The nuclear arsenals of Britain, France, China, Israel, India, and Pakistan are thought (
1–
3) to lie in the range of ~100 to 300 warheads each (
Fig. 1). Although the use of these weapons by any of these countries could produce a regional, and likely global, disaster, India and Pakistan are of special concern because of a long history of military clashes including serious recent ones, lack of progress in resolving territorial issues, densely populated urban areas, and ongoing rapid expansion of their respective nuclear arsenals. Here, we examine the possible repercussions of a nuclear war between India and Pakistan circa 2025 in which cities are one class of target, either by direct or collateral targeting. These repercussions have not been investigated previously. Because of the near-term regional effects of nuclear blast, thermal radiation, and prompt nuclear radiation, we find that perhaps for the first time in human history, the fatalities in a regional war could double the yearly natural global death rate. Moreover, the environmental stresses related to climate changes caused by smoke produced from burning cities could lead to widespread starvation and ecosystem disruption far outside of the war zone itself.
Nuclear arsenals of India and Pakistan
The United States and Russia account for around 93% of the world’s estimated 13,900 nuclear weapons. Seven other nuclear-armed nations are not bound by treaties that require them to divulge information, such as the number of strategic launchers and the number of warheads deployed on missiles, allowing estimates of the numbers of nuclear warheads and yields in their arsenals, but between them, the seven nations may now hold a total of 1200 warheads. As shown in
Fig. 1, India’s and Pakistan’s nuclear forces in 2019 each may contain 140 to 150 warheads, with a possible expansion to 200 to 250 warheads in each country by 2025 (
1,
3–
5). Britain (~215), France (~300), China (~270), and Israel (~80) have a similar number of weapons but have been maintaining relatively constant arsenals (
2). Estimates of the numbers of warheads possessed by India and Pakistan are based on the capacity of delivery systems that can be observed from remote sensing, rather than on the amount of enriched uranium and plutonium fuel that the countries may have produced.
Pakistan has nuclear-capable aircraft (F-16A/B and Mirage III/V) with ranges up to 2100 km, eight types of land-based ballistic missiles with possible ranges up to 2750 km, and two types of cruise missiles with ranges up to 350 km (
4,
6). All of India can be reached by the longest-range delivery systems. Since India has about 400 cities with more than 100,000 people (
7), Pakistan could potentially attack slightly more than one-third of all moderate- and large-sized cities in India with its current arsenal and more than two-thirds by 2025. Kristensen (
8) provides satellite images and locations for 10 facilities in Pakistan that may be locations of missile garrisons or nuclear-capable fighter-bombers. Pakistan is developing capabilities for sea-based nuclear weapons. According to Pakistani officials, Pakistan’s weapons are disassembled, and the parts are stored in several separate locations to reduce the possibility that terrorists might capture a usable weapon (
6). Using satellite images, expert studies, and local news reports, Kristensen and Norris (
2) identify nine locations in Pakistan where nuclear weapons may be stored.
On the basis of the sizes of weapons tested by Pakistan in 1998, it is estimated that the current weapons have yields of 5 to 12 kt (
6,
8). However, much higher yields are theoretically possible, which could greatly increase both casualties and global environmental effects. Pakistani scientists claim that all the weapons they tested in 1998 were uranium-based, boosted fission weapons that can have yields of hundreds of kilotons, without the need to develop more complex two-stage fission-fusion weapons. The 1998 tests did not demonstrate such high yields, and it is unknown whether Pakistan has been able to produce and deploy such high-yield warheads. Kristensen
et al. (
4) discuss the limited evidence of availability of tritium in Pakistan, which would be needed to allow boosted weapons to be produced. Advanced states are motivated to use boosted fission and two-stage weapons because they are smaller and lighter weight than fission weapons of the same yield, making them easier to deliver by missiles or aircraft. Boosted weapons also require less uranium or plutonium for a given yield.
