime climate of a land surrounded by ocean, which makes them susceptible to minor temperature changes.  

“The ice in the glacier is only just below zero degrees, so if you think about that, you don’t have to change temperature very much to bring it up to melting,” says Dr Heather Purdie, a glaciologist at the University of Canterbury.

“And [glaciers] are not just measuring temperature, they’re measuring precipitation, snow, cloudiness - all these other general climate parameters are homogenised into the glacier, and it’s providing an average picture of regional climate for the area that it’s in.

“If we kind of watch what they’re doing, we can see what’s happening. They’re a really good independent measurement of climate.”

It used to be that a glacial pace, in common usage, was synonymous with slowness. Today, a glacial pace is quick and unpredictable; ice that formed over thousands of years can dissolve and vanish over decades.

In New Zealand, this effect has been most obvious at Franz Josef and Fox glaciers on the West Coast, which are steep, accessible, and acutely sensitive to temperature changes.

Even among the world’s many glaciers, Franz and Fox are extraordinary, because of how closely they follow the climate. All glaciers have a “response time”, which is how long it takes for the glacier to respond to temperature changes.

For many large glaciers, it’s a couple of decades; larger glaciers can have response times as long as a century. These glaciers are still in the climate of the past, slowly catching up with the polluted climate of the present.

Franz Josef and Fox, due to their size and their maritime climate, have smaller response times. Franz’s is only three or four years, and Fox’s is five or six years, which means we can see how they’re being affected by today’s climate.

By following the shifts in the glacier, we can see what the slow burning climate signals are unable to tell us: What we are doing right now is transforming the natural world, which is changing right in front of us.

At the centre of Franz Josef glacier is a round, exposed rock face known as the “black hole”.

It first appeared as a small, black mark in the early 2000s after heavy rain, and has grown to become a glaring blot on the glacier.

When the ice flow reaches the black hole, it splits into two streams, cascading down each side before rejoining as one to flow down the tongue.

Its name has a fitting quality for a retreating glacier; the ice retracts further up the valley, as if being sucked into the black hole.

Within the next few decades, it is likely the glacier will end somewhere around the black hole, which is above where most of the glacier walks are done today. Projections for the glacier’s retreat in the future differ, and largely depend on the extent of warming, but it’s expected much of Franz Josef’s tongue will be gone by the end of the century.

By then, the glacier will be all but unrecognisable.

The first photograph of Franz Josef, taken around 1870, shows an enormous, jagged wall of ice thundering down the valley, nearly as tall as the surrounding mountains. Stand in the same spot today, and you see very little; the ice has thinned by many hundreds of metres and curled around a corner.

Franz Josef retreated drastically throughout the 20th century, as temperatures slowly started to warm. It lost about 3km of its length in total, exposing the gorge and the river flowing beneath for the first time in many centuries.

While the retreat in the 20th century was significant, it hit a higher gear around 2008. In just a decade, it has lost another 1.4km of its length, the fastest rate of retreat ever recorded at the glacier.

The result is that the glacier is likely the smallest it has been in a very long time. There is preliminary work which shows that very little of the rock at the front of the glacier has been exposed to the atmosphere before, meaning those studying it are the first humans to set foot there, on land that for millennia was buried deep in the ice.

From a ledge high above Franz Josef, Dr Brian Anderson, a glaciologist at Victoria University, looks down at the steep, ice-carved valley below.

Until recently, the gorge had been covered in ice, concealing everything beneath. Plants are now starting to grow where the glacier once filled the valley, which have left a shadow along the walls, marking where the glacier used to be.

“I’d never seen this gorge until the last decade, so I never knew what was under here,” Anderson says.

When he was growing up on the West Coast, Anderson spent a lot of time in the mountains. For much of his life, the glaciers were advancing, growing in response to a localised cooling period that lasted throughout the 1980s and 1990s, a period of growth that was both spectacular and unique.

When he started studying Franz Josef as a glaciologist, he visited every month for 12 years to measure stakes he had drilled into the ice, to see how far the glacier had moved.

It was an old-school style of glaciology, he says, similar to how the glacier was measured earlier in the century - on one trip, whilst scrambling around the forest, he found several of the stakes used to measure the glacier many decades earlier, which had since been forgotten in the forest that grew in the glacier’s wake.

When Anderson wrote his thesis about Franz Josef in the early 2000s, he projected a sharp retreat in the coming years, in response to the warming climate. When the retreat began in earnest in 2008, even he was surprised at how quickly the glacier started disappearing.

“It’s faster than I thought it could possibly retreat, to be honest, and it’s the fastest in the historic record,” he says.

He has started using more modern tools to chronicle the glacier’s movements. He has a network of nine strategically placed cameras taking photos every hour, which he stitches together into timelapses. They are in obscure spots off the beaten track so they won’t be disturbed, and lodged in boxes, mostly to protect them from kea. He has lost two cameras to lightning strikes, but the rest have survived, feeding a collection of what is now 100,000 photos.

