Nuclear Winter

Fallout consists of radioactive particles that settle out of the atmosphere, carrying hazardous radioactivity far from the site of its release. Nuclear winter is a model of the projected consequences of nuclear war, which may result in drastically reduced surface temperatures over several seasons or years. This model has dramatically altered thinking on the ability of humanity to survive a major nuclear exchange.

Fallout

A nuclear explosion occurs when the nuclei of a critical mass of susceptible isotopes interact to either break apart into smaller nuclei through nuclear fission or fuse together to form more massive nuclei through the process of nuclear fusion. Less than one-millionth of a second after detonation, a nuclear explosive changes from solid material into a rapidly expanding fireball. Eventually, it cools enough so that the bomb vapor condenses into the solid particles that incorporate the radioactive ashes of the nuclear explosive. Fallout is the name given to the radioactive particles that rain down from the debris cloud of a nuclear explosion. The length of time it takes for fallout to reach the ground depends largely on the size of the particles and how high they are lofted. Thus, fine particles can remain in the stratosphere for years, circling the globe many times. The longer these particles remain aloft, the weaker the radioactivity will be when the fallout reaches the ground. Half-lives of radioactive isotopes from nuclear weapons range from a fraction of a second to billions of years, but all radioactive isotopes obey a simple rule: for each factor-of-seven increase in time, the intensity of radioactivity decreases by approximately a factor of ten.

Short-lived isotopes are intensely radioactive for a brief time; long-lived isotopes are only weakly radioactive, but they are active for a long time. Those with intermediate half-lives are the most dangerous fallout isotopes. Two other important considerations are the type of radiation emitted and the human body’s affinity for certain elements. Fallout radiation is usually in the form of gamma rays or beta particles. Gamma rays are similar to X-rays, but they have higher energies and are more penetrating.

Radioactive fallout can also be produced by a nuclear reactor accident, as happened when the Soviet reactor at Chernobyl exploded in 1986. Operator error coupled with a poor design caused two chemical explosions to lift the thousand-ton cover plate from the reactor core, blasting radioactive particles more than seven kilometers into the air and starting several fires in and around the reactor. Emissions from the fires formed a radioactive cloud 1.5 kilometers (about 1 mile) high at one point. As the fire burned, radioisotopes continued to be released over a period of several days.

In 2011, a tsunami resulting from a major subsea earthquake off the coast of Japan severely damaged several reactors at the Fukushima nuclear power station. This caused the cooling systems to fail, and station workers were forced to use seawater to cool the reactor cores, which not only rendered the cores unusable but also produced several by-product explosions that further increased the damage and caused the release of radioactive contaminants into the atmosphere. The greatest fear with regard to nuclear explosions, whether due to bombs or industrial accidents, is that the rapid environmental changes that could result would initiate an artificial ice age that has been termed “nuclear winter.”

Earth’s Thermal Budget

The theory of nuclear winter proposes that massive quantities of smoke and dust injected into the atmosphere by multiple nuclear detonations would drive temperatures on Earth below the freezing point of water for extended periods and would diminish the amount of sunlight reaching the surface, creating a period of prolonged cold and darkness. Atmospheric circulation could distribute the smoke and dust veils from even a moderate-sized nuclear exchange to make the effects global rather than confined only to the regions of the detonations. Secondary effects from radioactive fallout and toxic substances carried by the smoke and dust veils also would be extremely widespread. The combined effects of nuclear winter could be sufficient to cause the eventual extinction of plant and animal life and possibly of humanity itself.

The key factor in the nuclear winter scenario is that the combined effects of smoke and dust injected into the atmosphere by multiple nuclear detonations could severely reduce the amount of sunlight reaching Earth’s surface, throwing the “thermal budget” of the ecosystem into imbalance and sending surface temperatures well into the subfreezing range. Over time and across latitudes, the thermal budget of Earth is more or less in balance, with the amount of thermal radiation arriving from the sun being roughly equal to the amount of energy radiated back into space. (Low latitudes characteristically receive more thermal radiation than they emit, and higher latitudes less, creating large-scale atmospheric and oceanic circulation patterns that balance the thermal budget and account for major weather patterns.)

