When Mount Pinatubo erupted in 1991, about a million tons of sulfur dioxide was pumped into the stratosphere, reacting with water to form a hazy layer that spread around the globe, and – by scattering and absorbing incoming sunlight – cooled the earth’s surface for almost two years. We could mimic this effect through stratospheric aerosol insertion – essentially launching material like sulfur dioxide or soot into the stratosphere.