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Table 12.1 Comparison between
Greenland ice-core spikes DVI Frost-ring years Match?
volcanic activity and occurrence of
frost rings in bristlecone pine trees in
Agung, 1963 Agung, 1963 1965 Yes
the western USA between 1600 and
Hekla, 1947 – – No
1965. The first column indicates
– – 1941 No
eruptions which produced a
Katmai, 1912 Katmai, 1912 1912 Yes
significant acidity spike in an ice core
– Santa Maria, 1902 1902 Yes
collected from Crete, Greenland.
Krakatau, 1883 Krakatau, 1883 1884 Yes
The second column indicates known
– – 1866 No
historical eruptions with significant
– Merapi, 1837 1837 Yes
Dust Veil Index (DVI) values occurring
– – 1831 No
during this time period. The third
– – 1828 No
column indicates years between 1600
Tambora, 1815 Tambora, 1815 1817 Yes
and 1965 in which frost-ring damage
– – 1805 No
was recorded in bristlecone pine trees
Laki, 1783 – – No
in the western USA. The final column
– – 1761 No
indicates whether there is coincidence
Katla, 1755 – – No
between eruptions and the occurrence
Lanzarote, 1730–36 – 1732 Yes
of frost rings.
– Tongkoko, 1680 1680 Yes
Pacaya, 1671 – – No
– Long Island, 1660 1660 Yes
Komagatake, 1640 Komagatake, 1640 1640 Yes
Unknown, 1601 – 1601 Yes
Data from Hammer, C.U., Clausen, H.B. and Dansgaard, W. (1980) Greenland ice-
sheet evidence of post-glacial volcanism and its climatic impact. Nature, 288,
230–235; LaMarche, V.C. and Hirschboeck, K.K. (1984) Frost rings in trees as
records of major volcanic eruptions. Nature, 307, 121–126.
of an eruption, it is not the most significant one in studies show that the concentration of aerosols
terms of climate change. Most ash particles injected in the atmosphere after an eruption declines over a
into the atmosphere have only a short residence period of 2–3 years. Another reason that the aero-

time (the length of time they spend in the atmo- sols are more important than the ash is because they
sphere). They are typically removed from the atmo- are about ten times more effective at scattering
sphere within days to weeks of the eruption. incoming sunlight. So the presence of the aero-
Smaller particles will stay in the atmosphere longer, sols causes much incoming sunlight to be scattered
but they are usually only a small fraction of the back out into space, reducing the amount of sun-
erupted mass and so their impact is minimal. light reaching the ground and causing surface cool-
Volcanic eruptions release gas as well as ash into ing. Aerosol droplets not only scatter sunlight, they
the atmosphere. Sulfurous gases, SO and H S, also absorb it. Absorption of sunlight (and longer
2 2
released during an eruption will combine with wavelength radiation coming from the Earth’s
water vapor in the atmosphere to form droplets or surface) by aerosols can cause significant warm-
aerosols of sulfuric acid (H SO ) which are typ- ing of the stratosphere. Satellite observations after
2 4
ically ∼1–2 µm in diameter. These acidic aerosols the 1991 Pinatubo eruption showed significant
are far more important in affecting climate than are stratospheric warming. The size of the aerosols is
the ash particles. This is in part because they have a an important factor in determining whether the
longer residence time than the ash. Aerosols form overall effect is one of cooling or warming. If the
in the atmosphere over a period of a few weeks radius of the aerosols is typically < 2 µm then cool-
after the eruption and, due to their small size, have ing dominates; for larger aerosols warming will
long fallout times (see eqn 8.3 and Fig. 8.4). Satellite dominate. The size of the aerosols depends to some