There has been considerable speculation about the role of volcanoes in the release to the atmosphere of greenhouse gases.
Going by the example of the Mt Pinatubo eruption (Philippines, 1991), the short term effect of a major volcanic event is to in fact cause lower temperatures. That volcanoes release large quantities of sulphur is self-evident to anybody who has ever been in a volcanic area (For example, the city of Rotorua in New Zealand's Volcanic Plateau, throughout which the "rotten egg" smell of hydrogen sulphide gas permeates everything!).
This results in an "aerosol" of fine particles of sulphuric acid in the atmosphere, which reflect the sun's rays - see the following link to a CSIRO discussion about this effect, and the result it has on surface temperatures.
(CSIRO = Commonwealth Scientific and Industrial Research Organisation)