Pakistan has produced tactical nuclear weapons for use on battlefields to counter the conventional weapons advantage of an invading Indian army. Their current arsenal probably includes 24 tactical weapons of unknown yield, but perhaps in the range of 5 to 12 kt (
6). Tactical and strategic weapons (which are used to attack targets distant from a battlefield) can overlap in yield. The yields of advanced boosted fission weapons can be adjusted across a large range from sub-kilotons to more than 100 kt. Tactical weapons may be less secure than strategic ones and may lower the threshold for nuclear weapons use (
6).
The 2018 arsenal of India is thought to contain 130 to 140 nuclear warheads, which may expand to 200 by 2025 (
5). Kristensen and Norris (
2) list five locations in India where nuclear weapons may be stored, but they estimate that there are others whose physical locations have not been identified.
India has nuclear-capable aircraft including Mirage 2000H and Jaguar IS/IB, with ranges up to 1850 km. It has four types of land-based ballistic missiles that have been deployed with ranges up to 3200 km and two others that are under development with ranges up to 5200 km. The range of these missiles allows India to reach all of Pakistan now, as well as all of China when its new missiles are deployed. India also has one deployed ship-based ballistic missile and two submarine-based missiles in development (
9). Since Pakistan has about 60 cities with more than 100,000 people, India could potentially attack each moderate- or large-sized city in Pakistan with two nuclear warheads using its current arsenal and four warheads if its arsenal grows to 250 weapons by 2025.
On the basis of the sizes of weapons tested by India in 1998, the current weapons may have yields of 12 to 40 kt. However, higher yields are possible. India claims to have tested a two-stage weapon in 1998, but the recorded yield did not indicate a successful design. Kanwal (
10), a retired Brigadier, examines the ideas of many Indian military leaders and suggests an Indian nuclear arsenal in 2011–2020 with 150 warheads, of which 134 have yields of 200 kt, whereas in 2021–2030, the arsenal might contain 200 warheads all of 200-kt yield. Although India does not need so many weapons to attack Pakistan, India is also concerned about China. China has about 360 cities with more than 100,000 inhabitants, so it is possible that India is sizing its nuclear forces in case of a nuclear conflict with China.
Scenario for war
Neither Pakistan nor India is likely to initiate a nuclear conflict without substantial provocation. India has declared a policy of no first use of nuclear weapons, except in response to an attack with biological or chemical weapons (
5). Pakistan has declared that it would only use nuclear weapons if it could not stop an invasion by conventional means or if it were attacked by nuclear weapons. Unfortunately, the two countries have had four conventional wars (1947, 1965, 1971, and 1999) and many skirmishes with substantial loss of life since the partition of British India in 1947. Therefore, the possibility of conventional war becoming nuclear is of concern.
Lavoy and Smith (
11) discuss three plausible scenarios for a nuclear war between India and Pakistan. India has conventional military superiority. India is also geographically much larger than Pakistan. One possible route to nuclear war involves a conventional conflict between India and Pakistan. If Pakistan perceived that India were about to successfully invade them, that would put pressure on Pakistan to launch its nuclear weapons before they were overrun by the superior conventional Indian forces. Another possibility for starting a nuclear conflict is that India or Pakistan could lose control of its command and control structures due to an attack on them by the other side or possibly an attack by terrorists from within India or Pakistan or from another country. In such a scenario, it is not clear who might be in control of the nuclear forces and what steps they might take. A third possibility for starting a nuclear conflict is that India or Pakistan might mistake an attack by conventional forces, or even military exercises, for an attack by nuclear forces.
To help evaluate the consequences of a nuclear conflict between India and Pakistan, table S1 provides a specific scenario for a war assumed to take place in 2025. Although this scenario has Pakistan first launching nuclear weapons, we do not mean to imply that they are more likely to do this than India. Because large numbers of weapons are assumed to be used by both sides, we would expect our results to be similar no matter how the war started. Moreover, we would expect the global outcomes projected here to apply equally well—with relevant recalibration for weapon sizes and targets and related smoke emissions—to any nuclear conflict between nuclear-armed states that involves a corresponding total yield detonated essentially in urban areas.
