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The Atmospheric and Climatic Consequences of Nuclear War 

CARL SAGAN 31oct83

The Cold and the Dark: The World after Nuclear War:
Conference on the Long-Term Worldwide Biological Consequences of Nuclear War
(Washington, D.C.) 31oct83


Photo: 1985 - The Atmospheric and Climatic Consequences of Nuclear War CARL SAGAN 1984 from The Cold and the Dark: The World after Nuclear War: Conference on the Long-Term Worldwide Biological Consequences of Nuclear War (Washington, D.C.) 31oct83

Carl Sagan [1985]


It is the Halloween preceding 1984, and I deeply wish that what I am about to tell you were only a ghost story, only something invented to frighten children for a day. But, unfortunately, it is not just a story. Our recent research[1,2] has uncovered the surprising fact that nuclear war may carry in its wake a climatic catastrophe, which we call "nuclear winter," unprecedented during the tenure of humans on Earth.

We stumbled upon these results by accident, by a circuitous route, by one of those circumstances common in science where studying something purely for its intellectual interest leads you to conclusions of surprising practical utility. For me, it began in 1971 with the Mariner 9 exploration of the planet Mars. Mariner 9 was the first spacecraft to orbit another planet. Its engineers guaranteed that it would work only for three months after orbital injection. The spacecraft arrived at Mars to find the planet completely covered with a global dust storm. After a month of photographing an almost entirely featureless disk, we began to worry seriously that by the time the dust would all settle out of the Martian atmosphere the spacecraft would no longer be working. The dust storm in fact took three months to dissipate, but the spacecraft worked far better than the engineers had said—and for the next year we were able to examine the planet pole to pole in the first detailed orbital reconnaissance of another planet. 

During those first three months, there was very little to look at except the dust in the atmosphere. There was an instrument on board the spacecraft called an infrared interferometric spectrometer, which had the ability to examine the atmosphere at various wavelengths and therefore to probe to different depths in the atmosphere—from very high altitudes down to the surface. We were able to see the temperature of the atmosphere and that of the surface change with time. The results showed that the atmosphere was considerably warmer than is usually the case on Mars, and the surface considerably colder. As the dust settled out, the atmosphere became cooler and the surface warmer—both approaching their usual, or "ambient," values. It was not difficult to understand the reasons for this. The winds had stirred a great deal of dust off the Martian deserts into the atmosphere. Sunlight was being absorbed by the high-altitude dust, thereby heating the atmosphere. But, by the same token, the sunlight was impeded from reaching the surface, and so the surface was cooled. An observer on Mars would have noticed, after the dust storm stirred, that cold and darkness were spreading over the planet. After many months (the dust storm had started several months before Mariner 9 arrived at Mars), the dust had mainly fallen out of the atmosphere, and conditions had returned to normal.

Such dust storms are a Martian commonplace, and have been noted by ground-based observers for more than a century. They characteristically arise in the same few locations on Mars, spread first in longitude, then in latitude, and in a matter of a few weeks at most typically cross the Martian equator into the other hemisphere. Now, the surface atmospheric pressure on Mars is about the same as that in the stratosphere of the Earth. Mars rotates, as the Earth does, once every twenty-four hours, and its axis of rotation is tilted to its orbital plane by just about the same angle as the Earth's. There are differences between Mars and Earth, of course— including the absence of oceans on Mars, and the fact that it is farther away from the sun. But it seemed to us that the Martian experience might be relevant to Earth.

A number of us, having little before us for the first three months after orbital injection but the dust storm, set to calculating by how much the atmosphere should be warmed and the surface cooled for a given amount of dust put up into the atmosphere. A rough calculation was not very difficult, and several different groups were able to understand not just qualitatively but quantitatively the temperature changes that the dust storm had brought temporarily to Mars. My colleagues (and former students) James B. Pollack and O. Brian Toon, both now at the NASA Ames Research Center, were eager to apply this kind of computational armamentarium to terrestrial problems. We set out trying to understand what happens to the climate of the Earth when a large volcano goes off and distributes stratospheric aerosols worldwide. In some cases, we know how much dust is put into the upper atmosphere, what the particle sizes of the dust are (generally smaller than a micrometer [a ten-thousandth of a centimeter]), and what the composition of the fine particles is (generally sulfuric acid and silicates). Because the stratosphere is very dry, rain does not carry these aerosols out; and because convection is very muted in the stratosphere, atmospheric motions tend not to carry the fine aerosols out. And so they slowly sink by their own weight—slowly because their sizes are so small—taking more than a year for the stratosphere to clear. At the same time, there are, for many volcanic explosions, measurements of a small but definite global temperature decline—for all volcanic explosions in the last few centuries, a cooling of a degree or less. We found[3] that we were able to calculate these temperature declines fairly accurately; the methods developed for Mars, and considerably extended since, worked quite well for Earth.

It was then proposed by Alvarez et al.[4] that the extinction of the dinosaurs and many other species 65 million years ago, at the boundary of the Cretaceous and Tertiary epochs, was due to the collision with the Earth of an asteroid 10 kilometers across, and the subsequent spewing of enormous quantities of fine dust into the atmosphere. Joined by Richard Turco of R&D Associates in Marina del Rey, California, Pollack and Toon calculated that a severe cooling and darkening event might have been attendant to such an asteroidal collision. I wish to stress, however, that our conclusions on the climatic consequences of nuclear war do not depend on this interpretation of the Cretaceous/Tertiary extinctions. The dinosaurs could have died of influenza without affecting the validity of our conclusions.

We had known, of course, that nuclear explosions put large amounts of fine dust into the atmosphere, and had talked on and off for a period of years about calculating what the climatic effects of this dust might be. At a meeting at Ames Research Center (devoted in part to the question of the origin of life) in 1981, we decided to go ahead with the calculations. The effort was further spurred a year later by word of some very interesting work[5] performed by Paul Crutzen of the Max-Planck-Institute for Chemistry in Mainz, Federal Republic of Germany, and John Birks of the University of Colorado. Crutzen and Birks had made a preliminary estimate of the amount of smoke from the burning of forests and cities that might be released into the atmosphere in a nuclear war. Clearly here was an additional important source of fine particles that might attenuate sunlight.

So now I come to the question of the effects of nuclear war. The immediate consequences of a single thermonuclear weapon explosion are well-known and well-documented[6]—fireball radiation, prompt neutrons and gamma rays, blast, and fires. The Hiroshima bomb that killed between 100,000 and 200,000 people was a fission device with a yield of about 12 kilotons (the explosive equivalent of 12,000 tons of TNT). A modern thermonuclear warhead uses a device something like the Hiroshima bomb as the trigger—the "match" to light the fusion reaction. A typical American thermonuclear weapon might have a yield of about 500 kilotons (or 0.5 megaton, a megaton being the explosive equivalent of a million tons of TNT). There are many weapons in the 9- to 20-megaton range in the strategic arsenals of the U.S. and the USSR today. The highest-yield weapon ever exploded is 58 megatons.[7]

Strategic nuclear weapons are those designed for delivery by ground-based or submarine-launched missiles, or by bombers, to tar-gets in the adversary's homeland. Many weapons with yields roughly equal to that of the Hiroshima bomb are today assigned to "tactical" or "theater" military missions, or are designated "munitions" and relegated to ground-to-air and air-to-air missiles, torpedoes, depth charges, and artillery. While strategic weapons often have higher yields than tactical weapons, this is not always the case.[8] Modern tactical or theater missiles (e.g., Pershing 2, SS-20) and aircraft (e.g., F-15, MiG-23) have sufficient ranges to make the distinction between "strategic" and "tactical" or "theater" weapons increasingly artificial. Both categories of weapons can be delivered by land-based missiles, sea-based missiles, and aircraft, and by intermediate-range as well as intercontinental delivery systems. Nevertheless, by the usual accounting, there are around 18,000 strategic and theater thermonuclear weapons and the equivalent number of fission triggers in the American and Soviet strategic arsenals, with an aggregate yield of about 10,000 megatons. The total number of nuclear weapons (strategic plus theater and tactical) in the arsenals of the two nations is close to 50,000, with an aggregate yield near 15,000 megatons. For convenience, we here collapse the distinction between strategic and theater weapons and adopt, under the rubric "strategic," an aggregate yield of 13,000 megatons. The nuclear weapons of the rest of the world—mainly Britain, France, and China—amount to many hundred war-heads and a few hundred megatons of additional aggregate yield.

