Nuclear Winter
Carl Sagan
Carl Sagan was horn in 1934 in New York. After
graduating with both a B.A. and a B.S. degree from the
University of Chicago, Sagan completed his M.S. in physics and
earned a Ph.D. in astronomy and astro-physics in 1960. Sagan
then taught astronomy at Harvard until 1968, when he became
profossor of astronomy and space sciences at Cornell University.
He was then appointed director of the laboratory for Planetary
Studies. His works include The Cosmic Connection (1973),
which received the Campbell Award for best science book; the
Pulitzer-prize winning Dragons of Eden (1977); Broca's
Brain (1979), on developments in neurophysiology; and Cosmos
(1980), which accompanied his widety-acclaimed television
series. In "The Nuclear Winter" (1983), Sagan explored
the unforeseen and devastating physical and chemical effects of
even a small-scale nuclear war on the earth's biosphere and life
on earth.
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Except for fools and madmen, everyone knows that nuclear war would he
an unprecedented human catastrophe. A more or less typical strategic
warhead has a yield of 2 megatons, the explosive equivalent of 2
million tons of TNT. But 2 million tons of TNT is about the same as
all the bombs exploded in World War II -- a single bomb with the
explosive power of the entire Second World War but compressed into a
few seconds of time and an area 30 or 40 miles across
In a 2-megaton explosion over a fairly large city, buildings would be
vaporized, people reduced to atoms and shadows, outlying structures
blown down like matchsticks and raging fires ignited. And if the bomb
were exploded on the ground, an enormous crater, like those that can
be seen through a telescope on the surface of the Moon, would be all
that remained where midtown once had been. There are now more than
50,000 nuclear weapons, more than 13,000 megatons of yield, deployed
in the arsenals of the United States and the Soviet Union -- enough to
obliterate a million Hiroshimas.
But there are fewer than 3000 cities on the Earth with populations of
100,000 or more. You cannot find anything like a million Hiroshimas to
obliterate. Prime military and industrial targets that are far from
cities are comparatively rare. Thus, there are vastly more nuclear
weapons than are needed for any plausible deterrence of a potential
adversary.
Nobody knows, of course, how many megatons would be exploded in a
real nuclear war. There are some who think that a nuclear war can be "contained,"
bottled up before it runs away to involve much of the world's
arsenals. But a number of detailed analyses, war games run by the U.S.
Department of Defense, and official Soviet pronouncements all indicate
that this containment may be too much to hope for: Once the bombs
begin exploding, communications failures, disorganization, fear, the
necessity of making in minutes decisions affecting the fates of
millions, and the immense psychological burden of knowing that your
own loved ones may already have been destroyed are likely to result in
a nuclear paroxysm. Many investigations, including a number of studies
for the U.S. government, envision the explosion of 5,000 to 10,000
megatons -- the detonation of tens of thousands of nuclear weapons
that now sit quietly, inconspicuously, in missile silos, submarines
and long-range bombers, faithful servants awaiting orders.
The World Health Organization, in a recent detailed study chaired by
Sune K. Bergstrom (the 1982 Nobel laureate in physiology and
medicine), concludes that 1.1 billion people would be killed outright
in such a nuclear war, mainly in the United States, the Soviet Union,
Europe, China and Japan. An additional 1.1 billion people would suffer
serious injufles and radiation sickness, for which medical help would
be unavailable. It thus seems possible that more than 2 billion
people-almost half of all the humans on Earth-would be destroyed in
the immediate aftermath of a global thermonuclear war. This would
represent by far the greatest disaster in the history of the human
species and, with no other adverse effects, would probably be enough
to reduce at least the Northern Hemisphere to a state of prolonged
agony and barbarism. Unfortunately, the real situation would be much
worse. In technical studies of the consequences of nuclear weapons
explosions, there has been a dangerous tendency to underestimate the
results. This is partly due to a tradition of conservatism which
generally works well in science but which is of more dubious
applicability when the lives of billions of people are at stake. In
the Bravo test of March 1, 1954, a 15-megaton thermonuclear bomb was
exploded on Bikini Atoll. It had about double the yield expected, and
there was an unanticipated last-minute shift in the wind direction. As
a result, deadly radioactive fallout came down on Rongelap in the
Marshall Islands, more than 200 kilometers away. Most all the children
on Rongelap subsequently developed thyroid nodules and lesions, and
other long-term medical problems, due to the radioactive fallout.
Likewise, in 1973, it was discovered that high-yield airbursts will
chemically burn the nitrogen in the upper air, converting it into
oxides of nitrogen; these, in turn, combine with and destroy the
protective ozone in the Earth's stratosphere. The surface of the Earth
is shielded from deadly solar ultraviolet radiation by a layer of
ozone so tenuous that, were it brought down to sea level, it would be
only 3 millimeters thick. Partial destruction of this ozone layer can
have serious consequences for the biology of the entire planet.
