When the Mount Pinatubo volcano erupted in the Philippines in 1991, it spewed a massive cloud of ash and sulfur into the air. The sulfate particles then scattered into the Earth’s stratosphere where, for the next two years, they reflected sunlight back into space. The particles cooled the planet by about 0.9 degrees Fahrenheit. 

In recent years, scientists desperate to stop global warming have looked back at this natural event and wondered: Could people recreate similar effects to help reverse rapidly rising global temperatures? 

Enter stratospheric aerosol injection, the process of releasing tiny reflective particles of sulfur dioxide into the stratosphere that reflect sunlight back into space in order to cool off the planet. The concept mimics the natural activity of volcanoes like Mount Pinatubo. But it is driven by humans.

Proponents of stratospheric aerosol injection, including start-ups and researchers investigating and experimenting with the process, call it “sunscreen for the earth” and argue that we can create a layer of protection to shield us from the hot rays of the sun. It is one of a growing variety of Earth-cooling techniques that fall under the conceptual umbrella of “solar geoengineering.” Other proposed solar geoengineering techniques range from creating light-reflecting clouds to deploying giant mirrors in space. In 2020, Xiulin Ruan, a professor of mechanical engineering at Purdue University in Indiana, unveiled a “whiter than white” specialized paint, designed for rooftops and roads, that can bounce 95% of the sun’s rays back into deep space, cooling the buildings beneath it.