See Figures GW3 and GW4. One source of variability is the variation of incoming solar radiation. See Figure GW28 . These variations are on time scales that are too long to have a serious impact on the decadal and centennial scales. Although there has been steady warming from the mid 1970's to the early 1990's, there was an interruption of this warming after the Mt. Pinatubo eruption.
Volcanic emissions reduce the amount of solar radiation reaching the surface.
Cooling after a major volcanic eruption has produced well documented climate cooling historically. Another potential cooling mechanism is anthropogenic release of sulfate aerosols. This is an industrial effluent linked to smog and acid rain. It has much the same effect as volcanic emissions, reducing incoming solar radiation. Inclusion of the sulfate aerosol effect in climate models has enabled a more realistic climatic reconstruction of the past hundred years.