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Extinction of the human species: What could cause it and how likely is it to occur?

Published online by Cambridge University Press:  07 March 2025

Sean ÓhÉigeartaigh Open the ORCID record for Sean ÓhÉigeartaigh [Opens in a new window]
Sean ÓhÉigeartaigh*
Affiliation:
University of Cambridge, Cambridge, UK
*
Corresponding author: Sean ÓhÉigeartaigh; Email: so348@cam.ac.uk
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Abstract

The possibility of human extinction has received growing academic attention over the last several decades. Research has analysed possible pathways to human extinction, as well as ethical considerations relating to human survival. Potential causes of human extinction can be loosely grouped into exogenous threats such as an asteroid impact and anthropogenic threats such as war or a catastrophic physics accident. In all cases, an outcome as extreme as human extinction would require events or developments that either have been of very low probability historically or are entirely unprecedented. This introduces deep uncertainty and methodological challenges to the study of the topic. This review provides an overview of potential human extinction causes considered plausible in the current academic literature, experts’ judgements of likelihood where available and a synthesis of ethical and social debates relating to the study of human extinction.

Topics structure

Topic(s)

Human factorsMass extinction

Subtopic(s)

Existential riskSocietal collapse

Keywords

anthropoceneanthropogenic impactsclimate changeexistential risksocietal collapse

Information

Type
Review
Information
Cambridge Prisms: Extinction , Volume 3 , 2025 , e4
Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
© The Author(s), 2025. Published by Cambridge University Press

Impact statement

Human extinction may seem unlikely, but cannot be ruled out as a possibility. We now know that there have been cataclysmic events in the past that might have wiped out humans were those events to occur now. Novel threats to human civilisation are now posed by our own activities, both in the form of climate change and environmental degradation and in the form of powerful weapons such as nuclear weapons and emerging technologies such as artificial intelligence (AI) and engineered biology. These risks are the subject of a growing body of research. By reviewing this scholarship, and the evidence for claims that have been made regarding various risks, this review aims to inform the reader of the current state of knowledge in the field. The review aims to be a resource for both academic readers and decision-makers on the state of knowledge regarding a variety of risks – how likely they are and how robust the evidence is. This synthesis will aid experts and decision-makers in assessing how much weight to give concerns regarding human extinction and how to prioritise efforts to reduce or mitigate these risks.

Introduction

The last half-century has seen growing scientific awareness of natural phenomena such as meteor impacts linked to cataclysmic impacts on Earth, some resulting in mass species extinctions. Scientific evidence has also accumulated around the potentially disastrous consequences of climate change, unsustainable resource use, and the development of powerful technologies, leading scholars to raise concerns that homo sapiens may also face imminent extinction threats (Rees, Reference Rees2003; Bostrom, Reference Bostrom2013; Ord, Reference Ord2020).

As a species, humans have many advantages in the face of such threats. Any catastrophe would need to eliminate virtually all humans across the wide range of environments we inhabit on Earth, leaving remaining humans unable to sustain themselves or reproduce (Avin et al., Reference Avin, Wintle, Weitzdörfer, Ó hÉigeartaigh, Sutherland and Rees2018). Our ability to learn about and respond to the threat would also need to be insufficient. However, there remain potential catastrophes sufficiently large that humans would presently be unable to avoid or defend against them. Developments in science and technology in the last century have made possible the creation of powerful weapons such as nuclear bombs and the tools to coordinate around their wide-scale use. It is likely that further technological breakthroughs will make even more devastating weapons possible and allow them to be used more effectively. Furthermore, it is possible that developments in science and technology may pose risks of accidental disaster or have unintended consequences, such as accelerating humanity’s degradation of its environment to the point of uninhabitability (Rees, Reference Rees2003).

Scholarship on human extinction risk frequently involves discussion of two other closely related terms. Existential risks are defined as threatening the premature extinction of Earth-originating intelligent life or the permanent and drastic destruction of its potential for desirable future development (Bostrom, Reference Bostrom2013).Footnote 1 This category includes extinction risks but also irrecoverable collapse short of full extinction, as well as some more esoteric dystopian scenarios. For the purpose of this review, I will refer to existential risk scenarios that relate to extinction risk, and so the term may be used interchangeably. Global catastrophic risks lack a single crisp definition, but generally refer to events or outcomes that could inflict serious damage to human well-being at a global scale, potentially threatening human civilisationFootnote 2 (Bostrom and Cirkovic, Reference Bostrom and Cirkovic2011). Some but not all global catastrophes would lead to human extinction.