Radiation from the sun reaches Earth mostly in the visible-light wavelengths of the electromagnetic spectrum, but thermal radiation emitted from the ground surface back into space is mostly in infrared wavelengths. If Earth did not have an atmosphere, the thermal budget would balance out at a level significantly below the freezing point of water. Fortunately, components of the atmosphere—primarily water vapor and carbon dioxide molecules—are excellent absorbers of infrared radiation, and they reradiate at least half of the infrared energy they absorb back toward the ground surface rather than out into space. The process, known as the greenhouse effect, is sufficient to increase the average temperature of a planet by several degrees. Consequently, it helps keep ambient temperatures in most places on Earth above the freezing point of water, making possible a biological regime based on liquid water. Any planetary-scale effect that would prevent sunlight from reaching the surface would diminish or even eliminate the greenhouse effect of those gases in the atmosphere, allowing temperatures to drop to the subfreezing levels that would otherwise exist.

Diminished Sunlight

Nuclear winter is thus a kind of “anti-greenhouse” effect. The greenhouse effect is produced by the small proportion of infrared-absorbing carbon dioxide, methane, water vapor, and other gases in the atmosphere that increases ambient temperatures on Earth by reducing the amount of infrared energy being emitted back into space from the ground surface. In the nuclear winter scenario, changes in the atmosphere reduce the amount of incoming solar radiation reaching the surface by absorbing greater quantities of solar radiation in the upper atmosphere, with no reduction in surface infrared radiation to maintain a thermal balance. Surface temperatures would drop significantly, and because any appropriate atmospheric agent responsible for such a reduction would have to absorb primarily visible light, continuous darkness would result at the surface.

While dangerous radioactive materials can be injected into the atmosphere by the failure of nuclear power installations, the nuclear winter scenario is much more closely associated with the wartime exchange of nuclear weapons. Depending on the extent of a general nuclear exchange, one hundred million to three hundred million metric tons of smoke and soot particles might be injected into the atmosphere, enough to diminish sunlight reaching the ground surface by more than 90 percent if it were distributed uniformly throughout the atmosphere. The smoke would come from immense firestorms generated by detonations over urban or large industrial centers, so powerful that they would generate hurricane-like convective winds similar to those experienced by the victims of the Allied incendiary bombing of Dresden, Germany, during World War II.

The convective energy of these firestorms would drive the smoke and soot clouds beyond the dense troposphere and up into the stratosphere. The higher these clouds were injected, the longer they would be likely to persist because smoke and dust are cleansed from the atmosphere mostly by rainfall, and there is little water vapor in the stratosphere. Research suggests that this material would remain aloft long enough to become distributed throughout the stratosphere, with global rather than merely regional repercussions. Firestorms in modern urban centers, burning not only structures and material life but also huge quantities of plastics, light metals, and a host of synthetic chemicals of still unknown potential, would be far more dangerous over much greater areas than anything previously experienced. Additional smoke accumulation would come from forests and grasslands ignited by nuclear explosions.

Effects of Nuclear Winter

The horrific loss of human life from the immediate effects of a nuclear conflict remains part of the nuclear winter scenario, but the secondary, longer-term losses are even more sobering. Deprived of heat and light from the sun, much of Earth would experience a state of subfreezing temperatures and continuous darkness, even at midday, that would last for many years. Among the survivors of the nuclear holocaust, cold, exposure, and extraordinary psychological stress would continue to increase the fatality rate. Over several successive seasons, the result would be the collapse of the natural food chain, the destruction of most plant life and the animal life that feeds on it, and mass starvation. All remaining order would likely break down as survivors fought for food, water, and shelter in a world that would no longer have the advanced infrastructure and technology that is taken for granted today.

Interruption of sunlight to the surface would result in a complete disruption of the process of photosynthesis in plants and inevitably lead to the mass failure of food crops around the world. Wild plant species would also be affected, and those susceptible to small changes in the environment would be quickly driven to extinction. Large-scale extinction of vertebrate species dependent on plant matter for food would soon follow. At sea, the phytoplankton population that anchors the food chain for many large species would collapse, and many of the larger species of aquatic life would face extinction as well.

