Other considerations. Although the probability curves used here to determine fatalities and total casualties caused by airbursts have been crudely “calibrated” by the experience during WWII, the current estimates for a modern regional conflict involves a number of uncertainties that are difficult to reduce. Among the principal unknowns are the target points and the number and size of weapons used. There are many possible scenarios for a war, which can only be speculated upon in advance. Moreover, local environmental conditions—winds, humidity, precipitation, and so on—must be assumed from a wide range of possibilities. However, the core factual basis for the present estimates has been established through independent studies cited above. One can also question the use of probability curves based on data from Hiroshima in determining the ability of people in 21st-century cities to survive a nearby nuclear explosion. The probability curves adopted here correspond to physical processes triggered by nuclear detonations (principally thermal radiation and blast) that are likely to be lethal even in modern buildings and settings (fig. S3). We do not differentiate casualties between mass fires in high winds, where conflagrations occur and fire spread is likely ( 24 , 25 ), and mass fires in low winds, where firestorms develop and limited spreading is expected, as there is insufficient information available to make such a distinction quantitatively. Fire spread is likely to increase the ultimate casualty number, but it may also allow more people to flee from the fire.

The fatalities and casualties outlined in table S2, Fig. 2 , and fig. S2 are computed, assuming airbursts used against urban targets, and that mass fires were started in each city, as occurred in Hiroshima. It is likely that some of the 45 strategic weapons assumed to be used against isolated military targets, and some of the 40 tactical weapons, will be exploded as ground bursts. The direct casualties and fatalities from ground bursts may be relatively small. However, ground bursts carry soil into the fireball, where very small radioactive particles can attach themselves to the dust particles. The relatively large dust particles are likely to fall out of the atmosphere within a few days, when the radioactive particles are still very dangerous. Large numbers of fatalities and casualties, potentially larger than the values given in table S2 and Fig. 2 , can be caused by exposure to this radioactive material within a few days of the explosions.

India would suffer two to three times more fatalities and casualties than Pakistan (table S2) because, in our scenario, Pakistan uses more weapons than India and because India has a much larger population and more densely populated cities. However, as a percentage of the urban population, Pakistan’s losses would be about twice those of India. In general, as shown in Fig. 2 , the fatalities and casualties increase rapidly even up to the 250th explosion due to the high population in India, whereas the rate of increase for Pakistan is much lower even for the 50th explosion.

During WWII, it is estimated that about 50 million people were killed, not considering those who died from disease and starvation over 6 years [e.g., ( 22 )]. Because of the dense populations of cities in Pakistan and India, table S2 shows that even a war with 15-kt weapons could lead to fatalities approximately equal to those worldwide in WWII and a war with 100-kt weapons could directly kill about 2.5 times as many as died worldwide in WWII, and in this nuclear war, the fatalities could occur in a single week. The world’s annual death rate from all causes is about 56 million people per year ( 23 ). Therefore, a war between India and Pakistan in our scenario with 15-kt weapons could kill the same number of people in a week as would die naturally worldwide in a year, effectively increasing the immediate global death rate by a factor of 50. A regional catastrophe would occur if India and Pakistan were to engage in a full-scale nuclear war with their expanding arsenals.

The population density in the target area affects the casualties, as well as the estimated fuel load. Table S3 lists the population and population densities for the densest urban areas attacked and the least dense. The population density in the target area usually declines as the yield increases because more suburban areas are included in the larger areas that are damaged by higher-yield weapons. In some cases, especially for low-population regions in Pakistan, the population may decrease with yield because different urban areas are chosen as the last target for differing yields. The highest population densities in table S3 are in the range of 37,000 to 80,000 people/km 2 . The population density in the area of the mass fire in Hamburg during WWII is estimated to have been about 20,000 people/km 2 ( 21 ). Similarly, the population density for the 150th weapon used on India is between 17,000 and 4900 people/km 2 and that for the 100th weapon used on Pakistan is between 8500 and 1600 people/km 2 . For reference, the population density of 1980s San Jose, California, a suburban city, was estimated to be about 1300 people/km 2 ( 16 ).

