To illustrate why choices that lead to extinction events are potentially special among ethical problems, we consider the following thought experiment. Amy has been given three options:

1. Peace;

2. Kill 99% of the population; or

3. Kill 100% of the population.

This is a trivial choice for Amy, as she, like most people, would intuitively choose to maximise the number of lives spared, selecting option 1. The controversy inherent to this problem is found in determining which pair of options have the greatest ethical difference: options 1 and 2; or options 2 and 3? The relative number of lives saved is greatest between options 1 and 2; however, Derek Parfit, who posed this problem [1], believes that the greatest ethical difference is between options 2 and 3. Option 3, as an extinction-level event, necessarily prevents all future lives that the population might ever produce.

Amy might be horrified by the magnitude of potential lives that will never exist under option 3, conservatively estimated as 10 million billion people [2]. Reducing the risk of extinction by a millionth of 1% is worth more than 100 million future human lives [2]. If Amy values the lives of her descendants, or other humans, or even all non-human life lost collaterally, Amy might feel an enormous imperative to reduce the risk of human extinction.

One frighteningly possible extinction-level threat is nuclear war. The aim of this article is to summarise the current literature on nuclear war risk, consequences and deterrence alternatives so that the reader can decide if the utility of nuclear weapons justifies their current proliferation.

Likelihood of nuclear war

The yearly likelihood of nuclear war has never been higher [3,4]. Past nuclear arms control agreements have expired or been abandoned, diplomatic tensions between nuclear states are high and nuclear weapon research and deployment continues unabated [4]. As of May 2019, 13 thousand nuclear weapons are estimated to exist, about two thousand of which are on high alert for immediate deployment in the US, Russia, Britain and France alone [5]. The United States, under the Trump administration, withdrew from the Joint Comprehensive Plan of Action (JCPOA), intended to curb Iran’s nuclear threat, and withdrew from the INF Treaty banning intermediate-range missiles [3]. Both treaties were hard-won attempts at nuclear de-escalation [3]. The Biden administration has expressed desire to rejoin the JCPOA, but has yet to act [3]. Russia, under Putin, and the United States, under the Obama, Trump and Biden administrations, continue to modernise their nuclear arsenals, justified by claims of escalation on the other side [3]. India and Pakistan expand their arsenals amid growing tensions [3]. Despite increased negotiations with North Korea in recent years, no meaningful progress has been made in denuclearising the volatile regime [3].

The primary use of nuclear weapons is to act as a deterrent against foreign nuclear attack—if they strike, we strike—and in this capacity they excel [6]. The possibility of retribution makes initial nuclear strikes against a nuclear-armed enemy generally inadvisable. Most nuclear states, including the US and Russia, have not ruled out launching a preemptive nuclear strike [4]. Few nuclear states have adopted an explicit ‘no first use’ policy [4]. However, nuclear weapons are ineffective deterrents against non-nuclear attack and adopting ‘no first use’ does not impact their nuclear deterrent capacity [6]. The blow-back effect of nuclear weapons on the firing state is likely so catastrophic [7–10] that it is doubtful that any non-nuclear aggression would render viable a nuclear response [6].

Even if sustained nuclear deterrence policy is successful, nuclear arsenals are vulnerable to human error, technology failure and cybersecurity threats from terrorists and rival states [4]. Dozens of near-miss nuclear launches and accidents have been recorded, many caused by human error or sensor malfunction [11]. Nuclear command chains remain subject to human error and escalation. Current and future nuclear weapons are conceivably targetable by cyber attack and there exists no unilateral policy between nuclear states for the response to such an event [4]. Stockpiles of weapons-ready nuclear materials are potentially vulnerable to terrorists [12], who conceivably have no deterrence obligations.

To reduce human error and increasing strategic capabilities, nuclear-armed states are considering acceding control of their arsenals to presumably less-fallible artificial intelligence (AI), which raises unique concerns. AI is vulnerable to hacking, alteration of training data, input manipulation and unprecedented events [13]. Additionally, AI may allow improved identification of enemy nuclear weapon launchers, preemptive striking of which would remove the capacity of the enemy to retaliate [13]. Thus, AI may reduce the capacity of nuclear deterrence and decrease global nuclear stability [13]. A counterargument is that AI will remain incapable of solving sufficiently complex defence problems with limited data [14]. However, even perception of AI’s capacity to remove nuclear deterrents, however untrue, might be enough to cause nuclear threat escalation [13]. Therefore, introduction of AI to nuclear weapon command chains could be significantly destabilising to nuclear safety [13].

