Global cooling is a phenomenon caused by the large quantities of small (less than 2µm diameter) aerosols released into the atmosphere by some volcanic eruptions. Eruption clouds are composed of Tephra, Volcanic Gases and the Trophospheric air that they mix with. The sulphur dioxide particles react with the water vapour to form the aerosols.
Wheras the tephra falls quite quickly, the volcanic gases stay in the atmosphere for a relatively long time (effects from the Pinatubo eruption were seen by astronauts for over a year)
The aerosol layer above thunder clouds over Central America about 3 weeks after the eruption of Pinatubo.
Photograph taken from the space shuttle Atlantis on mission STS-43
Not every volcanic eruption causes global cooling, however. If the aerosols released into the atmosphere are larger than 2µm then they have the same effect as a greenhouse gas, trapping some energy in the Earth's atmosphere. Whatever the effect of such aerosols, it is not permanent since they eventually fall back to the Earth. Pinatubo only reversed global warming for a few years, although it did demonstrate that volcanic eruptions have the ability to change the world climate noticeably.
Some of the effects of volcanic eruptions.
This site was created by Chris Garrett (cg6145@bris.ac.uk)
Last updated 6th April 1997