An even more marked increase in fatalities and casualties shown in Fig. 2 is due to increasing numbers of weapons and increasing yields. In Fig. 2 , the targets are graphed in decreasing order of the population density within the target area [refer to Methods and ( 16 )]. In the scenario outlined in table S1, Pakistan is assumed to use 150 strategic weapons on Indian urban targets and India is assumed to use 100 weapons on Pakistani urban targets. The calculations use the current population of India and Pakistan, not those for 2025, because it is not possible to forecast changing populations in individual target areas. Targets that are not in urban areas are not considered, but they would lead to additional fatalities and casualties. Table S2 lists the fatalities and casualties from the scenario given in table S1. About 50 million people would die if 15-kt weapons are used, almost 100 million if 50-kt weapons are used, and about 125 million if 100-kt weapons are used.

For 50 weapons of 15-kt yield exploding on both India and Pakistan, we find that the casualty estimates have risen relative to Toon et al. ( 16 ) from 22 to 27 million fatalities and from 44 to 45 million total casualties ( Fig. 2 ) due to the expanded urban populations in LandScan2016 ( 20 ) compared to LandScan2003 ( 18 ). These increases in fatalities and casualties are much less than the ~50% increase in urban population between 2000 and 2015 (fig. S4), suggesting that the size of the area that is urban increases more than the population density within the urban region.

We have recomputed the fatalities and casualties for the most recent Indian and Pakistani urban population counts using the approach discussed in Methods (see below) and in Toon et al. ( 16 ). Figure 2 illustrates the cumulative fatalities and cumulative total casualties as a function of the number of explosions and their yield derived using the LandScan2016 ( 20 ) population database. The corresponding fatalities calculated for individual targets are given in the Supplementary Materials (fig. S2). Cumulative fatalities (as well as overall casualties) are higher in India because it has a greater urban population. Fatalities are not linear with respect to the number, or yield, of the weapons used, because smaller cities (of which there are greater numbers) have lower populations, whereas higher-yield weapons on these targets would encounter low-density suburban or rural areas away from the city centers where lower-yield weapons concentrate most of their damage. Compared with India, Pakistani fatalities (fig. S2B) vary less with weapon yield above 15 kt, especially after the most densely populated 100 targets have been attacked, due to the relatively low populations of the remaining targets. India has many more moderate-sized cities than Pakistan, and fatalities continue to grow rapidly with yield above 15 kt, even for the 250th target (fig. S2A).

However, the urban populations of India and Pakistan are growing rapidly. The total urban populations of India and Pakistan are projected to increase by about 90% between 2000 and 2025, as shown in fig. S4 ( 19 ). The number of weapons possessed by the two countries is also thought to be increasing rapidly. By 2025, India and Pakistan could have three and five times, respectively, the number of weapons estimated by Toon et al. ( 16 ), and these would likely have higher yields than previously estimated ( 16 ).

Toon et al. ( 16 ) estimated that a war between India and Pakistan involving 50 nuclear weapons with 15-kt yield detonated as airbursts over the most densely populated cities of each nation would lead to about 22 million immediate fatalities and 44 million total casualties. Casualties include fatalities, severe injuries, and lesser injuries that can develop into more serious conditions, especially in the aftermath of a nuclear attack. At that time, it was assumed ( 16 ) that India had 85 (65 to 110) nuclear weapons and Pakistan had 52 (44 to 62), all with 15-kt yields. These casualty and fatality estimates were made using the LandScan2003 ( 18 ) population database together with the Gaussian probability distribution for fatalities and total casualties versus distance from ground zero shown in fig. S3 ( 16 ).

