Aerosols
Dr Paul Crutzen suggests reversing global warming by spreading tiny particles in the upper atmosphere to reflect the sun’s rays. This effect has already been shown to work in nature: fine sulphate particles, called aerosols, ejected by large volcanic eruptions like that of Mount Pinatubo in 1991, have produced periods of global cooling. And sulphate pollution from industry had similar consequences, helping to balance the warming effects of carbon dioxide until the 1990s, when pollution controls in many regions had the perverse effect of increasing warming.
“Plan B for global warming?,” The Economist