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"I got a phone call from two men who said we work as consultants for the CIA and we'd like to know if some other country was controlling our climate would we know about it," Professor Robock said.

"I told them after thinking a little bit we probably would because if you put enough material in the atmosphere to reflect sunlight we would be able to detect it and see the equipment that was putting it up there," he said.

"At the same time I thought they were probably also interested in if we could control somebody else's climate could they detect it," he added....