How Australians are making it rain

By Justine Davies 11 March 2010
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Cloud seeding is a controversial technique to generate rain. But does it work and what are the consequences?

THE RECENT DELUGE OVER Queensland has temporarily washed away many peoples’ concerns about drought. Certainly the heavy rains falling on Eastern states of Australia have meant that some of the water restrictions may at last be eased.

Across the other side of the continent in WA, however, the scenes couldn’t be more different. Perth is parched after the hottest summer for over a century and dismal rainfall across southwest and southeast Australia has meant continued water restrictions.

One study, published recently in the journal Nature Geoscience suggests the drought in WA is the worst in 750 years.

Many Australians are used to seeing ample evidence of water-saving measures – cars remain unwashed, lawns are dry and native plants have replaced their thirsty European relatives in our gardens. Although the situation in Eastern Australia has been eased temporarily, in the future, an increasing population combined with the continuation of poor rainfall will likely have much more deleterious effects on our lifestyles.

It is therefore unsurprising that some of the largest scientific trials of weather modification have taken place here in our arid continent.

CLOUDS ARE KNOWN TO be fickle beasts. They float over dry areas, refusing to part company with their water, only to arrive at airports and drop their cargo as hail that delays flights. However, the need to alter rainfall patterns is not a modern problem; finding ways to increase rainfall has long been a part of human culture and many dances have been performed and animals sacrificed to this end. But, modern science has now found a method of changing rainfall patterns which may really work.

Cloud seeding is a technique used by scientists to modify rainfall. Water in clouds needs to form heavy droplets to fall to the surface, but often the cloud droplets are just too small to effectively rain. The technique sees planes inject chemicals such as silver iodide and dry ice (carbon dioxide) into clouds to encourage ice crystals to form. In the right conditions, these icy crystals will grow quickly, become heavy enough to fall, and in their transit to earth the crystals are warmed, melt and fall as rain.

This is good in theory – we can seed clouds over deserts and also before the rain reaches areas where it will be particularly destructive – but does the technique work, and what are the long-term effects of this strategy?

Whether cloud seeding actually works is controversial; while it is known that the system does increase the amount of ice in clouds, it is difficult to ascertain whether this actually results in the alteration of rainfall patterns.

Unfortunately, there is no parallel world in which scientists can determine what would have happened if the cloud had not been seeded. It is impossible to perform controlled experiments where clouds of the same composition and temperature, in identical environmental conditions located in the same area are compared. Therefore much research has been based on computer simulations, which may not be applicable to the real world.

IN TASMANIA, THE SITUATION is different. Recently, associate professor Steven Siems of Monash University in Victoria, Australia, published results from a real-life study of the effects of more than four decades of cloud seeding in Tasmania. (see Steven talking about cloud seeding here on the ABC’s Catalyst).

He and his team compared monthly rainfall figures from an area that had been seeded with figures from surrounding non-seeded areas and found the process may have increased rainfall by 5 per cent each month (he states that this is a conservative estimate).

However, Steven cautions: “There could be other explanations for the increased rainfall – although we suspect that cloud seeding is a significant contributor.”

In terms of volume, the extra rainfall produced by cloud seeding in Tasmania equates to 5 extra litres per square metre per month. Over the course of a year, this makes cloud seeding a viable option in Tasmania. But, if we relate these statistics to other parts of the world, such as Scotland or Ethiopia, would a 5 per cent increase in rainfall make a difference?

It is difficult to say; Scotland had more than 1400 mm of rainfall in 2008, so as a simple calculation, an additional 5 per cent would increase this by approximately 70 mm. Conversely, Ethiopia’s annual rainfall can be as low as 450 mm – would an extra 5 per cent (22 mm) be enough to stop its crops from failing?

EACH INDIVIDUAL AREA NEEDS to be assessed on its own merits, explains Roelof Bruintjes of the National Centre for Atmospheric Research in the USA. “Under certain conditions cloud seeding can work,” he says. “The problem, though, is that no cloud is the same as another cloud, which makes doing experiments difficult and translating research from one place to another difficult”. Still, it looks like the method may be a useful way of helping our drought-ridden country, and ongoing research projects in southeast Queensland as well as the Snowy Mountains should give some more answers very soon.

But, even if cloud seeding proves successful, what are its adverse effects? There are very few studies in this area. We know that some of the chemicals used are toxic to fish, but the concentrations released by seeding are unlikely to be harmful. In addition, there have been concerns over the socio-economic impacts of the program in Tasmania.

There may be other possible, as yet unstudied or unreported problems, such as the collateral damage to areas in which weather patterns have been modified due to distant cloud seeding efforts.

So, although the use of cloud seeding has gained in popularity so much that Beijing reputedly used it during the Olympics, caution should be applied before we consider it a panacea for all our rainfall problems. We shouldn’t stop using grey water to water our vegetables just yet.

Justine Davies, a former medical doctor, lives in a remote part of Scotland. She now writes for publications including the UK’s Guardian newspaper, The Scotsman and the BBC. She specialises in writing about medicine, ecology, zoology and the environment.