SOUTH HANGAY PROVINCE, Mongolia — They call it the zud, a prolonged period of heavy snows and paralyzing cold that adds to the challenges of living on a treeless expanse nearly the size of Alaska. But this year’s zud followed a punishing summer drought that stunted the grass and left Munkhbat Lkhagvasuren’s herds emaciated and his family in debt after borrowing money for fodder.

As the snow piled waist high this winter and temperatures plunged to 40 below zero, Mr. Lkhagvasuren crammed two dozen of the weakest goats and sheep into his yurt. The unlucky ones, more than 1,000 animals, froze to death in a great heap outside his front door. “I tried everything but could not fight against nature,” he said tearfully in a recent interview, the stench of rotting flesh overpowering despite a devilish wind. “I am broken and lost.”

Mongolia and its 800,000 herders are reeling from the worst winter that anyone can remember. According to United Nations relief officials, nearly eight million cows, yaks, camels, horses, goats and sheep died, about 17 percent of the country’s livestock. Even if the spring rains arrive soon, 500,000 more animals are expected to succumb in the coming weeks.

“This is not only a catastrophe for the herders but for the entire Mongolian economy,” said Akbar Usmani, the resident representative for the United Nations Development Program. “We expect the ripple effects for months and years to come.”

The last serious zuds, three consecutive harsh winters between 1999 and 2002, sent thousands of destitute nomads streaming into the capital, Ulan Bator. A decade later, their tattered yurts still crowd bleak neighborhoods on the city’s fringe as the former herders struggle to fit into the modern world. The United Nations estimates that the current disaster may prompt as many as 20,000 herders to abandon their nomadic life and flee to the city.

“A lot of the herders have no skills so they usually end up breaking the law and falling into poverty,” said Buyanbadrakh, the governor of a small administrative district, known as a soum, who like some Mongolians uses a single name. He said 70 percent of the livestock in his soum, Zuunbayan-ulaan, were wiped out this year with at least 2,800 families losing their entire herds.

With so many desperate nomads selling off their remaining animals to survive, the price of meat has dropped by half in recent months. “People are taking it very hard,” he said. “Some have gone crazy.”

The disaster poses a challenge to a government already struggling to address the needs of the third of the population that lives in poverty. But it also raises a host of thorny questions about climate change, environmental degradation and whether the pastoral way of life that sustains many of the country’s 3 million people has a future.

Mongolians are fiercely proud of their millenniums-old nomadic ways, best personified by the deification of Genghis Khan, the 13th-century leader whose horseback warriors conquered much of Asia and Eastern Europe. Although mining and tourism are a growing portion of the Mongolian economy, a third of the population still depends entirely on husbandry for its livelihood. “The key question we have to ask is whether this way of life is sustainable,” said Mr. Usmani of the United Nations. “It’s a very sensitive issue.”

Despite the severe winter, one of the more sensitive long-term issues, oddly, is how to curb the explosive growth in livestock, which has quadrupled to 40 million head since the 1990 revolution that ushered in democracy and ended a socialist system that tightly controlled the size of the nation’s herds to prevent overgrazing. Environmentalists and government officials agree that the two decades of unbridled privatization and a boom in cashmere exports upended the traditional mix of livestock, which had long favored sheep over goats.

In the past, sheep made up 80 percent of small-animal herds and goats the rest. But as the price of cashmere soared over the last decade, that ratio reversed, with devastating results for the ecology of the steppe. Voracious eaters, goats often destroy the grass by nibbling at the roots. Their sharp hooves also damage fragile pasture by breaking up the protective tangle of grass and lichens, allowing the wind to sweep away topsoil and encouraging desertification.

The other wildcard is climate change, which many herders blame for the increasingly inhospitable weather. Winters are longer and colder, the winds blow stronger and the summers, they say, are drier. “I don’t know what happened to the mild spring rains that the grass needs to drink,” said Degkhuu, 62, a lifelong herder who lost his entire flock. “Now, when the rains come they are heavy and create flash floods.”

A recent World Bank study found that hundreds of rivers and lakes had disappeared in Mongolia, and the diversity of plant species had plummeted by a third since 1997, although researchers partly blamed the proliferation of goats.

For the moment, the government is focused on clearing the millions of dead animals that litter the grasslands and are beginning to decompose now that spring has finally arrived. A work-for-cash program, financed with a usd1.5 million grant from the United Nations, pays herders to gather the carcasses and bury them in pits. It is grim work, but those lucky enough to get a spot on the crews are happy for the income.

At best, the money will only delay a looming crisis among families who have run out of food and are saddled with bank loans they took on to buy emergency feed. Mr. Lkhagvasuren, 34, the herder who lost 1,000 animals, said he owed over usd1,800, a huge sum given that the average Mongolian earns usd3,200 a year. He said he lost most of his most prized animals — horses, cows and about 200 yaks — and that it would take at least a decade to replenish his herd of goats and sheep, about 100 of which survived.

As he sat in his yurt drinking salty milk tea and smoking tobacco rolled in a strip of newsprint, a crew dragged off the carcasses and heaved them into rickety trucks. “I can’t bear to watch,” he said.