Iran is experiencing a drought so intense that the country’s president has said the capital city, Tehran, might have to be evacuated. In an attempt to bring rain, aircraft began a cloud-seeding operation on 15 November that is planned to last through the traditional rainy season until May. But experts caution that this technique is challenging and unlikely on its own to make a major dent in the water crisis.

How bad is the drought in Iran?

Rainfall across Iran is 85 per cent below average, and Tehran has received only 1 millimetre of rain this year. Reservoirs in the capital and in nearby regions are in a “worrying state”, officials have said, with water capacity under 5 per cent at 32 dams. Satellite images some have dried up entirely.

Tehran residents have reportedly reduced water consumption by about 10 per cent, but that isn’t enough, officials say. Water pressure has been reduced at night, and authorities are planning to fine households and businesses that consume excessive water. If rainfall doesn’t pick up by December, the 14 million people who live in Tehran could have to start evacuating, the president has said.

What caused the drought?

Experts the climate crisis has contributed to the drought, which has already lasted for five years. Iran is experiencing its driest autumn in 50 years, and Tehran, which often gets snow in November, is seeing temperatures of 15°C (59°F) or more.

But poor management is the main cause of what a former Department of Environment official, calls Iran’s “water bankruptcy”. The government has massively expanded agriculture in dry areas, overtaxing water resources. Half a million illegal wells, many of them drilled by desperate farmers, have depleted the groundwater.

What is cloud seeding?

Cloud seeding was developed in the 1940s by scientists including Bernard Vonnegut, brother of the novelist Kurt Vonnegut. It involves dispersing particles that encourage the suspended water droplets in clouds to fall as rain. Although some have sprayed salt into low-lying clouds, many have focused on spreading chemicals, most commonly silver iodide, into higher, mixed-phase clouds. The supercooled liquid water droplets freeze on contact with this crystalline compound, forming ice crystals that grow heavy and fall as snow or rain.

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It is often hard to know how much rain or snowfall would have occurred without the cloud seeding, though.

“The effects are very difficult to show because of the large natural variability of clouds,” says at University Clermont Auvergne in France. “You look outside, you have a cloud field, and there are clouds that rain, and others don’t rain.”

An experiment in 2014 comparing two mountain ranges in Wyoming found that cloud seeding could strengthen precipitation by .

Can it solve the drought?

Iran, which previously accused Israel and the United Arab Emirates of stealing its rain through cloud seeding, now has its own programme that involves seeding agents from cargo planes, drones and , a term that typically refers to smoke furnaces on high mountains.

It said it seeded clouds on 15 November in the basin around Lake Urmia, which, over two decades, has dried up into a salt plain littered with rusty boats. Areas west of the lake received up to 2.7 centimetres of rain early the next morning, according to a run by the University of California, Irvine.

For a cloud-seeding campaign to replenish reservoirs, however, the clouds must contain a lot of water. That kind of cloud may be hard to find in arid Iran, where there aren’t many large water bodies to evaporate moisture into the air.

“Cloud seeding is often much more difficult during a drought because the atmosphere is so dry, and any clouds that are present may not have sufficient moisture,” says Karen Howard, a scientist at the US Government Accountability Office.

But masses of rain clouds have blown into Iran from the Black Sea in the past three days, even causing flooding in western provinces, including Ilam and Kurdistan, on 16 November.

Cloud seeding will be able to at least “squeeze out a few more drops” from weather systems like this, says at the University of Arizona. “It’s not going to lead to extreme things like flooding or solving widespread drought,” he says. “But it can help a little bit.”

Severe droughts that persist for years have grown hotter, drier and larger since the 1980s. These long-lasting droughts – some of which are extreme enough to be classified as “megadroughts” – can be especially devastating to agriculture and ecosystems.

Rising temperatures linked to climate change have increased the risk of drought because warmer air can hold more moisture, boosting evaporation from the land. Combined with changing precipitation patterns that lead to less rain, this can exacerbate and lengthen periods of drought – as witnessed in the recent worst-in-a-millennium megadroughts in parts of North and South America.

at the Swiss Federal Institute for Forest, Snow and Landscape Research and his colleagues identified more than 13,000 droughts that lasted at least two years between 1980 and 2018 to reveal long-term trends. They found that, since the 1980s, the most severe multi-year droughts have become even drier and hotter.

The droughts have also affected a larger part of the globe, with the area affected by the 500 most severe drought events underway in any given year expanding by about 50,000 square kilometres annually. “That’s an area bigger than Switzerland,” says Karger.

Satellite images of greenness in the areas affected by drought also showed some ecosystems became browner, indicating the drier conditions were having an effect. The most dramatic shift was in temperate grasslands, which are more sensitive to changes in water availability, while tropical and boreal forests showed a smaller response.

The researchers did not do a formal analysis to define how much human-caused climate change has contributed to the trend, but the patterns are consistent with what researchers expect to see with rising temperatures, says at Columbia University in New York, who was not involved with the research.

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The work highlights how long-term drought can have consequences just as severe as climate disasters like destructive wildfires or powerful hurricanes, says Cook. “For both people and ecosystems, the cumulative impact of droughts is really what matters.”

Grassland productivity drops by a larger amount than we had thought during extreme drought, according to experiments at 100 sites around the world. The finding suggests plants may struggle to cope with the more frequent and severe droughts that climate change is expected to bring.

at Colorado State University and her colleagues designed a shelter that can be placed over a piece of land, and topped with strips of plastic to divert some of the rain away from the vegetation below.

Working with other researchers around the world, Smith and her team were able to install such shelters at 100 grass or shrubland sites across six continents.

For each site, the team aimed to mimic what would be considered an extreme drought for the area – the type that would be seen once in every hundred years, says Smith. For example, a rainier site in Europe would have more plastic strips on the roof, compared with a drier site, to better simulate a drought.

After a year, the team found some of the experiments were successful at replicating drought conditions, and some less so due to higher-than-average rainfall in certain regions.

The 44 sites that experienced extreme drought saw plant growth reduced by 38 per cent for grasslands and 21 per cent for shrublands. “It was large,” says Smith, adding that the reductions in plant growth were significantly more severe than the researchers had observed in past studies.

They also found that more arid sites with less biodiversity are particularly vulnerable to drought. “The dry sites are already on the edge,” says Smith. ”They don’t have much of a buffer in their system to cope.”

Smith hopes that these insights can improve global climate models, which, up until now, have been underestimating the role of droughts in the carbon cycle.