Bulletin IT IS 5 MINUTES TO MIDNIGHT of the Atomic Scientists ® Feature Bulletin of the Atomic Scientists 68(5) 66–74 ! The Author(s) 2012 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav Self-assured destruction: DOI: 10.1177/0096340212459127 The climate impacts of http://thebulletin.sagepub.com nuclear war Alan Robock and Owen Brian Toon Abstract A nuclear war between Russia and the United States, even after the arsenal reductions planned under New START, could produce a nuclear winter. Hence, an attack by either side could be suicidal, resulting in self- assured destruction. Even a ÒsmallÓ nuclear war between India and Pakistan, with each country detonating 50 Hiroshima-size atom bombsÑonly about 0.03 percent of the global nuclear arsenalÕs explosive powerÑas air bursts in urban areas, could produce so much smoke that temperatures would fall below those of the Little Ice Age of the fourteenth to nineteenth centuries, shortening the growing season around the world and threaten- ing the global food supply. Furthermore, there would be massive ozone depletion, allowing more ultraviolet radiation to reach EarthÕs surface. Recent studies predict that agricultural production in parts of the United States and China would decline by about 20 percent for four years, and by 10 percent for a decade. The environmental threat posed by even a small number of nuclear weapons must be considered in nuclear policy deliberations. Military planners now treat the environmental effects as collateral damage, and treaties currently consider only the number of weapons needed to assure destruction of opposing forces. Instead, treaties must call for further reductions in weapons so that the collateral effects do not threaten the continued survival of the bulk of humanity. Proliferation cannot be treated as a regional problem. A regional conflict has the potential to cause mass starvation worldwide through environmental effects. Keywords agriculture, climate, nuclear war, nuclear weapons, nuclear winter, ozone depletion, proliferation, smoke n the early 1980s, as the arms race could produce so much smoke that it pushed the worldÕs total tally of would block out the sun, plunging the I nuclear weapons beyond 50,000, world into sub-freezing temperatures, nuclear winter changed everything. killing virtually all crops, and condemn- Scientists, including us, made the world ing the planet to mass starvation. aware of the environmental impacts of This shocking possibility, and the the smoke that would be generated by intense debate surrounding it, brought fires in cities and industrial areas in the the insanity of the continuing nuclear event of a nuclear war. A war between arms race to the front burner. The scien- the United States and the Soviet Union tific message, from research done jointly Robock and Toon 67 by American and Soviet scientists, was et al., 1983). Research in the past few based on primitive computer models of years has shown that SAD exists even the climate system, yet the physics were for the nuclear states with much smaller clear: If you block out enough sunlight, it arsenals: Britain, France, China, Israel, gets cold, dark, and dry at the surface, India, and Pakistan (Mills et al., 2008; and the destruction of the ozone layer Robock and Toon, 2010; Robock et al., allows deadly ultraviolet radiation to 2007b; Toon et al., 2007b). Yet policy penetrate the atmosphere. The scien- makers seem to be unaware of this situ- tistsÕ models motivated US President ation or are keeping it to themselves. Ronald Reagan and Soviet General When the policy implications of SAD Secretary Mikhail Gorbachev are considered, elementary planetary (Hertsgaard, 2000; Zubok, 2000) to hygiene demands a much more rapid reach agreement on a nuclear arms reduction in nuclear arsenals than cur- reduction treaty. rently planned, along with intensified The principle of mutually assured efforts to prevent further nuclear destruction (MAD) has presumably proliferation. been part of the reason that nuclear war has been avoided since those ter- Nuclear arsenals rible days in August 1945, when more than 250,000 residents of Hiroshima The total number of nuclear weapons and Nagasaki were incinerated by worldwide peaked at about 70,000 in nuclear explosions. MAD posits that, if 1986, when Reagan and Gorbachev country A attacks country B, the retali- agreed to reduce them (Figure 1). ation from country B will be so devastat- Currently, there are about 15,000 war- ing that a first strike would be suicidal heads (Norris and Kristensen, 2010). (e.g., Elkind, 2012). New START, signed by Presidents Nuclear winter theory tells us that it Barack Obama and Dmitry Medvedev would be suicidal for country A to in 2010, requires each side, within launch a full-scale nuclear attack on seven years of the treaty coming into country B regardless of whether country