No one knows, of course, how many warheads with what aggregate yield would be detonated in a nuclear war. Because of attacks on strategic aircraft and missiles, and because of technological failures, it is clear that less than the entire world arsenal would be detonated. On the other hand, it is generally accepted, even among most military planners, that a "small" nuclear war would be almost impossible to contain before it escalated to include much of the world arsenals.[9] ( Precipitating factors include command and control malfunctions, communications failures, the necessity for instantaneous decisions on the fates of millions, fear, panic, and other aspects of real nuclear war fought by real people.) For this reason alone, any serious attempt to examine the possible consequences of nuclear war must place major emphasis on large-scale exchanges in the 5,000- to 7,000-megaton range—between about a third and a half of the world strategic inventories—and many studies have done so.[10] Many of the effects de-scribed below, however, can be triggered by much smaller wars.

The adversary's strategic airfields, missile silos, naval bases, submarines at sea, weapons manufacturing and storage locales, civilian and military command and control centers, attack assessment and early-warning facilities, and the like are probable targets ("counterforce attack"). While it is often stated[11] that cities are not targeted "per se," many of the above targets are proximate to or collocated with cities, especially in Europe. In addition, there is an industrial-targeting category ("countervalue attack"). Modern nuclear doctrines require that "war-supporting" facilities be attacked. Many of these facilities are necessarily industrial in nature, and engage a workforce of considerable size. They are almost always situated near major transportation centers, so that raw materials and finished products can be efficiently transported to other industrial sectors, or to forces in the field. Thus, such facilities are, almost by definition, cities, or near or within cities. Other "war-supporting" targets may include the transportation systems themselves (roads, canals, rivers, railways, civilian airfields, etc.), petroleum refineries, storage sites and pipelines, hydroelectric and nuclear power plants, radio and television transmitters, and the like. A major countervalue exchange therefore might involve almost all large cities in the United States and the Soviet Union, and possibly most of the large cities in the Northern Hemisphere.[12] There are fewer than 2,500 cities in the world with populations of over 100,000 inhabitants, so the devastation of all such cities is well within the means of the world nuclear arsenals.

Recent estimates of the immediate deaths from blast, prompt radiation, and fires in a major exchange in which cities were targeted range from several hundred million" to—most recently, in a World Health Organization study in which targets were assumed not to be restricted entirely to NATO and Warsaw Pact countries—1.1 billion people.[13] Serious injuries requiring immediate medical attention (which would be largely unavailable) would be suffered by a comparably large number of people,[14] perhaps an additional 1.1 billion.[13] Thus it is possible that something approaching half the human population on the planet would be killed or seriously injured by the direct effects of a nuclear war. Social disruption; the unavailability of electricity, fuel, transportation, food deliveries, communications, and other civil services; the absence of medical care; the decline in sanitation measures; rampant disease and severe psychiatric disorders would doubtless claim collectively a significant number of further victims. But a range of additional effects—some unexpected, some inadequately treated in earlier studies, some uncovered by us only recently—makes the picture much more somber still. Destruction of missile silos, command and control facilities, and other hardened sites requires—because of current limitations on missile accuracy—nuclear weapons of fairly high yield exploded as ground bursts or as low air bursts. High-yield ground bursts will vaporize, melt, and pulverize the surface at the target area and propel large quantities of condensates and fine dust into the upper troposphere and stratosphere. The particles are chiefly entrained in the rising fireball; some ride up the stem of the mushroom cloud. Most military targets, however, are not very hard. The destruction of cities can be accomplished, as demonstrated at Hiroshima and Nagasaki, by lower-yield explosions less than a kilometer above the surface. Low-yield air bursts over cities or near forests will tend to produce massive fires, in some cases over a total area of 100,000 square kilometers or more. City fires generate enormous quantities of black smoke which rise at least into the upper part of the lower atmosphere, or troposphere (Fig. 1A). If firestorms occur, the smoke column rises vigorously, like the draft in a fireplace, and may (the question is still unresolved) carry some of the soot into the lower part of the upper atmosphere, or stratosphere. The smoke from forest and grassland fires would initially be restricted to the lower troposphere.

The fission of the (generally plutonium) trigger in every thermonuclear weapon and the reactions in the (generally uranium-238) casing added as a fission yield "booster" produce a witch's brew of radioactive products, which are also entrained in the cloud. Each such product, or radioisotope, has a characteristic half-life (defined as the time to decay to half of its original level of radioactivity). Most of the radioisotopes have very short half-lives, and decay in hours to days. Particles injected into the stratosphere, mainly by high-yield explosions (Fig. 1A), fall out very slowly—characteristically in about a year, by which time most of the fission products, even when concentrated, will have decayed to much safer levels. Particles injected into the troposphere by low-yield explosions (Fig. IA) and fires fall out more rapidly—by coagulation, gravitational settling, rainout, convection, and other processes—before the radioactivity has decayed to moderately safe levels. Thus, rapid fallout of tropospheric radioactive debris tends to produce larger doses of ionizing radiation than does the slower fallout of radioactive particles from the stratosphere.

Nuclear explosions of more than one megaton yield generate a radiant fireball that rises through the troposphere fully into the stratosphere (Fig. 1A). The fireballs from weapons with yields between 100 and 1,000 kilotons (1 ,000 kilotons = 1 megaton) will partially extend into the stratosphere. The high temperatures in the fireball chemically ignite some of the nitrogen in the air, producing oxides of nitrogen, which in turn chemically attack and destroy the gas ozone in the middle stratosphere. But ozone absorbs the biologically dangerous ultraviolet radiation from the sun. Thus, the partial depletion of the stratospheric ozone layer, or "ozonosphere," by high-yield nuclear explosions will increase the flux of solar ultraviolet radiation at the surface of the Earth (after the soot and dust have settled out). After a nuclear war in which thousands of high-yield weapons are detonated, the increase in biologically dangerous ultraviolet light might be several hundred percent.[1,2,10] In the more dangerous shorter wave-lengths, larger increases would occur. Nucleic acids and proteins, the fundamental molecules for life on Earth, are especially sensitive to ultraviolet radiation. Thus, an increase in the solar ultraviolet flux at the surface of the Earth is potentially dangerous to life.

These four effects—obscuring smoke in the troposphere, obscuring dust in the stratosphere, the fallout of radioactive debris, and the partial destruction of the ozone layer—constitute the four known principal adverse environmental consequences that would occur after a nuclear war is "over." There may well be others about which we are still ignorant. The dust and, especially, the dark soot absorb ordinary visible light from the sun, heating the atmosphere (Figs. 1B and IC) and cooling the Earth's surface.

Figure 1A. 

An approximate representation of the ordinary temperature structure of the Earth's atmosphere at northern (or at southern) midlatitudes. The surface, heated by the sun, has an annual temperature of 13 °C (56°F) on the average through the year. The temperature declines with altitude to a height (h) of about 13 kilometers (8 miles), where the temperature is - 55°C (- 67°F). These low temperatures are familiar to mountain climbers and airplane pilots. This lower region of the Earth's atmosphere, called the troposphere, is well-mixed by winds and turbulence and experiences rainfall. Thus, fine particles will be carried out or rained out of the troposphere comparatively rapidly.