Regional nuclear war casualty estimates. Even one nuclear weapon explosion in a city can do a great deal of damage. For example, in the most densely populated urban area in Pakistan, a 15-kt airburst at the optimum height to maximize blast damage could kill about 700,000 people (fig. S2B) and injure another 300,000. With a 100-kt airburst over the same region, roughly 2 million fatalities and an additional 1.5 million nonfatal casualties could occur. Similar numbers would result for nuclear explosions over large Indian cities (fig. S2A).

About 20 min after the Hiroshima nuclear explosion, a firestorm grew from the many small fires ignited directly or indirectly by the explosion. On the basis of the inflowing winds, the mass fire fully developed 2 to 3 hours after the explosion and died down around 6 hours after the explosion ( 15 ). The energy released in this mass fire may have been more than 1000 times greater than the energy released in the nuclear bomb blast ( 16 ). The area burned was about 11.4 km 2 according to Glasstone and Dolan ( 15 ) and 13 km 2 according to Ishikawa and Swain ( 17 ).

World War II experience. A considerable amount of information about the direct effects of nuclear explosions was gained from the nuclear attacks on Hiroshima and Nagasaki during World War II (WWII) and through the approximately 520 above-ground nuclear test explosions conducted before the 1963 Treaty banning nuclear weapons tests in the atmosphere, in outer space, and under water. Much of this information is summarized by Glasstone and Dolan ( 15 ) for generic topographical situations. Of course, the nuclear weapons tests took place in areas with little combustible material to prevent large-scale fires, so the tests provide little information about ignition of fires and fire behavior in urban areas. The area destroyed in the nuclear explosions over Japanese cities in WWII was greater in Hiroshima (yield, ~15 kt) than in Nagasaki (yield, ~20 kt), probably due to differences in topography ( 15 ). The bombed portion of Nagasaki is located in a valley, whereas Hiroshima is located in a flat terrain. Therefore, in reality, not all nuclear explosions follow the simple equations relating yields to destruction derived for flat terrain.

Global climate perturbations due to nuclear conflict between India and Pakistan: Global catastrophe

Turco et al. (26, 27) showed that smoke from fires started in cities by nuclear explosions could cool Earth’s climate so much that agriculture would fail globally, leading to mass starvation. These early studies are supported by current climate model simulations (28, 29). Following a full-scale nuclear war involving the United States, Europe, Russia, and China using current arsenals, Toon et al. (30) estimated that 180 Tg (1 Tg = 1 Mt = 1012 g) of black carbon (BC) could be generated by a total of 4400 explosions of 100-kt weapons in urban areas, about half the arsenals of Russia, China, Britain, France, and the United States, assuming a yield that is lower than the average yield. Robock et al. (28), using a modern global climate model and assuming 150 Tg of smoke emitted in a superpower nuclear war [consistent with (30)], predicted a full-blown nuclear winter, with temperatures in mid-latitude grain-growing regions held below freezing for several years, destroying much of the world’s agricultural productivity.

Robock et al. (31) and Toon et al. (16) showed that a conflict between India and Pakistan with 50 weapons of 15-kt yield used by each side that generated 5 Tg of BC would produce large climate changes as supported by additional studies with other models (13, 14, 32, 33). Mills et al. (13, 14) also found large ozone losses. These climate changes are large enough to significantly damage agriculture worldwide (34–36). Here, we compute the smoke-generated and climate changes for the scenario outlined in table S1 for possible Pakistani and Indian nuclear arsenals of 2025.