The last two decades have seen a growth of scholarship on this class of risksFootnote 3 including a number of recent books: The Precipice (Ord, Reference Ord2020) and edited volumes The Era of Global Risk (Beard et al., Reference Beard, Richards and Rios Rojas2023) and An Anthology of Global Risk (Beard and Hobson, Reference Beard and Hobson2024). The Precipice (Ord, Reference Ord2020) provides analysis of a range of risks to human extinction and the author’s assessment of their comparative likelihood. Drawing on the ethical framework of longtermism, it argues for the importance of mitigating these risks by emphasising the loss of moral value that would result from the extinguishing of humanity’s long-term future. Edited volumes The Era of Global Risk (Beard et al., Reference Beard, Richards and Rios Rojas2023) and An Anthology of Global Risk (Beard and Hobson, Reference Beard and Hobson2024) encompass a wider range of methodological, ethical, critical, and science policy perspectives on the study of individual risks and global catastrophic risk as a field. Other recent literature provides in-depth historical (Moynihan, Reference Moynihan2020; Torres, Reference Torres2023a,Reference Torresb), philosophical (Torres, Reference Torres2023a,Reference Torresb; Ware, Reference Ware2024) and political (Ware, Reference Ware2024) analyses of the concept of human extinction.

In contrast this review, to the extent possible, limits itself to an up-to-date analysis of the simpler question of “what could kill everyone, and how likely is it to happen?” I will provide a brief synthesis of the current state of knowledge on the most discussed exogenous and endogenous risks, and where available, experts’ likelihood estimates. Section 5 provides a brief overview of the complex social, ethical and political debates surrounding the topic, but a detailed treatment is outside the scope of the review.

Exogenous risks: Broadly unrelated to human activity

Five mass extinctions involving the loss of over 75% of species have occurred in the last 500 million years (Barnosky et al., Reference Barnosky, Matzke, Tomiya, Wogan, Swartz, Quental, Marshall, McGuire, Lindsey, Maguire and Mersey2011). In three – the end-Permian, end-Triassic and end-Cretaceous extinctions – flood basalt events are believed to have played a significant role (Bond and Grasby, Reference Bond and Grasby2017). These huge lava flows, lasting hundreds of thousands of years (Clapham and Renne, Reference Clapham and Renne2019), might conceivably pose extinction risk. However, they are estimated to occur once every 20 to 30 million years (although not at regular intervals) (Courtillot and Renne, Reference Courtillot and Renne2003). The Toba supervolcanic eruption 74,000 years ago was at one point thought to have come close to wiping out humans, although more recent evidence suggests this is unlikely (Yost et al., Reference Yost, Jackson, Stone and Cohen2018). Rougier et al. (Reference Rougier, Sparks, Cashman and Brown2018) suggest a frequency of 60,000–6 million years for Toba-level events; however, volcanologists remain sceptical that a single volcanic eruption could cause human extinction (Cox, Reference Cox2017).

Although the Cretaceous-Paleogene extinction event coincides with the Deccan flood basalt eruption (Keller, Reference Keller and Talent2012), the leading candidate for its cause is a different exogenous threat: the ten-kilometre-diameter asteroid that created the Chicxulub crater 66 million years ago (Chiarenza et al., Reference Chiarenza, Farnsworth, Mannion, Lunt, Valdes, Morgan and Allison2020). It is thought an impact of similar magnitude would threaten human extinction (Ord, Reference Ord2020) although some scholars believe some human populations would survive (Salotti, Reference Salotti2022). By identifying the trajectories of 95% of asteroids >1 km in diameter, asteroid-monitoring efforts have established bounds on the likelihood of a > 1-km asteroid impact in the coming century, estimated at 1 in 120,000 (Ord, Reference Ord2020).Footnote 4 Baum (Reference Baum2018) notes substantial uncertainty with regard to the potential for an asteroid impact to cause civilisational collapse and proposes that 100 m in diameter should be used as a threshold when considering asteroid impacts with potentially globally catastrophic consequences.

