In April 1815, Mount Tambora, a volcano on the island of Sumbawa in present-day Indonesia (then part of the Dutch East Indies), erupted in what is now considered the most powerful volcanic eruption in recorded human history.Hägerdal, Hans (2017), Held's History of Sumbawa. Amsterdam: Amsterdam University Press, p. 19-20, 141–2. This eruption, with a volcanic explosivity index (VEI) of 7, ejected of dense-rock equivalent (DRE) material into the atmosphere, and was the most recent confirmed VEI-7 eruption.

Although the Mount Tambora eruption reached a violent climax on 10 April 1815, increased steaming and small phreatic eruptions occurred during the next six months to three years. The ash from the eruption column dispersed around the world and lowered global temperatures in an event sometimes known as the Year Without a Summer in 1816. This brief period of significant climate change triggered extreme weather and harvest failures in many areas around the world. Several climate forcings coincided and interacted in a systematic manner that has not been observed after any other large volcanic eruption since the early Stone Age.

Chronology of the eruption

Mount Tambora experienced several centuries of dormancy before 1815, caused by the gradual cooling of hydrous magma in its closed magma chamber. Inside the chamber at depths between , the exsolution of a high-pressure fluid magma formed during cooling and crystallisation of the magma. An over-pressurization of the chamber of about was generated, with the temperature ranging from . In 1812, the volcano began to rumble and generated a dark cloud. On 5 April 1815, a giant eruption occurred, followed by thunderous detonation sounds heard in Makassar on Sulawesi away, Batavia (now Jakarta) on Java away, and Ternate on the Molucca Islands away. On the morning of 6 April, volcanic ash began to fall in East Java with faint detonation sounds lasting until 10 April. What was first thought to be the sound of firing guns was heard on 10 April in Trumon, Sumatra, more than away., cited by Oppenheimer (2003). New analysis and agreements in dates suggest the rumbles of Tambora were heard even farther in Nong Khai (modern-day Thailand) at away, Vientiane (modern-day Laos) at away, and perhaps Mukdahan (Thailand) at away.

At about 7:00 p.m. on 10 April, the eruptions intensified. Three plumes rose up and merged. The whole mountain was turned into a flowing mass of "liquid fire". Pumice stones of up to in diameter started to rain down around 8:00 p.m., followed by ash at around 9:00–10:00 p.m. Soon after, a violent whirlwind ensued, which hit the village of Saugur (now Sangar), blowing down nearly every house and carrying everything it encountered into the air, including large trees. Pyroclastic flows cascaded down the mountain to the sea on all sides of the peninsula, wiping out the village of Tambora, and affecting a total area on land of about . A moderate-sized tsunami struck the shores of various islands in the Indonesian archipelago on April 10th, with a height of up to in Sanggar around 10:00 p.m. A tsunami of in height was reported in Besuki, East Java, before midnight, and one of in height in the Molucca Islands. The total of tsunami related deaths has been estimated to be around 4,600. Loud explosions were heard until the next evening, 11 April. The ash veil spread as far as West Java and South Sulawesi. A nitrous odor was noticeable in Batavia, and heavy tephra-tinged rain fell, finally receding between 11 and 17 April.

The explosion had an estimated VEI of 7. An estimated of pyroclastic trachyandesite were ejected, weighing about 10 billion tonnes. This left a caldera measuring across and deep. The density of fallen ash in Makassar was . Before the explosion, Mount Tambora's peak elevation was about , making it one of the tallest peaks in the Indonesian archipelago. The explosion dropped its peak height by about one-third, to only .

The 1815 Tambora eruption is the largest observed eruption in recorded history, as shown in the table below. The explosion was heard at least away and possibly over away, and ash fell at least away.

Aftermath

All vegetation on the island was destroyed. Uprooted trees, mixed with pumice ash, washed into the sea and formed rafts up to across. Between 1 and 3 October, the British ships and James Sibbald encountered extensive pumice rafts about west of Tambora. Clouds of thick ash still covered the summit on 23 April. Explosions ceased on 15 July, although smoke emissions were observed as late as 23 August. Activity resumed in August 1819—a small eruption with "flames" and rumbling aftershocks, and was considered to be part of the 1815 eruption. This eruption was recorded at 2 on the VEI scale.

The eruption column reached the stratosphere at an altitude of more than . The coarser ash particles settled out one to two weeks after the eruptions, but the finer ash particles stayed in the atmosphere from a few months to a few years at altitudes of . Longitudinal winds spread these fine particles around the globe, creating optical phenomena. Prolonged and brilliantly colored sunsets and twilights were seen frequently in London between 28 June and 2 July 1815, and 3 September and 7 October 1815. The glow of the twilight sky typically appeared orange or red near the horizon and purple or pink above.

