UNITED NATIONS — Nigeria. Syria. Somalia. And now Iran.

In each country, in different ways, a water crisis has triggered some combination of civil unrest, mass migration, insurgency or even full-scale war.

In the era of climate change, their experiences hold lessons for a great many other countries.

The World Resources Institute warned this month of the rise of water stress globally, “with 33 countries projected to face extremely high stress in 2040”.

A water shortage can spark street protests: Access to water has been a common source of unrest in India.

It can be exploited by terrorist groups: Al-Shabab has sought to take advantage of the most vulnerable drought-stricken communities in Somalia.

Water shortages can prompt an exodus from the countryside to crowded cities: Across the arid Sahel, young men unable to live off the land are on the move. And it can feed into insurgencies: Boko Haram stepped into this breach in Nigeria, Chad and Niger.

Iran is the latest example of a country where a water crisis, long in the making, has fed popular discontent.

That is particularly true in small towns and cities in what is already one of the most parched regions of the world. Farms turned barren, lakes became dust bowls.

Millions moved to provincial towns and cities, and joblessness led to mounting discontent among the young. Then came a crippling drought, lasting roughly 14 years.

In short, a water crisis - whether caused by nature, human mismanagement, or both - can be an early warning signal of trouble ahead.

A panel of retired United States military officials warned in December that water stress, which they defined as a shortage of fresh water, would emerge as “a growing factor in the world’s hot spots and conflict areas”.

“With escalating global population and the impact of a changing climate, we see the challenges of water stress rising with time,” the retired officials concluded in the report by CNA, a research organisation based in Arlington, Virginia.

Climate change is projected to make Iran hotter and drier.

A former Iranian agriculture minister, Issa Kalantari, once famously said that water scarcity, if left unchecked, would make Iran so harsh that 50 million Iranians would leave the country altogether.

Is water the reason for the latest unrest in Iran?

Not entirely.

Water alone doesn’t explain the outbreak of protests that began in early January and spread swiftly across the country. But as David Michel, an analyst at the Stimson Center put it, the lack of water - whether it’s dry taps in the city, or dry wells in the countryside, or dust storms rising from a shrinking Lake Urmia - is one of the most common, most visible markers of the government’s failure to deliver basic services.

“Water is not going to bring down the government,” he said. “But it’s a component - in some towns, a significant component - of grievances and frustrations.”

Managing water, he said, is the government’s “most important policy challenge.”

How did it get this bad?

Like many countries, from India to Syria, Iran after the 1979 revolution set out to be self-sufficient in food. It wasn’t a bad goal, in and of itself. But as the Iranian water expert Kaveh Madani points out, it meant that the government encouraged farmers to plant thirsty crops like wheat throughout the country. The government went further by offering farmers cheap electricity and favourable prices for their wheat - effectively a generous two-part subsidy that served as an incentive to plant more and more wheat and extract more and more groundwater.

The result: “25 per cent of the total water that is withdrawn from aquifers, rivers and lakes exceeds the amount that can be replenished” by nature, according to Claudia Sadoff, a water specialist who prepared a report for the World Bank on Iran’s water crisis.

Iran’s groundwater depletion rate is today among the fastest in the world, so much so that by Mr Michel’s calculations, 12 of the country’s 31 provinces “will entirely exhaust their aquifers within the next 50 years”.

In parts of the country, the groundwater loss is causing the land to sink.

Water is a handy political tool, and to curry favour with their rural base, Iran’s leaders - and particularly the Islamic Revolutionary Guards Corps - dammed rivers across the country to divert water to key areas.

As a result, many of Iran’s lakes have shrunk. That includes Lake Urmia, once the region’s largest saltwater lake, which has diminished in size by nearly 90 per cent since the early 1970s.

Does climate change play a role?

According to the government, Iran expects a 25 per cent decline in surface water runoff - rainfall and snow melt - by 2030.

In the region as a whole, summers are predicted to get hotter, by two to three degrees Celsius at current rates of warming, according to the Intergovernmental Panel on Climate Change. Rains are projected to decline by 10 per cent.

A 2015 study by two scientists at the Massachusetts Institute of Technology predicted that, at current rates of warming, “many major cities in the region could exceed a tipping point for human survival”.

For the leaders of water-stressed countries, the most sobering lesson comes from nearby Syria.

Its drought, stretching from 2006 to 2009, prompted a mass migration from country to city and then unemployment among the young. Frustrations built up. And in 2011, street protests broke out, only to be crushed by the government of Bashar Assad.

It piled on to long-simmering frustrations of Syrians under Assad’s authoritarian rule. A civil war erupted, reshaping the Middle East.

Water, said Julia McQuaid, the deputy director of CNA, doesn’t lead straight to conflict.

“It can be catalyst,” she said. “It can be a thing that breaks the system.” THE NEW YORK TIMES