Damon Tabor runs through some extreme geo-engineering options:
In 1991, when Mount Pinatubo, a volcano in the Philippines, spewed some 20 million tons of sulfur dioxide into the atmosphere, the average global temperature dropped by 1°F over the next year. Hence the term "Pinatubo option," which refers to the process of enshrouding the planet in aerosol particles that reflect sunlight and thus cool the Earth. Even if it were possible to activate an actual volcano, the cooling effect from a single eruption would be short-lived and impossible to control. … More-dramatic plans call for dispatching flotillas of Navy warships to fire particle-packed artillery shells into the sky.
(Volcano made from wool, by Eszter Burghardt, part of her Wooly Sagas series)
The Dish 