The estimated number of deaths varies depending on the source. Zollinger (1855) puts the number of direct deaths at 10,000, probably caused by pyroclastic flows. On Sumbawa, 18,000 starved to death or died of disease. About 10,000 people on Lombok died from disease and hunger., cited by Oppenheimer (2003). Petroeschevsky (1949) estimated that about 48,000 people were killed on Sumbawa and 44,000 on Lombok.Petroeschevsky (1949): A contribution to the knowledge of the Gunung Tambora (Sumbawa). Tijdschrift van het K. Nederlandsch Aardrijkskundig Genootschap, Amsterdam Series 2 66, 688–703, cited by Oppenheimer (2003). Stothers in 1984 and several other authors have accepted Petroeschevsky's claim of 88,000 deaths in total. However, a 1998 journal article authored by J. Tanguy and others claimed that Petroeschevsky's figures were unfounded and based on untraceable references. Tanguy's revision of the death toll was based on Zollinger's work on Sumbawa for several months after the eruption and on Thomas Raffles's notes. Tanguy pointed out that there may have been additional victims on Bali and East Java because of famine and disease. Their estimate was 11,000 deaths from direct volcanic effects and 49,000 by post-eruption famine and epidemic diseases. According to Clive Oppenheimer, there were at least 71,000 deaths in total. Reid has estimated that 100,000 people on Sumbawa, Bali and other locations died from the direct and indirect effects of the eruption.Anthony Reid, 'Lessons of Tambora ignored, 200 years on', 25 April 2015, East Asia Forum, Australian National University, accessed 27 April 2015.

Disruption of global temperatures The eruption caused a volcanic winter. During the Northern Hemisphere summer of 1816, global temperatures cooled by . This cooling directly or indirectly caused 90,000 deaths. The eruption of Mount Tambora was the largest cause of this climate anomaly. While there were other eruptions in 1815, Tambora is classified as a VEI-7 eruption with a column tall, eclipsing all others by at least one order of magnitude.

The VEI is used to quantify the amount of ejected material, with a VEI-7 being . Every index value below that is one order of magnitude (meaning ten times) less. Furthermore, the 1815 eruption occurred during a Dalton Minimum, a period of unusually low solar radiation. Volcanism plays a large role in climate shifts, both locally and globally. This was not always understood and did not enter scientific circles as fact until the 1883 eruption of Krakatoa tinted the skies orange.

The scale of the volcanic eruption will determine the significance of the impact on climate and other chemical processes, but a change will be measured even in the most local of environments. When volcanoes erupt, they eject carbon dioxide (CO2), water, hydrogen, sulfur dioxide (SO2), hydrogen chloride, hydrogen fluoride and many other gases (Meronen et al. 2012). CO2 and water are greenhouse gases, which comprise 0.0415 percent and 0.4 percent of the atmosphere, respectively. Their small proportion disguises their significant role in trapping solar radiation and reradiating it back to Earth.

Global effects

The 1815 eruption released SO2 into the stratosphere, causing a global climate anomaly. Different methods have estimated the ejected sulfur mass during the eruption: the petrological method; an optical depth measurement based on anatomical observations; and the polar ice core sulfate concentration method, using cores from Greenland and Antarctica. The figures vary depending on the method, ranging from 10 to 120 million tonnes.

In the northern spring and summer of 1815, a persistent "dry fog" was observed in the northeastern United States. The fog reddened and dimmed the sunlight, such that sunspots were visible to the naked eye. Neither wind nor rainfall dispersed the "fog". It was later identified as a stratospheric sulfate aerosol veil. In the summer of 1816, countries in the Northern Hemisphere suffered extreme weather conditions, dubbed the "Year Without a Summer". Average global temperatures decreased by about , enough to cause significant agricultural problems around the globe. On 4 June 1816, frosts were reported in the upper elevations of New Hampshire, Maine (then part of Massachusetts), Vermont and northern New York. On 6 June 1816, snow fell in Albany, New York, and Dennysville, Maine. On 8 June 1816, the snow cover in Cabot, Vermont, was reported still to be deep. Such conditions occurred for at least three months and ruined most agricultural crops in North America. Canada experienced extreme cold during that summer. Snow deep accumulated near Quebec City from 6 to 10 June 1816.

The second-coldest year in the Northern Hemisphere since around 1400 was 1816, and the 1810s are the coldest decade on record. That was the consequence of Tambora's 1815 eruption and possibly another VEI-6 eruption in late 1808. The surface temperature anomalies during the summer of 1816, 1817 and 1818 were , and , respectively. Parts of Europe also experienced a stormier winter.

This climate anomaly has been blamed for the severity of typhus epidemics in southeast Europe and along the eastern Mediterranean Sea between 1816 and 1819. The climate changes disrupted the Indian monsoons, caused three failed harvests and famine, and contributed to the spread of a new strain of cholera that originated in Bengal in 1816.Peterson, Doug LAS News (Spring 2010) University of Illinois at Urbana-Champaign p. 11. Many livestock died in New England during the winter of 1816–1817. Cool temperatures and heavy rains resulted in failed harvests in the British Isles. Families in Wales travelled long distances as refugees, begging for food. Famine was prevalent in north and southwest Ireland, following the failure of wheat, oat and potato harvests. The crisis was severe in Germany, where food prices rose sharply, and demonstrations in front of grain markets and bakeries, followed by riots, arson and looting, took place in many European cities. It was the worst famine of the 19th century.

Effects of volcanism Volcanism affects the atmosphere in two distinct ways: short-term cooling caused by reflected insolation and long-term warming from increased CO2 levels. Most of the water vapor and CO2 is collected in clouds within a few weeks to months because both are already present in large quantities, so the effects are limited. It has been suggested that a volcanic eruption in 1809 may also have contributed to a reduction in global temperatures.

Impact of the eruption