In Altansukh Purev’s yurt, the trappings of a herder’s life lie in plain sight. In the corner are his saddle and bridle. By the door, he has left a milk pail. If you didn’t know better, you might think his horses and cattle were still grazing outside on the remote plains of outer Mongolia.

But they aren’t. Altansukh’s milk pail stands empty. There is no horse for him to saddle. His cattle are dead. And this tent, which once stood in the countryside, is now on the fringes of the Mongolian capital, Ulaanbaatar, surrounded by pylons, rubble and the husks of old cars. Altansukh, his wife and their four children may live among rural paraphernalia, but following a disastrously cold winter a few years ago, they were forced to move to the city to survive.

“We lost all our animals,” the 38-year-old says. “Thirty-nine out of 40 cows, almost 300 sheep. The cows wandered far away in the snow and never came back. And when we got up one morning, all the sheep had frozen to death. We had lost everything – so we decided to leave immediately for Ulaanbaatar.”

Look down on Ulaanbaatar from the hills at its edge, and you will see a central hotchpotch of new skyscrapers and crumbling Soviet tower blocks surrounded by an unplanned periphery of white yurts, or, as they are known in Mongolia, gers. Thousands and thousands of gers.

These are the homes of around 600,000 former herders who – like Altansukh – have migrated to the Mongolian capital in the past three decades. The scale of the migration is extraordinary: around 20% of the country’s people have moved to Ulaanbaatar, doubling the city’s population and significantly increasing its physical footprint.

“I arrived in Mongolia in 1998, and at that time there were still some areas of Ulaanbaatar that were totally empty,” says Mitsuaki Toyoda, the head of mission for Save the Children, which runs aid programmes in the city’s deprived suburbs. “But over the years it has just expanded and expanded.”

One partial explanation lies in the environment. Over the past 70 years, the average temperature in Mongolia has risen by 2.07 degrees, more than double the average global increase of 0.85 degrees over the past century. This has exacerbated a periodic weather phenomenon known in Mongolia as the dzud, which creates summers that are unusually dry, followed by spells during the winter that are unusually cold.

The dry summers make it harder to grow and harvest grass, while the harsher winters require an even bigger supply of fodder. When the cold hits, sheep and cattle have less fodder to feed on, causing widespread loss of life (for animals), and livelihood (for humans). Dzuds in 1999 and 2009 killed around 10 million and 8 million livestock respectively, and were followed by a spike in migration to Ulaanbaatar, government records show.

Aid workers fear further mass-movement in the coming months and years: another dzud is thought to be underway this winter.

Herding is a way of life for over a third of Mongolians, and of symbolic importance to the whole country. Thanks in part to rural-to-urban migration, it is now under threat.

But whether this migration is directly linked to climate change is a subject of debate. As the world warms, world leaders must prepare for the likelihood of refugees fleeing the weather rather than war – but the situation in Mongolia is too complex to be considered a straightforward test case.

“Climate change is responsible for some portions of the loss [of livestock] – but not all of it,” says Dr Batjargal Zamba, an adviser at Mongolia’s environment ministry. “It’s a combination of change to climate, and also a change to Mongolian economic activity and lifestyle.”

How much is down to the former, and how much to the latter?

The beginnings of an answer can be found about 370 miles (600km) south-east of Ulaanbaatar, on the edge of the Gobi desert, outside the ger of Begzsuren Nyangaa.

Begzsuren is 68, and started herding in 1956. Unlike Altansukh, he is still at it. So are his nine children, who all live within riding distance of his ger. One of them has the largest herd in the region, with 3,800 livestock, and another was once named the country’s herder of the year.

Staring from his tent, sipping tea boiled in milk, Begzsuren can see the effects of climate change in the valley outside. The land here has become much more obviously desert-like, he says. The grass grows shorter than it used to, and it is thinning out. There is also much less rain, and dried riverbeds can be found across the surrounding region.

“It’s a totally different picture now,” Begzsuren says. “When I was 17 or 18, we had a lot of rain, a lot of grass, and we could harvest the grass in the autumn. When I tell my children this, they don’t believe me! They say: how could this land produce that?”

The autumnal rains have virtually disappeared, he sighs. “I really miss those autumn days. Now the climate has changed so much – you can say we only have two seasons now.”

That’s a problem for herders. With little grass, there is less fodder to feed the animals over the winter. So when the cold snaps come, the cattle are at much greater risk of death.

But here in the Mongolian steppe, a changing climate isn’t the only challenge. Until the fall of the Soviet Union, Mongolia was a communist country. Herding was tightly managed by central government. Livestock was owned by collectives, officials decided where you lived, and there were restrictions on the number of animals in each herd. Most importantly, officials kept a central supply of fodder that they gave to herders during the harsh winters – meaning that when the dzuds came, the worst could be averted.

“Because of the socialist system, everything was well organised,” remembers Begzsuren. “Fodder was given to each family. There was a lot of winter preparation.”

But that all changed in 1990, when communism ended, and the state was opened up to the market. Many herders welcomed this: they could now own as many animals as they liked, and live where they wanted. The number of livestock had remained steady under communism – fixed at around 20 million for roughly half a century. Now the headcount shot up – to over 33 million by 1999, and 70 million by this summer, according to government statistics.

