Compiled by: Dr Franz L Kessler (shortened version to satisfy blog constraints)
 
Introduction
 
According to the Smithonian Institute, the world counts some 1500 volcanoes of recent eruption history, and some twenty volcanoes are in activity every moment. Studies suggest that vulcanicity is a rather constant activity, but centered in a number of volcanic belts only. It can be argued, though, that several periods of planets geological history have seen a higher level of activity compared to today.
 
 During all eruptions, volcanoes vent gasses (among these: H20, C02, H2S, F) that are either released into the atmosphere, or remain dissolved in the ocean, this being the case of many submarine eruptions.
 
If we consider volcanic eruptions on land, there are only a handful of known eruptions during the last couple of hundreds of years that temporarily affected world climate. The amount of solid material (ash), and gasses blown into the atmosphere, as well as gas composition are important parameters. Dust blown up in Plinian clouds is dense enough to stall a jet plane’s engines in altitudes of 30000 feet and above.   Both fine volcanic ash, and sulfurdioxide (S02) act as sun-ray absorbers and can cause a cooling of the atmosphere.
 
But only a few events are powerful enough to cause changes in climate recordable on a global scale. This article summarizes events (sorted in chronological order) related to the eruptions of the volcanoes: Laki (Iceland), Tabora, Krakatau ( both Indonesia), Katmai (Alaska), Surtsey (Iceland), Mt. St.Helens (USA), and Mt Pinatubo (Philipines).
 
 
This article is a selective compilation of data from the following sources:
 
- Smithsonian Institute : http://www.volcano.si.edu
 
- Wikipedia:    http://en.wikipedia.org/wiki
 
- Testimony of the Mt. Pinatubo ashfall in Malaysian Borneo
 
 
1. Laki
 
This volcano in Iceland erupted in 1783 with 130 craters opened with phreatomagmatic explosions because of the groundwater interacting with the rising basalt magma. The eight month emission of sulphuric aerosols resulted in one of the most important climatic and socially repercussive events of the last millennium.
In Great Britain, the summer of 1783 was known as the "sand-summer" due to ash fallout. The gases were carried by the convective eruption column to altitudes of about 15 kilometres (10 mi). The aerosols built up caused a cooling effect in the Northern Hemisphere.
 
The eruption continued until 7 February 1784, but most of the lava was erupted in the first five months. Grímsvötn volcano, from which the Laki fissure extends, was also erupting at the time from 1783 until 1785. The outpouring of gases, including an estimated 8 million tons of fluorine and estimated 120 million tons of sulphur dioxide gave rise to what has since become known as the "Laki haze" across Europe.
 
Consequences in Iceland
 
The consequences for Iceland were catastrophic. An estimated 25% of the population died in the famine and fluorine poisoning after the fissure eruptions ceased. Around 80% of sheep, 50% of cattle and 50% of horses died because of dental and skeletal fluorosis from the 8 million tons of fluorine that were released.
 
 
Benjamin Franklin recorded his observations in a 1784 lecture:
“During several of the summer months of the year 1783, when the effect of the sun's rays to heat the earth in these northern regions should have been greater, there existed a constant fog over all Europe, and a great part of North America. This fog was of a permanent nature; it was dry, and the rays of the sun seemed to have little effect towards dissipating it, as they easily do a moist fog, arising from water. They were indeed rendered so faint in passing through it, that when collected in the focus of a burning glass they would scarce kindle brown paper. Of course, their summer effect in heating the Earth was exceedingly diminished. Hence the surface was early frozen. Hence the first snows remained on it unmelted, and received continual additions. Hence the air was more chilled, and the winds more severely cold. Hence perhaps the winter of 1783-4 was more severe than any that had happened for many years.”
 
 
2. Mount Tambora (or Tomboro) is an active stratovolcano, also known as a composite volcano, on Sumbawa island, Indonesia. Sumbawa is flanked both to the north and south by oceanic crust, and Tambora was formed by the active subduction zones beneath it. This raised Mount Tambora as high as 4,300 m (14,000 ft), making it one of the tallest peaks in the Indonesian archipelago, and drained off a large magma chamber inside the mountain. It took centuries to refill the magma chamber, its volcanic activity reaching its peak in April 1815. The following1816 became known as the Year Without a Summer because of the effect on North American and European weather. Agricultural crops failed and livestock died in much of the Northern Hemisphere, resulting in the worst famine of the 19th century.
 
