The most substantive climatic influence from volcanic eruptions arises from the generation of atmospheric haze. The huge eruption columns administer sulfur-rich gases and ash particles into the stratosphere and troposphere and these clouds can envelop the globe within the weeks of the volcanic activity.
The small ash components reduce the amount of sunlight reaching the Earth's surface and lower average global temperatures. The sulfurous gases add with water in the atmosphere to produce acidic aerosols, which also absorb incoming solar radiation and reflect it back into space.