Smoke and BC (soot) emission estimates. As discussed by Toon et al. (16, 30), we compute the amount and properties of smoke lofted to the upper troposphere in a sequence of steps, which are outlined below. 1. We first assume that the area subject to fire ignition for a 15-kt nuclear explosion is the same as that observed in Hiroshima (13 km2). For different yields, we take the area subject to fire as proportional to the yield (15). 2. The fuel loading in the fire zone is determined using a recent population database (20) by allocating to each person in the area burned 11,000 kg of flammable material consisting of construction materials, furnishings, clothing, asphalt roofs, plastics, fuels, and other flammables in their homes, places of work, schools, stores, gas stations, and so on. This fuel allocation is based on studies of the quantities of combustible materials present in the developed world in the 1980s (27), as well as limited specific assessments of actual fuel availability in the relatively densely populated urban area of WWII Hamburg, Germany (various estimates yielding 12 to 47 g/cm2), and more sparsely populated 1990 residential San Jose, California (1.34 g/cm2) (14, 37). Reisner et al. (38) introduced a new technique to determine fuel loads in the United States using census data for urban fuels. Our estimated fuel load for their sparsely populated target location near Atlanta (0.87 g/cm2) is within about 20% of their value. We have also used urban data from Washington, DC, to project a fuel load of 4 g/cm2, which agrees within a few percentage with the mass per person estimated in (27). Larson and Small (39) suggested that, within the inner 2-km radius of urban cores in three classes of American cities circa 1980, fuel loadings were 23, 41, and 63 g/cm2. Fuel loads in the major cities of Pakistan and India—summarized in table S3—are generally predicted to be in that same range. Unfortunately, less information is available to test these fuel values for Pakistan and India. Although Toon et al. (16) suggest that fuel burdens might be only half as large in the less developed world as in the developed world, this result is skewed by the inclusion of rural areas in the overall estimate. More directly, fuel loadings in Indian office buildings were found to be similar to those in British office buildings (40). In summary, considering the urban fuel loading models and data currently available, we conclude that there is a general, if somewhat tentative, consistency among the various studies mentioned above. 3. With regard to fire behavior, we assume that either (i) a firestorm would develop following a nuclear detonation in some number of cities, as happened at Hiroshima (and following the conventional bombing of Hamburg during WWII, for example), or (ii) a large-scale spreading conflagration would evolve in other urban areas, as happened with the conventional bombing of Tokyo and other cities during WWII. Further, in either case (i) or (ii), we assume that similar total quantities of fuel would eventually be consumed, and similar amounts of smoke would be lofted, after taking into account fire behavior (see the discussion below and also item 4). One characteristic that is not explicitly factored into our calculations is the difference in the period of time each type of fire would last, in general being longer for a conflagration as compared to a firestorm. This factor is not significant for the present global climate analysis. Following Glasstone and Dolan (15), firestorms result when “many fires merge to form a single convective column ... rising from the burning area” and with “strong, fire-induced radial (inwardly directed) winds … virtually everything combustible within the firestorm area is eventually destroyed.” On the basis of WWII experience with 69 mass fires in Japan and others in Germany, Glasstone and Dolan (15) conclude that firestorms can occur under the following conditions: a fuel loading of at least 4 g/cm2, half the structures in an area aflame simultaneously, ambient winds less than 3.6 m/s, and a minimum burning area of about 1.3 km2. For a 15-kt explosion, the minimum required fire ignition area is exceeded by roughly an order of magnitude. Table S3 also indicates that fuel loads needed to establish firestorms are generally exceeded, except in the case of large-yield weapons detonated over smaller Pakistani cities, where the requisite fuel load may be exceeded only within the city center. Moreover, it is clear that wind speeds may exceed the threshold for firestorm formation in some places at certain times. The WWII mass fires were generally much smaller than those that would be started by nuclear weapons considered here, so these firestorm conditions may not be applicable. Mass fires, consisting of numerous fires burning simultaneously over a large area, may grow into massive conflagrations instead of firestorms when winds are high. Conflagrations have moving fire fronts and can continue to spread as long as there is sufficient fuel. High winds can drive and intensify such fires. Conflagrations, unlike firestorms, may be started at a single ignition point and are commonly associated with large forest fires burning along a widening frontal line. Conflagrations in forests generally consume readily ignitable fuels, such as the crowns of the trees and forest undergrowth, but not living tree trunks [for example, see (41, 42)]. However, nuclear conflagrations in urban areas would likely be much more intense—owing to the many simultaneous starting points and heavy, highly flammable fuel loading. Moreover, given their propensity to spread outside of the initial ignition zone, conflagrations in urban settings could eventually consume as much fuel as a stationary firestorm, and perhaps more. Intense conflagrations are also observed to deposit smoke in the upper troposphere, and even the lower stratosphere, presumably by inducing strong pyroconvection at the fire front (41–44). Accordingly, both firestorms and conflagrations ignited by nuclear fireballs may ultimately have similar impacts on fuel consumption and, depending on fire intensity, smoke injection heights. 