During an excavation in 2004, a team of archaeologists discovered cultural remains buried by the 1815 eruption. They were kept intact beneath the 3 m (10 ft) deep pyroclastic deposits. At the site, dubbed the Pompeii of the East, the artifacts were preserved in the positions they had occupied in 1815.
 
The 1815 eruption released sulfur 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 Tg S to 120 Tg S.
In the spring and summer of 1816, a persistent dry fog was observed in the northeastern U.S. 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 identified as a stratospheric sulfate aerosol veil. In summer 1816, countries in the Northern Hemisphere suffered extreme weather conditions, dubbed the Year Without a Summer. Average global temperatures decreased about 0.4–0.7 °C (0.7–1.3 °F), enough to cause significant agricultural problems around the globe. On 4 June 1816, frosts were reported in Connecticut, and by the following day, most of New England was gripped by the cold front. On 6 June 1816, snow fell in Albany, New York, and Dennysville, Maine. Such conditions occurred for at least three months and ruined most agricultural crops in North America. Canada experienced extreme cold during that summer. Snow 30 cm (12 in) deep accumulated near Quebec City from 6 to 10 June 1816.
 
1816 was the second coldest year in the northern hemisphere since AD 1400, after 1601 following the 1600 Huaynaputina eruption in Peru. The 1810s are the coldest decade on record, a result of Tambora's 1815 eruption and other suspected eruptions somewhere between 1809 and 1810 (see sulfate concentration figure from ice core data). The surface temperature anomalies during the summer of 1816, 1817 and 1818 were −0.51, −0.44 and −0.29 °C, respectively. As well as a cooler summer, parts of Europe experienced a stormier winter. This pattern of climate anomaly has been blamed for the severity of typhus epidemic in southeast Europe and the eastern Mediterranean between 1816 and 1819. Much livestock died in New England during the winter of 1816–1817. Cool temperatures and heavy rains resulted in failed harvests in the United Kingdom of Great Britain and Ireland. Families in Wales traveled 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. Due to the unknown cause of the problems, 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.
 
 
 
3. Krakatoa (Indonesian: Krakatau
The 1883 eruption ejected more than 25 cubic kilometres of rock, ash, and pumice and generated the loudest sound historically reported: the cataclysmic explosion was distinctly heard as far away as Perth in Australia . In the year following the eruption, average global temperatures fell by as much as 1.2 degrees Celsius. Weather patterns continued to be chaotic for years, and temperatures did not return to normal until 1888. The eruption injected an unusually large amount of sulfur dioxide (SO2) gas high into the stratosphere which was subsequently transported by high-level winds all over the planet. This led to a global increase in sulfurous acid (H2SO3) concentration in high-level cirrus clouds. The resulting increase in cloud reflectivity (or albedo) would reflect more incoming light from the sun than usual, and cool the entire planet until the suspended sulfur fell to the ground as acid precipitation.
 
Global optical effects
 
The dramatic skyline in Edvard Munch's The Scream (1893) is thought to be based on the global optical effects caused by the eruption and seen over Oslofjord, Norway. The eruption darkened the sky for days afterwards, and produced spectacular sunsets throughout the world for many months. British artist William Ashcroft made thousands of colour sketches of the red sunsets half-way around the world from Krakatoa in the years after the eruption. In 2004, researchers proposed the idea that the blood-red sky shown in Edvard Munch's famous 1893 painting The Scream is also an accurate depiction of the sky over Norway after the eruption. Munch said: "suddenly the sky turned blood red ... I stood there shaking with fear and felt an endless scream passing through nature." Also, a so called blue moon had been seen for two years as a result of the eruption.
 
 
4. Mount Katmai is a large stratovolcano (composite volcano) on the Alaska Peninsula in southern Alaska, located within Katmai National Park and Preserve. It is about 6 miles (10 km) in diameter with a central lake-filled caldera about 3 by 2 mi (4.5 by 3 km) in area. The over 60-hour-long eruption actually took place at a vent about 6 mi (10 km) to the west of Mt. Katmai (now marked by Novarupta dome) from which an estimated 30-35 km³ of ash flows and tephra were ejected rather than at Mt. Katmai itself. Approximately 12-15 km³ of magma was vented during the 1912 eruption producing about 35 km³ of tephra. An estimated 11-15 km³ of ash flow tuff traveled 12 miles (20 km) northwest covering an area of about 120 km² in what subsequently came to be known as the Valley of Ten Thousand Smokes. The ash flow tuff produced in the 1912 eruption is made up of a silica-rich volcanic rock called rhyolite. In fact, this is the only major Quaternary eruption of rhyolite to have occurred in Alaska. Maximum thickness of the ashflow is estimated to be about 800 feet (240 m). About 20 km³ of airfall tephra was carried east and southeast with a minor lobe to the north covering 77,000 km² with more than 1 in (2.5 cm) of ash. Light ash fall was reported as far away as the Puget Sound region 1,500 mi (2,400 km) away. Extremely fine ash blown into the stratosphere remained in suspension as aerosols for months and caused spectacular red sunsets in many parts of the world.
 