One of his time lapses shows a year of retreat at Fox Glacier in 2012, in which a large chunk of its tongue collapsed. It went viral, because it was a stark illustration of how dramatically a glacier can change in a remarkably small period of time.

“I see all these things that I didn’t really realise were changing," he says.

"Basically, everything’s moving, everything’s coming downhill, there are little rockfalls everywhere, it’ll rain a lot and it’ll flood and masses of ice will fall off the glacier, just all these things you wouldn’t necessarily notice just by visiting.”

His latest findings show something quite extraordinary: Franz Josef is advancing. Since the end of 2016, it has crept forward by about 80m, which would make it one of the few glaciers in the world that is growing, not shrinking.

It’s not much, Anderson says - an 80m advance after a 1400m retreat is one step forward after 18 steps backwards - and he has no doubt it will retreat again. Already, after a historically warm summer, the glacier appears to be leaking, with a large hole spurting water that wasn’t there last year.

But it’s a rare glimmer of hope in a field that may one day become redundant, once the glaciers are all but gone.

“How many advancing glaciers are there in the world that you can go and visit?” he says.

“It’s probably only a handful.”

He pauses as a helicopter passes overhead, the deafening sound of its rotors filling the empty space of the gorge, dropping off another load of tourists onto the ice, tiny black dots against the sprawling white ice.

Anderson is not hopeful that emissions can be curbed to keep the glacier looking like it does now; more retreat is inevitable.

It has already happened so quickly he can see the retreat through his own children. He walked onto the glacier with his first child, but not with his second. By then it was too late.

But right now, for a brief moment, the glacier is growing, and it still looks spectacular.

“It’s really special that the glacier’s advancing at the moment,” he says.  

“It’s probably not going to do it for very long, just because it’s so warm. But even though the big picture is one of retreat and a really obvious human cause for that, I think we also have to appreciate what we have, which is still really special.”

On February 22, 2011, shortly after midday, a sound like a rifle shot echoed through the mountains. Moments later, the front of New Zealand’s largest glacier was shaken loose, crashing into the lake beneath.

The broken ice was as long as 12 rugby fields, and weighed around 30 million tonnes. It broke into dozens of icebergs, which were visible from a NASA satellite, and slowly floated to the edge of the lake at its base.

It happened thirty minutes after an earthquake caused severe damage in Christchurch, several hundred kilometres away. The ice had been on the cusp of breaking off for months, but it was likely - although not definitively proven - the jolt of the earthquake that served as a final, tectonic kick.

There have been similar calving events since then. The local tourism operator has reported icebergs 50m high, shimmering blue giants floating above the gunmetal grey of the meltwater lake.

In the 1970s, Lake Tasman did not exist, but now it is 7km long and several hundred metres deep, growing by the day. Icebergs bob on its surface, which are circled by bright yellow boats from the local tourism venture, which offers boat tours to see the glacier.

Tasman Glacier has reached what is known as a tipping point: It has melted to such an extent that it cannot come back, due to the lake that has developed at its base. Because the glacier now ends in a lake, which cuts at the glacier’s base from below, it will continue to shed ice until it runs out of mass, regardless of what the climate does, effectively stuck in a cycle of self-annihilation.

Within the coming decades, Tasman Lake will grow to its maximum size, slightly longer than nearby Lake Pukaki. Many other glaciers in the Alps are developing lakes: some look like small, crystal puddles, high on a peak, but others are starting to look like Tasman, swallowing the glacier that feeds them.

“What we see at Tasman Glacier is that it’s crossed this threshold where a lake’s formed, then the glacier started calving into the lake, and as that’s happened the glacier’s retreating more and more quickly,” Anderson says.

“I think that’s probably never happened before at Tasman Glacier, and there are various geomorphic arguments for why that would be. It kind of indicates the climate that we’re in is unlikely to have been this warm before.”

Much like its counterparts on the other side of the divide, Tasman has remained a hotspot for tourism, in spite of its retreat.

There are several companies operating on the newly formed lake, offering boat tours and kayaking trips. Because foot access is so difficult, the boats offer services to mountaineers who need to cross the lake to climb the glacier.

In the same way helicopters came to the rescue at Franz Josef and Fox, boats have done the same at Tasman.

“From a tourism point of view, there’s other opportunities,” says Heather Purdie, the University of Canterbury glaciologist.

“We’re getting smaller real estate, if you like, for walking around on, but we’re getting this whole new lake.  It’s an evolving landscape and people have to adjust to it.”

Purdie, who used to be a glacier guide, has studied the consequences of glacial retreat on tourism.

There has been a trend worldwide called last chance tourism, in which landmarks seen to be threatened by climate change have a surge in popularity.

It’s not entirely clear if that’s happening here - research showed it was a factor for some people visiting Franz Josef, but not overwhelmingly so - but if it does, it will add further pressure on the communities around the glaciers to expand.

“We get a few people telling us that’s one of their reasons, but it’s not coming out really strongly yet. Over time, we are waiting to see if it happens,” Purdie says.