Moderate-sized thermonuclear explosions are capable of perforating the ozone layer of the atmosphere, and a significant number of such explosions could lead to massive ozone depletion. In addition to radioactive fallout from the explosions themselves—a hazard now judged to be more serious than previously believed as a result of the long-term suspension of matter in the atmosphere—survivors of a nuclear war could be exposed to harmful ultraviolet radiation from the sun that is ordinarily blocked by atmospheric ozone.

The impact of radioactive fallout, once presumed to be localized in the vicinities of the nuclear explosions themselves, would become global and prolonged as these substances circulate in the upper atmosphere. Water and food sources hundreds or thousands of miles distant from actual detonations would be contaminated by the atmospheric circulation of radioactive matter and toxic substances from burning cities. Human populations would also decline, making the species vulnerable to eventual extinction.

Possible consequences of nuclear winter include long-term climatic disruption. For example, the deposition of dust and soot over much of Earth’s surface could lower the albedo, or reflective value, of the surface, with unknown climatic consequences over time. Coastal areas—in contrast to continental interiors, where convection would be almost nonexistent for years—might suffer storms of unprecedented magnitude as relatively warm, humid air currents from over the oceans collided with chilled air masses on land.

Magnitude of Nuclear War

Phenomena associated with the nuclear winter scenario may vary considerably depending upon the magnitude of a nuclear exchange and the nature of selected targets; catastrophic results are possible far short of a full-scale thermonuclear war. In a “medium-range” exchange, for example, involving some 5,000 megatons of detonations, including one thousand ground bursts—the sort of exchange that might be envisioned should one superpower attempt to eliminate the first-strike capability of another—the resulting smoke and dust veils could create a global atmospheric inversion that would essentially stop wind currents near the surface. As the smoke and dust absorbed visible sunlight and blocked it from reaching the surface, the upper atmosphere would be heated as much as 30 to 80 degrees Celsius (54 to 144 degrees Farenheit). As the ground cooled, these hot layers would rise and expand, blanketing the planet in an unbroken cloud of smoke and dust. Even several months after the exchange, the lowest levels of the atmosphere would receive only enough radiation to drive very weak convective currents. This temperature inversion would inhibit the rise of moist air from the surface and thereby inhibit cleansing rainfall, increasing the time that the smoke and dust veils would remain suspended in the atmosphere.

One of the most significant features of the nuclear winter model is the ability of its climatic consequences to involve the tropics through atmospheric circulation. Tropical plant and animal species are far more vulnerable than others to even minor shifts in temperature or rainfall. Prolonged cold and darkness over the tropics could result in the extinction of many species on the planet. At the very least, the destruction of the forest regions of the tropics, major absorbing agents for solar energy, would further alter Earth’s thermal balance toward long-term subfreezing conditions. Widespread destruction of forests would also lead to further large-scale fires.

The nuclear winter model now predicts much more severe radiation hazards, spread over wider areas, than those estimated by earlier projections. Most radioactive isotopes in a nuclear explosion condense onto aerosols and dust sucked in by the initial fireball. Inasmuch as the model predicts very large quantities of dust aerosols in the atmosphere for longer periods of time, the dangers of transport or dispersal of radioactive materials would multiply, particularly with respect to intermediate and prolonged fallout. Although not immediately lethal, this fallout could lead to chronic radiation sickness, depressed immune systems, genetic defects, and death for millions from cancer or other radiation-induced causes.

Study of Nuclear Winter

Formulation of the nuclear winter scenario and its widespread discussion in the scientific, military, and political communities resulted largely from the work of a research team appointed in 1982 by the National Academy of Sciences (NAS). The NAS requested that Richard B. Turco, Owen B. Toon, Thomas P. Ackerman, James B. Pollack, and Carl Sagan—a team subsequently known as TTAPS, based on their last initials—investigate the possible effects of dust raised by nuclear detonations. The TTAPS team comprised scientists who already had investigated a variety of atmospheric effects of dust particles, including the consequences of volcanic explosions, the behavior of dust storms on Mars, and theories of mass extinction caused by asteroid or comet impacts in the geologic past.

