The stratospheric sulfate experiment has already had its proof of concept — courtesy of planet Earth. On June 15, 1991, Mount Pinatubo, which for months had been rumbling, belching, and terrorizing the main Philippine island of Luzon, finally blew its top in an explosion so powerful that it carried 500 feet of the mountain's peak along with it. It was the second-largest volcanic eruption of the 20th century, 10 times the size of the Mount Saint Helens explosion in 1980 and the first of its scale to occur with modern scientific technologies in place — especially satellites — to measure the global environmental and climatic effect.
Pinatubo's eruption didn't just unleash huge mud slides and lava flows; it also fired an ash stream 22 miles into the air, injecting 20 million tons of sulfur dioxide into the stratosphere. Over the following months, a massive haze gradually dispersed across the globe. Meanwhile, the sulfur dioxide component underwent chemical reactions to form a particulate known as sulfate aerosol (in essence, droplets of water and sulfuric acid), which absorbs sunlight and reflects some of it back into space.
The climatic effect of this volcanic eruption was rapid, dramatic, and planetary in scale. In a year, the global average temperature declined by half a degree Celsius, and researchers observed less summer melt atop the Greenland ice sheet.