These discoveries, and others like them, were made by chance. They
were largely unexpected. And now another consequence -- by far the
most dire -- has been uncovered, again more or less by accident.
The U.S. Mariner 9 spacecraft, the first vehicle to orbit another
planet, arrived at Mars in late 1971. The planet was enveloped in a
global dust storm. As the fine particles slowly fell out, we were able
to measure temperature changes in the atmosphere and on the surface.
Soon it became clear what had happened:
The dust, lofted by high winds off the desert into the upper Martian
atmosphere, had absorbed the incoming sunlight and prevented much of
it from reaching the ground. Heated by the sunlight, the dust warmed
the adjacent air. But the surface, enveloped in partial darkness,
became much chillier than usual. Months later, after the dust fell out
of the atmosphere, the upper air cooled and the surface warmed, both
returning to their normal conditions. We were able to calculate
accurately, from how much dust there was in the atmosphere, how cool
the Martian surface ought to have been.
Afterwards, I and my colleagues, James B. Pollack and Brian Toon of
NASA's Ames Research Center, were eager to apply these insights to the
Earth. In a volcanic explosion, dust aerosols are lofted into the high
atmosphere. We calculated by how much the Earth's global temperature
should decline after a major volcanic explosion and found that our
results (generally a fraction of a degree) were in good accor4 with
actual measurements. Joining forces with Richard Turco, who has
studied the effects of nuclear weapons for many years, we then began
to turn our attention to the climatic effects of nuclear war. [The
scientific paper, "Global Atmospheric Consequences of Nuclear
War," was written by R. P. Turco, 0. B. Toon, T. P. Ackerman, J.
B. Pollack and Carl Sagan. From the last names of the authors, this
work is generally referred to as "TTAPS."]
We knew that nuclear explosions, particularly groundbursts, would
lift an enormous quantity of fine soil particles into the atmosphere
(more than 100,000 tons of fine dust for every megaton exploded in a
surface burst). Our work was further spurred by Paul Crutzen of the
Max Planck Institute for Chemistry in Mainz, West Germany, and by John
Birks of the University of Colorado, who pointed out that huge
quantities of smoke would be generated in the burning of cities and
forests following a nuclear war.
Croundburst -- at hardened missile silos, for example -- generate
fine dust. Airbursts -- over cities and unhardened military
installations -- make fires and therefore smoke. The amount of dust
and soot generated depends on the conduct of the war, the yields of
the weapons employed and the ratio of groundbursts to airbursts. So we
ran computer models for several dozen different nuclear war scenarios.
Our baseline case, as in many other studies, was a 5000-megaton war
with only a modest fraction of the yield (20 percent) expended on
urban or industrial targets. Our job, for each case, was to follow the
dust and smoke generated, see how much sunlight was absorbed and by
how much the temperatures changed, figure out how the particles spread
in longitude and latitude, and calculate how long before it all fell
out in the air back onto the surface. Since the radioactivity would be
attached to these same fine particles, our calculations also revealed
the extent and timing of the subsequent radioactive fallout.
Some of what I am about to describe is horrifying. I know, because it
horrifies me. There is a tendency -- psychiatrists call it "denial"
-- to put it out of our minds, not to think about it. But if we are to
deal intelligently, wisely, with the nuclear arms race, then we must
steel ourselves to contemplate the horrors of nuclear war.
The results of our calculations astonished us. In the baseline case,
the amount of sunlight at the ground was reduced to a few percent of
normal-much darker, in daylight, than in a heavy overcast and too dark
for plants to make a living from photosynthesis. At least in the
Northern Hemisphere, where the great preponderance of strategic
targets lies, an unbroken and deadly gloom would persist for weeks.
Even more unexpected were the temperatures calculated. In the
baseline case, land temperatures, except for narrow strips of
coastline, dropped to minus 25 Celsius (minus 13 degrees Fahrenheit)
and stayed below freezing for months -- even for a summer war.
(Because the atmospheric structure becomes much more stable as the
upper atmosphere is heated and the low air is cooled, we may have
severely underestimated how long the cold and the dark would last.)
The oceans, a significant heat reservoir, would not freeze, however,
and a major ice age would probably not be triggered. But because the
temperatures would drop so catastrophically, virtually all crops and
farm animals, at least in the Northern Hemisphere, would be destroyed,
as would most varieties of uncultivated or domesticated food supplies.
Most of the human survivors would starve.
In addition, the amount of radioactive fallout is much more than
expected. Many previous calculations simply ignored the intermediate
time-scale fallout. That is, calculations were made for the prompt
fallout -- the plumes of radioactive debris blown downwind from each
target-and for the long-term fallout, the fine radioactive particles
lofted into the stratosphere that would descend about a year later,
after most of the radioactivity had decayed. However, the
radioactivity carried into the upper atmosphere (but not as high as
the stratosphere) seems to have been largely forgotten. We found for
the baseline case that roughly 30 percent of the land at northern
midlatitudes could receive a radioactive dose greater than 250 rads,
and that about 50 percent of northern midlatitudes could receive a
dose greater than 100 rads. A 100-rad dose is the equivalent of about
1000 medical X-rays. A 400-rad dose will, more likely than not, kill
you.
