Geo-engineering a cooler planet. A whole range of scientists are looking at engineering technologies to mitigate climate change. We could for example reflect more solar radiation back into space by making more reflective clouds; or we could trap carbon dioxide out of the atmosphere and bury it underground, to diminish the “greenhouse” effect. Would such technologies work, and would there be side effects? And who would decide whether to do this, and when, if ever, to stop?