Which year was called 'the year without a summer'?

Answer

The 1815 eruption of Mount Tambora cooled global weather and caused crop failures.

Context

Mount Tambora, on the island of Sumbawa in present-day Indonesia, erupted in April 1815 in the largest eruption of the past several centuries. The blast threw huge quantities of sulfur dioxide into the stratosphere, where it formed sulfate aerosols that reflect sunlight. Those aerosols spread around the world and lowered average temperatures by a fraction of a degree to about a degree Celsius in the following year, with larger regional effects.

The effects were strongest in 1816. In New England and eastern Canada, frosts and snow fell in the summer months, and the failed harvests pushed up food prices. Europe saw cold, wet weather and poor harvests, and parts of Asia reported disrupted monsoons. Records show hunger, food riots and migration, including movement of New England farmers westward, though historians debate how much of that was due to Tambora versus the colder phase of the late Little Ice Age and other volcanic eruptions around 1809.

The weather also had cultural effects: the gloomy summer of 1816 kept Mary Shelley and her friends indoors near Lake Geneva, where she began writing Frankenstein. The eruption itself is thought to have killed tens of thousands of people locally, and estimates of the death toll vary. The phrase 'the year without a summer' applies mainly to the Northern Hemisphere.