1816 had no summer because a volcano erupted the year before

In June 1816, snow fell in New England and Europe's harvests failed. The cause was an eruption on an Indonesian island a year earlier, on the other side of the world.

In April 1815, Mount Tambora, on the island of Sumbawa in what is now Indonesia, blew itself apart. It is the most recent eruption confirmed at 7 on the Volcanic Explosivity Index, and nothing on that scale has happened since.

The climax came on 10 April. The explosions were heard at least 2,600 kilometres away. On Java, people took them for cannon fire, and a detachment of troops was marched out to help a post they thought was under attack. The mountain lost about a third of its height, from roughly 4,300 metres to 2,851.

The eruption itself killed around 10,000 people. Many more died on Sumbawa and neighbouring Lombok in the hunger and disease that followed. Estimates of the total vary widely, but they run to tens of thousands.

A haze around the world

Tambora also sent sulfur high into the stratosphere, above the weather, where it formed a haze that reflected sunlight back into space. Global average temperatures fell by about 0.4 to 0.7 °C, and the worst of it came the following year.

That sounds small. It was enough. In New England and upstate New York, frost killed crops in May 1816, and in early June snow fell in Albany and in Maine. A dry fog dimmed the Sun so much that sunspots could be seen with the naked eye. Many farming families gave up and moved west.

Europe had its coldest summer in a record running from 1766 to 2000. Harvests failed, famine followed, and food riots broke out across the continent. Typhus, spreading out of Ireland, killed more than 65,000 people between 1816 and 1819.

The cold left a mark on literature too. That June, endless rain kept Mary Shelley, Percy Shelley, Lord Byron and John Polidori indoors at the Villa Diodati by Lake Geneva. Byron proposed a contest to write the most frightening story. Mary Shelley's answer became Frankenstein, and Polidori's The Vampyre grew out of a fragment Byron wrote. Byron also wrote a poem, Darkness, after a day so dim that the birds went to roost at noon.

It shows how thin the margin can be. Less than a degree of cooling, a few frosts at the wrong moment, and a year's harvest across two continents was gone.