5. Surtsey (Icelandic: "Surtur's island") is a volcanic island off the southern coast of Iceland. It was formed in a volcanic eruption which began 130 meters below sea level, and reached the surface on 14 November 1963. The eruption may have started a few days earlier and lasted until 5 June 1967, when the island reached its maximum size of 2.7 km². Since then, wind and wave erosion has seen the island steadily diminish in size: As of 2007, its surface area is 1.4 km² in size.
 
 
The 1980 eruption of Mount St. Helens, a volcano located in Washington state, in the United States, was a major volcanic eruption. The May 18, 1980, event was the most deadly and economically destructive volcanic eruption in the history of the United States. Fifty-seven people were killed and 200 homes, 47 bridges, 15 miles (24 km) of railways and 185 miles (300 km) of highway were destroyed. U.S. President Jimmy Carter surveyed the damage and stated it looked more desolate than a moonscape.
A volcanic ash column rose high into the atmosphere and deposited ash in 11 U.S. states. At the same time, snow, ice, and several entire glaciers on the mountain melted, forming a series of large lahars (volcanic mudslides) that reached as far as the Columbia River, nearly fifty miles (eighty kilometers) to the south. Less severe outbursts continued into the next day only to be followed by other large but not as destructive eruptions later in 1980.
 
7. Mount Pinatubo is an active stratovolcano located on the island of Luzon in the Philippines, at the intersection of the borders of the provinces of Zambales, Tarlac, and Pampanga. Ancestral Pinatubo was a stratovolcano made of andesite and dacite. Before 1991, the mountain was inconspicuous and heavily eroded. It was covered in dense forest, which supported a population of several thousand indigenous people, the Aeta, who had fled to the mountains from the lowlands when the Spanish conquered the Philippines in 1565. The volcano's eruption in June 1991 produced the second largest terrestrial eruption of the 20th century. The 1991 eruption had a Volcanic Explosivity Index (VEI) of 6, and came some 450-500 years after the volcano's last known eruptive activity (estimated as VEI 5, the level of the 1980 eruption of Mount St. Helens), and some 500-1000 years after previous VEI 6 eruptive activity.
 
The effects of the eruption were felt worldwide. It ejected roughly 10 billion metric tons of magma, and 20 million tons of SO2, bringing vast quantities of minerals and metals to the surface environment. It injected large amounts of aerosols into the stratosphere—more than any eruption since that of Krakatoa in 1883. Over the following months, the aerosols formed a global layer of sulfuric acid haze. Global temperatures dropped by about 0.5 °C (0.9 °F), and ozone depletion temporarily increased substantially. The ash cloud from the volcano covered an area of some 125,000 km² (50,000 mi²), bringing total darkness to much of central Luzon. Almost all of the island received some ashfall, which formed a heavy, rain-saturated snow-like blanket. Tephra fell over most of the South China Sea and ashfall was recorded as far away as Vietnam, Cambodia and Malaysia. As the clouds reached Malaysian Borneo, “ a haze span over the land, like none had ever witnessed. This haze penetrated every house, every room, and left a millimeter-blanquet of very fine grey ash everywhere (Obong Jau).”
 
Conclusions
 
Estimated duration of cooling                  Scale
 
Laki                2-5 years                          global
Tambora        2-5 years                          global
Karakatoa      1-3 years                          global
Katmai           1-3 years                          Northern Hemisspere
Surtsey          1-3 years                          Northern Hemissphere
St. Helens      1-3 years                          Northern Hemissphere
Pinatubo        2-5 years                          global
 
Dust and gases from major volcanic eruptions can affect world’s climate for a number of years, resulting in a temporary cooling, a change of weather patterns that affect harvests in (some) positive and (mostly) negative ways. The toll on human society and nature is frightening. The question of how many Tambora-style eruption would balance the global warming, remains unanswered. No doubt there will be enormous volcanic eruptions sooner or later on this planet, but it would be unwise to reserve too much hope that such events – catastrophic, random, unpredictable- will solve our man-made problems.
 
© 2008 by Franz L Kessler