In 2015, Russia announced the development of an autonomous underwater nuclear drone, Oceanic Multipurpose System Status-6, which would deliver a large nuclear warhead at 100 km/h [13]. Status-6 is planned to circumvent the difficulties associated with remote guidance of such a high-speed underwater device using AI and conceivably could bypass antisubmarine defences, introducing asymmetry and thus instability to nuclear deterrence [13].

Consequences of nuclear war

Nuclear weapons possess enormous destructive potential. A nuclear war between the US and Russia would see 150-450 million people dead from the initial blasts and radioactive fallout [15]. A smaller regional nuclear war between India and Pakistan with projected 2025 nuclear weapon stockpiles would initially kill 50-125 million people [16]. An adequate humanitarian response to a nuclear war is currently impossible [17–20].

Nuclear weapons may may implicitly violate international humanitarian law, which requires warring parties to distinguish military and civilian targets and avoid harming the latter [20]. In 2011, the International Red Cross and Red Crescent Movement’s Council of Delegates stated, ‘it is difficult to envisage how any use of nuclear weapons could be compatible with the requirements of international humanitarian law, in particular the rules of distinction, precaution and proportionality’ [21].

Nuclear strikes likely cause widespread fires, which release soot into the atmosphere [22], potentially blocking sunlight and lowering global temperature. A nuclear war between the US and Russia with current strategic arsenals is theorised to result in nuclear winter, a dramatic, long-term reduction in global temperatures caused by 150 million tons of smoke injected into the upper atmosphere [7]. After nuclear exchange, continental North America and Eurasia temperatures are predicted to drop by at least 20°C for up to three years [7]. Summer temperatures below −4°C during years two and three post-war would render agriculture impossible for the US, Russia, Eastern Europe and much of China [7]. Global temperatures are predicted to remain 0.5-1°C below normal 15 years after nuclear exchange [7].

Some humans might survive nuclear winter, but this is not guaranteed [9]. If fewer than the minimum viable human population, estimated between 150 and 40 000 people, can gather in one place, human extinction is reasonably likely, especially with the added pressures of nuclear winter, lost infrastructure and potentially damaged agricultural land [23].

A much smaller regional nuclear war between India and Pakistan, involving the release of 250 nuclear weapons, would release 16-36 million tons of soot [16]. This would reduce global temperatures by 2-5°C over 10 years and decrease world agricultural output by 15-30% [16], potentially starving over one billion people [24]. A follow-up study [25] to an older study [26] simulating the release of 100 nuclear weapons between India and Pakistan found that the subsequent fires would not loft sufficient soot into the upper atmosphere to cause significant global cooling. However, the follow-up study has been criticised for simulating unrealistic urban infrastructure, fire type and atmospheric effects [27].

In addition to the global cooling effects, nuclear war would cause significant and enduring environmental damage. Global nuclear war is predicted to cause a 75% reduction in global ozone coverage, resulting in UV Index values over 35 in tropical regions for four years [28]. Regional nuclear war is predicted to cause a 25% reduction in global ozone with a recovery time of 12 years [28]. Global cooling from nuclear war is expected to generate an artificial El Niño event, lasting up to seven years and decreasing Pacific phytoplankton growth by 40%, with massive implications for marine life [29].

Nuclear threat reduction

The US and Russia each have 1600 nuclear warheads deployed and a further 4000 warheads stockpiled [5]. Such quantities of nuclear weapons far exceed the pragmatic limit required to achieve nuclear deterrence in the worst case scenario, 100 warheads per state actor [8]. If the US launched 1000 nuclear weapons at foreign targets, the best case outcome with no retaliation would still leave 140 thousand US citizens dead by starvation due to global agricultural loss [8].