Computer modeling represents the only feasible means of studying the consequences of a nuclear exchange. The TTAPS researchers used advanced models of atmospheric circulation, particle microphysics, and radiative-convective behavior. Also used were models of nuclear-exchange scenarios of varying size and geographical complexity. Other researchers, including Paul J. Crutzen at Germany's Max Planck Institute for Chemistry and John W. Birks at the University of Colorado, persuaded the TTAPS team to include in their research the possible effects of smoke from massive fires caused by nuclear explosions.

Prior to the TTAPS research, little was known about potential large-scale smoke-and-dust pollution except as the consequence of volcanic explosions. The TTAPS team was able to study firsthand results of the eruption of El Chichón in 1982, which generated a significant dust veil and possibly some short-term surface temperature reduction in some areas. The much larger explosion of Krakatoa in 1883 caused visible atmospheric effects around the world and, possibly, minor surface-temperature changes. Historically, several volcanic explosions far larger than Krakatoa are known from the eighteenth and nineteenth centuries, some of them comparable in energy to several multimegaton nuclear detonations. Yet none generated more than minor multiyear climatic effects.

The TTAPS team concluded that earlier attempts to judge the possible effects of nuclear explosions based on records and observations of volcanic explosions were misleading. No matter how large, volcanic events were episodic and localized. The clouds of soil and dust particles, or aerosols, created by these explosions consist mostly of relatively large and bright, light-scattering particles that can create dazzling displays but do not absorb much visible light. By including in their model the smoke and soot from fires generated by nuclear explosions, however, the TTAPS team identified the potential source of aerosols of very dark and very fine particles with average diameters smaller than the typical wavelength of infrared radiation. These smoke and soot particles would have a very high absorptive capacity for visible light but would not correspondingly increase infrared absorption in the atmosphere, thus bringing about surface darkness and temperature reduction.

One of the most unsettling results of the TTAPS research was to show that a full-scale nuclear exchange would not be needed to bring about the most serious nuclear winter effects. Presuming that such an exchange would entail a total explosive yield of about twenty-five thousand megatons, nuclear winter could be triggered by much smaller conflicts, in the range from three thousand to five thousand megatons, depending on the nature of the targets. Prior to the 1963 international treaty banning atmospheric testing of nuclear weapons, scenarios of nuclear conflict centered on massive atmospheric explosions of thermonuclear weapons in the twenty- to forty-megaton range, principally over cities and refinery areas. Many of these weapons were to have been delivered by strategic bombers.

Subsequent development of ballistic missiles using multiple reentry warhead configurations of one to five megatons each, directed initially at the enemy’s own missile silos, greatly increased the anticipated number of ground-level detonations in a nuclear war scenario, thereby increasing the expected amount of material injected into the atmosphere by the explosions. Ironically, the strategic use of larger numbers of considerably smaller-yield warheads makes a nuclear winter catastrophe even more likely, even if the nuclear exchange was limited to targeted strikes concentrated on missile silos and military installations.

The study of nuclear winter and its effects continued into the twenty-first century. According to a study released in 2022 in Nature Food which drew on forty years of research, a global nuclear conflict would likely result in a 63 percent reduction in the Earth’s population, mainly due to caloric deficit. The resulting climate change would be apocalyptic to all life on Earth, but would more greatly impact countries that relied on imports of food for their population’s survival. These findings brought to light the continued importance of diplomacy and political agreements in the nuclear age to prevent global nuclear conflict.

Beyond global conflict, though, another potential catalyst for nuclear winter entered the scene as technology advanced. Researchers and scientists have been wary of the potential of artificial intellegence (AI) contributing to a nuclear winter. Beyond the science-fiction interpretations involving robots taking over the planet, AI poses a true threat if used nefariously, especially because of the binary codes used that can be rapidly copied and deployed. Because of this, the Center for AI Safety, along with AI experts, journalists, policymakers, and the public, released a one-sentence agreement to recognize the potential risks of AI and the importance in preventing it from causing harm. The statement reads as follows: "Mitigating the risk of extinction from AI should be a global priority alongside other societal-scale risks such as pandemics and nuclear war." The effect of such action is recognizing the dangers of AI and the need to collaborate to harness AI's capabilities globally.