The cold, the dark and the intense radioactivity, together lasting
for months, represent a severe assault on our civilization and our
species. Civil and sanitary services would be wiped out. Medical
facilities, drugs, the most rudimentary means for relieving the vast
human suffering, would be unavailable. Any but the most elaborate
shelters would be useless, quite apart from the question of what good
it might be to emerge a few months later. Synthetics burned in the
destruction of the cities would produce a wide variety of toxic gases,
including carbon monoxide, cyanides, dioxins and furans. After the
dust and soot settled out, the solar ultraviolet flux would be much
larger than its present value. Immunity to disease would decline.
Epidemics and pandemics would be rampant, especially after the billion
or so unburied bodies began to thaw. Moreover, the combined influence
of these severe and simultaneous stresses on life are likely to
produce even more adverse consequences -- biologists call them
synergisms -- that we are not yet wise enough to foresee.
So far, we have talked only of the Northern Hemisphere. But it now
seems - unlike the case of a single nuclear weapons test -- that in a
real nuclear war, the heating of the vast quantities of atmospheric
dust and soot in northern midlatitudes will transport these fine
particles toward and across the Equator. We see just this happening in
Martian dust storms. The Southern Hemisphere would experience effects
that, while less severe than in the Northern Hemisphere, are
nevertheless extremely ominous. The illusion with which some people in
the Northern Hemisphere reassure themselves -- catching an Air New
Zealand flight in a time of serious international crisis, or the like
-- is now much less tenable, even on the narrow issue of personal
survival for those with the price of a ticket.
But what if nuclear wars can be contained, and much less than
5000 megatons is detonated? Perhaps the greatest surprise in our work
was that even small nuclear wars can have devastating climatic
effects. We considered a war in which a mere 100 megatons were
exploded, less than one percent of the world arsenals, and only in
low-yield airbursts over cities. This scenario, we found, would ignite
thousands of fires, and the smoke from these fires alone would be
enough to generate an epoch of cold and dark almost as severe as in
the 5000 megaton case. The threshold for what Richard Turco has called
The Nuclear Winter is very low.
Could we have overlooked some important effect? The carrying of dust
and soot from the Northern to the Southern Hemisphere (as well as more
local atmospheric circulation) will certainly thin the clouds out over
the Northern Hemisphere. But, in many cases, this thinning would be
insufficient to render the climatic consequences tolerable -- and
every time it got better in the Northern Hemisphere, it would get
worse in the Southern.
Our results have been carefully scrutinized by more than 100
scientists in the United States, Europe and the Soviet Union. There
are still arguments on points of detail. But the overall conclusion
seems to be agreed upon: There are severe and previously unanticipated
global consequences of nuclear war-subfreezing temperatures in a
twilit radioactive gloom lasting for months or longer.
Scientists initially underestimated the effects of fallout, were
amazed that nuclear explosions in space disabled distant satellites,
had no idea that the fireballs from high-yield thermonuclear
explosions could deplete the ozone layer and missed altogether the
possible climatic effects of nuclear dust and smoke. What else have we
overlooked?
Nuclear war is a problem that can be treated only theoretically. It
is not amenable to experimentation. Conceivably, we have left
something important out of our analysis, and the effects are more
modest than we calculate. On the other hand, it is also possible-and,
from previous experience, even likely-that there are further adverse
effects that no one has yet been wise enough to recognize. With
billions of lives at stake, where does conservatism lie-in assuming
that the results will be better than we calculate, or worse?
Many biologists, considering the nuclear winter that these
calculations describe, believe they carry somber implications for life
on Earth. Many species of plants and animals would become extinct.
Vast numbers of surviving humans would starve to death. The delicate
ecological relations that bind together organisms on Earth in a fabric
of mutual dependency would be torn, perhaps irreparably. There is
little question that our global civilization would be destroyed. The
human population would be reduced to prehistoric levels, or less. Life
for any survivors would be extremely hard. And there seems to be a
real possibility of the extinction of the human species.
It is now almost 40 years since the invention of nuclear weapons. We
have not yet experienced a global thermonuclear war -- although on
more than one occasion we have come tremulously close. I do not think
our luck can hold forever. Men and machines are fallible, as recent
events remind us. Fools and madmen do exist, and sometimes rise to
power. Concentrating always on the near future, we have ignored the
long-term consequences of our actions. We have placed our civilization
and our species in jeopardy.
Fortunately, it is not yet too late. We can safeguard the planetary
civilization and the human family if we so choose. There is no more
important or more urgent issue.
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