One of the most important issues facing humanity is the rise in temperature of the planet. One current line of investigation for the reversal of global warming is that of using one or more of a suite of geoengineering (or climate engineering) techniques known as solar radiation management (SRM) in order to reflect sunlight back into space. The Paris Agreement of 2015, COP21, invited further research into this kind of geoengineering solution. One idea is to artificially emulate the effect of large volcanic eruptions, which can certainly lead to global cooling. Here, medieval history offers a perspective from which to help understand the challenges that geoengineering may present and inform our choices. The closest natural parallel to stratospheric aerosol injection (SAI) geoengineering are volcanic eruptions and case studies of their climatic (and subsequent societal) impacts are much needed. By studying historical explosive volcanism, medieval history provides a laboratory for understanding the climatic and societal impacts of geoengineering in the form of reports of extreme weather and societal stresses such as subsistence crises and even conflict arising from scarcity induced resource competition. We argue that this history must be taken seriously in the discussion about whether to proceed with solar geoengineering. The twentieth and twenty-first centuries have been volcanically quiescent relative to earlier centuries, but this can change at any time. In particular, advocates of a geoengineered solution have to appreciate the relevance of the question: what will happen if the planet experiences another period in which one or more sulphur-rich VEI 5 to 7 eruptions occur, if we have already laden the stratosphere with sulphates artificially? The medieval experience of such eruptions can point to an answer that serves as a warning.