Geoengineering seeks to alter the Earth itself to respond to the climate crisis. Aerosols injected into the stratosphere by nature during volcanic eruptions can sometimes produce dramatic cooling effects, such as the “year without a summer” in 1816 following the eruption of Mt. Tambora in Indonesia. Solar radiation management seeks to add anthropogenic aerosols to the stratosphere to overcome the most severe effects of global warming such as massive heat waves. Carbon dioxide removal from the atmosphere can be done using photosynthetic plants or with engineered devices to capture it using various chemical processes and specialized machinery. Up to a trillion new trees would have to be planted to significantly mitigate climate change. Carbon dioxide captured by engineered devices must be stored or sequestered away from the atmosphere for periods of at least a century and the longer the better. All these options are under consideration by governments, research institutions, and venture capital investors. When combined with an energy switch away from fossil fuels, geoengineering techniques promise a way to mitigate the worst aspects of climate change.