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Summary and Conclusions
The Committee on the Atmospheric Effects of Nuclear Explosions
addressed the following charge: (1) determine the manner in which the
atmosphere of the earth would be modified by a major exchange of
nuclear weapons and, insofar as the current state of knowledge and
understanding permits, give a quantitative description of the more
important ofithe changes, and (2) recommend research and exploratory
work appropriate to a better understanding of the question.
The committee was not asked to (and did not) address the related
but distinct questions of the extent of radioactive fallout or the
biological or social implications of postwar atmospheric modification.
Recent calculations by different investigators suggest that the
climatic effects from a major nuclear exchange could be large in
scale. Although there are enormous uncertainties involved in the
calculations, the committee believes that long-term climatic effects
with severe implications for the biosphere could occur, and these
effects should be included in any analysis of the consequences of
nuclear war. However, the committee cannot subscribe with confidence
to any specific quantitative conclusions drawn from calculations based
on current scientific knowledge. The estimates are necessarily rough
and can only be used as a general indication of the seriousness of what
might occur.
Despite the early state of understanding of these matters, the
possibility of severe degradation of the atmosphere after a major
nuclear exchange is of sufficient national and international concern
that a major effort to narrow the scientific uncertainties should be
given a high priority.
BACKGROUND
It is widely understood that any major nuclear exchange would be
accompanied by an enormous number of immediate fatalities;
nevertheless, a much larger fraction of the human population would
survive the immediate effects of a nuclear exchange. This study
addresses current knowledge about the nature of the physical
environment the survivors would have to face.
1
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The realization that a nuclear exchange would be accompanied by the
deposition into the atmosphere of large amounts of particulate matter
is not new. However, the suggestion that the associated attenuation of
sunlight might be so extensive as to cause severe drops in surface air
temperatures and other major climatic effects in areas that are far
removed from target zones is of rather recent origin. That perception
has grown out of a number of recent investigations. Crutzen and Birks
(1982) recognized that the amount of smoke from the fires ignited by
nuclear blasts could be of crucial importance, and Alvarez et al.
(1980) hypothesized that the massive species extinctions of 65 million
years ago were part of the aftermath of the lofting of massive
quantities of particulates resulting from the collision of a large
meteor with the earth. Others have recognized the similarity between
the Alvarez dust hypothesis and the effects of nuclear war (see
Appendix).
The consequences of any such changes in atmospheric state would
have to be added to the already sobering list of relatively
well-understood consequences of nuclear war, including prompt
radiation, blast, and thermal effects, short-term regional radioactive
fallout, inadequate medical attention for surviving casualties, and the
long-term biological effects of global fallout. Long-term atmospheric
consequences imply additional problems that are not easily mitigated by
prior preparedness and that are not in harmony with any notion of rapid
postwar restoration of social structure. They also create an entirely
new threat to populations far removed from target areas, and suggest
the possibility of additional major risks for any nation that itself
initiates use of nuclear weapons, even if nuclear retaliation should
somehow be limited.
TEE COMMITTEE ' S BASELINE CASE
To provide a framework for its study, the committee first constructed a
baseline war scenario, made up of assumptions concerning the nature of
the weapon exchange. The baseline scenario (see Chapter 3 for greater
detail) was selected so as to be representative of a general nuclear
war: one-half--about 6500 megatons (Mt)--of the estimated total world
arsenal would be detonated. Of this, 1500 Mt would be detonated at
ground level. Of the other 5000 Mt that would be detonated at
altitudes chosen so as to maximize blast damage to structures, 1500 Mt
would be directed at military, economic, and political targets that
coincidentally lie in or near about 1000 of the largest urban areas.
All explosions would occur between 30°N and 70°N latitude.
The committee also chose, on the basis of a review of the
scientific literature, a set of baseline physical parameters to use in
calculating the effects of the baseline weapon exchange. Each baseline
parameter was chosen to lie well within the spectrum of scientifically
plausible values, values in the middle ranges of plausibility being
preferred.
There are three immediate consequences of a major nuclear exchange
that could have a significant impact on the subsequent state of the
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atmosphere. Large amounts of dust could be lofted high into the
atmosphere; large fires could be initiated; and large amounts of
undesirable chemical species could be released. Some of the key
parameters ~
assumed for the baseline case follow.
