Mt Pinatubo
Gases from the 1991 eruption of Mt Pinatubo in the Philippines had a global impact on temperature for several years following the event.
Credit: iStock
The eruption of Mt Pinatubo in 1991 was one of the two largest volcanic eruptions in the 20th century. In the approximately nine hours of the primary eruption, the volcano spewed between 15 and 30 million tons of sulfur dioxide gas in addition to huge quantities of volcanic ash, which covered some areas more than a foot deep. Other gases emitted in significant quantities included water, carbon dioxide, hydrogen sulfide, hydrochloric acid, and hydrofluoric acid.
Mt Pinatubo’s eruption was notable for several reasons. First, the volcano stood at a relatively high elevation, which allowed the volcanic gases to be projected into the high troposphere and low stratosphere. Second, the mix of gases from the eruption included an exceptionally high sulfur content. Neither of these factors was present in the 1980 eruption of Mount St. Helens, so only the eruption of Mt Pinatubo produced unusual climate effects.
Typical of most volcanoes, the Mt Pinatubo ash was mad up of relatively large particles. Because they were black, they absorbed the heat from sunlight and contributed to warming the earth as well as to some spectacular sunsets in the year or so after the eruption. Because the large ash particles were also relatively heavy, most of the ash settled out of the atmosphere in about a year. In contrast, the sulfur dioxide gas traveled through the atmosphere as an aerosol, and therefore it remained suspended in the atmosphere for several years following the eruption before it was completely removed. These tiny droplets each acted like a tiny beach umbrella, shading the Earth from the sun’s rays. As a result, after the ash had been removed, the sulfur-based gases had a cooling effect on the climate. In 1992 and 1993, the trend of increasing global temperatures was reversed temporarily until the sulfates were also removed from the atmosphere through natural processes.
Sulfur dioxide also contributed to the depletion of the ozone layer, and the Antarctic ozone hole was particularly large in 1992 and 1993. Sulfur dioxide aerosols were not directly responsible for the reduction in ozone, but because they facilitated the action of the chlorine catalysts released by CFC’s, they exacerbated the situation.
More information about the eruption of Mt Pinatubo can be found at http://earthobservatory.nasa.gov/Features/Volcano/