4. An important assumption in the present work is that all of the available fuel in the initial target-area fire zone is consumed when a firestorm develops. Although it is clear that this would be an upper limit, several factors mitigate toward this result. For example, accounts from WWII urban firestorms, such as those in Hiroshima and Hamburg, are consistent with nearly complete fuel consumption. Firefighting and suppression in nuclear attack zones would be effectively impossible, allowing fires to burn to completion. In addition, blast waves would release and disperse highly flammable fuels from storage tanks of all sizes, as well as piping and pipelines, and shatter and expose otherwise shielded fuels such as framing and building contents, leading to a more violent conflagration. Accordingly, the massive size and intensity of nuclear urban fires would most likely incinerate or pyrolyze a much larger fraction of available fuel than with smaller-scale localized combustion. On the other hand, it is also likely that in blast-damaged regions of a city center, some otherwise available fuel would be covered by rubble and would not completely burn. In a nuclear airburst, reinforced concrete structures within the 140-kPa (20 psi) blast overpressure region can be destroyed. However, if the height of burst is optimized to produce such a blast pressure, the area of such destruction for a 15-kt airburst represents roughly 14% of the area within the 400,000 J m−2 (~10 cal cm−2) fire ignition zone, and for a 100-kt blast, roughly 8%. Since, in most cases, the fuel density would be greater in the high overpressure zone, a larger fraction of the total fuel in the fire zone would be effectively buried—perhaps 20% or more, depending on the precise targets and weapons used. Owing to other sources of uncertainty in the fuel consumption estimation and the difficulty in determining a reasonable fuel sequestration factor due to rubble, we have ignored this effect in the current analysis until more information is available. In the case of conflagrations, we allow that 50% of the fuel within the initial ignition zone would be burned, but that fire spread outside the area affected by thermal pulse would effectively double the fuel eventually consumed (24, 25). These assumptions are not inconsistent with a significant impact on fuel consumption due to rubble formation in the blast zone. 5. We use an average BC (or soot) emission factor for burned fuel based on studies summarized by Turco et al. (26), yielding 0.02 g BC/g fuel burned. The less-absorbing organic carbon fraction of smoke that is typically mixed with the BC is ignored here. Other independent estimates of the total mass of emitted smoke may or may not include the mass of organic carbon in addition to BC. Accordingly, some care must be taken in comparing smoke estimates from different sources, as well as those quoted in assessments of impacts. The measured BC fraction of smoke can range widely from close to 90% to less than a few percentage, depending on the material burned and the flaming conditions that apply. For example, flaming combustion in forest fires may have a modest BC component, whereas the smoldering smoke has very little BC. On the other hand, burning fossil fuels have very high BC content. Our adopted average BC emission factor above has been derived by considering the range of fuel types and combustion conditions expected under nuclear attack scenarios (16, 26, 27, 45). 6. Considering several studies summarized in (16, 27), we assume that smoke generated by all nuclear bomb fires is initially injected into the 300- to 150-hPa pressure region of the upper troposphere (~9 to ~13.5 km). For latitudes from the equator to 35°N in the area of India and Pakistan, the cold point tropopause is in the 16- to 19-km altitude range (46). Therefore, we do not inject any smoke directly into the stratosphere. However, any smoke that might stabilize in the lower troposphere may be lofted too high. 7. On the basis of limited observations of pyrocumulus clouds (16), we assume that 20% of the BC is removed by rainfall during injection into the upper troposphere. Further smoke is rained out by the climate model before the smoke is lofted into the stratosphere by solar heating of the smoke. The fraction of the injected mass that is present in the model over 15 years is shown in fig. S5. In the first few days after the injection, 10 to 15% of the smoke is removed in the climate model before reaching the stratosphere. Therefore, in total, 30 to 35% of the smoke is removed by rainfall before it enters the stratosphere.

