Scientists who have produced the first robust proof that cloud seeding can increase long-term rainfall are urging developing countries considering the technology to be cautious.
Cloud seeding involves injecting clouds with chemicals that encourage water vapour to form ice crystals heavy enough to fall, melting on their way to produce rain. Chemicals can be injected into clouds using aircraft or by launching rockets.
The researchers — led by Steven Siems, an associate professor from Monash University, Australia — examined more than four decades of cloud seeding experiments in Tasmania and found rainfall was at least five per cent higher over seeded areas.
But co-author Anthony Morrison points out that clouds in Tasmania contain vast amounts of supercooled liquid water and are unusually clean — making them particularly suitable for cloud seeding.
And Siems wants more research, saying, ''There could be other explanations for the increased rainfall — although we suspect that cloud seeding is a significant contributor.''
He told SciDev.Net that promoting cloud seeding to developing countries is "probably not a good thing to do".
"There are many, many unscrupulous people in the field of weather modification who up until now have promoted some methods without any proper scientific evidence. Developing countries are particularly at risk here," says Siems.
The technique ''remains controversial, especially because in the early days unrealistic claims were made about its success'', says Johannes Verlinde, associate professor of meteorology at US-based Pennsylvania State University.
Another reason for the controversy, he says, is that no two clouds are alike, making it difficult to compare clouds to prove it really works.
Siems cautions that developing countries should carefully consider whether cloud seeding is right for them and avoid other unproven techniques.
Roelof Bruintjes, of the National Centre for Atmospheric Research in Colorado, United States, agrees, and says that there are many companies promoting techniques such as 'ionisation' — where charged particles in the air are claimed to act as nuclei for rain drops to form — that have not been scientifically proven.
The problem, he says, ''is that people are desperate and in some cases are willing to try anything''.
However, he also says cloud seeding may be an economical way to enhance water resources in some developing countries. Bruintjes' own organisation is helping Mali monitor cloud seeding experiments.
But he "would advise all governments considering cloud seeding to conduct tests first to see if it is going to work for their country".
The research was published in the June issue of the Journal of Applied Meteorology and Climatology.
Journal of Applied Meteorology and Climatology 48, 1267 (2009)