The amount of dust (Chapter 4) that would be deposited in the
stratosphere is related to total megatonnage. ~ ~~
that the total amount lofted is 0.3 teragrams (1 Tg = 1012 g ~
106 metric tons) per megaton detonated. Eight percent of the mass of
dust would be of submicron size, which remains aloft for long periods.
The 1500 Mt in ground bursts would raise about 15 Tg of submicron dust
into the stratosphere, where it could reside for more than a year.
During that time, the solar radiation through that dust, and into the
lower atmosphere, would be reduced.
The analysis of fires and smoke is complex (Chapter 5~. The 5000
Mt of air bursts would initiate vigorous fires in cities and forests
over areas where the thermal radiation incident on combustible material
was 20 calories per square centimeter (cal/cm2) or greater, a number
well in excess of that known to be adequate to ignite the fuels at
hand. In the city-scale urban conflagrations that would ensue, the
baseline assumption is that three-quarters of the combustible material
in affected areas would be consumed. (Nearly complete consumption of
combustible materials is typical of large city-wide fires for which
data are available.) Although many of the urban fires would probably
spread beyond the 20 cal/cm2 ignition zone, no additional fuel burden
from that spreading is assumed in the baseline case. Of the material
that burned in cities, the baseline case assumes that some 4 percent
(limited data suggest values lying between 1 percent and 6 percent)
would be converted to smoke particles in a range of submicron sizes
that would absorb and scatter sunlight very effectively.
Certain processes may, however, diminish the optical effects of the
smoke at this stage. During the burning of the urban and/or forest
fires, the very fine smoke particles would undergo some coagulation in
the rising plume. Over regions where the ambient ground-level humidity
was high, the condensation of moisture entrained in the plume could
incorporate some of the smoke. There is little empirical evidence to
suggest extensive scavenging of the smoke by these processes, but in
the baseline case, 50 percent of the smoke is assumed to be removed
from the plumes of urban fires in this manner.
On the basis of available information on plume dynamics, it is
assumed that, shortly after deposition the smoke from the ensemble of
fires would be uniformly distributed vertically (mass per unit height)
between 0 and 9 km over the entire affected area; the local vertical
distribution would be nonuniform, however, because the altitudes of the
smoke plumes would vary from one fire to another and would also vary
with the time-dependent intensity of the fire. Although under special
meteorological circumstances some of the smoke might be deposited at
altitudes significantly higher than 9 km, this effect is ignored in the
baseline case. Initially, and for some weeks, the smoke would have a
very nonuniform horizontal distribution, but would be distributed
throughout the troposphere of the northern temperate zone.
The committee assumes
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The injection of nitrogen oxides from the nuclear clouds into the
upper atmosphere would lead to a depletion of the ozone column, which
would be restored in about 2 years (Chapter 6~.
The atmospheric implications of the baseline case (and of the
results of other groups' analyses) are presented in Chapter 7. The
committee expects that solar radiation passing through the
stratospheric layer of nuclear dust would be absorbed in the upper
regions of the smoke layer. The smoke layer would heat up, and since
little solar radiation would reach lower levels, the air over land
surfaces would cool.
Although a few types of natural events can provide marginally
relevant information on aspects of the problem (see also Chapter 8),
much of our understanding of the atmospheric response to large amounts
of airborne particulates will come from model simulations. These
models are validated within relatively small natural variations, so
their predictive capability is limited for these large perturbations.
Only preliminary estimates can be made of the rate of spreading of
particulates over initially clear latitudes and of the rate of removal
of particulates.
The duration and magnitude of atmospheric effects would depend on
how long the absorbing particulates remained aloft. There is
especially large uncertainty associated with long-term removal
processes for smoke that survives the early scavenging. Low-altitude
precipitation processes might remove the low-altitude smoke, that below
4 km (the normal range for smoke), rather efficiently. But at high
altitudes, the increased air temperature and the low humidity could
lead to a removal rate in the 4- to 9-km range that would be slower
than the removal rate in today's troposphere. The baseline assumes
that removal rates would be at least comparable to normal removal rates
in the lower atmosphere (below 5 km), but would be slightly slower than
normal in the upper troposphere (5 to 10 km), with about one-half of
the initial particulates removed from the lower atmosphere in 3 days,
and from the upper atmosphere in 30 days.* It is unlikely that the
average residence times for postwar smoke would be much less than these
values, and quite possible that the mean residence time in the upper
troposphere would be longer.