Many scenarios of an India-Pakistan conflict in 2025 are possible, ranging from no nuclear weapons deployed to as many as 500 nuclear weapons—many with yields above 100 kt—detonated. We chose the scenario outlined in table S1 as plausible following advice from a number of military and policy experts. In addition, the information presented in this paper and the Supplementary Materials can be used as a basis to compute the results for other scenarios. The main determinants of casualties and climate effects are the number of weapons used, the yield of the weapons, and the targets for the weapons, each of which is unknown in advance. The discussion in the following paragraphs exemplifies scenario factors that have been widely considered in the literature concerning conflicts between India and Pakistan, which might be varied in alternative scenarios including the role of the number of potential targets in choosing the sizes of arsenals; the characteristics, such as failure rates, of available weapons and delivery systems; the events that might lead to an escalating nuclear conflict; resolution of the Kashmir problem that might lessen the likelihood of a dangerous confrontation; the importance of urban targets in contributing to fatalities and climate effects owing to high population densities and fuel loadings; the difficulty of preventing a conflict from going nuclear because of the destabilizing effects of tactical nuclear weapons on both sides; the importance of Indian concerns about China in making it difficult for Pakistan and India to reduce their nuclear stockpiles; and the possible role of the disproportionate sizes of the countries, militaries, and populations of India and Pakistan in motivating the initial use of nuclear weapons.
In the scenario outlined in table S1, we assumed that each country would have 250 nuclear weapons in 2025 (
5,
9). We also adopted a highly simplified scenario in which only urban targets are considered, and these are attacked using airbursts. Many military or strategic targets in rural areas are likely to be attacked as well, but these would involve smaller populations and lower fuel loading, which would not add significantly to the near-term fatalities or smoke emissions. Therefore, we do not specifically track them in our scenario. Likewise, some targets, such as buried military facilities, might attract ground bursts, which would produce significant radioactive fallout and many additional fatalities—effects that are not explicitly considered in this work.
India has one of the largest conventional militaries in the world, with about 1.4 million active duty personnel. India has not deployed tactical nuclear weapons. Indian nuclear strategy requires that a significant number of high-yield bombs be held back in case China joins a war on the side of Pakistan (
10). Because Pakistan is a small country with only about 60 cities with more than 100,000 people, India would not need all of its 250 weapons to destroy Pakistan’s cities.
We assume that India will keep 100 nuclear weapons in its arsenal to deter China from entering the war. Chinese involvement would greatly amplify the destruction discussed below. As China expands its presence in Pakistan as part of the China-Pakistan Economic Corridor, which is an element of China’s broader “Belt and Road Initiative,” the odds of a Pakistani-Indian war spreading to China would appear to be increasing.
Of India’s 150 weapons that can be used against Pakistan, we assume that about 15% will fail. In this case, failure is primarily due to the weapons not being delivered or failing to explode. Most urban targets in Pakistan are so large that precise targeting is not needed to hit them. Therefore, our scenario suggests 125 weapons actually exploding. We further assume that there are 25 targets in Pakistan that are isolated military bases or industrial facilities located in regions with low populations and little combustible material. We do not include these in computing fatalities or environmental damage. Therefore, we assume that India has 100 strategic nuclear weapons to use on urban countervalue targets or military counterforce targets that are located within urban areas, such as military bases, industrial facilities, oil refineries, nuclear weapons facilities, and airports.
Pakistan also has one of the largest militaries in the world, with about half as many active duty personnel as India has. We assume that, in 2025, Pakistan will have 50 tactical weapons with yields of 5 kt to be used against an invading Indian army. We assume that 20% of these will fail or be overrun by the Indian Army. Many of these tactical weapons might be used in sparsely populated areas with little flammable material. Accordingly, we only consider the remaining 200 strategic weapons when computing fatalities or smoke created from fires. Of these 200 strategic weapons, we assume that 15% will fail to be delivered to the target but that the remaining 170 will be detonated over their targets. We further assume that 20 of these explosions will be over isolated military, nuclear, or industrial areas. The balance, 150 weapons, will thus be used against India’s urban countervalue targets and military counterforce targets located within urban areas.
