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OCR for page 174
8
Use of Climatic Effects of
Volcanic Eruptions and
Extraterrestrial Impacts on the
Earth as Analogs
Very large explosive volcanic eruptions and asteroid or meteor impacts
can inject large amounts of dust high into the atmosphere. It is
important therefore to assess the extent to which data provided by such
events can be useful in the attempt to understand the atmospheric
modification that would follow a nuclear exchange. The committee did
not, however, find any unambiguous evidence provided by volcanic and
impact events to support or refute a conclusion that nuclear war may
seriously affect the world's climate. No recent natural events have
been energetic enough to provide more than a small atmospheric
perturbation; furthermore, the only investigations of earlier, larger
events, whose goals included dust lofting estimates have been those
associated with the hypothesis that a very large meteor caused the
extinction at the Cretaceous-Tertiary boundary some 65 million years
ago. That event would have been more energetic than the baseline
exchange by a factor of more than 104. Nevertheless, an account is
included here of those aspects of volcanic and natural impact events
that, if there were more data available, would be pertinent.
It is important to consider the similarities and differences
between volcanic explosions and nuclear explosions, and between
extraterrestrial impacts and nuclear explosions. One must keep in mind
that substantial unknowns exist in our understanding of the effects of
these events on the terrestrial ecosystem, which parallel in many ways
the uncertainties in our understanding of the potential effects of
nuclear war. For example, it is only for the past 100 years that a
reliable record exists of the optical depths of clouds of volcanic
· ~
origin.
VOLCANIC ERUPTIONS
Since large volcanic eruptions could introduce quantities of material
into the atmosphere comparable to those from a nuclear war, it is
pertinent to ask if we can use evidence from volcanic explosions to
empirically determine the climatic effects that would be caused by a
nuclear war. It turns out that volcanoes do not prove very useful in
this regard.
174
OCR for page 175
175
The largest explosive volcanic eruptions in the last 200 years have
occurred at Tambora, Indonesia, in 1815; at Krakatau, Indonesia, in
1883; and at Katmai, Alaska, in 1912 (Simkin, 19811. Each of these
eruptions produced 1016 to 1017g of 700 ° to 900°C volcanic
fragments within a few hours to a few~days. All three eruptions
produced large ash clouds that reached into the stratosphere. The
total thermal energies of these eruptions amounted to 1019 to 102°
joules (J), of which only about 1 percent was converted to mechanical
energy (as steam blasts, the initial velocities of ejected material,
and the buoyant lift of the ash cloud). This volcanic mechanical
energy (1017 to 1018 J) is equivalent to 25 to 250 Mt of nuclear
energy. However, the release of volcanic energy takes place over
minutes to days, and hence the power (energy release per second) of
historic volcanic eruptions has been much less than would be the power
of a single-megaton nuclear explosion. Nevertheless, some volcanic
blasts devastate areas similar in size to areas that would be affected
by nuclear explosions, and they loft large amounts of dust and gases
into the stratosphere.
Prehistoric volcanic eruptions have produced thick blankets of
explosive volcanic debris called ash flows covering areas of up to
10,000 km2 with masses of up to 2 x 1018 g. Most of the enormous
thermal energy in these huge eruptions (2 x 1021 J. equivalent to
500,000 Mt) is retained in the hot fragmental debris blanket and is
slowly dissipated over tens to hundreds of years.
In addition to energy release patterns, another important
distinction between volcanic explosion clouds and nuclear explosion
clouds involves composition. Volcanic clouds contain large quantities
of sulfur gases. However, both volcanic explosion clouds and
near-surface nuclear explosion clouds contain silicate glass and
silicate mineral particles.
Very large volcanic eruptions occur infrequently, on the average of
about once every 10,000 to 100,000 years. They occur as locally
isolated events, whereas the 25,000 potential nuclear explosions
assumed in this report occur nearly simultaneously over large areas.
Historic volcanic explosions have not generated large forest or
brush fires, and the thick ash flow blankets of very large prehistoric
eruptions would have tended to smother fires. Hence the ~soot" problem
that results from multiple nuclear explosions probably has no
counterpart in volcanic eruptions.
