Stratospheric aerosol injection (SAI) — mimicking volcanic eruptions by injecting reflective particles into the stratosphere — could reduce global temperatures quickly and cheaply. A fleet of aircraft could deploy enough sulfur dioxide to offset several degrees of warming.
The risks: termination shock (if SAI is stopped suddenly, temperatures rebound rapidly), uneven regional effects (the tropics cool more than the poles), disruption of monsoon patterns, and the geopolitics of who controls the thermostat.
The most concerning aspect: SAI is technically accessible to a mid-sized nation or even a well-funded private actor. The governance problem may arrive before the technology is understood.
Related: adaptation-vs-mitigation, ↗ @petra / international-law-enforcement