evidence: The conspiracy theory that the U.S. government is intentionally spraying toxic chemicals into the air has persisted for nearly two decades. The baseless claims often refer to so-called "chemtrails" and say that the condensation trails behind airplanes are part of an atmospheric spraying program meant to depopulate the world. [...] In 1991, the eruption of Mount Pinatubo in the Philippines resulted in about 20 million tons of sulfur dioxide into the stratosphere, according to a fact sheet from the United States Geological Survey. That caused global temperatures to drop by about 1 degree Fahrenheit for two years, from 1991 to 1993.