The eruption of the Mt. Pinatubo volcano on 15 June 1991 in the Philippines put between 15 and 20 megatons of SO2 into the stratosphere. Robock (2002) gives an overview of the climatic effects of this major volcano and how climate data taken for several years following the eruption have been used to validate climate models. In the Northern Hemisphere surface temperatures were 3°C warmer in winter 1991-92 and dropped 2°C below normal in the summer of 1992.
Sulfate particles (SO4) formed from SO2 emitted by the volcano increased ice particle concentrations in the lower stratosphere, which accelerated ozone loss due to heterogeneous chemical reactions with anthropogenic chlorine. This is the same process that leads to seasonal ozone loss over Antarctica.
Global warming was retarded for several years following the Pinatubo eruption due to cooling effect of the volcanic aerosol (Robock, 2002). Reasonably accurate estimates of sulfate loading in the stratosphere together with global measurements of surface and stratospheric temperatures have provided a natural "field experiment" that climate modelers have been able to simulate with high accuracy. This provides confirming evidence of the ability of global climate models to simulate natural climatic behavior and the climatic effects of anthropogenic contributions to atmospheric chemistry.
Reference
Robock, A., 2002: The climatic aftermath. Science 1242-1244