Because of the massive repercussions of nuclear war on the aggressing nation, nuclear deterrence of conventional attacks lacks credibility [6]. A cap of 50 nuclear weapons worldwide is required to reasonably guarantee safety from nuclear winter [9]. The goal of nuclear deterrence might be accomplished with alternative weapons of mass destruction, such as non-contagious biological weapons, high-altitude neutron bombs and nuclear electromagnetic weapons [9], which, while similarly catastrophic, avoid the extinction risk posed by nuclear weapons. It should be noted that contagious biological weapons, such as weaponised smallpox or Ebola, are of comparable extinction risk to nuclear winter [9].

In 2017, the UN overwhelmingly voted to adopt the Treaty on the Prohibition of Nuclear Weapons (TPNW), which, as of 22 January 2021, outlawed the use, possession and development of nuclear weapons worldwide [30]. No nuclear weapon possessor states voted on the treaty [6]. The TPNW currently has 86 nation signatories and 56 states parties that have ratified the treaty [31].

Critics of the TPNW believe that a nuclear weapons ban will have little effect compared to a gradual disarmament negotiation process, particularly if no nuclear weapon states sign the treaty [21,32]. However, others believe that banning nuclear weapons might help delegitimise their use in warfare or deterrence, noting, ‘historically, the prohibition of a weapon has usually preceded its elimination’ [21]. Strengthening the norm against nuclear weapon use could reduce nuclear war risk by ‘raising the threshold required for states to decide to use nuclear weapons’ [11].

A global campaign for non-nuclear NATO members to sign the TPNW, if effective, will reduce the deterrence requirements of the US [6]. Australia, a NATO member, has not yet signed the treaty [31]; despite this, 38 Australian cities, including Canberra, Melbourne and Sydney, but not Brisbane, have endorsed the treaty [33]. The current federal opposition party, the Australian Labor Party, has resolved to sign the treaty [34].

It is up to the reader to decide if the extinction-level risks posed by nuclear weapons, arguably a paramount threat of our time, justify their existence as deterrents in an increasingly unstable global landscape when diplomatic solutions and alternative, winter-safe, deterrents exist; if not, what steps must be taken to reduce the nuclear threat to all current and future life?

References

[1] D. Parfit, Reasons and Persons (Oxford: Clarendon Press, 1984).

[2] N. Bostrom, Existential Risk Prevention as Global Priority, Global Policy 4, 15 (2013).

[3] J. Mecklin, This is your COVID wake-up call: It is 100 seconds to midnight, Bulletin of the Atomic Scientists (2021).

[4] House of Lords Select Committee on International Relations, Rising nuclear risk, disarmament and the Nuclear Non-Proliferation Treaty, No. 338, UK Parliament, 2019.

[5] H. M. Kristensen and M. Korda, Status of World Nuclear Forces, Federation of American Scientists, 2021.

[6] S. Fetter and J. Wolfsthal, No First Use and Credible Deterrence, Journal for Peace and Nuclear Disarmament 1, 102 (2018).

[7] J. Coupe, C. G. Bardeen, A. Robock, and O. B. Toon, Nuclear Winter Responses to Nuclear War Between the United States and Russia in the Whole Atmosphere Community Climate Model Version 4 and the Goddard Institute for Space Studies ModelE, Journal of Geophysical Research: Atmospheres 124, 8522 (2019).

[8] J. M. Pearce and D. C. Denkenberger, A National Pragmatic Safety Limit for Nuclear Weapon Quantities, Safety 4, 25 (2018).

[9] S. D. Baum, Winter-safe Deterrence: The Risk of Nuclear Winter and Its Challenge to Deterrence, Contemporary Security Policy 36, 123 (2015).

[10] A. Robock and O. Brian Toon, Self-assured destruction: The climate impacts of nuclear war, Bulletin of the Atomic Scientists 68, 66 (2012).

[11] S. D. Baum, R. de Neufville, and A. M. Barrett, A Model for the Probability of Nuclear War, No. 18-1, Global Catastrophic Risk Institute, 2018.

[12] Nuclear Threat Initiative, The Nuclear Threat, (2015).

[13] E. Geist and A. J. Lohn, How Might Artificial Intelligence Affect the Risk of Nuclear War?, RAND Corporation, 2018.

[14] R. Loss and J. Johnson, Will Artificial Intelligence Imperil Nuclear Deterrence?, War on the Rocks (2019).

[15] Future of Life Institute, The Risk of Nuclear Weapons, (2016).

