Caused by a volcanic eruption in Indonesia:
Tambora has been linked with drastic weather changes in North America and Europe the following year, with frosts in June and heavy rains throughout the summer in many areas. This led to food shortages, which may have prompted westward migration in America and, in a Europe barely recovered from the Napoleonic Wars, led to widespread famine.more at Wikipedia.
and notice that part about Europe recovering from the Napoleonic wars? LINK Five million dead? Who knows.
notice about a half of the "French military" casualties were from disease, and many were of "allies" where Napoleon pressured local gov'ts to join him. Half of Napoleon's "Grand Army" that invaded Russia were not French.
BBC story here which meant fewer young men to farm the fields...
yes, there were major famines in Europe in the 1800's...this was only one of several (more occured in the 1840's).
One result of the cold weather was that a bunch of hippies sat around and wrote stories: Teenager Mary Shelley wrote Frankenstein, and probably influenced culture more than her poet husband, who no body reads except under duress in school.