The effects of gases and dust from volcanic eruptions on climate
have been a subject of speculation ever since Benjamin Franklin alleged
that the Laki eruption in Iceland in 1783 had caused a "dry fog" in
Europe with attendant cold weather and poor crops (Humphreys, 1940~.
Some investigators have concluded that increased volcanism was a major
cause of the Pleistocene ice age (Kennett and Thunnel, 1977~; others
have argued that apparent worldwide average temperature drops of up to
1°C in the year following the large explosive eruptions of Tambora
Volcano (Indonesia, 1815) and Krakatau Volcano (Indonesia, 1883) either
could be errors of measurement (or analysis), or if real, could be
coincidental to normal fluctuations in average world temperature
(Landeberg and Albert, 19741.
OCR for page 176
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Most present investigators of the influence of volcanic eruptions
on climate (Pollack et al., 1976; Robock, 1978; Toon and Pollack, 1982)
agree that there is a measurable effect and that it is largely caused
by the sulfuric acid aerosol particles that form in the stratosphere
from sulfur dioxide gases in volcanic ash clouds that reach
stratospheric altitudes.
Actual sampling of the amount of dust and gases from volcanic
eruptions reaching the stratosphere began following the eruption of
Agung Volcano (Indonesia, 19631. Data on silicate dust (mainly
volcanic glass and small silicate mineral fragments) and sulfur gases
reaching the stratosphere from recent eruptions of Agung, Fuego
(Guatemala), Mount St. Helens, and E1 Chichon (Mexico) volcanoes are
listed in Table 8.1. Rough estimates of the amounts of dust and sulfur
gases reaching the stratosphere from the eruptions of Tambora,
Krakatau, and Agung are given in Table 8.2. Since the Tambora and
Krakatau eruptions produced 10 to 100 times more volcanic debris than
the Agung, Fuego, Mount St. Helens, and E1 Chichon eruptions, it is
apparent from the tables that larger volcanic eruptions do not generate
linearly proportional amounts of dust and gases that reach the
stratosphere. One probable reason for this lack of proportionality is
that larger eruptions produce denser ash clouds, which are less
buoyant. Both Settle (1978) and Wilson et al. (1978) have demonstrated
by theory and observation of historic volcanic eruptions that the
height of explosive eruption clouds increases as the rate of emission
of fragmental volcanic material increases. However, Wilson et al.
calculate that eruption rates of about 106 m3/s will generate
maximum ash cloud heights of 55 km. Larger eruption rates will produce
dense clouds that will fall back under their own weight before reaching
this maximum altitude.
This lack of linear scaling is important in considering the
possible climatic effects of extremely large volcanic explosions in
prehistoric times. In the last 2 million years, there have been six
explosive volcanic eruptions in the western United States (three at
Yellowstone; two at Valles Caldera, New Mexico; and one at Long valley,
California) that have produced 100 to 2000 km3 of fragmental volcanic
material (ash flows) during apparent time intervals of a few hours to
weeks (Francis, 1983~. An estimate for the total of both fine dust and
sulfate aerosols injected into the stratosphere by a single very large
volcanic eruption is 1015 g; however, there is a high degree of
uncertainty in this estimate. Linearly proportional scaling of the
1013 g of stratospheric dust and aerosols produced by the E1 Chichon
eruption up to a Yellowstone-type eruption yields an estimate of 1016
g as an upper limit. The approximately 1014 g fallout of sulfate
from the Tambora eruption (Table 8.2) provides an apparent lower
limit. However, large sulfate loadings may be self-limiting due to the
nonlinear dependence of growth rate and sedimentation of injected
sulfur. Therefore it is not clear that volcanoes have ever exceeded
the apparent lower limit. Loading the global stratosphere with 1015
g of fine dust and sulfate aerosols from a great volcanic explosion
would produce a worldwide average temperature drop of about 10°C for
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179
TABLE 8.2 Comparison of Estimates of Fine Dust, Aerosols, and Sulfate
Fallout
Eruption, Total
Year, and <2-pm
Volumea DUstb (g)
Total Global SO4
Aerosols (g ~Falloutd (g
Tambora, 1815 1.S x 1013 to ? 1.5 x 1014
About 150 km3 1.2 x 1018 (?) (7.5)
(150)
Krakatau, 1883 2 x 1018 to 3 x 1013 5.5 x 1013
About 20 km3 1.6 x 1014 (~3) (~3)
(20)
Agung, 1963 1 x 1011 to 0~9 x 1013 2 x 1013
About 1 km3 8 x 1013 (1) (1)
(1)
aVolumes of eruptions are expressed as bulk volumes of near-source
and distal ejecta.