It is hoped that the committee's baseline case will provide a
useful point of departure for those who wish to identify and assess the
environment that would prevail following a major nuclear exchange.
*If the smoke particles acquired electrical charges, the coagulation
and smoke removal times could be affected. However, there is no
evidence from large historical fires that electrical activity intense
enough to be observed was operative. Furthermore, there is no evidence
that the sometimes large and visible electrical effects in intense
natural events (e.g., tornadoes and volcanoes) influence the dynamics
of the storm. Thus, having no reliable basis on which to do otherwise,
the committee has disregarded potential electrical effects.
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NOTES ON THE NATE ED SIGNIFICANCE OF CERTAINTY
As may be clear from this brief description of the baseline case, there
are many points in the analysis at which there is a wide range of
parameter values that are consistent with the best current scientific
knowledge. Any estimate of the overall atmospheric response will
involve a compounding of the effects of these uncertainties.
Obviously, calculations made under these conditions cannot be read as a
scientific prediction of the effects of a nuclear exchange; rather,
they represent an interim estimate from which the reader can infer
something of the potential seriousness of the atmospheric degradation
that might occur.
Some reviewers of earlier drafts of this report cautioned that even
the most qualified numerical results produced under these conditions
could be misinterpreted, and some suggested that at present the only
scientifically valid conclusion would be that it is not at this time
possible to calculate the atmospheric effects of nuclear war. The
committee believes, however, that an appropriately qualified,
preliminary quantitative treatment of the problem is warranted on two
grounds. First, given the enormous human stakes that may be involved,
it may not be advisable to wait until a strong scientific case has been
assembled before presenting tentative results; there is a danger that a
report that reached no conclusions at all would be misconstrued to be a
refutation of the scientific basis for the suggestion that severe
atmospheric effects are possible. Second, a quantitative approach to
the problem is the best way to ensure that all important factors are
systematically considered, and quantification helps distinguish the
important factors from the less important ones in the overall
analysis. Such results are necessary to the orderly allocation of
resources to the most pertinent research questions.
The findings of this report depend in rather large measure on a
still limited body of scientific inquiry, some of which is not yet
fully documented. Attention to the subject is so recent, in fact, that
some of the underlying analysis has not yet undergone the peer review
process that precedes publication in most scientific journals.
The reader should appreciate the possibility that further research
may well invalidate some of the estimates discussed in this report. As
recently as 1975, when the National Research Council report Long-Term
Worldwide Effects of Multiple Nuclear ~e~pcn~ Deto~atiop~ appeared,
plausible weapon use scenarios differed significantly from those
envisaged today, and the crucial importance of fires and smoke had not
then been recognized. It follows that the findings presented in this
report differ from those of the 1975 report. Furthermore, the
pervasive uncertainties in the data and the limited validity of the
atmospheric models used to date imply that some future study, conducted
at a time when the data and models have been improved, could produce
quite different analyses and conclusions. It is possible that improved
understanding of some mechanisms (e.g., early scavenging) could so
affect the results that the atmospheric degradation would be shown to
be weaker than that estimated in the baseline case, but the same
uncertainty also makes it a clear possibility that the exchange could
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produce a degradation that would be greater than, and would last longer
than, that estimated in the baseline case.
In short: the committee's findings are clearly and emphatically of
an interim character.
A vigorous research effort is now needed. Nevertheless, one cannot
expect that long-term nuclear effects will be characterized with great
precision or confidence in the next few years. Many uncertainties
cannot be narrowed because they depend on human decisions that can be
made, or changed, long after any particular prediction has been
issued. These include, for example, the total yield of the exchange,
individual warhead yields, the mix of targets, the mix of altitudes at
which the bursts would occur, and the season of the year in which the
exchange would occur. In addition, there are obvious limits to the use
of large-scale experiments in this field, and the evolution of
atmospheric models will require some time.
Many significant uncertainties, however, can be narrowed by further
study. In particular, the heights to which smoke is deposited in
city-scale fires, the early smoke removal by coagulation and
condensation in the fire plume, the extent of continued buoyant rising
of sun-heated opaque clouds, and the dynamical response of the
atmosphere, first to patchy high-altitude solar absorption and then to
the heating of more broadly distributed but still heavy smoke cover,
have received only scattered and recent attention.
