As Indonesia raised the warning level for the Mount Sinabung eruption it is worth noting that the last volcanic eruptions of 5 or greater on the Volcanic Explosivity Index (VEI) scale were the 1991 eruptions of Mount Pinatubo (VEI of 6) and Mount Hudson (VEI of 5).
The currently ongoing Mount Sinabung eruption (pictures here) gives cause for concern, as all Indonesian volcanoes do, but there is no certainty as to what intensity it may finally reach:
Jakarta Post: The volcanic ash spewed by Mount Sinabung in Karo regency, North Sumatra, disrupted a number of flights from Kuala Namu International Airport in Deli Serdang regency on Sunday.
In Bandung, the Center for Volcanology and Geological Hazard Mitigation (PVMBG) increased the volcano’s status to “awas” (beware), the highest level of the four-level alert system.
During the 19th century VEI eruptions of 5 or greater ocurred every 11 years on average with the Krakatoa eruption being the greatest at VEI 6 in 1883. Through the 20th century, an eruption of intensity 5 or greater came at intervals varying from 1 year upto 23 years with an average interval of just under 7 years. The Novarupta (1912) and Mount Pinatubo (1991) eruptions were the two classified at VEI6.
- 1902 Santa Maria
- 1907 Kudach
- 1912 Novarupta
- 1913 Colima
- 1918 Katla
- 1932 Cerro Azul
- 1933 Kharimkotan
- 1956 Bezymianny
- 1963 Mount Agung
- 1980 Mount St. Helens
- 1982 El Chichón
- 1991 Mount Pinatubo
- 1991 Mount Hudson
So far in this century the 2010 Eyjafjallajökull eruption in Iceland “only” reached a VEI intensity of 4. The 2012 Mt. Etna eruption was rated a 3+.
The last VEI 5 level eruption was in 1991 and a major eruption of intensity 5 or greater is now overdue. The interval of 22 years is almost as long as the longest interval between major eruptions in the 20th century. Of course even VEI 6 is still a great deal less than the VEI 8 of Toba or Yellowstone.
The VEI is an index taking both the volume of tephra ejected as well as the height to which it is ejected into account. While there is no evidence that the impact of volcanic eruptions is more than a temporary perturbation of the underlying climate cycle, these perturbations may well last a year or two. The effect on climate is usually that of cooling.
The most significant climate impacts from volcanic injections into the stratosphere come from the conversion of sulfur dioxide to sulfuric acid, which condenses rapidly in the stratosphere to form fine sulfate aerosols. The aerosols increase the reflection of radiation from the Sun back into space, cooling the Earth’s lower atmosphere or troposphere. Several eruptions during the past century have caused a decline in the average temperature at the Earth’s surface of up to half a degree (Fahrenheit scale) for periods of one to three years. The climactic eruption of Mount Pinatubo on June 15, 1991, was one of the largest eruptions of the twentieth century and injected a 20-million ton (metric scale) sulfur dioxide cloud into the stratosphere at an altitude of more than 20 miles. The Pinatubo cloud was the largest sulfur dioxide cloud ever observed in the stratosphere since the beginning of such observations by satellites in 1978. It caused what is believed to be the largest aerosol disturbance of the stratosphere in the twentieth century, though probably smaller than the disturbances from eruptions of Krakatau in 1883 and Tambora in 1815.
But an eruption of VEI 5 or greater within the next year or two could well accelerate the underlying global cooling cycle that may be underway (and caused primarily by solar effects – direct and indirect).
The diagram below comes from a guest post by Dan Pangburn at The Hockey Schtick.
Global temperature anomaly and major volcanic eruptions
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