For decades, “geoengineering” was a forbidden subject in climate circles. Talking about adjusting the Earth’s energy balance by, for example, using mirrors, white roofs or aerosols to reflect solar radiation away from the planet and back into space could legitimize a strategy that should be a last resort. Fiddling with the Earth’s thermostat could have unintended effects on natural systems. Moreover, geoengineering might not address all the problems associated with rising carbon dioxide levels, such as ocean acidification. Better simply to stop emitting heat-trapping gases.