busing the method of Murrow et al. (1980), assumes an average bulk
density of fine ash of 1 g/cm3. This volume represents the total
mass of <2-pm-diameter dust ejected into the atmosphere. Only a
fraction of this dust entered the stratosphere. Evidence from Mount
St. Helens eruptions (Rose and Hoffman, 1982) demonstrates that a
large, but as yet undetermined, portion of the fine dust will be
quickly removed by particle aggregate formation.
CDeirmendjian (1973~. Refers to stratospheric loading.
dHammer et al. (1980~. Refers to stratospheric loading.
NOTE: Figures in parentheses represent relative quantities of dust,
aerosol, and sulfate fallout, with Agung (1963) as base figure.
SOURCE: Rampino and Self (1982~.
several months. This hypothetical climate anomaly is calculated by
scaling upward the optical effect of the E1 Chichon eruption.
In Table 8.1 it can be seen that the silicate loading following the
eruption of Mount St. Helens was dominated by large ash particles and
the submicron mass was probably smaller than the 1013 g of submicron
dust estimated for the baseline case. Likewise, the E1 Chichon
silicate mass loadings consisted of large particles and the estimates
are unreliable since no in situ measurements were made in the E1
Chichon cloud for several months, at which time the silicate dust
loadings were very small (about 1012 g).
Unfortunately, the possibly large mass loadings of stratospheric
submicron debris for prehistoric giant volcanic eruptions cannot be
OCR for page 180
180
substantiated from current data. Very little study has been devoted to
the environmental effects of giant volcanic eruptions. Clearly, the
environmental effects may have been severe even thousands of kilometers
from the volcanoes since substantial quantities of ash were deposited
at such distances. The greatest volcanic eruptions, even if they do
put as much as 1016 g into the atmosphere, would not be expected to
cause effects similar to those of the much more powerful meteorite
posited by Alvarez et al. as discussed by Toon et al. (1982~. Major
biological extinctions due to volcanic eruptions are then neither
expected nor detected in the geologic record. Severe global climatic
changes that would pose problems for modern society can neither be
substantiated nor excluded on the basis of our current limited
knowledge of prehistoric volcanic eruptions.
In summary, large explosive volcanic eruptions may be reasonable
analogs for some atmospheric effects of a nuclear war, but not enough
is known about these eruptions to provide useful guidelines. Clearly,
studies of the amount of fine dust and aerosols that actually reaches
the stratosphere are warranted. Likewise, a better knowledge of the
environmental impact of previous eruptions is needed.
EXTRATERRESTRIAL IMPACTS
Alvarez et al. (1980) suggested that the abrupt extinction of many
species of marine plankton and other organisms at the end of the
Cretaceous period {about 65 million years ago) was a consequence of the
impact of an extraterrestrial body of about 10 km in diameter that
lofted quantities of cratering dust particles into the atmosphere.