The troposphere (and what we know as "weather") ends at the tropopause, at about 13 kilometers. Above it is the stratosphere. There, temperatures are more nearly constant with altitude; vertical winds and turbulence are mild; rainfall nonexistent; and fine particles are removed very slowly. Smoke from fires is mainly restricted to the troposphere and the soot particles are carried out comparatively rapidly. Dust from high-yield ground bursts—at silos and other hardened installations—is injected to a considerable extent into the stratosphere and falls out comparatively slowly. The explosive yield just barely able to inject material into the stratosphere is about 100 kilotons, as shown. The fireball and stabilized cloud from a 1-megaton (MT) explosion rise almost entirely into the stratosphere.

Figures 1B and 1C 

When the upper air is heated (through the absorption of sunlight by fine particles raised in the nuclear war), the surface is cooled, because the same particles prevent sunlight from reaching the surface. In Figure 1B, calculated from TTAPS results, the structure of the Earth's atmosphere at northern midlatitudes 30 days after a baseline nuclear war is shown (Table 1, Case 1). As in Figure 1A, the vertical axis represents height (h) and the horizontal axis indicates air temperature in degrees centigrade. In Figure 1C, the new temperature structure is shown after 120 days. In both cases the familiar atmospheric structure (Fig. 1A) has vanished, the temperature of the lower atmosphere is more constant with altitude, and a new temperature inversion region has appeared.

Just as for temperature inversions over cities such as Los Angeles, the altered temperature structure is very stable, and particles that have reached these altitudes are removed much more slowly than would ordinarily be the case. Since the influence of this temperature inversion is not yet included in the TTAPS calculations (the calculations are not "fully interactive"), the time scales for normal conditions to recover, shown in Figure 2, may be severe underestimates. In the 30-day case, the region in which the temperature hardly varies with altitude has reached the ground, and in this sense nuclear war can be said to bring the stratosphere down to Earth.

Comparison of these figures also helps explain why the fine particles tend to stream, after a while, across the equator into the Southern Hemisphere. Consider, e.g., an altitude of 10 kilometers in the Northern Hemisphere. A few weeks after the baseline war, the temperatures there are around 0°C (Fig. 1B). At the same altitude, in the as-yet dust- and smoke-free Southern Hemisphere (Fig. 1A), the temperatures are 50° colder. Parcels of air, and the particles they contain, will flow " downhill," from hotter regions to colder ones. In physics, fluxes tend to follow gradients. The large temperature contrasts will induce rising southward motion in the Northern Hemisphere and sinking northward motion in the Southern Hemisphere. The net effect may be to spread the dust-laden air globally and to lift it even further above the surface.



All four of these effects have been treated in our recent study,[1] known from the initials of its authors as TTAPS. For the first time it is demonstrated that severe and prolonged low temperatures, the "nuclear winter," would follow a nuclear war. (The study also ex-plains the fact that no such climatic effects were detected after the detonation of hundreds of megatons during the period of U.S./USSR atmospheric testing of nuclear weapons, ended by the Limited Test Ban Treaty in 1963: The explosions were sequential over many years, not virtually simultaneous, and, occurring over scrub desert, coral atolls, tundra, and wasteland, they set no fires.) The new results have been subjected to detailed scrutiny, and many corroboratory calculations have now been made, including at least two in the Soviet Union.

Unlike many previous studies, the effects do not seem to be restricted to northern midlatitudes, where the nuclear exchange would mainly take place. There is now substantial evidence that the heating by sunlight of atmospheric dust and soot over northern midlatitude targets would profoundly change the global circulation (see legend to Figs. 1B and 1C). Fine particles would be transported across the equator in weeks, as is the case on Mars, bringing the cold and the dark to the Southern Hemisphere. (In addition, some studies" suggest that over 100 megatons would be dedicated to equatorial and Southern Hemisphere targets, thus generating fine particles locally.) While it would be less cold and less dark at the ground in the Southern Hemisphere than in the Northern, massive climatic and environmental disruptions may be triggered there as well.

In our studies, several dozen different scenarios were chosen, covering a wide range of possible wars, and the range of uncertainty in each key parameter was considered (e.g., to describe how many fine particles are injected into the atmosphere). Five representative cases are shown in Table 1, ranging from a small, low-yield attack exclusively on cities, utilizing, in yield, only 0.8 percent of the world strategic arsenals, to a massive exchange involving 75 percent of the world strategic arsenals. "Nominal" cases assume the most probable parameter choices; "severe" cases assume adverse parameter choices, but still in the plausible range.

TABLE 1. Five Representative Nuclear Exchange Scenarios, TTAPS 

A  Total Yield (Megatons)
B  Percentage Yield, Surface Bursts
C  Percentage Yield, Urban or Industrial Targets
D  Warhead Yield Range (Megatons)
E  Total Number of Explosions

Case				 A	  B	 C	D	  E
1.  Baseline case, countervalue  5,000 	  57 	 20 	0.1—10 	  10,400
    and counterforce a

11. 3,000 megaton nominal,	 3,000 	  70 	 0 	1—10 	  2,150
    counterforce only b,c

14. 100 megaton nominal,	 100 	  0 	 100 	0.1 	  1,000
    countervalue only d

16. 5,000 megaton "severe," 	 5,000 	  100  	 0 	5-10 	  700
    counterforce only b,e

17. 10,000 megaton "severe," 	 10,000	  63 	 15 	0.1-10 	  16,160 
    countervalue and 
    counterforce, d,e

a  In the baseline case, 12,000 square kilometers of inner cities are burned; 
   on every square centimeter an average of 10 grams of combustibles are burned,
   and 1.1 percent of the burned material rises as smoke. Also, 230,000 square 
   kilometers of suburban areas burn, with 1.5 grams consumed at each square 
   centimeter and 3.6 percent rising as smoke. 
b In this highly conservative case, it is assumed that no smoke emission occurs,
  that not a blade of grass is burned.
c Only 25,000 tons of fine dust is raised into the upper atmosphere for every 
  megaton exploded.
d In contrast to the baseline case, only inner cities burn, but with 10 grams 
  per square centimeter consumed and 3.3 percent rising as smoke into the high 
e  Here, the fine (submicrometer) dust raised into the upper atmosphere is 
   150,000 tons per megaton exploded.

Predicted continental temperatures in the Northern Hemisphere vary after nuclear war according to the curves shown in Figure 2. The high heat capacity of water guarantees that ocean temperatures will fall at most by a few degrees. Because temperatures are moderated by the adjacent oceans, temperatures in coastal regions will be less extreme than in continental interiors. However, the very sharp temperature contrast between the frozen continents and the only slightly cooled oceans will produce continuing storms of unprecedented severity along coastlines, and the preferential rainout and washout of radioactivity there indicate that neither continental interiors nor coast-lines will be spared. The temperatures shown in Figure 2 are average values for Northern Hemisphere land areas, with no account yet taken of the influence of the oceans or the initial patchiness of the clouds.

Even much smaller temperature declines are known to have serious consequences. The explosion of the Tambora volcano in Indonesia in 1815 was the probable cause of an average global temperature decline of less than 1°C, due to the obscuration of sunlight by the fine dust propelled into the stratosphere. The hard freezes the following year were so severe that 1816 has been known in Europe and America as, respectively, "the year without a summer," and "eighteen-hundred-and-froze-to-death." A 1°C cooling would nearly eliminate wheat growing in Canada.[15] Small global changes tend to be associated with considerably larger regional changes. In the last thousand years, the maximum global or Northern Hemisphere temperature deviations have been around 1°C. In an Ice Age, a typical long-term global temperature decline from preexisting conditions is about 10°C. Even the most modest of the cases illustrated in Figure 2 give temporary temperature declines of this order. The baseline case is much more adverse. Unlike the situation in an Ice Age, however, the global temperatures after nuclear war would plunge rapidly and probably take only months to a few years to recover, rather than thousands of years. No new Ice Age is likely to be induced by the nuclear winter, at least according to our preliminary analysis.

