In the case of the 1991 Mt. Pinatubo eruption, the sulfate aerosol cloud resulted in a radiative forcing of about -0.5 W/m2, which lasted for a few months.  This coincided with a global cooling on the order of a few tenths of degree C, which lasted a year or so. Other historical volcanic eruptions are also associated with anomalies in the climate record.  The most noteworthy is the enormous 1815 eruption of the Tambora volcano.  This is referred to as the “year without a summer” in Europe and North America.  The cooling effect was probably on the order of 5ºC. However, the global cooling effects of the volcanic eruptions we are familiar with last at most a few years.