The world is losing its battle against climate change. Greenhouse gas emissions rose to record levels last year, as countries lagged in meeting their already inadequate pledges under the Paris Agreement. Based on the current trajectory, the warming Earth will blow well past the 2-degrees Celsius ceiling widely agreed to be the maximum acceptable increase in average global temperatures before catastrophic impacts set in. In the face of this looming threat, climate change mitigation and adaptation efforts are necessary but insufficient. Humanity must also consider a third option it has long resisted: geoengineering, or the deliberate, large-scale manipulation of the planetary environment.

Geoengineering takes various forms, but most fall into one of two categories: carbon-dioxide removal and solar radiation modification. The first one, also known as negative emissions, entails the permanent removal of CO2 from the atmosphere and its subsequent storage, either in plants, underground or beneath the ocean floor. Solar radiation modification denotes the deployment of technologies to alter the amount of radiation entering or leaving Earth’s atmosphere. Unlike carbon removal, solar radiation modification does not remove greenhouse gases. Rather, it reduces the heat that they trap. ...