he initial and primary cause of the year with out summer, was the eruption of Mt. Tambora a volcano located near Bali Indonesia that cast over a million tons of volcanic ash and sulfur dioxide into the stratosphere. This volcano eruption happened on April 10 1815 , and was the largest in the last 1300 years immediately claiming 12,000 lives. The eruption is estimated to be 100 time more powerful than the 1980 eruption of Mount St. Helens. As the Mt. Tambora eruption was big enough to send ash and sulfur 10 miles above the Earth surface, the sulfur dioxide reacted with water vapor to form sulfate acid. Sulfate acids that lingers above the altitude of rain dose not get washed out and over the next year reflected sunlight causing the Earth to cool. The ash fallout was as much as 1300km away.
Earth had already been experiencing a little ice age due to a period of low solar activity from 1790-1830 known as the Dalton Minimum. 1816, in particular had the lowest sunspot activity to date since solar activity recording had begun.
The eruption triggered global climate change with rain and cold causing dramatic crop failures in the northern hemisphere, followed by famine thought out most of Europe. This was at a time when the majority of the worlds population depended on subsistence agriculture, living from harvest to harvest. Agriculture crop failed though out Europe and North America in 1816 and 1817, due to lack of sun or frost. April 12 1816, it began snowing in Quebec City , on April 18 a news report said there was snow up to four feet deep. There was heavy snow in June 7-8, with New England having snow drifts 18-20 inches high, ice and snow covered lakes and rivers as far south as Pennsylvania.
Along with millions of people on the brink of starvation, and thousands of rural refugees political and social unrest hit a all time high. Prices of commodities skyrocketed with no relief in site. With the increased cost of livestock feed, it became unaffordable for people to feed them and farm animals were slaughtered for food including horses even though the horse was the principal means of transportation. The cost of travel went up and may have been the inspiration for a German by the name of Karl Drais, to invent the bicycle in 1818. Across the Atlantic in Europe, France and Germany struggled with increasing food cost. The rains in the Netherlands destroyed the hay and grain crops and farmers fearing their livestock would die of starvation also did a mass slaughter. Meanwhile in Ireland, there was the people were faced with lost wheat crops and the potato harvest also failed. Riots broke out in counties across England as the cost of food increased and massive shortages caused more famine. Switzerland one of the most hardest hit countries in Europe, experienced 130 days of rain from April to September 1816. When crops failed thousands died of starvation. Late season rains ravaged India, and the crops failed casing wide spread famine. With a combination of famine and communities that became overcrowded with mass migration, it lead to a new deadly strain of cholera that became the world's first pandemic. By the winter of 1816 the pandemic in the Indian Sub Content, quickly killed 10,000 people and spread though out the world. Could a massive volcano eruption like Mt Tambora cause world food shortages, starvation, pandemics, and political unrest today? The world's population in 1816 was just over 1 billion, today the worlds population is 7.8 billion, with the majority of the population in major city centers. The percentage of people with a greenhouse in their backyard growing their own produce to last 3 years is probably below 1 percent. Very few people with the exception of a few hoarders, have foods stores that would last 3 years, and with wide spread crop failures over three years food shortages certainly occur, followed by famine and starvation. Pandemics are already present, and its anyone's guess would new pandemics would come about with famine though out the world. Volcanoes are still active in the world and eruptions are certainly not under the control of humans. 715 million years ago and 635 million years ago Earth turned into a frozen snowball when volcanoes blocked out the sun. Humans are not in control of the greater forces of nature and this could happen again.