Two hundred years ago the most powerful eruption in modern history made itself felt around the world. It could happen again at almost any time.
For a different view on the effects of volcanic eruptions on climate, see this post on "Watts Up With That?" from 2012:
Missing the Missing Summer
The author makes the case that average temperatures were not noticeably affected by the eruption of Mount Tambora.
There is a lot of interesting discussion in the comments about volcanoes, average temperatures, crop yields, etc. Many of the commenters provided links to additional information.
See also my earlier posts on this subject:
Mount Tambora - 200 Years Ago
1800 and Froze to Death