mirrors orbiting outer space; giant reflectors between the Earth and the Sun; trillions of helium balloons in the air. Does this read like a sci-fi script to you? These are some of the rather elaborate solutions to counter the rising carbon dioxide ( co2 ) levels in the atmosphere. These ideas may sound too far-fetched, but they do make a point. "People look at this kind of thing to get a sense of the greenhouse effects,' says Michael MacCracken, head of the Office of the Global Change Research Programme. "The main thing we are learning is