The quickest and most obvious effect of volcanic eruptions comes from dust that is released into the atmosphere. Large dust particles settle quickly and therefore only affect the climate for hours or days by blocking the sun and cooling the immediate area. However, smaller dust particles may reach as high as the stratosphere, cooling a larger area around the volcano by staying there for weeks or months.
The most significant effect on the climate from volcanic eruptions comes from sulfur and not carbon dioxide. When volcanoes that contain sulfur erupt, the sulfur released into the stratosphere mixes with the small amount of water found there and creates a reflective sulfuric acid haze. These particles reflect the sun's rays back in the opposite direction and can remain in the stratosphere for up to two years, cooling the Earth for so long that an overall drop in the average global temperature can be measured.
Fortunately, the carbon dioxide released by any one volcanic eruption is not significant enough to cause global warming. Most of the carbon dioxide released is absorbed by plants and has little to no warming effect. Any immediate warming is so small that it is greatly overshadowed by the cooling effects of dust particles and sulfur gas.
In the short term, volcanic eruptions can cause even significant cooling for up to two years; however, in the long term, the effects can be quite the opposite. This is where multiple volcanic eruptions and carbon dioxide begin to play a role. According to the Center for Education Technologies, multiple large volcanic eruptions over thousands of years can actually increase greenhouse gas levels enough to cause observable global warming.