It all means the cost could often be outweighing the output, especially when using airborne methods. Ground-based methods, on the other hand, which rely on generators sending silver iodide or another trigger up into the clouds via air currents, are cheaper but far less predictable. "Airborne seeding is pretty efficient, but it's also very expensive, so that's why people do the ground-based seeding," says Friedrich. It's also impossible to know what the outcome will be of wider, consistent climate modification, in China or elsewhere. "It is very difficult to assess, let alone predict, regional climate impacts and remote anomalies from weather modification operations," says Manon Simon, a lecturer at the University of Tasmania, who's done extensive research on the potential geopolitical implications of China's weather modification. It's particularly hard to know whether long-term programmes may result in more frequent or intense droughts o...