Environmental consequences of nuclear war

Any of several targeting strategies might be employed in a nuclear conflict. For example, in a “rational” war, a few weapons are deployed against symbolically important targets. Conversely, a “counterforce” war entails a massive attack against key military, economic, and political targets. We consider a “countervalue” strategy in which urban areas are targeted, mainly to destroy economic and social infrastructure and the ability to fight and recover from a conflict. In any case, when the conflict involves a large number of weapons, the distinction between countervalue and counterforce strategies diminishes because military, economic, and political targets are usually in urban areas.

Box 1 on page 38 describes how we estimate casualties (fatalities plus injuries) and soot (elemental carbon) emissions; figure 1 shows results. The figure gives predicted casualties and soot injected into the upper atmosphere from an attack on several possible target countries by a regional power using 50 weapons of 15-kiloton yield, for a total yield of 0.75 megaton. The figure also provides estimates of the casualties and soot injections from a war based on envisioned SORT arsenals. In the SORT conflict, we assume that Russia targets 1000 weapons on the US and 200 warheads each on France, Germany, India, Japan, Pakistan, and the UK. We assume the US targets 1100 weapons each on China and Russia. We do not consider the 1000 weapons held in the UK, China, France, Israel, India, Pakistan, and possibly North Korea. (Box 2 on page 40 provides information on the world’s nuclear arsenals.) The war scenarios considered in the figure bracket a wide spectrum of possible attacks, but not the extremes for either the least or greatest damage that might occur.

As figure 1 shows, a war between India and Pakistan in which each uses weapons with 0.75-Mt total yield could lead to about 44 million casualties and produce about 6.6 trillion grams (Tg) of soot. A SORT conflict with 4400 nuclear explosions and 440-Mt total yield would generate 770 million casualties and 180 Tg of soot. The SORT scenario numbers are lower limits inasmuch as we assumed 100-kt weapons; the average SORT yield would actually be larger. The results can be relatively insensitive to the distribution of weapons strikes on different countries because attacks on lower-population areas produce decreased amounts of soot. For instance, 100 weapons targeted each on France and Belgium leads to about the same amount of soot as 200 on France alone. On the other hand, using fewer weapons on densely populated regions such as in India and China would reduce soot generation.

The 4400 explosions that we considered are 1000 more than are possible with the lower SORT limit. However, even if the US and Russia achieve that lower limit, more probable weapons yields would produce soot emissions and casualties similar to those just described. Because of world urbanization, a SORT conflict can directly affect large populations. For example, with 1000 weapons detonated in the US, 48% of the total population and 59% of the urban population could fall within about 5 km of ground zero; 20% of the total population and 25% of the urban population could be killed outright, while an additional 16% of the total population and 20% of the urban population could become injured.

Figure 2 illustrates how the number of casualties and fatalities and the amount of soot generated in China, Russia, and the US rises with an increasing number of 100-kt nuclear explosions. In generating the figure, we assumed regions were targeted in decreasing order of population within 5.25 km of ground zero, as described in box 1. Attacks on China had the most dire effects because China has many highly populated urban centers. Indeed, attacks on a relatively small number of densely populated urban targets generate most of the casualties and soot. For example, 50% of the total soot produced by a 2000-weapon attack would result from 510 detonations on China, 547 on Russia, or 661 on the US. A single US submarine carrying 144 warheads of 100-kt yield could generate about 23 Tg of soot and 119 million casualties in an attack on Chinese urban areas or almost 10 Tg of soot and 42 million casualties in an attack on Russian cities.

In the late 1980s, Brian Bush, Richard Small, and colleagues assessed soot emissions in a nuclear conflict. ⁹ Their work, independent of the studies with which two of us (Toon and Turco) were engaged, involved a counterforce attack on the US by the USSR. They assumed 500-kt weapons aimed at 3030 specific targets such as US Army, Navy, and Air Force bases, fuel storage locations, refineries, and harbors, but not missile silos or launch-control facilities. Cities were not explicitly attacked in their counterforce scenario, but in the end, 50% of the US urban areas were destroyed.

Bush and colleagues estimated 37 Tg of smoke emissions, which contain not only light-absorbing black soot but also nonabsorbing organics and other compounds whose effects on climate are smaller than that of soot. Using our methodology for estimating fire emissions, which includes accounting for soot that is rained out, we calculate their result as being equivalent to about 21 Tg of soot emission. In our simulated countervalue attack with 1000 weapons of 100-kt yield, we found that 28 Tg of soot was generated. Our burned area is somewhat larger, which accounts for the greater soot emission. In short, both scenarios affect similar urban areas and generate similar amounts of soot.

However, Bush and colleagues assumed 3 times as many weapons and 15 times the total explosive yield that we assumed. Because of multiple targeting and overlap of detonation zones, their scenario has a built-in fire ignition redundancy factor of about 8.7; our model has negligible redundancy. In fact, their analysis of 3030 specific targets identified only 348 unique, non-overlapping detonation sites in the US. That substantial level of overkill is symptomatic of the enormous excesses of weapons deployed by the superpowers in the 1980s.

