"Keith’s specific program would begin with balloon tests in the lower stratosphere (8 miles up) releasing just 100 grams of sulfuric acid—about the amount of particles in a few minutes of normal jet contrail. “If those studies confirm safety and effectiveness,” Keith said, “then we could begin gradual deployment as early as 02020 with three business jets re-engineered for high altitude. By 02030 you could have about ten aircraft delivering a quarter million tons of sulfur per year at a cost of $700 million.“ The amount of sulfur being released might be up to a million tons by 02070, but that would still be only one-eighth of what went into the stratosphere from the Mt. Pinatubo volcanic eruption in 01991, and one-fiftieth of what enters the lower atmosphere from our current burning of fossil fuels. By then we may have developed more sophisticated particles than sulfate. It could be diamond dust, or alumina, or even something like a nanoscale “photophoretic” particle designed by Keith that would levitate itself above the stratosphere." - Todd Hoff