Sooner or later we will experience another huge volcanic eruption. The 1815 eruption of Tambora caused the "Year without a Summer." Daily minimum temperatures were abnormally low in the northern hemisphere from late spring to early autumn. Famine was widespread because of crop failures (Source:
http://volcano.und.edu/vwdocs/volc_images/southeast_asia/indonesia/tambora.html
) This is what we are ultimately facing (only multiply by 1000).

http://www.roperld.com/HomoSapienEvents.htm
Eruption of Toba, Sumatra volcano and the ensuing cold millennium which killed almost all Hss (estimate of 10,000 left) This was right at a minimum temperture of about 7 degrees C below current temperature. The eruption took the temperature down to about 9 degrees C below current temperature for about 1000 years.

http://en.wikipedia.org/wiki/Lake_Toba
The Toba eruption (the Toba event) occurred at what is now Lake Toba about 71,500 ± 4000 years ago. It had an estimated Volcanic Explosivity Index of 8, making it the most recent supervolcano eruption and probably the largest volcanic eruption within the last two million years. Bill Rose and Craig Chesner of Michigan Technological University deduced that the total amount of erupted material was about 2800 cubic km (670 cubic miles) — around 2000 km³ of ignimbrite that flowed over the ground and around 800 km³ that fell as ash, with the wind blowing most of it to the west.

This eruption was the last of a series of at least three caldera-forming eruptions which have occurred at the volcano. Earlier calderas were formed around 700,000 and 840,000 years ago.

Source:
http://volcano.und.edu/vwdocs/volc_images/southeast_asia/indonesia/toba.html