Geology 309 - Lecture 26
Volcanoes and Climate
Atmospheric gases and their effects on climate
for a list of volcanic gases and how they may have an effect on
climate (list shown in class)
of sulfate aerosols and other particles from eruption
column, and how they affect
incoming solar radiation
- SO2 gas appears to be the most important volcanic
gas responsible for affecting Earth's climate after an eruption.
- Sulfur dioxide gas is converted to sulfuric acid aerosol
layers by chemical reactions between the gas and H2O vapor in
SO2 + H2O + 1/2 O2 = H2SO4
- The aerosol remains in suspension long after solid ash particles fall
to earth - many years.
- Aerosols "hang out" 15-25 km above Earth's surface in the stratosphere
Aerosol effect on incoming sunlight for
some major eruptions
- The amount of SO2 emitted from a volcanic eruption is
very dependent on the magma type. More mafic magmas, more
- The higher the eruption column (stratospheric vs. tropospheric),
the more aerosol formation from SO2, and hence the greater the
See below for discussion of these sulfate aerosols
in ozone destruction
Fine ash falls too quickly (weeks to months) to significantly
cool the atmosphere over an
extended period of time, no matter how large the eruption.
Laki eruption in Iceland (1783): first time anyone make a connection between
an eruption and worldwide climatic events. Benjamin Franklin
was first to hypothesize about relationship between acidic haze that
appeared around the world in 1783/84 and the eruption in Iceland
This relationship was further explored after eruption of Krakatau in
Indonesia in 1883.
Impact of some major historic eruptions
Table of historic eruptions and associated climatic information
More detailed description of the
above eruptions, and their effects on climate.
Volcanoes and Ozone Depletion
As concern grew over depletion of ozone in the stratosphere scientists
examined the role of volcanoes. They noted that the gases emitted by most
eruptions never leave the troposphere, the layer in the atmosphere from the
surface to about 10km. The troposphere can be scrubbed by
rain. It is the gases (especially Cl) that
reach stratosphere that are a problem; for example
the eruption of Pinatubo in 1991. But, interestingly, there was
no increase in stratospheric Cl from Pinatubo.
Volcanoes account for about 3% of chlorine in the stratosphere. Methyl
chloride produces about 15% of the chlorine entering the stratosphere. The
remaining 82% of stratospheric chlorine comes from man-made sources, mostly
in the form of chlorofluorocarbons.
So volcanoes do not play a big direct role in ozone
depletion. However, they
may play a harmful indirect role. Scientists
have found that sulfate aerosols produced by major volcanic
eruptions accelerate ozone destruction.
The particles themselves do not directly destroy ozone but they do provide a
surface upon which chemical reactions can take place. This enhances
chlorine-driven ozone depletion. Fortunately, the effects from volcanoes
are short lived and after two or three years, the aerosol particles settle
out of the atmosphere.
Study of ozone amounts before and after the 1991 eruption of Mt. Pinatubo
show that there were significant decreases in lower stratospheric ozone.
The amount of ozone in the 16-28 km region was
some reduced by 33% compared to pre-eruption amounts. A similar reduced
amount of ozone was measured in the summer of 1992.
How do we go backwards to affects of volcanoes?
- written records - the classics
- Ice cores - acidity profiles from Greenland
ice cap. H2SO4 aerosols
fall onto permanent ice fields and then sealed by next snowfall. Can count
years, similar to tree rings. Have record of eruptions in last 10,000
years from Greenland ice cap. Good record from Peruvian Andes also.
Sometimes ash layers are present in ice cores. But
acidity information is more important for yielding climate information.
- Tree rings. Dendrochronology. Can look at frost damage in mature
bristlecone pines in western US (nearly 5000 years old). Frost damage
doesn't occur from ordinary winters, but from sustained freezing during
Related web pages and sources of information