Uncertainty in smoke parameters. It is clear that imprecise knowledge regarding fire ignition and growth, and smoke composition, emission, and lofting, which are closely related to fuel loading and consumption, introduces significant uncertainty into all nuclear war climatic scenarios. Although all of these uncertain factors have been discussed extensively in the literature [e.g., (16, 26, 27, 47)], some of the key parameters have not yet been sufficiently constrained to provide final assurance in climate predictions. Moreover, the parameterization of nuclear-initiated fires used in this work is, by necessity, highly simplified and not specific to any particular potential target. Nevertheless, there has been sufficient vetting of the physics and chemistry of potential nuclear warfare—including actual experience with nuclear attacks on cities in addition to large-scale testing, studies of basic processes under laboratory and field conditions, and theoretical modeling and analysis at all relevant spatial and temporal scales—that we consider the results presented here to be the most realistic currently possible. There have been contrary assessments of the possible impacts of nuclear attacks on the global climate and environment. For example, most recently, a high-resolution modeling study (38) purported to demonstrate that a nuclear fire initiated by a 15-kt explosion in India or Pakistan would not loft enough smoke into the upper troposphere to contribute to widespread effects. However, that conclusion was based on a single simulation of such a detonation over a sparsely populated area about 8 km from the city center of Atlanta, Georgia. Significantly, the adopted fuel loading in the affected area (1.07 g/cm2 in the ignition zone) was about one order of magnitude smaller than that in the most sparsely populated urban area considered in the present study, i.e., the 100th city attacked in Pakistan (refer to table S3). Accordingly, the preliminary findings in (38) are not representative of the fires that need to be considered in assessing the potential impacts of a conceivable nuclear conflict having regional or global extent.