Stellar phenomena such as supernova explosions or gamma ray bursts could also render the Earth’s environment less habitable or uninhabitable through major depletion of the ozone layer; such events happen in the Earth’s vicinity once every 250–500 million years (Melott and Thomas, Reference Melott and Thomas2011). Increases in solar luminosity will render the Earth uninhabitable to animal species within several billion years. While this would not seem survivable for present human civilisation, on such timescales, humans may have developed adaptations or spread beyond the Earth.

Using the track record of survival for the genus Homo, Snyder-Beattie et al. (Reference Snyder-Beattie, Ord and Bonsall2019) estimate annual probability of extinction from exogenous risks of below 1 in 870,000 per year. However, Baum (Reference Baum2023) cautions that this estimate does not take into account the interaction between threats Homo historically faced and the various systemic fragilities of modern human civilisation.

Endogenous threats: Caused or closely linked to human activity

Climate change and environmental and ecological degradation

Anthropogenic activity is leading to an unprecedented acceleration of global temperature. Scenarios involving greater temperature rise are predicted to result in increasingly severe droughts, flooding, heatwaves, famines, disease spread and loss of biodiversity, with major global human mortality implications (Lee et al., Reference Lee, Calvin, Dasgupta, Krinmer, Mukherji, Thorne, Trisos, Romero, Aldunce, Barret and Blanco2023). The likelihood of catastrophic outcomes depends in part on the rate at which global CO2 emission reduction targets are met and on the Earth’s climate sensitivity. The sixth assessment report of the Intergovernmental Panel on Climate Change (IPCC) narrows the “likely” range of climate sensitivity to 2.5–4 °C, meaning an expected temperature rise in this range for a doubling of CO2 levels compared to pre-industrial levels (Lee et al., Reference Lee, Calvin, Dasgupta, Krinmer, Mukherji, Thorne, Trisos, Romero, Aldunce, Barret and Blanco2023). This suggested a reduction of the likelihood of the global catastrophic scenarios that would be associated with greater temperature rises. However, there remains substantial uncertainty around many aspects of climate sensitivity, including the impact of positive feedback loops such as possible methane release from thawing permafrost and the potential for tipping points to be passed resulting in a self-perpetuating change in the Earth’s system (Armstrong McKay et al., Reference Armstrong McKay, Staal, Abrams, Winkelmann, Sakschewski, Loriani, Fetzer, Cornell, Rockström and Lenton2022).

Some extreme possibilities associated with high temperature and CO2 emission scenarios have yet to be ruled out; for example, a recent model predicted stratocumulus cloud break-up in 1300 ppm scenarios leading to an additional 8 °C rise, although the authors note substantial uncertainties remain (Schneider et al., Reference Schneider, Kaul and Pressel2019). The potential for climate change to bring about outcomes as severe as human extinction or civilisational collapse remains the subject of academic debate. Ord (Reference Ord2020) and MacAskill (Reference MacAskill2022)Footnote 5 conclude that the direct risk of human extinction from climate change appears very low. Kemp et al. (Reference Kemp, Xu, Depledge, Ebi, Gibbins, Kohler, Rockström, Scheffer, Schellnhuber, Steffen and Lenton2022) argue that potential worst-case outcomes for climate change, including global catastrophe or human extinction, remain “dangerously unexplored,” highlighting that in addition to direct impacts, climate change may exacerbate other global threats.

Climate change is exacerbating ongoing anthropogenic biodiversity loss and environmental degradation. Other drivers of biodiversity loss include habitat destruction and fragmentation, pollution and overexploitation of species and resources (IPBES, Reference Brondizio, Settele, Díaz and Ngo2019). Dasgupta (Reference Dasgupta2021) reviews the extensive dependence of human society on ecosystem services; substantial global harm is to be expected from the degradation of these services, although a direct path to human extinction is not outlined. Surveying the literature Kareiva and Carranza (Reference Kareiva and Carranza2018) note a “growing scepticism regarding the strength of evidence linking trends in biodiversity loss to an existential risk for humans,” arguing that in the absence of unlikely circumstances such as 90% of biodiversity being lost, human civilisation would survive. Ord (Reference Ord2020) argues that ongoing environmental damage is a source of unknown threats to humanity and that more research is needed.

