When we think of tackling climate change, we think about reducing CO2 emissions. Whilst this is essential, it is no longer enough. MacMartin and Ricke examine the failures of current carbon dioxide removal (CDR) strategies, all of which fail to satisfy three essential criteria: scalability, economic viability, and lack of detrimental local impacts. In the face of future uncertainty, there is another tactic – in addition to mitigation and carbon dioxide removal – that Macmartin and Ricke propose: ‘solar geoengineering’, an approach aiming to reduce global warming by decreasing the amount of incoming energy from the sun. Such ideas were first discussed in the 1960s, but remained mostly on the fringe until 2006. At its most basic level, geoengineering seeks to modify the radiation balance of Earth by increasing the amount of energy sent back into space, and this chapter discusses how it might be possible to accomplish this – from mimicking the cooling effect that occurs after large volcanic eruptions to enhancing the formation of reflective low clouds over the ocean. However, these approaches present both technical challenges and significant questions to be addressed – from physical effects to broader societal issues impacting ethics and international relations. If CO2 emissions continue unabated, there will be an increase in the amount of geoengineering required to compensate, and future generations would be committed to maintaining it indefinitely. In order for it to be effective, geoengineering must be considered as a supplement and not a substitute, used alongside CO2 reduction, and accompanied by a development in the capacity of international governance to make sound decisions.