Subsequent work by Alvarez and others has led to the discovery at the
Cretaceous-Tertiary boundary of a characteristic claystone layer
enriched in certain noble metals at about 60 additional localities
around the globe. Between one-fourth and one-half of the late
Cretaceous plant taxa recognizable from pollen became extinct at the
claystone layer, where it has been observed in North America. These
new observations greatly strengthen the initial hypothesis of Alvarez
et al. that a body of about 10 km in diameter did strike the earth at
the end of the Cretaceous and that this impact may have caused the
extinction of species. The extinction may have been a consequence of
the darkening of most of the earth's surface by cratering debris
suspended in the atmosphere (Alvarez et al., 1982; Toon et al., 1982),
of an increase in surface temperature after the debris had settled due
to an increase in the water vapor content of the atmosphere (Emiliani
et al., 1981), of the production of large quantities of NOX in the
impact fireball (Lewis et al., 1982), or of the interaction of physical
and biological effects. The suggestion that a global veil of dust was
the cause of the extinction of species at the end of the Cretaceous
spurred the present concern that the dust and soot produced in a
nuclear war might have similarly deleterious effects.
Shoemaker (1983) estimates that the cumulative frequency of impacts
by extraterrestrial bodies with radii >r varies approximately as
r~2, at least for bodies in the kilometer size range. At a
representative velocity of 20 km/s, the impact of a 10-km body would
OCR for page 181
181
release an amount of energy of the order of 108 Mt. The Cretaceous-
Tertiary claystone layer, where it is recognized, is about 2 cm thick
on average, and the total mass of a global deposit with this thickness
is about 1019 g. Impacts of bodies of 5-km diameter or larger occur
about 4 times as frequently as impacts of 10-km bodies, or at a rate of
10-7 yr~l, with impact energies of 107 Mt; for 2-km-diameter
bodies or larger the rate is 6 x 10-7 yr~1 and the energy is 106
Mt. The masses of ejecta scale approximately with yield; scaling from
the Cretaceous-Tertiary boundary claystone layer gives 1018 g and
1017 g (not necessarily all of it submicron sizes) globally dispersed
ejecta for the 5-km and 2-km bodies, respectively. The resultant clay
layers would be roughly 0.2 and 0.02 cm thick, respectively, and would
not have been detected in the geologic record.
Although the particle size distribution in impact-generated dust
clouds is unknown and the clouds could have had much smaller submicron
fractions than the nuclear clouds, estimates of impact-produced dust
could be from one to several orders of magnitude larger than the
2 x 1014 g of smoke and 2 x 1013 g of submicron dust generated in
the baseline nuclear war. The impact energies are also orders of
magnitude greater than the 6500-Mt yield of the baseline nuclear war.
Impacts roughly comparable to the baseline war in energy release and in
dust lofted require objects of about 500-m diameter, and occur roughly
once every 105 yr.
Mass extinction events comparable to that at the end of the
Cretaceous are fairly rare events in the history of the earth. Global
mass extinctions that have been recognized at the taxonomic level of
families of organisms have recurred at a mean interval of about 30
million years (Raup and Sepkoski, 1984~. There is some evidence that
mass extinctions might have been produced by the impact of bodies as
small as 5 km in diameter, but the impact of 2-km bodies appears not to
have left an easily recognized imprint on the succession of life forms
recorded by tOSSllS. It is clear that traceable catastrophes or the
magnitude of the Cretaceous-Tertiary extinction are only produced by
impacts with energy releases substantially exceeding those of a
possible near-term global nuclear exchange. On the other hand, quite
severe perturbations of the environment that did not succeed in
producing extinction of many species--and with durations of a few years
or less--cannot be easily detected in the stratigraphic record.
Therefore it is not, at present, known whether impacts with energies in
the range of 104 to 106 Mt had atmospheric effects similar to or
even more severe than those projected for the baseline nuclear war.
Further studies of the environmental effects of large asteroidal
impacts as well as studies of the debris lofted by asteroid impacts may
be valuable for establishing analogs for the nuclear war case. Toon et
al. {1982) have shown that impact events that produce masses of dust as
small as 1017 g should have produced light levels and low temperatures
very similar to those of 101 -g impacts. Moreover, light levels low
enough to cause failure of photosynthesis may occur with injections of
no more than 1016 or 1017 ~ of dust. Hence the impacts of bodies 2
km in diameter or somewhat smaller may have produced both physical and
biological effects that would be detectable in the stratigraphic record
by means of a directed intensive search.
OCR for page 182
182
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Representative terms from entire chapter:
nuclear war