Because of the obscuration of the sun, the daytime light levels could fall to a twilit gloom or worse. For more than a week in the northern midlatitude target zone, it might be much too dark to see, even at midday. In Cases 1 and 14 (Table 1), hemispherically averaged light levels fall to a few percent of normal values, comparable to that at the bottom of a dense overcast. At this illumination, many plants are close to what is called the compensation point, the light level at which photosynthesis can barely keep pace with plant metabolism. In Case 17, illumination, averaged over the entire Northern Hemisphere, falls in daytime to about 0.1 percent of normal, a light level in which most plants will not photosynthesize at all. For Cases 1 and especially 17, full recovery to ordinary daylight takes a year or more (Fig. 2).

Figure 2. In this figure, the average temperature of Northern Hemisphere land areas (away from coastlines) is shown varying with time after a nuclear war. The temperature is shown on the vertical axis, in degrees centigrade at left and in degrees Fahrenheit at right. The "ambient" temperature is averaged over all latitudes and seasons. Thus, normal winter temperatures at north temperate latitudes will be lower than is shown, and normal tropical temperatures will be higher than shown. The upper dashed horizontal line shows the average temperature of the Earth (13°C or 56°F), and the lower dashed horizontal line shows the freezing point of pure water (0°C or 32°F). The horizontal axis measures the time after the nuclear exchange in days from the beginning of the war to almost a year later. Each curve represents a different nuclear war scenario, ranging from 100 megatons (MT) total yield expended in the war to 10,000 MT. The ameliorating influence of the oceans (probably producing temperature declines about 50 to 70 percent of those shown here), as discussed in the text, is not included.

The cases shown here, from a much larger compilation in the TTAPS reports, are described further in Table 1. They include a mix of counter-value attacks on industries and cities, in which the main effect is smoke carried to the troposphere from fires, and counterforce attacks on missile silos, in which (very conservatively) no smoke is assumed to be produced but large quantities of dust are injected high into the atmosphere. Cases described as "nominal" assume the most likely values of parameters (such as dust particle size or the frequency of firestorms) that are imperfectly known. Cases marked "severe" represent adverse but not implausible values of these parameters.

In Case 14 the curve ends when the temperatures come within a degree of the ambient values. For the four other cases, the curves are shown ending after 300 days, but this is simply because the calculations were not extended further. In these four cases the curves will continue in the directions they are headed. Very roughly, Case 1 is the sum of Cases 11 and 14. Case 16 envisions an exchange limited to surface bursts of fairly high yield designed to destroy silos, and a high percentage of resulting fine dust. Following is a further description of the five cases:

Case 1 is the TTAPS baseline case in which 4,000 megatons are dedicated to counterforce attacks by the two sides and 1,000 megatons are allocated for cities and environs. The main effect is from the soot generated in urban conflagrations. The temperature minimum of -23°C (-9°F) is reached a few weeks after the exchange, and temperatures return to the freezing point after about three months. Recovery to ambient conditions, however, does not occur for more than a year, because of the slow fallout of stratospheric dust.

Case 11: Here the U.S. and/or the USSR detonate a total of 3,000 megatons on missile silos and other targets far from cities and forests. Fires are (unrealistically) assumed to be negligible. The land temperatures drop over a period of three months. Since the dust is removed very slowly from the stratosphere, it takes more than a year for the temperatures to recover their usual (ambient) values.

Case 14: Here the exchange is limited to only 100 megatons employed exclusively in low-yield air bursts over cities. In this calculation there is no dust produced—only smoke from the burning cities, very little of which reaches the stratosphere. The minimum temperature of -23°C (-9°F) is reached after a few weeks, and normal temperatures are attained after about 100 days. As the soot settles, sunlight begins to penetrate to the surface. One hundred megatons corresponds to about 0.8 percent of the strategic nuclear arsenals of the U.S. and the USSR.

Case 16 is a 5,000-megaton exchange in which mainly silos are at-tacked, in which more fine dust is raised per megaton of yield than in the conservative Case 11, and in which there is negligible burning of cities. Here, minimum temperatures are not reached for four months, when temperatures have dropped to -25°C (-13°F). Because the large amounts of dust placed in the stratosphere fall out very slowly, it takes more than a year for the land temperatures to return to the freezing point and much longer than that for normal temperatures to be reached.

Case 17: In this case about three-quarters of the strategic arsenals of the U.S. and the USSR are expended in a mix of attacks on silos and cities. After more than two months, minimum temperatures of -47°C (-53°F) are reached—temperatures characteristic of the surface of Mars. The soot falls out comparatively rapidly and the slowness of the recovery is due to stratospheric dust. The temperatures return to the freezing point only after about a year. 

As the fine particles fall out of the atmosphere, carrying radioactivity to the ground, the light levels increase and the surface warms. The depleted ozone layer now permits solar ultraviolet light to reach the Earth's surface in increased proportions. For the 5,000-megaton baseline case, we find that the prompt fallout, the plumes of radioactivity that are carried downwind of targets, gives a radiation dose for 30 percent of Northern Hemisphere midlatitude land areas of about 250 rads. In addition, there is a dose of about 100 rads delivered more or less uniformly over the Northern Hemisphere. This is a combination of external emitters and ingested radioactive materials. The prevailing wisdom establishes a mean lethal whole-body dose of ionizing radiation, for healthy adults, of between about 400 and 500 rads. This is with the help of comprehensive medical care. But for children and the elderly, for those suffering from disease or other assaults from the nuclear war environment, and especially in the absence of competent medical care, the mean lethal dose is reduced considerably—perhaps to 350 rads or even less. Thus, the radioactive fallout—especially in the northern midlatitudes, which have the greatest population density on the planet—would, by itself, be extremely dangerous in a postnuclear-war environment. The relative timing of the multitude of adverse consequences of a nuclear war is shown in Table 2.Perhaps the most striking and unexpected consequence of our study is that even a comparatively small nuclear war can have devastating climatic consequences, provided cities are targeted (see Case 14 in Figure 2; here, the centers of 100 major NATO and Warsaw Pact cities are burning). There is an indication of a very approximate threshold at which severe climatic consequences are triggered—by 100 or more nuclear explosions over cities, for smoke generation, or around 2,000 to 3,000 high-yield surface and low air bursts at, for example, missile silos, for dust generation and ancillary fires. Fine particles can be injected into the atmosphere at increasing rates with only minor effects until these thresholds are crossed. Thereafter, the effects increase rapidly in severity.[16] But these estimates of threshold are extremely rough.

As in all calculations of this complexity there are uncertainties. Some factors tend to work toward more severe or more prolonged effects,[17]; others tend to ameliorate the effects." The detailed TTAPS calculations described here are one-dimensional; that is, they assume the fine particles to move vertically by all the appropriate laws of physics, but neglect the spreading in latitude and longitude. When soot or dust is moved away from the reference locale, things get better there and worse elsewhere. In addition, fine particles can be trans-ported by weather systems to other locales, where they are carried more rapidly down to the surface. This would ameliorate obscuration not only locally, but globally. It is just this transport away from northern midlatitudes that involves the equatorial zone and the Southern Hemisphere in the effects of the nuclear war. It would be helpful to perform an accurate three-dimensional calculation on the general atmospheric circulation following a nuclear war. Preliminary estimates' suggest that the general circulation might moderate the low-temperature excursions of our calculations in continental interiors by some 30 percent, lessening somewhat the severity of the effects, but still leaving them at catastrophic levels (e.g., a 30°C rather than a 40°C temperature drop). To provide a small margin of safety, we neglect this correction in our subsequent discussion.