The yields of modern Indian and Pakistani weapons are unknown and not easily constrained. India detonated a ~40-kt yield weapon in 1998, which, they claimed, was a two-stage bomb. Kanwal (
10) suggests that this design could produce 200-kt yields. Pakistan claimed that its weapons tested in 1998 used boosted fission. Possibly, these could also produce yields of 200 kt. Given the lack of reliable information about yield, we will explore the consequences of using strategic weapons with yields of 15, 50, and 100 kt.
Our scenario, as outlined in table S1, begins with a terrorist attack on the Indian government, similar to the one that occurred on 13 December 2001, but with massive fatalities among members of India’s government. As happened in January 2002, we assume that India and Pakistan mobilize their troops within a few weeks of the terrorist attack. Indian troops would likely be dispersed along the border and in Kashmir. Skirmishes would break out, resulting in deaths on both sides. Similar skirmishes happened in 2002 and now occur with regularity, most recently with a conflict in the Kashmir region beginning with a terrorist event on 14 February 2019. In the 2002 confrontation, the United States, Russia, and other countries intervened, eventually convincing India and Pakistan to end the confrontation, which had continued into the summer of 2002 until Pakistan agreed to control terrorist groups within its borders.
A crisis simulation exercise in Sri Lanka during 2013 organized by the U.S. Naval Postgraduate School and involving retired senior military and civilian analysts from India and Pakistan found that “a limited war in South Asia will escalate rapidly into a full war with a high potential for nuclear exchange” (
12). In our scenario, with the Indian government having been severely damaged, the Indian Army brings a number of tanks to the border and crosses into Pakistan and also crosses the Line of Control in Kashmir. On day 1 of the nuclear conflict, Pakistan uses 10 tactical atomic bombs with 5-kt yield inside its own borders with low air bursts against the Indian tanks (table S1).
The conflict continues on day 2 when Pakistan uses another 15 tactical weapons with 5-kt yield on the battlefield, whereas India detonates two air bursts against the Pakistani garrison in Bahawalpur and deploys 18 other weapons to attack Pakistani airfields and nuclear weapons depots, partially degrading Pakistani retaliatory capabilities. Nevertheless, on day 3, Pakistan responds with a barrage of nuclear ballistic and cruise missiles on garrisons, weapon depots, naval bases, and airfields in 30 locations in Indian cities (30 air bursts with 15- to 100-kt yield each) plus another 15 tactical bursts with 5-kt yield. India also uses 10 strategic weapons against Pakistani military bases on day 3. Because of panic, anger, miscommunication, and protocols, escalation cannot be stopped now. On days 4 to 7, cities in India are hit with 120 strategic weapons, and those in Pakistan are struck with 70 air bursts with 15- to 100-kt yield. In total, Pakistan’s urban areas are hit with 100 nuclear weapons using airbursts, and India’s urban areas are hit with 150 nuclear weapons using airbursts. In addition, Pakistan has used 40 tactical nuclear weapons successfully and 20 strategic weapons successfully on targets not in urban areas, whereas India has used 25 strategic weapons successfully on targets not in urban areas.
In previous simulations (
13,
14), all of the smoke produced during the nuclear exchange (as described below) was initially distributed uniformly over a broad area of India and Pakistan in January 1. Here, the smoke is injected above individual targeted urban regions (at the grid scale of the climate model) on the day of the detonations. Hence, the smoke injection varies in location and time in accordance with the evolution of the specific war scenario (e.g., as illustrated in fig. S1 for the scenario with 50-kt weapons). Further, in the present climate simulations, the smoke injection is assumed to start on 15 May and extend over the duration of the exchange (e.g., 6 days for the case in fig. S1). We did not evaluate the sensitivity of the results to the time of year the war begins. In (
14), it was found that a war initiated on 1 January or 15 May made little difference to the ultimate climatic effects. On the other hand, a war occurring in Northern Hemisphere summer might lead to enhanced impacts initially, as implied by earlier nuclear winter studies.