On June 15, 1991, the eruption of Mount Pinatubo in the Philippines produced the second-largest terrestrial eruption of the twentieth century. Beyond the local destruction, the volcano executed an accidental planetary geoengineering experiment. The eruption blasted approximately 17 to 20 million tons of sulfur dioxide gas high into the stratosphere, far above the weather layer where rain could wash it out.
Over weeks, this gas reacted with water vapor to create a persistent global haze of microscopic sulfuric acid aerosol droplets. These aerosols acted as tiny mirrors, reflecting incoming solar radiation back into space. The global stratospheric aerosol optical depth surged by orders of magnitude, causing a measurable drop in global average surface temperatures by roughly 0.
5 to 0. 6 degrees Celsius (1 degree Fahrenheit) over the subsequent two years. Climate scientists tracked this aerosol layer using satellite remote sensing, mapping its dispersion across both hemispheres.
The resulting atmospheric cooling curves provided crucial empirical validation for global climate models, helping researchers understand how radiative forcing works. Pinatubo demonstrated the profound power of stratospheric particulate injection, a mechanism now studied extensively in modern solar radiation management geoengineering proposals.