Then there are holes in the clouds. Very few accessible targets are in the Atlantic and Pacific oceans. If such moving clear patches (an "Atlantic" hole and a "Pacific" hole) were to appear at regular intervals over most places in the Northern Hemisphere, the effects of cold and dark would be somewhat lessened. However, fires set, for example, in western North America or in Eurasian taigas would continue burning, some perhaps for weeks, and new fires would be set: Delayed launches may be directed at targets temporarily within a hole to aid satellite verification of target destruction. In addition, the winds at different altitudes move at different velocities, and a patch at one altitude may be over or under a thick cloud layer at another altitude. The dust injected into the stratosphere by the Mexican volcano, El Chichón, in its explosion on April 4, 1982, took ten days to reach Asia, two weeks to reach Africa, and circumnavigated the globe in three weeks—leaving a thin ribbon of particles behind it about 10° of latitude wide. (In a few months, about 10 to 20 percent of the stratospheric debris had been transported to the Southern Hemisphere.) When there are many sources of particles instead of one, the holes will close still faster. For these reasons, it seems unlikely that moving holes would remain unfilled or uncovered for more than a week or two, or that large-scale patchiness could ameliorate the climatic effects in a major way.

Further work is needed on many other aspects of the problem: for example, on possible small-scale patchiness; on possible quick freezes (as suggested by Covey et al.: see Stephen Schneider's remarks, this volume, pp. 89-94); on how fast individual smoke plumes spread (the particles in dense clouds coagulate and sediment out faster than in diffuse clouds); on local atmospheric circulation near coastlines and implications for rainout (see Georgiy Golitsyn's remarks, this volume, pp. 87-89); and on diurnal temperature variations and induced motions in early soot clouds. Some of these effects might improve conditions somewhat; others would make them somewhat more severe. There are also effects that tend to make the results much worse: For example, in our calculations we assumed that rainout of fine particles occurred through the entire troposphere. But under realistic circumstances, at least the upper troposphere may be very dry, and any dust or soot carried there initially may take much longer to rain out. There is also a very significant effect deriving from the drastically altered structure of the atmosphere, brought about by the heating of the clouds and the cooling of the surface. This produces a region in which the temperature is approximately constant with altitude in the lower atmosphere and topped by a massive temperature inversion (Figs. 1B and 1C). Particles throughout the atmosphere would thereafter be transported up or down very slowly—as in the present stratosphere. This is a second reason that the lifetime of the clouds of soot and dust may be much longer than we have calculated. If so, the worst of the cold and the dark might be prolonged for considerable periods of time, conceivably for more than a year. We neglect this effect in subsequent discussion, as well as many others—e.g., multiburst phenomena in which a first nuclear explosion enhances the extent of the burning and the altitude of soot transport from a second nuclear explosion.

TABLE 2 Schematic Summary of the Biological Effects of the Baseline (5,000-Megaton) Nuclear War a


at Risk

at Risk

at Risk

Rate for
at Risk


Blast H M L H M-H
Thermal Radiation M M L M M-H
Prompt Ionizing Radiation L L L H L-M
Fires M M L M M
Toxic Gases H M L L L
Dark H H M L L
Cold H H H H M-H
Frozen Water Supplies H H M M M
Fallout Ionizing Radiation H H L-M M M-H
Food Shortages H H H H H
Medical System Collapse H H M M M
Contagious Diseases M M L H M
Epidemics & Pandemics H H M M M
Psychiatric Disorders H H L L L-M
Increased Surface UV Light H H M L L
Synergisms ? ? ? ? ?

a  A schematic representation of the time scale for many of the effects is presented; the effects are most severe when the thickness of the horizontal bar is greatest. "Synergisms" is a potentially significant category in which the total result is greater than the sum of the component effects. Most synergisms are entirely unknown. At right is an indication of the risks to American/Soviet populations, to Northern Hemisphere populations, to Southern Hemisphere populations, and to the entire human community of the various effects listed. H, M, and L stand for high, medium, and low, respectively. In the last column only, L represents zero to a million deaths, M a million to a few hundred million deaths, and H more than a few hundred million deaths. (Chart prepared by Mark Harwell and the author.)

Nuclear war scenarios are possible that are much worse than the ones we have presented. For example, if command and control capabilities are lost early in the war—by, say, "decapitation" (the early surprise attack on civilian and military headquarters and communications facilities)—then the war conceivably could be extended for weeks as local commanders make separate and uncoordinated decisions. At least some of the delayed missile launches could be retaliatory strikes against any remaining adversary cities. Generation of an additional smoke pall over a period of weeks or longer following the initiation of the war would extend the magnitude, and especially the duration, of the climatic consequences. Or it is possible, within the boundaries of plausibility, that more cities and forests would be ignited than we have assumed, or that smoke emissions would be larger, or that a greater fraction of the world arsenals ( tactical as well as strategic weapons) would be committed. Less severe cases, within the same boundaries, are of course possible as well.

These calculations therefore are not, and cannot be, assured prognostications of the full consequences of a nuclear war. Many refinements in them are possible and are being pursued. But there seems to be general agreement on the overall conclusions: In the wake of a nuclear war there is likely to be a period, lasting at least for months, of extreme cold in a radioactive gloom, followed—after the soot and dust falls out—by an extended period of increased ultraviolet light reaching the surface.[18]


There has been a systematic tendency for the effects of nuclear weapons and nuclear war to be underestimated. The yield of the first nuclear explosion near Alamogordo, New Mexico, on July 16, 1945, was underestimated by almost all those who designed and constructed the weapon. The extent of fallout from early thermonuclear weapons tests was underestimated; the impairment or destruction of satellites by nuclear weapons explosions in space was a surprise; the depletion of the ozonosphere by high- yield bursts was unanticipated; and nuclear winter was for many—ourselves included—an astonishment. What else have we overlooked?

One, possibly serious, additional effect is the production of toxic gases by city fires. It is now a commonplace that in the burning of modern tall buildings, more people succumb to toxic gases than to fire. Ignition of many varieties of building materials, insulation, and fabrics generates large amounts of such pyrotoxins, including carbon monoxide, cyanides, vinyl chloride, oxides of nitrogen, ozone, dioxins, and furans. Because of differing practices in the use of such synthetics, the burning of cities in North America and Western Europe would probably generate more pyrotoxins than cities in the Soviet Union, and cities with substantial recent construction more than older unreconstructed cities. In nuclear war scenarios in which a great many cities are burning, a significant pyrotoxin smog might persist for months. The magnitude of this danger is unknown.

Another probably very significant and almost unevaluated consequence of nuclear war is what are called synergisms. A very simple example follows from the compromise of the human immune system by both prompt ionizing radiation and ionizing radiation from fallout, as well as from the enhanced post-nuclear winter ultraviolet flux. At the same time that survivors will be much more vulnerable to disease, medical services will have collapsed; insect predators such as birds will have been preferentially killed by the cold, the dark, and the radiation; insects will have proliferated enormously because they can resist these environmental assaults and because the predators that keep them in check will have been greatly reduced in numbers; the radiation may produce particularly virulent forms of microorganisms carried by the insect vectors; and hundreds of millions or billions of corpses will be beginning to thaw. There are many other cases where the interaction of several of the environmental assaults listed in Table 2 will result in a net adverse consequence much more severe than the simple sum of the component effects. Almost all synergisms are of unknown magnitude; however, almost all of them will have an incremental adverse consequence.

So if the weight of historical evidence and the nature of synergisms imply that the consequences of nuclear war would be even more severe than the present nuclear winter analysis indicates, where does conservatism lie? Is it a proper posture, considering the unprecedented stakes in the answer, to assume that the effects of nuclear war will be less severe than is currently estimated, or more?

