Explosive volcanic eruptions can inject large quantities of dust and sulphur dioxide gas into the atmosphere. If the eruption is powerful enough for this material to reach the upper atmosphere, the veil of pollution may remain for several years, gradually spreading to cover much of the globe. This volcanic pollution reflects the incoming stream of energy from the Sun back into space, thus cooling the Earth's surface. This effect is evident in observed data. Depending on the magnitude and location of the volcano, an individual eruption can cause global cooling of two to three tenths of a degree Celsius over the following year or two. The large 20th century eruptions of Agung, El Chichon and Pinatubo are clearly evident in the observed data, and their effects are reproduced by global climate models when this natural forcing mechanism is included in simulations. Since the timing and location of volcanic eruptions cannot be predicted, it is not possible to incorporate their year-to-year effects in future climate projections. In the long-term, their influence has been, and will continue to be, slight.