CONCLUSIONS
The general conclusion that the committee draws from this study is the
following: a major nuclear exchange would insert significant amounts
of smoke, fine dust, and undesirable chemical species into the
atmosphere. These depositions could result in dramatic perturbations
of the atmosphere lasting over a period of at least a few weeks.
Estimation of the amounts, the vertical distributions, and the
subsequent fates of these materials involves large uncertainties.
Furthermore, accurate detailed accounts of the response of the
atmosphere, the redistribution and removal of the depositions, and the
duration of a greatly degraded environment lie beyond the present state
of knowledge.
Nevertheless, the committee finds that, unless one or more of the
effects lie near the less severe end of their uncertainty ranges, or
unless some mitigating effect has been overlooked, there is a clear
possibility that great portions of the land areas of the northern
temperate zone (and, perhaps, a larger segment of the planet) could be
severely affected. Possible impacts include major temperature
reductions (particularly for an exchange that occurs in the summer)
lasting for weeks, with subnormal temperatures persisting for months.
The impact of these temperature reductions and associated
meteorological changes on the surviving population, and on the
biosphere that supports the survivors, could be severe, and deserves
careful independent study.
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A more definitive statement can be made only when many of the
-
uncertainties have been narrowed, when the smaller scale ohenomena are
-
better understood, and when atmospheric response models have been
constructed and have acquired credibility for the parameter ranges of
this phenomenology.
The committee also draws several more specific conclusions:
1. In an extensive nuclear exchange, explosions over urban areas
and forests would ignite many large fires. Massive smoke emissions are
an important aspect of nuclear warfare that have only recently been
recognized. For the major 6500-Mt nuclear war considered here, fires
could release massive amounts of smoke into the troposphere over a
period of a few days. Much of the smoke might be removed by
meteorological processes within several weeks, depending on feedback
effects, but significant amounts could remain for several months.
During its tenure in the atmosphere, the smoke would gradually spread
and become more uniformly distributed over much of the northern
hemisphere, although some patchiness would be likely to persist. Light
levels could be reduced by a factor of 100 in regions that were covered
with the initial hemispheric average smoke load, causing intense
cooling beneath the particulate layer and unusually intense heating of
the under layer. While large uncertainties currently attend the
. . . _ . . . _ . . .
estimates or smoke emissions, and Of their optical and physical
consequences, the baseline case implies severe atmospheric consequences.
2. The production of smoke from fires, and the implied effects on
the atmosphere, is more directly linked to the extent of detonation
over urban areas than to the aggregate yield of a nuclear exchange.
The industrialized nations of the world have concentrated a large
proportion of their resources and combustible fuels in the vicinity of
the central areas of their large cities. Any war scenario that
subjects these city centers to nuclear attack, even one employing a
very small fraction of the existing nuclear arsenal, could generate
nearly as much smoke as in the 6500-Mt baseline war scenario.
3. The climatic impact of soot is very sensitive to its lifetime
in the perturbed atmosphere and the uniformity of its distribution.
The lifetime of soot is highly uncertain, particularly in the upper
troposphere. The perturbation itself would produce severe new effects,
many of which could tend to increase the residence time of the soot.
Although the lofted soot (and dust) would rapidly spread around the
latitude band of injection, the distribution could be uneven for
several months, with continent-size patches of lesser and greater
density, particularly near the southern edge of the affected zones.
4. In the baseline nuclear war scenario, hundreds of teragrams of
dust would be injected into the atmosphere from surface detonations. A
significant fraction of the dust consisting of particles with radii
less than one micron (1 um) would be expected to remain aloft for
months. About one-half of these submicron particles would be injected
into the stratosphere and would produce some long-term reduction of
sunlight at the earth's surface, even after smoke and dust at lower
altitudes were removed. This stratospheric dust alone would lead to
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perceptible reductions in average light intensities, and continental
surface temperatures would fall measurably. In a plausible scenario
that involves more ground burst attacks against very hard targets than
are assumed in the baseline case, the possible dust effects are several
times larger.