Smoke emission scenarios. Because our global climate model has limited spatial and temporal resolutions compared to the scales of individual nuclear blast zones and fires, the smoke emissions determined for various attack scenarios have been inserted into the climate model, consistent with model resolution and the smoke parameterization described earlier. Figure 3 shows the cumulative mass of BC that is inserted into the 300- to 150-hPa region (after rainout), with targets number-ordered by population. The BC emitted by individual targets is illustrated in fig. S2, which shows that, depending on yield, 10 to 25 targets in Pakistan and 15 to 125 cities in India could each produce more than 0.1 Tg of BC. Nuclear explosions in Pakistan generate far less BC than those in India for the same yield owing to the lower populations in Pakistan and the less dense urban areas after approximately the 10 most populated cities are considered. The total BC emitted from a war in which 50 weapons with a 15-kt yield are used to attack each country is about 8.7 Tg. Toon et al. (16) estimated that 6.6 Tg would be generated using the LandScan2003 database. The 30% increase in predicted BC emissions between 2003 and 2016 is due to the growing urban populations over this period, as shown in fig. S4 (by ~50% between 2000 and 2016). Fig. 3 Mass of black carbon (BC) injected into the atmosphere after prompt rainout (300- to 150-hPa region) for a given number of targets ordered by the population. Indian targets are given as dotted lines, whereas Pakistan targets are given as solid lines. Color coding designates yield. For the scenario in table S1 with 100 nuclear weapons used by India on Pakistan and 150 nuclear weapons used by Pakistan on India, there are (Fig. 3) 16.1 Tg of BC injected into the upper troposphere (11 from India and 5.1 from Pakistan) for yields of 15 kt, 27.3 Tg (19.8 from India and 7.5 from Pakistan) for 50-kt weapons, and 36.6 Tg (27.5 from India and 9.1 from Pakistan) for 100-kt weapons. These injection amounts are after considering the 20% removal of smoke by precipitation in the rising pyrocumulus. These BC injections are of considerable concern for the climate. The greatest known natural injection of BC into the stratosphere of ~6 × 10−3 Tg occurred during August 2017 from forest fires in British Columbia (42, 43). These fires led to radiatively forced rise of the smoke from 12 to above 23 km in about 2 months, radiatively driven hemispheric distribution of the smoke in the stratosphere, as well as temperature changes in the smoky layer due to heating by smoke, and ozone changes in the smoke due to vertical transport of low ozone air from the troposphere. The amount of BC in our 15-kt scenario is almost 3000 times more than in this forest fire injection. In forest fires, only a small fraction of the fuel is consumed. The values for fuel burned in the British Columbia forest fire (42) are 10 to 25% of the fuel load expected in boreal forests. In addition, the accessible fuel loading is substantially lower in forests than in urban areas. In total, the fuel burned in the urban areas in our 15-kt scenario is about 60 times greater than estimated for typical forest fires. Our BC emission fractions are also about 50 times greater than in the forest fire case because the materials burned in urban mass fires produce more BC than does burning organic forest material in line fires.