It is no longer true that the really serious effects of nuclear war would be restricted to the combatant nations. The biology in equatorial latitudes, for example, is much more vulnerable to even small temperature declines than the biology in more northerly or more southerly latitudes. Agriculture—at least in the Northern Hemisphere, which produces the bulk of the export grain on the planet—would be devastated even by a "small" nuclear war. The propagating ecological consequences all over the Earth are likely to be severe and if, as our and many other studies now show, the cold and the dark move to the Southern Hemisphere, nuclear war implies an unprecedented global catastrophe. It is no longer possible to imagine that nations far from the conflict could merely sit the war out, and inherit a postwar environment freed of the annoyances of big power politics. Instead it seems much more likely that there are no sanctuaries from nuclear war anywhere on Earth. This is one of many implications of the new studies for doctrine, policy, and international politics. A discussion of these subjects is beyond the scope of this meeting and these Conference proceedings, but I have made a preliminary discussion of such implications elsewhere.[19]

If cities are targeted, we see (Fig. 2) that even a war involving only 100 megatons (in 1,000 100-kiloton bursts over 100 or more major cities) could trigger the nuclear winter. But 100 megatons is less than one percent of the global strategic arsenals. Figure 3 shows the growth in the number of strategic weapons in the American and Soviet arsenals as a function of time. The shaded area represents, very roughly, the threshold region in which, it now appears, nuclear winter could be triggered. Well below the threshold region no combination of communications failures, computer errors, miscalculation, psychopathic leaders, or any other exigency could trigger the climatic catastrophe. The United States crossed that threshold—of course without knowing it—in the early 1950s. The Soviet Union crossed that threshold—again without knowing it—in the middle 1960s. In all this time the leaders of the United States, the Soviet Union, and other nations have been making fundamental decisions about the life and death of everyone on the planet without knowing what the consequences of nuclear war would be, and while supposing that the consequences would be much more modest than now appears to be the case. And the global arsenals, now about twenty times the nuclear winter threshold, are growing. Britain, France, and China have strategic arsenals at least approaching threshold. Other nations are accumulating nuclear weapons or nuclear weapons capability. The curves in Figure 3 are steepening still more.

And so we return to Halloween. This meeting on the "World after Nuclear War" is being held, because of circumstances as mundane as the availability of Washington hotels, on October 31. Halloween is celebrated today as a festival of ghosts and goblins and things we know are not real. The horrors of nuclear war, on the other hand, are not fantasies, not projections of our unconscious, but realities that we must deal with in the world of personal emotions and practical politics. Nuclear war is very much worth worrying about and not just on October 31.

Still, if you had to hold such a meeting on a date with some symbolic significance, Halloween seems to be an appropriate choice. It was originally, in pre- Christian times, a Celtic festival called Samhain. It marked the beginning of winter. It was celebrated by the lighting of vast bonfires. And it was named after and consecrated to the Lord of the Dead. The original Halloween combines the three essential elements of the TTAPS scenario: fires, winter, and death. Nuclear weapons are made by human beings. The global strategic confrontation of the United States and the Soviet Union has been devised and carried out by human beings. There is nothing inevitable about these matters. If we are sufficiently motivated, we can extricate the human species from this trap that we have foolishly set for our-selves. But time is very short.

Figure 3. The history of the strategic (and theater) nuclear arms race. Three regions are shown in the diagram: a lower region in which nuclear winter might not be triggered, an upper region in which it almost certainly could be triggered, and a transition region, shown shaded. The boundaries of this region are more uncertain than shown, and depend, among other things, on targeting strategy. But the threshold probably lies between a hundred and a few thousand contemporary strategic weapons.

Between 1945 and the present, the growth of American and of Soviet stockpiles is shown as the dark solid lines. The alternating dots and dashes show the sum of these two arsenals, which is also close to the total world arsenals. While the distinction between tactical weapons and strategic or theater weapons is beginning to be blurred, the former are not counted in this compilation. The decline in U.S. strategic stockpiles in the 1960s mainly reflects the growing predominance of ballistic missiles over bombers. Not all published sources are in perfect agreement on these numbers. The data used here were taken from Harold Brown (1981), "Report of Secretary of Defense to the Congress on the FY 1982 Budget, FY 1983 Authorization Request and FY 1986 Defense Programs," and "National Defense Budget Estimates, FY 1983," Office of the Assistant Secretary of Defense, Comptroller, March 1982, among other sources. The dashed lines at the right of the figure represent extrapolations of present trends.


This article would not have been possible without the high scientific competence and dedication of my co-authors of the TTAPS report, Richard Turco, Brian Toon, Thomas Ackerman, and James Pollack. I am also grateful, for stimulating discussions and/or careful reviews of an earlier version of this article, to Hans Bethe, Mark Harwell, John P. Holdren, Eric Jones, Carson Mark, Theodore Postol, Joseph Rotblat, Stephen Schneider, Edward Teller, and Albert Wohlstetter; and deeply appreciate the encouragement, suggestions, and critical assessments provided by Lester Grinspoon, Steven Soter, and, especially, Ann Druyan. Shirley Arden, Mary Maki, Mary Roth, and Joanne Vago provided, with their usual high competence, essential logistical services in the preparation of this paper and/or in the organization of the preparatory conference in Cambridge, Massachusetts. Finally, I thank my fellow members of the Committee on the Long-Term Worldwide Consequences of Nuclear War.


DR. VIKAS SAINI (Board of Directors, Nuclear Free America): I had two questions about the assumptions of the model. The first one is on the effects in the Southern Hemisphere: Is that, strictly speaking, transfer effects from detonations on the Northern Hemisphere or did you include targets in the Southern Hemisphere?

DR. SAGAN: No, we are not assuming any significant targeting in the Southern Hemisphere. In the Ambio scenario there are something like 100 megatons targeted in the Southern Hemisphere and tropical latitudes. Dust and smoke from such targets will arrive in the south faster than aerosols transported from the Northern Hemisphere. Any Southern Hemisphere targeting makes our effects still worse.

DR. SAINI: The second one had to do with some unforeseen effects of nuclear weapons detonations and the relationship to the Van Allen radiation belt. I was wondering if you knew about that and would comment on what seems to be one of the most disturbing aspects of our current situation; that is, the militarization of space.

DR. SAGAN: The imminent introduction of weapons into space is a policy question which is inappropriate for this meeting. It is certainly true that if you explode a nuclear weapon at the appropriate altitude, you have injected charged particles into the Van Allen radiation belt. But I do not think that has any climatic effects of the magnitude we are talking about here.

DR. GEORGE B. FIELD (Professor of Applied Astronomy at Harvard University and senior scientist at Smithsonian Astrophysical Observatory): I would like to request a point of clarification. In the last few minutes you gave a small amount of hope to those who would think in terms of arms control. You said that if we could limit the number of nuclear weapons both in the United States and in the Soviet Union to 1,000, we could avert some of the dire consequences which you described. On the other hand, earlier in your talk you examined the scenario in which there was an exchange of only 100 such nuclear weapons and the effects in that scenario were, in fact, dire.

DR. SAGAN: I am sorry for any confusion. In that case I was talking about 100 megatons, in weapons each of 100-kiloton yield. So I was talking about 1,000 weapons. There is no inconsistency.

DR. FIELD: Is that the marginal case in your view?

DR. SAGAN: Somewhere around there. It could be less for cities being targeted, and it might have to be rather more for high-yield counterforce attacks on silos. [This is discussed in greater detail in Ref. 19.]

DR. LARRY SMARR (Associate Professor of Astronomy and Physics, University of Illinois): The recent EPA and Science reports on the greenhouse effect mentioned the warming effects of CO2. I presume enormous quantities of CO2 will be a by-product of fires. In what sense have you taken these into account and in what sense can the warming effect of the CO2 oppose the cooling effect of the dust?

DR. SAGAN: I am glad you raised this question because it is a potential source of confusion; that is, two recent reports, one of which says burning fossil fuels puts gases into the atmosphere which heat the Earth, and another which you just heard, saying that nuclear war puts particles into the atmosphere that cool the Earth. Perhaps someone might think that the two effects cancel each other out. That is not our conclusion for more reasons than one. First, the CO2 put up even with all this burning is simply not enough to make any significant contribution to the greenhouse effect. The current value of 0. 03 percent of the Earth's atmosphere by volume of CO2 represents about three orders of magnitude more CO2 than would be released in the burning of cities and forests.