Under scenarios of unsustainable anthropogenic activity, wide-scale pollution, resource depletion, soil erosion, deforestation and other consequences of human activity may make global civilisation considerably less resilient, but a direct link to full extinction appears implausible. Concerns have been raised about the widespread release of chemicals that may be harmful to human health; in particular endocrine disruptors, which have been linked to fertility decline (Gonsioroski et al., Reference Gonsioroski, Mourikes and Flaws2020; Swan and Colino, Reference Swan and Colino2022). While it is premature to consider these extinction threats, more research is needed on their impacts on populations and reproductive health. Conversely, concerns have been raised for half a century that rising global population might lead to civilisational collapse (Ehrlich and Ehrlich, Reference Ehrlich and Ehrlich2009); however, research increasingly suggests that global population will peak and begin to decline within this century (Adam, Reference Adam2021).

Biological agents

Infectious disease has been a contributing factor in 3.7% of species extinctions known to have occurred between the years 1,500 and 2008 (Smith et al., Reference Smith, Acevedo-Whitehouse and Pedersen2009). In humans, the 1918 Spanish influenza outbreak killed 1–5% of world population (Johnson and Mueller, Reference Johnson and Mueller2002, Spreeuwenberger et al. Reference Spreeuwenberg, Kroneman and Paget2018).Footnote 6 Koch et al. (Reference Koch, Brierley, Maslin and Lewis2019) estimate that epidemics killed 90% of the indigenous population of the Americas as the result of diseases carried over by European colonists in the fifteenth and sixteenth centuries.

It is considered very unlikely that a naturally occurring pandemicFootnote 7 would be a primary cause of human extinction (Ord, Reference Ord2020), and the author is aware of no credible work presenting plausible pathways. Such a disease would need both to spread to nearly all humans across the world and be of extremely high lethality or cause infertility at extremely high rates. However, it seems plausible that pandemic disease could play a contributing role in combination with other factors in an extinction scenario.

A common concern is that advances in the biological sciences and biotechnology could enable the creation of more dangerous viruses and other forms of biological agent than exist in nature (Millett and Snyder-Beattie, Reference Millett and Snyder-Beattie2017). Biological weapon development and use are prohibited under the Biological Weapons Convention; however, leading states have developed major biological weapons programmes in the past, and it is conceivable that such directions might be pursued in future or are presently being pursued clandestinely (Riedel, Reference Riedel2004). Further, advances in these sciences lower the barrier to entry to the development of dangerous agents in terms of cost, expertise and equipment needed, potentially bringing globally catastrophic capabilities within the reach of small groups in future. While groups willing to bring about global catastrophe or even extinction would be expected to be rare, there have been notable examples of groups motivated by near-apocalyptic ideology such as the Aum Shinrikyo cult,Footnote 8 which attempted to weaponise Anthrax and carried out successful local nerve agent attacks (Lifton, Reference Lifton1999; Torres, Reference Torres2016). Gopal et al. (Reference Gopal, Bradshaw, Sunil and Esvelt2023) outline two scenarios leading to civilisational collapse: a “wildfire” scenario in which one or more pandemic agents leads to disruption of food, water, power supply and law enforcement and a “stealth” scenario in which an initially asymptomatic agent with long incubation time is spread to a majority of humanity. They consider both scenarios most likely to involve accidental or deliberate release of human-modified agents. Direct targeting of humans is only one plausible pathway to global harm; Monica Schoch-Spana et al. (Reference Schoch-Spana, Cicero, Adalja, Gronvall, Kirk Sell, Meyer, Nuzzo, Ravi, Shearer, Toner and Watson2017) note the potential pathogens targeting widespread eradication of food sources; and other forms of environmental harm are plausible.

