Scientists reviewing the first phase of a cloud seeding trial in the Snowy Mountains have found the technique has "no adverse environmental effects to date ", but warn that the technology should not be implemented before longer-term effects are known.
The electricity firm Snowy Hydro began seeding clouds in 2004 as part of a double-blind trial to test whether the technique would increase snowfall in the Snowy Mountains, in southern New South Wales.
In a mid-term review of the project released late last week, the Natural Resources Commission (NRC) — which provides independent advice to the NSW Government — concluded the technique increases snowfall in the targeted area by 14%.
"It's a really important piece of science — to me it's a good thing that we have got this quality of science being done in Australia," says NRC Commissioner Dr John Williams.
But in its report the NRC noted that a "key uncertainty" was the potential long-term accumulation and impact of silver iodide and indium trioxide, chemicals used in the cloud seeding process.
"Although no adverse environmental effects have been detected to date, it is an important matter for future risk analysis to understand the ultimate fate of these seeding chemicals," says the NRC.
How to seed a cloud
Cloud seeding is a weather modification technique, which adds additional particles — in this case the seeding agent silver iodide — to suitable clouds to encourage snow fall.
The project, which currently covers an area of around 2000 square kilometres, uses 23 pairs of ground-based generators along the western side of the mountain range which disperse the silver iodide and a tracing agent — indiuim trioxide.
As part of the double-blind trial protocol, Snowy Hydro seeds less than 50% of suitable clouds with silver iodide to allow comparisons to be made between seeded and unseeded storms.
Seeding is only carried out in winter when precipitation over the Snowy Mountains is largely associated with moist westerly weather systems, says Snowy Hydro.
To make snow, super-cooled liquid water contained within clouds needs to form ice crystals, a process which usually occurs when they interact with airborne dust or other ice crystals. The silver iodide has similar physical properties to natural ice crystals.
The NRC review found no evidence to suggest either the seeding or the tracing agents have accumulated in sampled soils, sediments, water or moss in areas tested.
There was also no evidence that the trial has affected snow habitats or had an adverse effect on rainfall in downwind areas.
But Williams says more time is needed to ascertain the long-term impact of the accumulation of silver iodide, noting its potential to enter the food chain of this pristine environment.
He says it would not be prudent to end the trial and make cloud seeding operational until the monitoring program can detect where the silver iodide and indium trioxide are accumulating.
He also points out that the data was collected from 2004-2009 during a drought, meaning there were smaller numbers of suitable clouds available for seeding, and that the amount of run-off could vary during times of naturally higher snow fall.
Snowy Hydro has also not yet reported on the likely increases in runoff water and available water in dams, the review noted, but it is expected to report on this in the next phase of the trial.
Professor Gary Jones, CEO of the eWater CRC in Canberra, ACT, and an independent expert adviser to the NRC, says future investigations should focus on where the silver iodide is accumulating.
"We just want to know where it's going so we can monitor it better in the future," says Jones.
"We are all as environmental scientists very cautious about this."
While the project is due to finish in 2014, Snowy Hydro is keen to press ahead with an operational cloud seeding program next year, noting it has the support of many stakeholders including tourism operators and irrigator groups.
Snowy Hydro spokesman David Hogan says that scientific investigations can continue during an operational program, "and along with regular review the interests of the environment are extraordinarily well protected".
In a fully operational program, the firm anticipated around 50 kilograms of silver iodide would be used each year over a target area of 2150 square kilometres, which equates to around 23 grams of silver iodide per square kilometre.
"In the old money, this is roughly the equivalent of the silver found in a 1966 fifty cent piece per square kilometre," he says.
"It is also important to understand that silver and indium are naturally occurring, and ubiquitous in the environment — monitoring before the trial commenced confirmed both to be present at every location sampled, sometimes at quite high concentrations. "
Hogan says there is now no plausible reason to delay a move to a complete operation.
"It is extremely difficult to continue to justify the enormous expense of a research trial, when the outcomes have been so compelling," he says.