Figure 3 indicates changes in global average precipitation and temperature as a function of soot emission, as calculated with the help of a modern version of a major US climate model. ^6,8  A relatively modest 5 Tg of soot, which could be generated in an exchange between India and Pakistan, would be sufficient to produce the lowest temperatures Earth has experienced in the past 1000 years—lower than during the post-medieval Little Ice Age or in 1816, the so-called year without a summer. With 75 Tg of soot, less than half of what we project in a hypothetical SORT war, temperatures would correspond to the last full Ice Age, and precipitation would decline by more than 25% globally. Calculations in the 1980s had already predicted the cooling from a 150-Tg soot injection to be quite large. ³ Our new results, however, show that soot would rise to much higher altitudes than previously believed—indeed, to well above the tops of the models used in the 1980s. As a result, the time required for the soot mass to be reduced by a factor of e is about five years in our simulations, as opposed to about one year as assumed in the 1980s. That increased lifetime causes a more dramatic and longer-lasting climate response.

The temperature changes represented in figure 3 would have a profound effect on mid- and high-latitude agriculture. Precipitation changes, on the other hand, would have their greatest impact in the tropics. ⁶ Even a 5-Tg soot injection would lead to a 40% precipitation decrease in the Asian monsoon region. South America and Africa would see a large diminution of rainfall from convection in the rising branch of the Hadley circulation, the major global meridional wind system connecting the tropics and subtropics. Changes in the Hadley circulation’s dynamics can, in general, affect climate on a global scale.

Complementary to temperature change is radiative forcing, the change in energy flux. Figure 3 shows how nuclear soot changes the radiative forcing at Earth’s surface and compares its effect to those of two well-known phenomena: warming associated with greenhouse gases and the 1991 Mount Pinatubo volcanic eruption, the largest in the 20th century. Since the Industrial Revolution, greenhouse gases have increased the energy flux by 2.5 W/m ² . The transient forcing from the Pinatubo eruption peaked at about −4 W/m ² (the minus sign means the flux decreased). One implication of the figure is that even a regional war between India and Pakistan can force the climate to a far greater degree than the greenhouse gases that many fear will alter the climate in the foreseeable future. Of course, the durations of the forcings are different: The radiative forcing by nuclear-weapons-generated soot might persist for a decade, but that from greenhouse gases is expected to last for a century or more, allowing time for the climate system to respond to the forcing. Accordingly, while the Ice Age–like temperatures in figure 3(a) could lead to an expansion of sea ice and terrestrial snow-pack, they probably would not be persistent enough to cause the buildup of global ice sheets.

Agriculture responds to length of growing season, temperature during the growing season, light levels, precipitation, and other factors. The 1980s saw systematic studies of the agricultural changes expected from a nuclear war, but no such studies have been conducted using modern climate models. Figure 4 presents our calculations of the decrease in length of the growing season—the time between freezing temperatures—for the second summer after the release of soot in a nuclear attack. ^6,8  Even a 5-Tg soot injection reduces the growing season length toward the shortest average range observed in the midwestern US corn-growing states. Earlier studies concluded that for a full-scale nuclear conflict, “What can be said with assurance … is that the Earth’s human population has a much greater vulnerability to the indirect effects of nuclear war [including damage to the world’s agricultural, transportation, energy, medical, political, and social infrastructure], especially mediated through impacts on food productivity and food availability, than to the direct effects of nuclear war itself.” As a result, “The indirect effects could result in the loss of one to several billions of humans.” ⁴  

Because the soot associated with a nuclear exchange is injected into the upper atmosphere, the stratosphere is heated and stratospheric circulation is perturbed. For the 5-Tg injection associated with a regional conflict, stratospheric temperatures would remain elevated by 30°C after four years. ^6–8  The resulting temperature and circulation anomalies would reduce ozone columns by 20% globally, by 25–45% at middle latitudes, and by 50–70% at northern high latitudes for perhaps as much as five years, with substantial losses persisting for an additional five years. ⁷ The calculations of the 1980s generally did not consider such effects or the mechanisms that cause them. Rather, they focused on the direct injection of nitrogen oxides by the fireballs of large-yield weapons that are no longer deployed. Global-scale models have only recently become capable of performing the sophisticated atmospheric chemical calculations needed to delineate detailed ozone-depletion mechanisms. Indeed, simulations of ozone loss following a SORT conflict have not yet been conducted.

Scientific debate and analysis of the issues discussed in this article are essential not only to ascertain the science behind the results but also to create political action. Gorbachev, who together with Reagan had the courage to initiate the build-down of nuclear weapons in 1986, said in an interview at the 2000 State of the World Forum, “Models made by Russian and American scientists showed that a nuclear war would result in a nuclear winter that would be extremely destructive to all life on Earth; the knowledge of that was a great stimulus to us, to people of honor and morality, to act in that situation.” Former vice president Al Gore noted in his 2007 Nobel Prize acceptance speech, “More than two decades ago, scientists calculated that nuclear war could throw so much debris and soot into the air that it would block life-giving sunlight from our atmosphere, causing a ‘nuclear winter.’ Their eloquent warnings here in Oslo helped galvanize the world’s resolve to halt the nuclear arms race.”