The capability to re-engineer biology more fundamentally may create unprecedented risks. In 2024, a group of leading biological scientists called for a halt to research to develop “mirror bacteria,” designed to have an opposite chirality to that of all forms of existing life (Adamala et al., Reference Adamala, Agashe, Belkaid, Bittencourt, Cai, Chang, Chen, Church, Cooper, Davis, Devaraj, Endy, Esvelt, Glass, Hand, Inglesby, Isaacs, Wilmot, Jones, Kay, Lensky, Liu, Medzhitov, Nicotra, Oehm, Pannu, Relman, Schwille, Smith, Suga, Szostak, Talbot, Tiedje, Venter, Winter, Zhang, Zhu and Zuber2024). The group argues that it is likely these synthetic life forms would both resist predation and bypass immune systems in humans, animals and plants, potentially leading to risk “of unprecedented scope and scale” due to invasive spread and widespread lethal infection.

While robust likelihoods are not possible here, Millett and Snyder-Beattie (Reference Millett and Snyder-Beattie2017) deem the likelihood of full human extinction primarily from a bioweapon or engineered biological agent as “extremely low” but not ruled out, although Ord (Reference Ord2020) estimates as high as 1 in 30 for the coming century. Again for extinction, scenarios involving one or multiple pathogens in combination with other factors may be most plausible.

Nuclear war

The most credible global catastrophe scenarios resulting from nuclear war involve large-scale nuclear weapon exchange bringing about a catastrophic climate effect known as a nuclear winter. In these scenarios, smoke from burning cities, industrial facilities and oilfields enters the upper atmosphere, disrupting sunlight for a period of years and leading to wide-scale crop failures and famine. Disagreement exists between scientific teams modelling the climatic impact of nuclear exchanges. Xia et al. (Reference Xia, Robock, Scherrer, Harrison, Bodirsky, Weindl, Jägermeyr, Bardeen, Toon and Heneghan2022) estimate that over 2 billion people could die from nuclear war between Pakistan and India and over 5 billion between Russia and the USA. However, Reisner et al. (Reference Reisner, Koo, Hunke and Dubey2019) predict more limited generation and dispersal of soot, leading to less severe outcomes.

The size of nuclear warhead arsenals has been reduced from a combined global peak of over 70,000 in the 1980s (Kristensen and Norris, Reference Kristensen and Norris2013), but ~12,512 still exist, with the USA holding 5,244 and Russia holding 5,889 (Kristensen and Korda, Reference Kristensen and Korda2023). Recent years have seen increasing geopolitical tensions between nuclear powers, heightening of political rhetoric around nuclear weapons use, the non-renewal of key treaties such as New START (Williams, Reference Williams2023) and the development of new technology that might upset strategic balance (Bajema and Gower, Reference Bajema and Gower2022), leading to concerns that the risk of nuclear war remains high. A nuclear war appears unlikely to bring about extinction directly, but would severely undermine global ability to respond to other threats.

Artificial intelligence

Three plausible pathways to global catastrophe mediated by artificial intelligence (AI) have been proposed (Hendrycks et al., Reference Hendrycks, Mazeika and Woodside2023). The first relates to AI being used by humans to enact more powerful offensive warfare than in previous history or contributing to military escalation scenarios (Maas et al., Reference Maas, Matteucci, Cooke, Beard, Rees, Richards and Rios-Rojas2023). The second relates to AI accelerating development of other sciences and technologies such as bioscience (Sandbrink, Reference Sandbrink2023) relative to human understanding and governance, increasing any risks posed by these developments. The most discussed scenario, and that most directly linked to human extinction, involves the development and subsequent loss of control of artificial general intelligence (AGI) with autonomy and superhuman cognitive and strategic planning ability. The creation of AGI that is fully aligned with human goals and values is considered by many AI scientists a key challenge for future AI development (Russell, Reference Russell2019); a highly capable system that was not fully aligned might, for example, perceive human oversight as an obstacle to be bypassed and pursue actions (for example, large-scale modification of the Earth’s environment) that might lead to human extinction as a result of indifference to human survival. A rapidly growing number of research leaders consider the development of AGI feasible in coming decades or even years, and an open letter signed by many leading technologists in 2023 stated “Mitigating the risk of extinction from AI should be a global priority alongside other societal-scale risks such as pandemics and nuclear war.”Footnote 9 The topic remains controversial, with many research leaders considering such outcomes highly unlikely.Footnote 10 A recent survey of 2,778 AI authors found that between 37.8% and 51.4% (depending on framing of the question) of respondents gave at least a 10% probability to advanced AI causing human extinction or an equivalently bad outcome (Grace et al., Reference Grace, Stewart, Sandkuhler, Thomas, Weinstein-Raun and Brauner2024).