[16] O. B. Toon, C. G. Bardeen, A. Robock, L. Xia, H. Kristensen, M. McKinzie, R. J. Peterson, C. S. Harrison, N. S. Lovenduski, and R. P. Turco, Rapidly expanding nuclear arsenals in Pakistan and India portend regional and global catastrophe, Science Advances 5, eaay5478 (2019).

[17] International Red Cross and Red Crescent Movement, Nuclear weapons – an intolerable threat to humanity, (2018).

[18] National Academies of Sciences, Engineering, and Medicine, Exploring Medical and Public Health Preparedness for a Nuclear Incident: Proceedings of a Workshop (The National Academies Press, Washington, DC, 2019).

[19] R. P. Gale and D. G. Nathan, Physicians and the Threat of Nuclear War, The ASCO Post (2019).

[20] N. Wells, Nuclear Weapons and the Future of Humanity, Australian Institute of International Affairs, 2017.

[21] J. Borrie, T. Caughley, T. G. Hugo, M. Løvold, G. Nystuen, and C. Waszink, A Prohibition on Nuclear Weapons, United Nations Institute for Disarmament Research, 2016.

[22] P. Yu, O. B. Toon, C. G. Bardeen, Y. Zhu, K. H. Rosenlof, R. W. Portmann, T. D. Thornberry, R.-S. Gao, S. M. Davis, E. T. Wolf, J. de Gouw, D. A. Peterson, M. D. Fromm, and A. Robock, Black carbon lofts wildfire smoke high into the stratosphere to form a persistent plume, Science 365, 587 (2019).

[23] S. Baum, S. Armstrong, T. Ekenstedt, O. Häggström, R. Hanson, K. Kuhlemann, M. Maas, J. Miller, M. Salmela, A. Sandberg, K. Sotala, P. Torres, A. Turchin, and R. Yampolskiy, Long-term trajectories of human civilization, Foresight 21, 53 (2019).

[24] L. Xia, A. Robock, M. Mills, A. Stenke, and I. Helfand, Decadal reduction of Chinese agriculture after a regional nuclear war, Earth’s Future 3, 37 (2015).

[25] J. Reisner, G. D’Angelo, E. Koo, W. Even, M. Hecht, E. Hunke, D. Comeau, R. Bos, and J. Cooley, Climate Impact of a Regional Nuclear Weapons Exchange: An Improved Assessment Based On Detailed Source Calculations, Journal of Geophysical Research: Atmospheres 123, 2752 (2018).

[26] A. Robock, L. Oman, G. L. Stenchikov, O. B. Toon, C. Bardeen, and R. P. Turco, Climatic consequences of regional nuclear conflicts, Atmospheric Chemistry and Physics 7, 2003 (2007).

[27] A. Robock, O. B. Toon, and C. G. Bardeen, Comment on “Climate Impact of a Regional Nuclear Weapon Exchange: An Improved Assessment Based on Detailed Source Calculations” by Reisner et al., Journal of Geophysical Research: Atmospheres 124, 12953 (2019).

[28] C. G. Bardeen, D. E. Kinnison, O. B. Toon, M. J. Mills, F. Vitt, L. Xia, J. Jägermeyr, N. S. Lovenduski, K. J. N. Scherrer, M. Clyne, and A. Robock, Extreme Ozone Loss Following Nuclear War Results in Enhanced Surface Ultraviolet Radiation, Journal of Geophysical Research: Atmospheres 126, e2021JD035079 (2021).

[29] J. Coupe, S. Stevenson, N. S. Lovenduski, T. Rohr, C. S. Harrison, A. Robock, H. Olivarez, C. G. Bardeen, and O. B. Toon, Nuclear Niño response observed in simulations of nuclear war scenarios, Communications Earth & Environment 2, 18 (2021).

[30] United Nations, Treaty on the Prohibition of Nuclear Weapons, 2017.

[31] International Campaign to Abolish Nuclear Weapons, Signature/ratification status of the Treaty on the Prohibition of Nuclear Weapons, (2021).

[32] M. Costlow, The Nuclear Ban Treaty is Way Off Target, War on the Rocks (2017).

[33] International Campaign to Abolish Nuclear Weapons, Australian cities behind the ban, (2021).

[34] International Campaign to Abolish Nuclear Weapons, Australia, (2021).