In 2006, the atmospheric scientist and Nobel prize winner Paul Crutzen broke what was until then a taboo within his community. As the leading figure in the community of atmospheric scientists, Crutzen advocated for geo-engineering, the deliberate modification of the Earth’s climate, by injecting sulfur dioxide into the stratosphere. This project takes Crutzen’s 2006 statement as the starting point to ask: Why was public discussion about geoengineering taboo within the scientific community? Geoengineering meant deliberate, large-scale intervention of global reach in the Earth’s climate system, modification of the Earth’s climate to slow down global warming. In the current context of anthropogenic climate change, geoengineering has become almost exclusively climate engineering. The origins of geoengineering, however, lie further back than the term used from the 1970s on. Other terms, like terraforming or planetary engineering, encompassed in science and literature what was later understood as geoengineering. Beginning in the 1950s and 1960s, I trace large-scale environmental engineering projects of global reach that were proposed and discussed in that period, and examine the scientific and political culture that made these projects possibly viable despite their highly speculative nature.