In this paper, I study the generative role of speculative promise in climate engineering research. My analysis operationalizes Alfred Nordmann's call for a 'forensics of wishing', a variety of technology assessment which scrutinizes the politics of anticipation in technoscience. Using scientific articles and reports as primary sources I trace the uptake and contestation of bold claims made by atmospheric scientist Paul Crutzen a decade ago. In 2006, Crutzen called for dedicated research on stratospheric albedo enhancement as a method to cool the planet. A back-of-the-envelope calculation invoking the eruption of the Mount Pinatubo volcano as a case to be mimicked served to illustrate the method. In the paper, I concentrate on the reception of this idea by fellow climate scientists. Besides fundamental objections being made to Crutzen's climate paradigm, less idealized models appeared to produce much less promising calculations. The initial claims however kept re-appearing as well and continue to exert influence as idealized models take the idea further. Speculation thus continues to be an important driver of the research. In the conclusion, I discuss aspects of the worldview underlying the proposal, drawing on environmental humanities literature on the human condition in the Anthropocene.