But there were also downsides. State support vanished, leaving herders to deal with the dzuds on their own. As private traders, they also needed to move closer to markets in order to turn a profit. Coupled with a rise in the number of livestock, this meant that more animals were grazing on less land. The climate may be getting worse, but political and social change has made Mongolians less able to deal with it, and therefore more likely to migrate to Ulaanbaatar.

ust as importantly, they are now allowed to do so. Under communism, herders could move to find better grasslands, but they could not migrate to the city. After 1990, this restriction was lifted – allowing people to leave herding, whether or not they were affected by the worsening climate. And for some, the dzuds are not the only push factor. Many also head to the capital in search of better schooling for their children.

“We considered both,” says Narmandakh Sainjargal, the wife of Altansukh and mother of their four children. “We lost all our animals, and a lot of other migrant families lost animals as well. But we also felt that it might be easier to find education in Ulaanbaatar.”

The situation in Sukhbaatar, Begzsuren’s home province, shows how migration can still be curbed when herders receive the kind of support that they lost back in the early 90s.

On the face of it, Sukhbaatar should be one of the main areas of origin for migrants arriving in Ulaanbaatar. A bumpy drive with the region’s deputy governor shows how badly it has been affected by recent weather patterns.

“Look at all this,” says Amarsanaa Byambadorj, squatting down in the desert. “It’s July, the height of summer, so the grass should be at its greenest and longest. But instead it’s very short, and it’s almost yellow.”

Nearby we find a sickening sight: a pile of dead livestock, rotting in the sun. More than a million animals died in last winter’s dzud, and this area in south-east Mongolia was one of the worst affected. Around 10 of them lie here in the dirt.

There used to be many more, until Save the Children cleared them away during the spring. “In March, it was terrible,” says Amarsanaa. “There were carcasses everywhere, and you could smell the stench.”

More than 10% of the 400,000 livestock in the area were killed, he says, and some families lost all their herd. The deputy governor introduces Byambadorj Zayabaatar, a 30-year-old who lost all his 320 animals during a cold snap towards the end of the winter. “I could see the animals were so cold, so in the beginning I would bring them inside the ger,” remembers Byambadorj. “But after a while there were too many, so I had to leave them outside. And they died before my eyes.”

Yet very few people from this area have considered moving to Ulaanbaatar. That is partly because there is little history of rural-to-urban migration from this area – most migrants in Ulaanbaatar come from western Mongolia. But it’s also because the government and various international agencies are increasingly stepping in to provide the outside help that had been lacking since the fall of communism.

This winter, Mongolia’s National Emergency Management Agency partnered with the UN development programme and NGOs such as Save the Children to provide fodder and other support to the worst affected areas.

Save the Children has been particularly active in Sukhbaatar. It paid for the worst-affected families to replace their dead livestock with animals from neighbouring herds that had largely survived the winter. The programme has been so successful that it has tempted the few families who had already left for the city to consider returning. “We have received many comments from herder families saying that if you continue to restock people’s herds, we would like to come back,” says Amarsanaa, the deputy governor.

Additionally, home-schooling programmes have been provided to herder families who live in remote areas, far from school, and who might otherwise consider migrating to Ulaanbaatar to further their children’s education.

“This way, they can live their nomad life, and the kids can still live at home – without the family having to move anywhere,” says Javzandulam Myagmarjav, a local librarian who will help run the project until funding runs out in 2017. “I’m quite sad it’s ending,” she says. “It helps herder families, and the long-term benefit will be that there is less migration to Ulaanbaatar.”

Yet even if people stop coming to Ulaanbaatar, the scale of recent migration will still have changed the capital beyond comprehension, and left it with a myriad of unsolved social challenges. Roughly half the population are migrant families living in the tent districts on the edge of the city – unable to access many of its services.

In Altansukh’s neighbourhood, there is no running water, mains electricity, sewage or central heating. And it’s a similar situation in all the migrant districts. To keep warm during the winter, migrant families are therefore forced to burn whatever they can find – and the fumes make their neighbourhoods more polluted than Delhi.

The newcomers find the city disorienting, with the houses so close together, and society so focused on money. In the countryside, you can live off the meat and milk from your herd, and use their hides to build your home. But in the city, everything must be bought from other people.

“We always miss the countryside,” sighs Altansukh’s wife, Narmandakh Sainjargal. “As herders you don’t spend any money. You have your animals and they give you everything you need. But in the city you have to pay every day for something – transport, books for school. It’s very strange to be here. We have paid such a lot of money for this small piece of land, whereas in the countryside you don’t have to pay anything, and you have all this space.”

With their skills little suited to urban life, many struggle to find work, making the hidden costs of state education too much for some families. Those who can send their children to school find that the teachers have little time for them: classes are so overcrowded that sometimes teachers run three shifts a day.

“Some new schools have been built, ” says Toyoda, head of mission for Save the Children, which has helped to fund their construction. “But it’s not enough. Not much has changed since [the 1990s] in terms of the living conditions there. If you’re from the countryside with just secondary education and no relevant work experience, then what proper job can you get? If you don’t have a proper job then it’s very difficult to get a bank loan. So you can’t purchase an apartment. Our conclusion is that the first generation of migrants will live in the ger districts for the rest of their lives.”

And back in Sukhbaatar, one of the country’s oldest herders can’t rule out more migrants joining them. “If nature keeps changing, if the climate keeps changing, we can’t say it won’t happen,” says Begzsuren. “If natural disasters keep on happening, people won’t have any choice but to move.”