I recently wrote about the Tunguska meteorite and its putative impact on the global climate, but there are also the effects of volcanoes to consider.
According to a recent posting on the scenta volcanoes news page at Sciencebase, The eruption of Pinatubo in 1991 led to a cooling of the Earth of 0.5 Celsius, while the eruption of Mount Tambora led to 1815 being labelled as the “Year Without Summer” leading to great crop failures worldwide. Check out the scenta volcanoes site for more on this and other volcanic news.