force, to reduce deployed warheads to B responds in kind. The resulting cli- a maximum of 1,550. However, each mate changes, triggered by smoke, long-range bomber counts as one war- would be so damaging to food and head no matter how many warheads it water supplies that infrastructure break- has, and the treaty does not limit the down would assure starvation in the much larger number of weapons that attacking country as well as the rest of are in storage or reserve. While the the world. Call it self-assured destruc- United States and Russia possess the tion, or SAD. bulk of the global nuclear arsenal, as of We now know that SAD has existed 2010 France had 300 nuclear weapons, since the 1950s. This was made explicit the United Kingdom had 225, China had in the 1980s by the first work showing 180, Pakistan had between 70 and 90, the potential for nuclear winter after a Israel and India each had between 60 war between the United States and and 80, and North Korea was thought Russia (Aleksandrov and Stenchikov, to have fewer than 10 nuclear weapons 1983; Robock, 1984, 1989; Turco (Norris and Kristensen, 2010). By 2012, 68 Bulletin of the Atomic Scientists 68(5) Figure 1. A history of nuclear weapons Source: Norris and Kristensen (2010) the arsenals of India and Pakistan had suggestions by a graduate student in pol- grown by an estimated 20 weapons itical science that the burning of forests each (Kristensen and Norris, 2011, 2012). and grasslands could cause changes in Studies of the effects of nuclear weap- continental weather (Lewis, 1979). We ons began soon after their invention. and our colleagues then discovered These studies were largely based on that smoke from urban fires posed an the military view that damage to speci- even greater global hazard in the form fied targets had to be assured to main- of climate anomalies, defined as a tain a deterrent. Hence, only the most Ònuclear winter,Ó capable of causing certain and quantifiable effectsÑsuch the worldwide collapse of agriculture as blast and prompt radiationÑwere (Aleksandrov and Stenchikov, 1983; considered. Robock, 1984; Turco et al., 1983). A nuclear war would also threaten much Nuclear winter of the worldÕs population by causing societal chaos and the loss of transpor- That changed in 1982, when the journal tation and energy production. of the Royal Swedish Academy of Modern climate models not only Sciences, Ambio, published a ground- show that the nuclear winter theory is breaking article (Crutzen and Birks, correct, but also that the effects would 1982) that identified the issue of smoke last for more than a decade (Robock generated by nuclear-ignited forest fires et al., 2007a, 2007b) because of an unex- as a global concern, following earlier pected phenomenon: Smoke would rise Robock and Toon 69 Figure 2. Global precipitation and temperature changes as a function of smoke emitted Sources: Robock et al. (2007a, 2007b); Toon et al. (2008) to very high altitudesÑnear 40 kilo- within the range of arsenals planned meters (25 miles)Ñwhere it would be for the coming decade (Toon et al., protected from rain and would take 2008). For instance, the use of 4,000 more than a decade to clear completely. weapons (the rough total for US and As a consequence, the smokeÕs climate Russian arsenals in 2017 under New impacts would be more extreme than START), each with a yield of 100 kilo- once thought. For example, the new tons (a typical yield for submarine models show that a full-scale nuclear weapons, but at the low end for conflict, in which 150 million tons of most nuclear weapons), against urban smoke are lofted into the upper atmos- or industrial targets would produce phere, would drastically reduce precipi- about 180 million tons of soot. A single tation by 45 percent on a global average, US submarine carrying 144 weapons of while temperatures would fall for sev- 100-kiloton yield could produce 23 mil- eral years by 7 to 8 degrees Celsius on lion tons of smoke if these weapons were average and would remain depressed by used on densely populated Chinese 4 degrees Celsius after a decade (Robock cities. et al., 2007a). Humans have not experi- enced temperatures this low since the Regional nuclear war last ice age (Figure 2). In important grain-growing regions of the northern The United States and Russia are not the mid-latitudes, precipitation would only countries capable of wreaking decline by up to 90 percent, and tem- worldwide climate havoc. All of the peratures would fall below freezing and nuclear statesÑexcept North Korea, remain there for one or more years. with its relatively small arsenalÑif The number of weapons needed to involved in a nuclear war, have the initiate these climate changes falls destructive power needed to alter the 70 Bulletin of the Atomic Scientists 68(5) global environment (Robock et al., over the mid-latitudes of both hemi- 2007b).