In June 1991, Mount Pinatubo in the Philippines erupted for nine hours, ejecting volcanic ash, water vapor, and at least 15 to 20 million tons of noxious sulfur dioxide gas into the stratosphere. Within two hours, the gas transformed into tiny sulfate mists or aerosols that formed bright clouds. Those clouds spread across the entire Earth and persisted for a year, effectively reducing global temperatures by 0.4 to 0.5 degrees Celsius between 1992 and 1993. Once these cooling aerosols fell out of the stratosphere two years later, global temperatures rose again.