Solar Variability

Over long periods of time (thousands of years) the orbit of the earth around the sun changes due to many factors including the gravitational pull of other planets. The changes in the orbit change the amount of solar radiation that the earth receives. These variations cause major changes in the climate which cause long periods of cooling. These periods of glaciation occur at a solar minimum, and cover much of the Northern hemisphere, referred to as Ice Ages. These solar minimums have been well documented to have occurred regularly over millions of years and the agreement between the Earth orbit around the sun and the Ice Ages is quite good. 

The major climatic affect that volcanoes have is due to the aerosol particles that are exploded into the troposphere by large eruptions. Even though there are many more small eruptions, unless the aerosols (and ash) make it out of the troposphere to the stratosphere, it will be rained or snowed out in a short time. The aerosols that do reach the stratosphere can have two affects. If they are larger than about 2 microns, they allow incoming solar radiation to reach the Earth's surface but block outgoing heat radiation--this will contribute to global warming. On the other hand, if the aerosols are smaller than about 2 microns, they tend to block the incoming solar radiation--leading to global cooling. Pinatubo, for example, caused about a 1/2 degree C cooling of the Earth for the year or so following its big 1991 eruption.