Tambora and Cholera

The rains, droughts, famine, and disease that afflicted much of the world in the years following the eruption of Mount Tambora in April 1815 were not isolated disasters.

Instead, they formed a cascade of interconnected environmental and humanitarian crises driven by the volcano's impact on global climate and atmospheric circulation. Although the eruption of Tambora is best remembered for causing the "Year Without a Summer" in Europe and North America, its effects were truly global, extending deep into the Indian Ocean basin and becoming one of history's first global pandemics.

Tambora's colossal eruption was the largest volcanic eruption in recorded history, with a Volcanic Explosivity Index (VEI) of 7. It ejected an estimated 50 to 60 million tonnes of sulphur dioxide (SO₂) into the stratosphere[1]. There, the gas oxidised into sulphate aerosols that remained suspended for several years. These aerosols reflected incoming solar radiation back into space, reducing global temperatures by approximately 0.4 to 0.7°C while disrupting atmospheric circulation patterns and precipitation systems[2].

Among the most significant consequences was the disruption of the Indian Summer Monsoon, one of the world's largest climate systems. Tree-ring records and ice-core data indicate that the volcanic aerosol cloud weakened the land-sea temperature contrast that drives the monsoon, resulting in delayed rainfall and widespread drought across much of the Indian subcontinent during 1816 and 1817[3]. Crop failures followed, contributing to famine and economic hardship across British India.

However, drought represented only the first phase of the climatic disturbance. When the monsoon finally arrived, rainfall became unusually intense and erratic. Rivers overflowed, floodwaters spread across the Ganges-Brahmaputra delta, and stagnant pools formed throughout Bengal. These rapid transitions between prolonged drought and extensive flooding dramatically altered freshwater and estuarine ecosystems[4].

Scientists know (now) that Vibrio cholerae, the bacterium responsible for cholera, is a naturally occurring aquatic microorganism closely associated with plankton, copepods (a group of small crustaceans), shellfish, and estuarine environments. Its abundance is strongly influenced by water temperature, salinity, nutrient availability, and seasonal flooding[5]. Changes in these environmental conditions can lead to explosive increases in bacterial populations.

Some have argued that the extraordinary environmental conditions following Tambora's eruption may have provided ideal circumstances for the emergence and spread of a particularly virulent pandemic strain of Vibrio cholerae. Modern genomic studies suggest that the strain responsible for the First Cholera Pandemic emerged in the lower Ganges delta around this period, although whether Tambora directly triggered its evolution remains uncertain[6].

Most researchers therefore regard the eruption not as the sole cause of the pandemic, but as an important environmental catalyst that amplified transmission by reshaping the ecology of Bengal's waterways.

Effects on humans
The human consequences were immense. In August 1817, cholera began spreading rapidly beyond its traditional endemic zone in the Ganges delta. By September it had reached Calcutta (now Kolkata), then one of the busiest ports in the British Empire. From there, commercial shipping routes, river transport, military campaigns, and expanding colonial trade networks carried the disease across the Indian subcontinent and into Southeast Asia[7].

By 1818 cholera had reached Bombay (now Mumbai) on India's west coast. During the following decades, successive pandemic waves spread through Afghanistan, Persia, the Ottoman Empire, Russia, Europe, East Africa, and eventually even North America. Between 1817 and the end of the nineteenth century, six major cholera pandemics swept the globe, killing many millions of people and permanently changing (perceptions of) public health, urban sanitation, and epidemiology.

Cholera and Vibrio cholerae
Cholera is an acute intestinal infection caused by toxigenic strains of Vibrio cholerae, principally serogroups O1 and O139. Infection occurs through contaminated water or food, and produces severe watery diarrhoea that can rapidly lead to dehydration, shock, and death if left untreated. In severe cases, an infected person may lose more than one litre of fluid per hour[8].

Today, despite effective modern treatments such as oral rehydration therapy and antibiotics for severe cases, cholera remains endemic in many parts of Africa and Asia. The World Health Organization estimates between 1.3 and 4 million cases annually, resulting in approximately 21,000 tot 143,000 deaths worldwide, although the true burden is probably higher because of underreporting.

[1] Self et al: Magma volume, volatile emissions, and stratospheric aerosols from the 1815 eruption of Tambora in Geophysical Research Letters – 2004. See here.
[2] Robock: Volcanic eruptions and climate in Reviews of Geophysics - 2000
[3] Oman et al: High-latitude eruptions cast shadow over the African monsoon and the flow of the Nile in Geophysical Research Letters – 2006. See here.
[4] Stommel, Stommel: The Year without a Summer in Scientific American – 1979
[5] Colwell: Global climate and infectious disease: The cholera paradigm in Science - 1996
[6] Mutreja et al: Evidence for several waves of global transmission in the seventh cholera pandemic in Nature – 2011. See here.
[7] Pollitzer Cholera. World Health Organization Monograph Series No. 43.- 1959. See here.
[8] Harris et al: Cholera in The Lancet – 2012. See here.