“It’s really interesting, this idea of last chance tourism, that people are attracted to things that may disappear. If anything the glaciers are getting smaller, but people are still turning up, they still want to see them. It may be that some of the motivation is because they’re mindful about climate change and glacial retreat, so [maybe] people are turning up because they want to see one of those or take their kids to see that in case it’s harder in a few years time.”

The vast majority of our glaciers are in the clouds, where they are surveyed from a small, single-engine plane soaring through the Alps at the end of summer.

Scientists have been keeping track of the end of summer snowline on the Southern Alps since 1977.

The survey effectively shows how much snow and ice remains throughout the year. A higher snowline means more snow and ice melted on the mountain than it gained, suggesting the glacier lost mass.

The end of summer snowline is a good proxy for the health of the glaciers as a whole; and based on recent results, the diagnosis is terminal.

This year’s blistering summer prompted the bleakest survey yet. Large swathes of the mountains are barren, and a record number of glaciers lost all of their snow.

The mountains are now dotted with lakes, which have grown gradually as the glaciers above them melt.  Some are small, crystal puddles, burrowed into hollows near the peak, but others are increasingly large. Hooker, Mueller, and Douglas glaciers all have large lakes, which, like Tasman, once did not exist.

For many years, the snowline was stable. On average, between 1977 and 2012, the snowline ended at 1815m, which kept many of the thousands of glaciers with permanent snow throughout the year.

When the snowline rises, the mass of ice and snow on the Alps becomes smaller. In recent history, the Southern Alps likely reached their maximum snow and ice mass in the late 19th century, with between 100 and 140 cubic kilometres covering the Alps.

By 1977, that had reduced to around 54 cubic kilometres. Today, it is around 40 cubic kilometres, roughly a third of what was once there.

When it gets warmer, it is likely our largest mountain range, the spine of the South Island, will lose much more snow and become a shadow of its former self.

“If, by 2040, we increase temperatures by a degree, we push the snowline up to 1910m,” says Dr Jim Salinger, a climate scientist at University of Auckland

“If we push it up 2C, it goes to 1995m; if we push it up 3C we have a snowline of 2085m. If you have that [3C], I would imagine you would have about 25 per cent of the snow and ice that we have today.”

Even with 1C of warming, an impossibly optimistic target, the mountains would lose significant volumes of snow and ice, which will have a huge effect on the glaciers, Salinger says.

“They’re going to retreat very quickly.”

The Māori name for Franz Josef is Kā Roimata a Hine Hukatere, which means The Tears of Hine Hukatere.

Hine was a snow maiden who fell in love with Wawe, a warm-blooded man. Every time they touched, his warm hands would melt her frozen skin, so Hine, an experienced mountaineer, took Wawe up the mountain to see Aorangi, the frozen son of the sky father Rakinui, for guidance. Wawe had been forbidden from climbing the mountain and was blown off the edge by a vengeful Tāwhirimātea, the God of wind, sending Wawe plunging to his death. Hine Hukatere wept for Wawe, and her tears froze as they dripped down the valley forming what many now call Franz Josef glacier.

The glacier’s existence, in that telling, is the product of grief. But in its time, particularly at its peak, it has been seen as something joyful.

There used to be a farmhouse overlooking the glacier, with a large orchard, a garden, and cattle grazing the fields. When James Cowan, a prominent travel writer, visited in the early 20th century, mist hung above the farmhouse in the cold air, dissolving into smoke from the fireplace.

“The soft dreamy tenderness of it all, the home of man, the curling blue smoke, the mountains of white and purple, make up a vision of wonder whose many tones and tints blend into a sweetly ordered whole,” he wrote.

“Let the landscape artist seek that scene by Waiau’s gorge, and paint it - if he can.”

Stand on Franz Josef Glacier on a warm summer’s day and evidence of its doomed fate is obvious. It appears to be leaking, polar blue meltwater dripping down the glacier like sweat. Deep bellows echo through the valley when slabs of ice fall off its steeper slopes.

The glacier guides have to dig new paths every day, because of the way the glacier is in constant movement, building its own deep crevasses and sharp overhangs and caves that look like they were hollowed by hand.

And yet, the withering glacier is more popular than ever.  There aren’t enough helicopters to take tourists over the broken towers of ice at the top of its tongue to land on the snowfield, which still seems to sprawl impossibly large.

Even at the first viewing platform, which will one day be out of sight of the glacier, the melting ice stills draws the crowd.

A toddler with his parents stares down the gloomy valley, with its low-hanging cloud and the white wisp of the glacier - he may be the last generation to see this glacier from where he stands. As the climate continues to warm, the likelihood of last chance tourists, wanting to bid their final goodbyes to the glaciers, arriving in large numbers grows.

For now, it is here. Edeltraud Mueller, the German tourist, looks at the glacier, retreating from the valley it carved over thousands of years, now returning to the mountains.

“It’s very beautiful,” she says.

“It’s extraordinary.”