In June 1816, snow fell in parts of New England. In July and August, frosts killed crops in places that should have been at the height of the growing season. In Europe, cold and constant rain ruined harvests and brought hunger to many regions. People at the time called it "eighteen hundred and froze to death." Later, it became known as the Year Without a Summer.
The cause was half a world away, and nobody at the time understood it.
The eruption
In April 1815, Mount Tambora, a volcano on the island of Sumbawa in what is now Indonesia, erupted. It is considered the largest volcanic eruption in recorded history. The explosion and the pyroclastic flows that followed killed many thousands of people on Sumbawa and nearby islands, and many more died afterward from famine and disease in the region. Estimates of the total deaths vary widely, but they are often given in the tens of thousands.
The eruption also sent enormous amounts of ash and sulfur gases high into the atmosphere. In the stratosphere, sulfur dioxide forms tiny droplets, called sulfate aerosols, that reflect some sunlight back into space. Spread around the globe by winds, these aerosols can cool the Earth's surface for a year or more.
The cold year
The effects were felt most strongly in the Northern Hemisphere in 1816.
North America. New England and eastern Canada saw unusual cold snaps throughout the summer, including frosts and snow in June. Farmers replanted crops several times. Crop failures pushed up food prices. Historians have connected the hard times with increased migration from New England to the western frontier in the years that followed.
Europe. Western and central Europe suffered a cold, wet summer. Harvests failed in many areas. Food shortages, rising grain prices and unrest followed in several countries. Some historians describe the crisis that followed as the last major subsistence crisis in the Western world.
Asia. Records from China, among other places, suggest disrupted weather and failed harvests in some regions as well.
Frankenstein on a rainy holiday
One of the most famous results of the cold summer was literary. In June 1816, a group of writers was staying near Lake Geneva in Switzerland, including Lord Byron, Percy Bysshe Shelley, Mary Godwin (soon to be Mary Shelley) and Byron's physician, John Polidori. Kept indoors by persistent storms, they agreed to a contest to write ghost stories.
Mary Shelley began the story that became Frankenstein; or, The Modern Prometheus, published in 1818. Polidori wrote The Vampyre, published in 1819 and often cited as an early influence on vampire fiction. Byron wrote the poem "Darkness," which describes a world where the sun has gone out.
What took so long to understand
People in 1816 did not know about Tambora's connection to their weather. News traveled slowly, and the link between large eruptions and global climate was not understood. Scientific work in the twentieth century, including studies of ice cores, tree rings and historical weather records, connected the 1815 eruption to the cold anomalies of 1816.
Researchers have also pointed out that the period was already relatively cool, in part because of other volcanic activity in the preceding years. Tambora appears to have made a cold period much worse.
Reading the record carefully
This is a good example of how history and science work together, and also of how to read claims carefully.
Some popular accounts make every cold or hungry event of 1816 a result of Tambora. Historians and climate scientists tend to be more cautious. Weather varies, and not every cold spell in 1816 was caused by the eruption. The best-supported picture is that Tambora's aerosols cooled the Northern Hemisphere summer significantly, which made crop failures much more likely across several regions.
Death tolls are another area where numbers vary. Figures for deaths on Sumbawa and Lombok, and for hunger in Europe, come from incomplete records. Good sources give ranges and explain their uncertainty.
Why it is remembered
The Year Without a Summer is remembered because it touches so many things at once: geology, climate, farming, migration, politics and literature. It is also a reminder that a single event in one part of the world can affect people far away in ways they cannot see.
Volcanoes are still studied closely for this reason. When Mount Pinatubo in the Philippines erupted in 1991, scientists measured a temporary drop in average global temperatures in the following year or so. It was much smaller than the effect of Tambora, but it confirmed the mechanism.
So when you read that snow fell in New England in June 1816, you are reading about an eruption in Indonesia fourteen months earlier, carried around the world in a thin layer of haze that nobody alive at the time could have recognized.