Also let me stress that the CO2 greenhouse effect is a long-term trend. There is no undoing it on time scales of decades. What we are talking about here is a sudden low-temperature nuclear war pulse in the system which then has a few years' decay time, superimposed on this very slow temperature increase from the burning of fossil fuels.

DR. ARNOLD W. WOLFENDALE (Professor of Physics, University of Durham, England): My question relates to the important topic of peer review. Clearly, anything that is new and startling needs review by many peers. The excellent 1975 report from the National Academy of Sciences produced a rather more favorable consideration. I am wondering whether the authors of that report are being consulted or asked for their comments on your results?

DR. SAGAN: The question of peer review is essential. That is why we have delayed so long in the public announcement of these dire results. The results that you have heard today have gone through a five-day meeting at the American Academy of Arts and Sciences in Cam-bridge, Massachusetts, in April 1983, of close to one hundred atmospheric scientists, nuclear physicists, and biologists—representing individuals of many different political persuasions, including representatives of the government weapons laboratories.

Both the physical paper I described and the biological paper which Dr. Ehrlich will describe have also gone through the peer review process for publication in the professional journal Science.[1,2]

In addition, there have been some six or eight other separate studies —two of them in the Soviet Union, trying to confirm or find fault with our results. They all corroborate our results.

DR. WOLFENDALE: Does that mean that the authors of the 1975 report have retracted their conclusions?

DR. SAGAN: I very much hope that the new National Academy panel will address that important issue. Let me say very quickly the reason for the differences between our nuclear winter results and those of the 1975 Academy study. First, the climatic effects were addressed by arguments from analogy with the Krakatoa volcanic explosion, not from any attempt actually to model the effects. In 1883, it was argued, a volcano went off that had as its only global effects a temperature decline of a half a degree or so, and pretty sunsets all over the world. The total explosive energy in that event was (perhaps) comparable to the total yield we are talking about in a nuclear war; so why worry?

That argument neglects several facts: First, the vast bulk of the material ejected in the Krakatoa explosion fell right there in the Sunda Straights. Second, volcanic ejecta, mainly silicates and sulfuric acid, have very much lower absorption coefficients than the dark smoke generated by nuclear war. Third, particle size distribution functions are different and, fourth, we are talking about thousands of simultaneous sources of fine particles. Krakatoa was a single event. There are other significant differences as well. Fold all that in, and the Krakatoa event is consistent with the calculations reported here.

DR. ROBERT EHRLICH (Chairman, Department of Physics, George Mason University, Virginia): The fact that a 100-megaton attack, less than 1 percent of the arsenals, gives such catastrophic results indicates that the main cause of the climatic problem is due to the smoke that would be generated from fires that are burning in cities. I am just wondering if you have considered—in a nuclear attack involving all cities of populations of more than 100,000 in the Northern Hemisphere—what is the likelihood that indeed half the area of cities would go up in smoke and also would burn for many weeks and months? And does your estimate of that probability agree with other people's estimates?

DR. SAGAN: Yes. This is one of the many parts of our study to which Dr. Turco has brought his considerable expertise. I think the answer is a week, perhaps; months, no. Very substantial burning occurs be-cause the fuel loading densities in cities are so enormously high.

MR. RALPH NADER (Consumer Advocate): Carl, let me ask you about the technical import of your findings. Assuming a successful first strike by Adversary A against Adversary B, at what level would a successful first strike, given your calculation, invite suicide for the aggressor?

DR. SAGAN: Or, put another way, what about a subthreshold first strike, below that nuclear winter threshold of maybe a thousand warheads? Would an effective first strike be self-deterring? I think I have to decide, Ralph, forgive me, that this is in the policy area. I don't want to discuss it at length; but I think that to take out all major fixed strategic targets reliably, you have to exceed the nuclear winter threshold. [highlighted by mindfully.org]

MR. NADER: I think you are drawing too fine a line. My question basically was in terms of the ricochet effect. To put it more simply, what would be the threshold of a ricochet effect on the first launch, first-strike period?

DR. SAGAN: We have an excellent chance that if Nation A attacks Nation B with an effective first strike, counterforce only, then Nation A has thereby committed suicide, even if Nation B has not lifted a finger to retaliate.

MASON RUMNEY (Executive Secretary, First Steps Foundation): I have one question. Where did you get the idea that the 100-megaton attack would be against cities where the fuel is, instead of against ICBM sites where it isn't?

DR. SAGAN: This is merely one of a wide range of possible scenarios.

DR. HERBERT SCOVILLE, JR. (President, Arms Control Association; Former Deputy Director, Central Intelligence Agency): What pro-portion of the long-term effect requires the smoke getting up into the stratosphere?

DR. SAGAN: Normally fires do not inject soot up into the stratosphere, and we have not assumed that they do to any significant degree. Virtually all of our smoke effects are tropospheric effects. In the baseline case, we have assumed the smoke in the lower troposphere to be subject to fairly quick rainout.

Is there a circumstance, likely or unlikely, in which the smoke plume reaches into the stratosphere? In that case the effects 'are much worse, much more prolonged than we calculated. We have not assumed any significant stratospheric soot. In at least some knowledge-able opinion, including that of George Carrier of Harvard, it is an unlikely effect. I would myself say that it is still an open question.

DR. MICHAEL J. PENTZ (Dean of the Faculty of Science, The Open University at Milton Keynes, United Kingdom, and Chair, SANA, Scientists against Nuclear Arms): I have a question which relates to Table 1 of the main paper, the set of scenarios that you studied. I was very interested in Numbers 11 and 16. Can you ex-plain the underlying assumptions, that is, regarding the 3,000-megaton counterforce attack and the more severe 5,000-megaton counter-force attack? The figure that interests me is the figure under the column Percentage Yield, Urban or Industrial targets, which you quote as zero in both cases.

The reason I puzzle over this is that SANA recently did a computer model of a primarily counterforce attack on targets in the United Kingdom involving about 343 targets and a total yield of 220 mega-tons, mixed ground bursts and air bursts. It was immediately apparent to us that a high proportion of such counterforce targets are either in the centers or near major cities and densely populated areas. I guess that is fairly typical for most of Europe. That is why I am worried about zero. Perhaps there is a decimal point which you could put in to count Great Britain and Europe into the picture.

DR. SAGAN: Everything you say, except for the misplaced decimal point, is correct. What we have been trying to do is in the usual scientific tradition of the separation of variables. We are saying: Imagine a pure counterforce attack in the multi-thousand-megaton range. What effects would it produce if there were no burning of a single tree or a single house? It is a lower limit to the effects.

So the way to look at that, I think, is to examine the 5,000-megaton baseline case, Case 1, which adds in the burning of cities as well. 

DR. PENTZ: To the extent of 20 percent only?

DR. SAGAN: Yes, indeed.

DR. PENTZ: I can see that could be realistic for the location of major counterforce targets in the U.S.A. or perhaps for the Soviet Union. But it would not be realistic for Britain.

DR. SAGAN: Absolutely right. Therefore it follows that the European situation is considerably worse than we have said. This is still another example of how conservative our calculations are.

MS. MYRTLE JONES (President, Mobile Bay Audubon Society): This is a timely conference, and your article in Parade yesterday [October 30, 1983] was very well put together and helped me to understand what you were saying today. You lightly touched on the fact that you had gone before Congress this morning. I was wondering if that was both Houses, and what kind of reception you got?

DR. SAGAN: This was a private meeting with members of both Houses just to give them some feeling for the new results. I would say that they were interested.

MS. JONES: Were they positively interested?

DR. SAGAN: I am not sure what that means. But there is no doubt that nuclear winter has strong policy implications, although, when we began the study, we had no idea that would be the case.