Many researchers have evaluated the consequences of single nuclear explosions, and a few groups have considered the results of a small number of explosions. But our work represents the only unclassified study of the consequences of a regional nuclear conflict and the only one to consider the consequences of a nuclear exchange involving the SORT arsenal. Neither the US Department of Homeland Security nor any other governmental agency in the world currently has an unclassified program to evaluate the impact of nuclear conflict. Neither the US National Academy of Sciences, nor any other scientific body in the world, has conducted a study of the issue in the past 20 years.

That said, the science community has long recognized the importance of nuclear winter. It was investigated by numerous organizations during the 1980s, all of which found the basic science to be sound. Our most recent calculations also support the nuclear-winter concept and show that the effects would be more long lasting and therefore worse than thought in the 1980s.

Nevertheless, a misperception that the nuclear-winter idea has been discredited has permeated the nuclear policy community. That error has resulted in many misleading policy conclusions. For instance, one research group recently concluded that the US could successfully destroy Russia in a surprise first-strike nuclear attack. ¹⁰ However, because of nuclear winter, such an action might be suicidal. To recall some specifics, an attack by the US on Russia and China with 2200 weapons could produce 86.4 Tg of soot, enough to create Ice Age conditions, affect agriculture worldwide, and possibly lead to mass starvation.

Lynn Eden of the Center for International Security and Cooperation explores the military view of nuclear damage in her book Whole World on Fire.  ¹¹ Blast is a sure result of a nuclear explosion. And military planners know how to consider blast effects when they evaluate whether a nuclear force is capable of destroying a target. Fires are collateral damage that may not be planned or accounted for. Unfortunately, that collateral damage may be capable of killing most of Earth’s population.

Climate and chemistry models have greatly advanced since the 1980s, and the ability to compute the environmental changes after a nuclear conflict has been much improved. Our climate and atmospheric chemistry work is based on standard global models from NASA Goddard’s Institute for Space Studies and from the US National Center for Atmospheric Research. Many scientists have used those models to investigate climate change and volcanic eruptions, both of which are relevant to considerations of the environmental effects of nuclear war. In the past two decades, researchers have extensively studied other bodies whose atmospheres exhibit behaviors corresponding to nuclear winter; included in such studies are the thermal structure of Titan’s ambient atmospheres and the thermal structure of Mars’s atmosphere during global dust storms. Like volcanoes, large forest fires regularly produce phenomena similar to those associated with the injection of soot into the upper atmosphere following a nuclear attack. Although plenty remains to be done, over the past 20 years scientists have gained a much greater understanding of natural analogues to nuclear-weapons explosions.

Substantial uncertainties attend the analysis presented in this article; references 5 and 8 discuss many of them in detail. Some uncertainties may be reduced relatively easily. To give a few examples: Surveys of fuel loading would reduce the uncertainty in fuel consumption in urban firestorms. Numerical modeling of large urban fires would reduce the uncertainty in smoke plume heights. Investigations of smoke removal in pyrocumulus clouds associated with fires would reduce the uncertainty in how much soot is actually injected into the upper atmosphere. Particularly valuable would be analyses of agricultural impacts associated with the climate changes following regional conflicts.

For any nuclear conflict, nuclear winter would seriously affect noncombatant countries. ¹² In a hypothetical SORT war, for example, we estimate that most of the world’s population, including that of the Southern Hemisphere/would be threatened by the indirect effects on global climate. Even a regional war between India and Pakistan, for instance, has the potential to dramatically damage Europe, the US, and other regions through global ozone loss and climate change. The current nuclear buildups in an increasing number of countries point to conflicts in the next few decades that would be more extreme than a war today between India and Pakistan. The growing number of countries with weapons also makes nuclear conflict more likely.

The environmental threat posed by nuclear weapons demands serious attention. It should be carefully analyzed by governments worldwide—advised by a broad section of the scientific community—and widely debated by the public.

Much of the research we have summarized is based on computations done by Charles Bardeen of casualties and the amount of soot generated in several hypothetical nuclear attacks. We thank our colleagues Georgiy Stenchikov, Luke Oman, Michael Mills, Douglas Kinnison, Rolando Garcia, and Eric Jensen for contributing to the recent scientific investigation of the environmental effects of nuclear conflict on which this paper is based. This work is supported by NSF grant ATM-0730452.

Brian Toon is chair of the department of atmospheric and oceanic sciences and a member of the laboratory for atmospheric and space physics at the University of Colorado at Boulder. Alan Robock is a professor of atmospheric science at Rutgers University in New Brunswick, New Jersey. Rich Turco is a professor of atmospheric science at the University of California, Los Angeles.