Principal Terms

aerosol: a suspension of solid or liquid particles in a gas

greenhouse effect: various mechanisms for increasing the absorptive capacity of radiated energy by the atmosphere

half-life: the time it takes for any initial quantity of a radioactive isotope to decay to one-half of that initial amount

isotope: atoms of an element having the same number of protons but a different number of neutrons

thermal budget: the balance of incoming and outgoing radiative energy on Earth

Bibliography

Badash, Lawrence. A Nuclear Winter’s Tale: Science and Politics in the 1980s. Cambridge: MIT P, 2009. Print.

Byrne, John, and Steven M. Hoffman, eds. Governing the Atom: The Politics of Risk. New Brunswick: Transaction, 1996. Print.

Chernousenko, Vladimir M. Chernobyl: Insight from the Inside. Berlin: Springer, 1991. Print.

Diermendjian, Diran.“Nuclear Winter”: A Brief Review and Comments on Some Recent Literature. Santa Monica: RAND Corporation, 1988. Print.

Drew, Dennis M., et al. Nuclear Winter and National Security: Implications for Future Policy. Maxwell Air Force Base: Air UP, 1986. Print.

Ehrlich, Paul R., et al. The Cold and the Dark: The World after Nuclear War. New York: Norton, 1984. Print.

Ehrlich, Paul R., et al. “Long-Term Biological Consequences of Nuclear War.” Science 222.4630 (1983): 1293–1300. Print.

Fanchi, John R. Energy: Technology and Directions for the Future. Burlington: Academic, 2004. Print.

Fanchi, John R., and Christopher J. Fanchi. Energy in the 21st Century. 3rd ed. Hackensack: World Scientific, 2013. Print.

Fisher, David E. Fire and Ice: The Greenhouse Effect, Ozone Depletion, and Nuclear Winter. New York: Harper, 1990. Print.

Greene, Owen. Nuclear Winter: The Evidence and the Risks. Cambridge: Polity, 1985. Print.

Harwell, Mark. Nuclear Winter: The Human and Environmental Consequences of Nuclear War. New York: Springer, 1984. Print.

Izrael, Urii Antonievich. Radioactive Fallout after Nuclear Explosions and Accidents. Oxford: Elsevier, 2002. Print.

Robock, Alan, et al. “Climatic Consequences of Regional Nuclear Conflicts.” Atmospheric Chemistry and Physics 7.8 (2007): 2003–12. Print.

Sagan, Carl. “Nuclear War and Climatic Catastrophe: Some Policy Implications.” Foreign Affairs 62.2 (1983): 257–92. Print.

Sagan, Carl, and Richard Turco. A Path Where No Man Thought: Nuclear Winter and the End of the Arms Race. New York: Random, 1990. Print.

“Statement on AI Risk.” Center for AI Safety, www.safe.ai/work/statement-on-ai-risk. Accessed 30 July 2024.

Sutton, Gerard K., and Joseph A. Cassalli, eds. Catastrophe in Japan: The Earthquake and Tsunami of 2011. Hauppauge: Nova, 2011. Print.

Takada, Jun. Nuclear Hazards in the World. New York: Springer, 2010. Print.

Toon, Owen B., Alan Robock, and Richard P. Turco. “Environmental Consequences of Nuclear War.” Physics Today 61.12 (2008): 37–42. Print.

Turco, Richard P., et al. “The Climatic Effects of Nuclear War.” Scientific American Aug. 1984: 33–43. Print.

Turco, Richard P., et al. “Nuclear Winter: Global Consequences of Multiple Nuclear Explosions.” Science 222.4630 (1983): 1283–92. Print.

Turner, Ben. “'Nuclear Winter' From a US-Russia Conflict Would Wipe Out 63% of the World's Population.” Live Science, 19 Aug. 2022, www.livescience.com/nuclear-war-could-kill-5-billion-from-famine. Accessed 14 Apr. 2023.

Zimmer, Daniel, et al. “Today's AI Threat: More Like Nuclear Winter Than Nuclear War.” Bulletin of the Atomic Scientists, 11 Feb. 2024, thebulletin.org/2024/02/todays-ai-threat-more-like-nuclear-winter-than-nuclear-war/#post-heading. Accessed 30 July 2024.