No one who lived through the extreme climate of 1816 understood what caused the months of seemingly endless rain in Europe, the June snowstorms that hit New England and the Ohio Valley, or the prolonged drought in the eastern U.S. that convinced many farmers to sell their land and move west. All they knew was the weather was against them. Many blamed God: in America religious revivals intensified, particularly in Puritan New England; in Europe a succession of "prophets" foretold of a rapidly approaching apocalypse. Those with a more secular philosophy invoked an array of natural forces to explain the shifting climate, including earthquakes, sunspots, icebergs, deforestation and changes in the Earth's magnetic field. Meteorology was in its infancy -- the first reliable weather forecasts were 140 years off -- so few contemporary scientists, many of them amateurs, could comprehend the changing conditions.
The true cause of "The Year Without Summer," as 1816 came to be known, lay half the world away on the island of Subawa, near Java in Indonesia. Mount Tambora erupted in April 1815 with such force that the British governor of the area, Sir Thomas Stamford Raffles, dispatched troops to investigate the loud noises he believed to be cannonfire. Tambora was thought to be extinct, but the 1815 eruption ranks as one of the four strongest in the last 10,000 years. It was roughly 100 times more powerful than Mount Saint Helens in 1980. Within 24 hours, the ash cloud had grown to the size of Australia; Tambora produced enough ash to cover Vermont to a depth of 12 feet. As the streams of molten lava ploughed into the ocean, the force generated tsunami waves up to 15 feet high. The rapidly cooling lava produced fields of floating pumice stone; some were three miles wide. Between 70,000 and 90,000 people died in Indonesia in the eruption and its aftermath, many from famine when the ash poisoned their crops.
For global climate, the height of the eruption was more important than its power. The plume of ash and gases extended more than 18 miles into the atmosphere, into the stratosphere. When eruptions are limited to the lower atmosphere, the troposphere, rain droplets can remove the ash, dust and sulfuric acid from the air in weeks. Clouds rarely reach the dry stratosphere, however, so particles can remain there for months or years before enough water sticks to them to make them heavy enough to fall. The 1815 eruption threw an estimated 55 million tons of sulfur-dioxide gas into the stratosphere, where it formed 100 million tons of sulfuric acid. Within a few months, the strong stratospheric winds dispersed the tiny droplets around the globe.
Because the droplets reflected and scattered sunlight, to the naked eye the cloud of sulfuric acid might have appeared as a thin, barely perceptible haze. Many people never noticed it. (Benjamin Franklin remarked on a similar cloud after the 1783 eruption of the Icelandic volcano Laki. He thought it might block sunlight, but few paid attention to his theory.) The cloud reflected only one half of one percent of the Sun's energy. That was enough to cool the average global temperature by two or three degrees Fahrenheit in 1816, however. Our planet's climate is finely balanced; small changes in global climate can alter weather patterns over continents or regions, magnifying the effects and the impacts on human life and the environment.
As the acid veil cooled temperatures, it also changed the path of the Atlantic jet stream -- the "conveyor belt" of winds, high in the troposphere, which steer storm systems. Normally in summer, the jet stream flows north from the Gulf of Mexico, across the eastern U.S., then west over the Atlantic Ocean to northern Europe and Scandinavia. This brings the familiar warm, sticky air to the U.S. east coast, but keeps most of Europe fairly dry. In the abnormally cold summer of 1816, however, the jet stream slowed and developed "U"-shaped bends, winding its way north and south instead of east to west.
In place of mild Gulf air, New Englanders suffered cold Arctic blasts from northern Canada. A powerful storm in early June dropped as much as foot of snow on southern Ontario, Quebec and New England. Frosts plagued the region throughout the season. Without the typical summer thunderstorms to bring rain, crops failed in a drought that stretched into autumn. Many farmers in New England and the mid-Atlantic sold their lands and moved west, including the family of the young Joseph Smith, who would found the Church of Latter-Day Saints.
Over Europe, the jet stream dipped far south, bringing a succession of unseasonably strong rain and wind storms. The wet and cold weather spread a typhoid epidemic in Ireland, which killed more than 60,000. In Switzerland, it rained on 130 out of 152 days between April and August. Flooding rivers forced thousands from their homes. Bands of displaced peasants roamed from town to town in search of food and charity, finding little of either. Trapped indoors by the relentless poor weather, the novelist Mary Shelley, her poet husband Percy Shelley and their friend and fellow poet Lord Byron began to tell ghost stories to one another, a popular pastime of the day. It was there, surrounded by the dark, gloomy conditions, that Mary conceived the idea for Frankenstein.
Many governments were slow to react to the mounting catastrophe. Fearing that a spike in grain prices would set off a revolt, and mindful of the recent French Revolution, European governments restricted grain exports and suppressed news of poor harvests. Neither policy helped the situation. Eventually, they were forced to provide some charity to the poor -- the first instance of widespread, systematic welfare assistance by western governments.
While the eruption of Tambora produced disastrous impacts on global weather patterns and society, mercifully those impacts were relatively short lived. After another, less extreme, cold summer in 1817, most of the stratospheric acid veil had dissipated. It would not be until the mid-1950s, while monitoring the Cold War nuclear tests, that scientists verified the effects of stratospheric particles on our climate -- the "nuclear winter" scenario resembles the effects of a volcanic eruption -- and thus establish what caused the extreme climate of the Year Without a Summer.