Volcanic Sulfur Aerosols Affect Climate and the Earth's Ozone Layer
Volcanic ash vs sulfur aerosols
The primary role of volcanic sulfur aerosols in causing short-term changes in the world's climate following some eruptions, instead of volcanic ash, was hypothesized by scientists in the early 1980's. They based their hypothesis on the effects of several explosive eruptions in Indonesia and the world's largest historical effusive eruption in Iceland.
Scientists studied three historical explosive eruptions of different sizes in Indonesia--Tambora (1815), Krakatau (1883), and Agung (1963). They noted that decreases in surface temperatures after the eruptions were of similar magnitude (0.18-1.3 °C). The amount of material injected into the stratosphere, however, differed greatly. By comparing the estimated amount of ash vs. sulfur injected into the stratosphere by each eruption, it was suggested that the longer residence time of sulfate aerosols, not the ash particles which fall out within a few months of an eruption, was the paramount controlling factor (Rampino and Self, 1982).
In contrast to these explosive eruptions, one of the most severe volcano-related climate effects in historical times was associated with a largely nonexplosive eruption that produced very little ash--the 1783 eruption of Laki crater-row in Iceland. The eruption lasted 8-9 months and extruded about 12.3 km3 of basaltic lava over an area of 565 km2. A bluish haze of sulfur aerosols all over Iceland destroyed most summer crops in the country; the crop failure led to the loss of 75% of all livestock and the deaths of 24% of the population (H. Sigurdsson, 1982). The bluish haze drifted east across Europe during the 1783-1784 winter, which was unusually severe.
Clearly, these examples suggested that the explosivity of an eruption and the amount of ash injected into the stratosphere are not the main factors in causing a change in Earth's climate. Instead, scientists concluded that it must be the amount of sulfur in the erupting magma.
The eruption of El Chichon, Mexico, in 1982 conclusively demonstrated this idea was correct. The explosive eruption injected at least 8 Mt of sulfur aerosols into the atmosphere, and it was followed by a measureable cooling of parts of the Earth's surface and a warming of the upper atmosphere. A similar-sized eruption at Mount St. Helens in 1980, however, injected only about 1 Mt of sulfur aerosols into the stratosphere. The eruption of Mount St. Helens injected much less sulfur into the atmosphere--it did not result in a noticeable cooling of the Earth's surface. The newly launched TOMS satellite (in 1978) made it possible to measure these differences in the eruption clouds. Such direct measurements of the eruption clouds combined with surface temperatures make it possible to study the corrleation between volcanic sulfur aerosols (instead of ash) and temporary changes in the world's climate after some volcanic eruptions.
Volcanic interactions with the atmosphere
Figure modified by K.
McGee et. al.,
from R. Turco, in Volcanism and
Climate Change,
1992
The most significant impacts from large explosive eruptions come from the conversion of sulfur dioxide (SO2) to sulfuric acid (H2SO4), which condenses rapidly in the stratosphere to form fine sulfate aerosols. The aerosols increase the reflection of radiation from the Sun back into space and thus cool the Earth's lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere.
Ozone depletion promoted by volcanic sulfur aerosols.
The sulfate aerosols also promote complex chemical reactions on their surfaces that alter chlorine and nitrogen chemical species in the stratosphere. This effect, together with increased stratospheric chlorine levels from chlorofluorocarbon (CFC) pollution, generates chlorine monoxide (ClO), which destroys ozone (O3).USGS Global Change Research Program
References
Sigurdsson, H., 1982, Volcanic pollution and climate--the 1783 Laki eruption: American Geophysical Union, EOS Transactions, v. 10 August 1982, p. 601-602.Rampino, M. R., and Self, S., 1982, Historic eruptions in Tambora (1815), Krakatau (1883), and Agung (1963), their stratospheric aerosols, and climatic impact: Quaternary Research, v. 18, p. 127-143.
Self, S., Zhao, Jing-Xia, Holasek, R.E., Torres, R.C., and King, A.J., 1996, The atmospheric impact of the 1991 Mount Pinatubo eruption, in Newhall, C.G., Punongbayan, R.S. (eds.), 1996, Fire and mud: Eruptions and lahars of Mt. Pinatubo, Philippines, Philippine Institute of Volcanology and Seismology, Quezon City and University of Washington Press, Seattle, 1126 p.
McGee, K.A., Doukas, M.P., Kessler, R. and Gerlach, T., 1997, Impacts of volcanic gases on climate, the environment, and people: U.S. Geological Survey Open-File Report 97-262, 2 p.