5. It is not possible at this time to estimate the most probable
average temperature changes at the surface caused by smoke and dust
lofted in the baseline case; nor would such a single value, even if
available, meaningfully describe the situation. In addition to the
large uncertainties in many of the critical physical parameters and the
inherent limitations of the models available for computer simulations,
the available calculations reflect wide seasonal and geographical
differences. Recent general circulation model simulations that
incorporate simplifying assumptions indicate that a baseline attack
during the summer might decrease mean continental temperatures in the
northern temperate zone by as much as 10° to 25°C, with temperatures
along the coasts of the continents decreasing by much smaller amounts.
In contrast, an attack of the same size during the winter, according to
these simulations, might produce little change in temperature in the
northern temperate zone, although there could be a significant drop in
temperatures at more southern latitudes.
6. The nitrogen oxides deposited in the stratosphere by nuclear
detonations would reduce the abundance of ozone. For the 6500-Mt
nuclear war, the northern hemisphere ozone reduction could become
substantial several months after the war. Estimates based on current
stratospheric structure suggest that the amount of ozone reduction
would decrease by one-half after about 2 years. At the time of maximum
ozone reduction, the biologically effective ultraviolet intensity
(using the DNA action spectrum) at the ground would be approximately
one and one-half times the normal levels. Initially, the presence of
dust and smoke particles in the atmosphere would provide a measure of
protection at the surface from the enhanced ultraviolet radiation.
This protection would gradually diminish as the particles were removed.
7. This study has concentrated on the possible effects that a
nuclear war could have on the northern hemisphere, primarily within the
mid-latitude region (30°N to 70°N) where the nuclear exchange would be
concentrated. It is particularly difficult to assess the potential
effects of the baseline war on the atmosphere of the northern tropics
and southern hemisphere. Although southern hemisphere effects would be
much less extensive, significant amounts of dust and smoke could drift
to and across the equator as early as a few weeks after a nuclear
exchange. A large rate of transport across the equator driven by
heating in the debris cloud cannot be ruled out. Indeed, such
heating-enhanced cross-equatorial circulation has been found for spring
and summer months in computer simulations.
8. Some prehistoric volcanic eruptions and impacts from
extraterrestrial bodies have released energies corresponding to levels
that would be released in a major nuclear exchange and may have lofted
massive amounts of dust; however, neither type of event provides a
useful direct analog to the nuclear case because neither type involved
the production of highly absorbing soot particles. Furthermore, the
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atmospheric consequences of prehistoric natural events of these
proportions are not known, and their effects on the fossil record, if
any, have not been sought in any systematic way. Accordingly,
available knowledge about prehistoric volcanic and impact events
provides neither support nor refutation of the committee's conclusions.
9. All calculations of the atmospheric effects of a major nuclear
war require quantitative assumptions about uncertain physical
parameters. In many areas, wide ranges of values are scientifically
credible, and the overall results depend materially on the values
chosen. Some of these uncertainties may be reduced by further
empirical or theoretical research, but others will be difficult to
reduce. The larger uncertainties include the following: (a) the
quantity and absorption properties of the smoke produced in very large
fires; (b) the initial distribution in altitude of smoke produced in
large fires; (c) the mechanisms and rate of early scavenging of smoke
from fire plumes, and aging of the smoke in the first few days; (d) the
induced rate of vertical and horizontal transport of smoke and dust in
the upper troposphere and stratosphere; (e) the resulting perturbations
in atmospheric processes such as cloud formation, precipitation,
storminess, and wind patterns; and (f) the adequacy of current and
projected atmospheric response models to reliably predict changes that
are caused by a massive, high-altitude, and irregularly distributed
injection of particulate matter. The atmospheric effects of a nuclear
exchange depend on all of the foregoing physical processes ((a) through
(e)), and their ultimate calculation is further subject to the
uncertainties inherent in Iffy.
REFERENCES
Alvarez, L.W., W. Alvarez, F. Asaro, and H.W. MichaeL (1980)
Extraterrestrial cause for the Cretaceous-Tertiary extinction.
Science 208:1095-1108.
Crutzen, P.J., and J.W. Birks (1982) The atmosphere after a nuclear
war: Twilight at noon. Ambio 11:114-125;.
National Research Council (1975) Long-Term Worldwide Effects of
Multiple Nuclear Weapons Detonations. Washington, D.C.: National
Academy of Sciences.
Representative terms from entire chapter:
nuclear war
