Not every crazy idea, say dropping out of Harvard to start a software firm, is a bad one. But you don't have to be Bill Gates to place your bets that way.
Consider atmospheric geoengineering -- pumping reflective particles into the stratosphere to reflect sunlight -- seen as a way to cut the effects of global warming. In 1991, the eruption of Mt. Pinatubo in the Philippines cooled the atmosphere's average temperature worldwide almost one degree Fahrenheit, a kind of "global dimming," serving as an inspiration for the idea. Such high-altitude aerosols, different from the ones found in spray cans, can play a big role in climate.
A 2006 paper in the journal Science, for example, written by the eminent atmospheric scientist Tom Wigley of the National Center for Atmospheric Research, suggested that annually blasting roughly 500,000 tons of sulfur (about 7% of yearly sulfur production) into the stratosphere every year for three decades would prevent global ...