Climate simulations. We have conducted a series of simulations using a configuration of the National Science Foundation/Department of Energy (DOE) Community Earth System Model (CESM) that is similar to that used in (48) to simulate the climate and atmospheric chemistry after the asteroid impact that killed the nonavian dinosaurs and many other species 66 million years ago by igniting most of Earth’s land biomass and injecting about 15,000 Tg of BC into the upper atmosphere. A brief outline of this model is given in Methods. Figure 4 shows the visible wavelength aerosol optical depth and the changes in solar energy at Earth’s surface. There are results for six BC injections including the three scenarios defined in table S1 using possible yields of 15-, 50-, or 100-kt weapons, resulting in BC injections of 16.1, 27.3, and 36.6 Tg, respectively. The 5-Tg case is based on estimates made in 2008 for Indian and Pakistan arsenals at that time (13, 14, 16, 31–36). The 46.8-Tg case would result from 250 weapons of 100-kt yield used against urban areas in India and Pakistan, which is likely an upper limit for a conflict between India and Pakistan, unless they have weapons with yields that are higher than 100 kt. By way of contrast with earlier nuclear winter scenarios, the green curves in Fig. 4 correspond to an injection of 150 Tg of BC over Russia and the United States, based on a scenario for a major nuclear war between these two superpowers (28–30). Fig. 4 Changes in amount of atmospheric aerosol and of solar energy at Earth’s surface after nuclear exchange. Visible wavelength aerosol optical depth versus time (A) and the change in shortwave surface energy relative to normal as a function of time (B) for varying amounts of BC emitted in the nuclear exchange. Color coding designates the BC injection. The primary mechanism leading to climate changes after a nuclear conflict is absorption of solar radiation by smoke from burning cities. The direct solar beam is diminished in proportion to the inverse of the exponential of the aerosol optical depth. The initial global average aerosol optical depths range from less than 0.1 to greater than 2 for the cases considered in Fig. 4A. After 9 years, the 150-Tg optical depth is about equal to the initial optical depth of the 5-Tg case. The optical depth in the 150-Tg case is lower than some of the other cases after 10 years because the larger BC emission has led to the formation of larger particles via coagulation, and these have been more rapidly removed by sedimentation. The downward solar energy (Fig. 4B) reaching the surface declines in proportion to the increase of optical depth. The solar energy reaching the surface before the war is about 160 W m−2. The fractional energy losses in Fig. 4B range from ~20 to 40% (~32 to ~64 W m−2) for our conflict scenario (table S1) over the range of possible yields of 15-, 50-, or 100-kt weapons. For reference, the maximum average solar radiative loss following the Mt. Pinatubo volcanic eruption in 1991 was about 4 W m−2 (49), whereas the radiative reductions proposed for climate geoengineering schemes to offset global warming due to greenhouse gas emissions are of a similar magnitude. In addition, by comparison, a full-scale nuclear war between Russia and the United States might produce a peak solar radiation loss at the surface of ~75% (120 W m−2) (28). With a loss of solar radiation at the surface, the surface cools and evaporation, convection, and precipitation are reduced. Figure 5A indicates simulated global average precipitation losses from 15 to 30% for our scenario over the range of possible yields of 15-kt (16.1 Tg of BC), 50-kt (27.3 Tg), or 100-kt (36.6 Tg) weapons. A war between the United States and Russia could reduce precipitation by nearly 60%. Figure 5B shows that the global average surface temperature drops between 1.25° and 6.5°C over several years for our scenario. These perturbations reach their peak about 3 years after the conflict and are near the peak value for about 4 years. It takes more than a decade for temperatures and precipitation to return to normal. The Last Glacial Maximum, 20,000 years ago, had a global temperature decline of about 3° to 8°C relative to preindustrial temperatures, but these temperature decreases persisted for thousands of years (50). Fig. 5 Temporal variation in global precipitation and temperature following a nuclear conflict. (A) Global average precipitation and (B) global average temperature, expressed as a percentage of control run values. Color coding designates the amount of BC emitted. The vertical purple bar represents the range of temperatures during the height of the Last Glacial Maximum about 20,000 years ago. Illustrations of postconflict temperature and precipitation anomalies over the major landmasses and oceans are presented in figs. S6 and S7. The average global land temperature (fig. S6B) declines by as much as ~4° to ~8°C for the present war scenario over the range of yields between 15 and 100 kt (BC emissions between 16 and 36 Tg). In contrast, annual average temperature decreases over land had been predicted to reach ~18°C for a full-scale nuclear winter. In the current scenario, globally averaged ocean surface temperatures (fig. S6A) decline by ~1 to almost 3°C for the range of yields assumed, whereas predicted anomalies reached ~6°C in the case of a superpower nuclear conflict. The ocean temperatures are expected to decrease in a layer extending roughly to the average thermocline depth [for example, as discussed in (48) for even larger smoke injections inferred at the geologic boundary marking the extinction of the dinosaurs]. Although cooling and precipitation reduction are global in scale, these changes vary regionally to a large extent. Postconflict temperature anomalies over land and ocean surfaces for the 50-kt (27.3 Tg) scenario are illustrated in fig. S6C, showing that cooling of the Northern Hemisphere continents is stronger than that of the Southern Hemisphere; temperature drops greater than 10°C occur across North America and Europe north of about 30° latitude, with cooling up to 5°C over all continents; ocean temperatures decrease in many regions by an average of 5°C, with greater reductions in the Northwest Atlantic. Similar spatial patterns of temperature anomalies were found for larger and smaller soot injections. Postconflict precipitation anomalies over land and oceans for the 50-kt (27.3 Tg) scenario are illustrated in fig. S7. Increased precipitation occurs in some areas, mainly because these regions are currently under the descending branches of the Hadley circulation. The descending air normally suppresses rainfall, but global cooling weakens the Hadley circulation, leading to more rainfall on average. Of greater significance to surviving populations are the large decreases in rainfall predicted over densely populated regions such as India and central China where precipitation almost ceases. The U.S. Northeast and Midwest lose more than 50% of their rainfall. Although not illustrated here, and contrary to the response of temperatures at the surface, stratospheric air temperatures increase sharply because of sunlight absorption by injected BC (31–33). Such heating has previously been shown to cause large depletions of stratospheric ozone (13, 14). It might be worth noting at this point that climate geoengineering proposals are based on reducing solar insolation by injecting stratospheric particles—such as sulfuric acid aerosol—that mainly scatter sunlight rather than absorbing it specifically to avoid the heating and ozone loss problem. However, sulfuric acid particles may still lead to ozone depletion through surface-catalyzed chemical reactions [e.g., (13, 14)].