MR. J. SALATUN (Air Vice-Marshal [retired], Indonesian Air Force, and member of Parliament, Jakarta): I have two questions.

Number 1, despite the pessimism, we must not forget that we now live in the thirty-eighth year after World War II, with nuclear bombs and without another world war. So my question is, what is the likelihood of nuclear war?

DR. SAGAN: Prophecy is a lost art. If there were any accurate way of making that prediction, it would be most important. But look at how poorly we can predict even the most minor aspects of world politics, such as which small nation will be invaded tomorrow.

To, therefore, expect some exact prognostications about the likelihood of nuclear war, I think is asking too much. It is certainly true that we have gone thirty-eight years without a nuclear war. Who knows, we might be able to survive for some longer period of time. But would you want to bet your life on it? I do not guarantee that this is a perfect analogy, but the situation reminds me of a man falling from the top of a high building, saying to an office worker through an open window as he passes by, "So far, so good."

MR. SALATUN: Question Number 2 is: What would you say of the possibility that your findings will trigger a new effort and simply force destruction?

DR. SAGAN: I guess that is a policy issue as well. May I ask you, Vice-Marshal, what do you think is the likelihood, as a result of the knowledge of nuclear winter, the realization that Indonesia is fundamentally threatened even if not a single nuclear weapon falls in its territory, that Indonesia will suddenly become much more interested in the great power nuclear confrontation?

MR. SALATUN: Well, all we can do is pray to God that it will not happen. But meanwhile we should prepare for the worst.

DR. SAGAN: In my opinion, you can do a lot more than just pray.

DR. GERALD O. BARNEY (President, Barney and Associates, Inc.): In the course of conducting the Global 2000 Report to the President, it became very apparent to me, and I think many others, that it is important when major studies are done to provide access to the de-tailed models that have been used in preparing them because often there are things buried in the computer models that are not immediately understandable in the papers that are published reporting the results.

I am wondering if the actual model that has been used in this work will be available and what would be the procedure for obtaining tapes or copies of the detailed program?

DR. SAGAN: It is a perfectly legitimate request and, of course, we would welcome such requests. A very much longer discussion of the TTAPS results is being prepared, which will give a great deal more of the results. But I am sure we would be happy to provide what you are asking.

Let me again stress, however, that all of those independent calculations used quite different codes. Since they all converged on the same solution, I do not think that our results are dependent on some quirk internal to the computer program. But, of course, every segment of the program should be explorable.

H. JACK GEIGER, M.D. (Professor of Community Medicine, City College of the City University of New York): I have a concern based on some experience about the ingenuity with which those whose task it is to defend the ideas of the winnability and survivability of nuclear wars may attempt to reinterpret or distort these data, particularly with regard to concepts such as threshold. What are the elements in reaching threshold as you define it: total number of weapons, total yield, or some mixed function of those?

DR. SAGAN: It is a mixed function of them, and also strongly involves targeting strategy. Notice that, with present accuracies and arsenals, when you start getting much below 20 kilotons you run into significant difficulties destroying hardened targets. I think there really is a lower cutoff under present conditions if the various nations are imagining preserving the option of plausible counterforce attack.

DR. ED PASSERINI (President, Carrying Capacity, Inc., Washing-ton, D.C.; Professor of Humanities and Environment, University of Alabama): This kind of follows Jack's question. There is a movement toward smaller yields and more precise targeting. Do you see the necessity of doing a follow-up study to look at what the effect would be of a subthreshold strike which is very precisely targeted?

DR. SAGAN: Well, as I was saying to Ralph Nader, I am very dubious about the possibility of a subthreshold attack, with the pre-sent configuration of yields and accuracy, having a plausible capability for a preemptive first strike on fixed targets. [Discussion of such future possibilities is made in Ref. 19.]

DR. FRANCIS B. PORZEL (Foundation for Unified Dynamics): I cannot pass up the opportunity to tell you that it has been almost to the hour thirty-one years since the first hydrogen bomb was fired.

I think it would help the report a great deal if you could relate to past experiences, to the atomic tests. Looking at the graphs, I note there were several periods during the fifties when the Soviet Union and the United States held test operations which were approaching the 100-megaton range in total; Bravo alone was 14 megatons for the first one in 1954.

You mentioned that the model was one-dimensional so it would not be applicable to this. But would you care to comment on what would be the caution that you would have to exercise with your model if one attempted to apply it to that experience?

DR. SAGAN: Put another way, what does the model predict for the atmospheric nuclear weapons explosions in the fifties? And the answer is it predicts no detectable effect. The reason is, remember, that the 100 megatons has to be dedicated to igniting about 100 city fires. That is not what you did. You had dust but no soot. The easiest way to describe this is through the concept of optical depth. The transmitted light through a pure absorbing overcast is roughly e, the base of natural logarithms, to the power minus optical depth. When the optical depth is around a tenth, the attenuation is one minus optical depth. It is very small.

When the optical depth gets up to 1, which you were never near in the fifties, then the attenuation becomes significant. And when the optical depth is around 10 the attenuation becomes severe. Because this is a nonlinear process, what happened in the fifties, we predict, should have no climatic effects and none were observed. But what is happening in our calculations is an optical depth of many. The consequent effects will be significant.

MS. MARION EDEY (Executive Director, League of Conservation Voters): My question is: What are the effects of the ozone layer in the Southern Hemisphere?

DR. SAGAN: My understanding is that the holes in the ozonosphere move rapidly and propagate into the Southern Hemisphere from the Northern Hemisphere.

PHILLIP GREENBERG: The views expressed today have moved me to make a brief comment. I am struck by the decision to avoid policy discussions and under the circumstances I can expect it and under-stand it.

By the same token I think we will all understand that there are certain policy implications that flow from this work and I note in many cases, on the part of people asking questions and on the part of you at the podium, a tendency to question the conservatism of the assumptions. But I think it would be a mistake for even those of you in the scientific community to become too absorbed in the question of conservatism of assumptions. Because while that is appropriate for a scientific paper, in the policy arena, when one considers high-consequence events, even if they are low probability, then the question of conservatism becomes reversed.

So I would simply say that I think that it is important in the discussions, and no doubt a criticism that you will have to bear from your colleagues who perhaps have a different point of view from a policy perspective, to remember that conservatism is different from scientific or policy viewpoints.

DR. SAGAN: I quite agree. It is a commonplace in crisis management as well as in actuarial statistics that what is important is not just the probability of the event and not just the cost of the event if it occurs, but the product of the two. We are very well aware of that and in fact have so far encountered very little criticism along the lines you mention. 

DR. THOMAS C. HUTCHINSON (Professor, Department of Botany, University of Toronto, Canada): How much of the oceans in the Northern Hemisphere are likely to be frozen by one year of minus 25 degrees centigrade (—13°F)?

DR. SAGAN: In freshwater systems, the typical depth of freezing will be a meter, a meter and a half, something like that. There should certainly be more ice floes in the ocean, but there is absolutely no chance that the oceans, per se, will freeze because of their high heat capacity and high thermal inertia.

So perhaps there are a few things that won't go wrong among the vast litany of things that will, should we be so foolish as to permit a nuclear war to happen.

THE COLD AND THE DARK: The World after Nuclear War
Foreword by LEWIS THOMAS
The Conference on the Long-Term Worldwide Biological Consequences of Nuclear War
New York . London
Copyright © 1984 by Open Space Institute, Inc.: The Center on the Consequences of Nuclear War
The Atmospheric and Climatic Consequences of Nuclear War © 1984 by Carl Sagan
All rights reserved. Published simultaneously in Canada by Stoddart, a subsidiary of General Publishing Co. Ltd., Don Mills, Ontario.
Conference held Oct. 31-Nov. 1, 1983 at the Sheraton Washington Hotel in Washington, D.C.

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