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CHINA is trying to modify the weather on a grand scale. The nation’s scientists want to increase snowfall over an area three times the size of Spain, leading to enough extra meltwater to fight drought and bolster agriculture. The project could have a massive impact – if it works.

The idea is to prompt clouds to release their moisture by seeding them with silver iodide particles. Burner devices placed at the base of ridges on the Tibetan Plateau are designed to produce a powerful updraft of hot air that lofts the particles into the clouds, where they should encourage ice crystals to form and fall as snow.

The researchers estimate this could boost the flow of meltwater to rivers in the region by up to 10 trillion litres a year. This would greatly benefit food production – poor water management caused by water shortage results in droughts and floods, and currently reduces harvests by 20 million tonnes annually (see “Water ways”).

The scale of the project is far larger than any previous attempt at controlling the weather. A report in the says 500 burners have already been deployed, at a cost of $8000 each, but the total number deployed could eventually reach tens of thousands. China has tried cloud seeding before, but it was always on a more local level, delivering the particles via mobile rocket launchers mounted on trucks in response to a specific crisis, like a drought, hailstorm or fog, or to ensure favourable conditions at national events like the Beijing Olympics.

“The cloud-seeding scheme could create up to an extra 10 trillion litres of water a year”

These efforts were not always guided by results and produced few research papers, says Roelof Bruintjes, chair of the World Meteorological Office’s Expert Team on Weather Modification. But that is starting to change.

“The Chinese have been doing cloud seeding for a long time, but it’s only in the last 10 to 15 years that they’ve taken a scientific approach,” he says. “We are training some of their scientists and we’re trying to get them to be more quantitative.”

Others remain sceptical that seeding produces any extra snow. Chien at National Taiwan University says cloud seeding projects are an example of Chinese ““. They give the appearance of the government responding to the needs of the people and reinforce the state’s apparent power, even if the effectiveness is unproven, he says.

Unclear skies

That isn’t stopping other nations jumping on board. According to the World Meterological Office, there were , up from 42 in 2011.

Much of this research is sponsored by the United Arab Emirates, which since 2001 has aimed to revitalise the field and improve water security. Sufian Farrah of the UAE Research Program for Rain Enhancement Science says the country is funding cloud chamber experiments, studies of the electrical properties of clouds, and how best to use drones and balloons for seeding, , Wyoming and Idaho, some of which were funded by the UAE, show that the type of cloud seeding China is attempting in Tibet has the potential to improve snowfall, says Bruintjes. “The amount of increase has been estimated to be in the range of 5 to 15 per cent, however there is a lot of natural variability,” he says.

That variation explains why proving the impact of cloud seeding is so difficult. A new drug can be trialled on thousands of patients to smooth out variation and tease out statistical evidence that it works, but running similar trials for cloud seeding is impractical.

However, a small US . The team used radar and probes to watch ice crystals form. “This study documents the whole chain of events from end to end, from seeding to snowfall, for the first time,” says Bruintjes.

Suppose, then, that China’s huge experiment really can help the country’s drought problem. What are the consequences of meddling with the weather? One obvious concern is the potential environmental or health impacts of pumping silver iodide into the sky.

, but its use in cloud seeding doesn’t seem to have had harmful effects. That is because the quantities of seeding material are relatively small compared with the amounts already in the environment, says Farrah. According to , it would take 500 years of seeding to deposit 1 gram of silver iodide in an area the size of a basketball court. Measurements of silver concentrations in water from seeded rain clouds in the US have been well within accepted safety levels.

There is also the question of where the extra water is coming from – does more for China mean less for its neighbours? “Nobody has shown a tremendous negative impact on surrounding areas,” says Bruintjes, but he notes that .

If it turns out that cloud seeding steers water across national borders, neighbours will probably need to hammer out deals on how to share this natural resource. “There’s only so much water,” says , a UK-based water management consultant. “If you are effectively taking atmospheric water, then that raises issues of water allocation.”

There is precedent for sharing water amicably – rivers that run through multiple countries are subject to international allocation agreements about who can take how much water. Sayers says he is not aware of such discussions taking place over atmospheric water yet.

Any impact large-scale seeding has on neighbouring countries could also be used maliciously. During the Vietnam war in the 1960s, US forces attempted to use cloud seeding to make the Ho Chi Minh Trail impassable, an effort known as . It failed, but a modern version could do better – or rather, worse, if you are on the receiving end.

“If the Chinese can control weather and climate, that is one massively powerful economic weapon,” says Jim Dale, a consultant at meteorology firm .

Following Project Popeye, in 1977 many nations signed up to the , banning the use of weather modification for warfare. China joined the treaty in 2005.

Such concerns aside, if China really has a working weather-control machine, it could be the start of a water revolution. Farrah says his team will meet with researchers from the Chinese cloud seeding programme this summer to discuss and share information. He believes that we are close to seeing a real impact from the technology. “Cloud seeding could be applied on a large scale within five years,” he says.

Unfortunately, it is not the solution to water shortages everywhere around the world, because it cannot turn a clear sky into a snowstorm. “Nobody can make a cloud,” says Bruintjes. “Thus, if there are no clouds, seeding cannot work.”

This article appeared in print under the headline “China’s plan to make it rain”