Physics experiments and unknown future technological developments

Concerns have been raised that particle physics experiments might have the potential to create consequences that could destroy the Earth: for example, creating a black hole, strangeletsFootnote 11 or triggering a phase transition that would rip the fabric of space (Rees, Reference Rees2021). According to Rees, the most favoured physics theories imply that the risks from such experiments “within our current powers” are zero; however, alternative theories imply the possibility of these catastrophic outcomes. Collisions between cosmic ray particles of much higher energies occur frequently without catastrophic consequence, offering some evidence against such concerns (Hut and Rees, Reference Hut and Rees1983). Kent (Reference Kent2004) discusses and critiques risk analysis estimates for the RHIC supercollider, for which probabilities of catastrophe ranged from 10−5 to 2 × 10−8 (Dar et al., Reference Dar, De Rújula and Heinz1999; Jaffe et al., Reference Jaffe, Busza, Wilczek and Sandweiss2000).

Concerns have also been raised about theorised but as yet undeveloped technologies such as atomically precise manufacturing (Phoenix and Drexler, Reference Phoenix and Drexler2004). More generally, many scholars have noted that some of the most powerful and potentially destructive technologies have been developed relatively recently in human history (Rees, Reference Rees2003; Bostrom, Reference Bostrom2013; Ord, Reference Ord2020). Given expectations of continued scientific advancement, it is likely that as-yet undeveloped technologies will pose catastrophic risks in coming decades and centuries and plausibly represent the greatest direct risk of human extinction.

Multiplicative stresses and civilisational vulnerability

A common theme across many of the risks discussed is that they may cause global catastrophe but are unlikely to wipe out the human species in isolation. However, it is realistic to expect that some catastrophes may trigger further catastrophic consequences across the categories above or otherwise occur in combination. The severity of some past major mass extinctions likely resulted from reinforcing interactions between multiple stresses, some playing out over thousands of years. The Permo-Triassic saw a cascade of global warming, ocean acidification and anoxia, and methane and hydrogen release Bond and Grasby (Reference Bond and Grasby2017). The Cretaceous-Paleogene saw the catastrophic consequences of the Chixhulub impact devastating a global environment already stressed by the Deccan Traps (Keller, Reference Keller and Talent2012).

Returning to modern times and shorter timescales, Kareiva and Carranza (Reference Kareiva and Carranza2018) argue that “it is the interconnections of stresses and the way we respond to environmental shocks that promulgates the greatest existential risk.”. Rees (Reference Rees2003) draws attention to the fragile nature of global supply chains: a catastrophe that resulted in higher order impacts, such as disruption of power and food supply chains, would quickly render cities uninhabitable. A civilisation reeling from one catastrophe might also be less resilient to further threats: a planet reeling from nuclear war may not be able to muster the resources to embark on a project to divert an asteroid. Nonetheless, total human extinction remains a very high bar, even for multiple of these threats in combination.

Analysing overall civilisational vulnerability and higher-order impacts of global-scale catastrophes is challenging. There is growing work in this direction. Baum and Handoh (Reference Baum and Handoh2014) propose a “Boundary Risk for Humanity and Nature” framework that combines concepts from global catastrophic risk and the planetary boundaries framework. This intersection is further explored in studies addressing the relationship between the sustainable development goals and existential risk (Cernev and Fenner, Reference Cernev and Fenner2020) and the role of planetary boundary breach in global catastrophe scenarios (Cernev, Reference Cernev2022). Jehn (Reference Jehn2023) examines the role of planetary boundary breach in exacerbating or mitigating the cascading impacts of nuclear war.

Liu et al. (Reference Liu, Lauta and Maas2018) present a taxonomy of existential hazards grouped by how global vulnerability and exposure to the hazards plays out. Avin et al. (Reference Avin, Wintle, Weitzdörfer, Ó hÉigeartaigh, Sutherland and Rees2018) analyse severe global catastrophic risk mechanisms by critical systems affected, global spread mechanism and prevention and mitigation failure. Cotton-Barratt et al. (Reference Cotton-Barratt, Daniel and Sandberg2020) propose a three defence layer model against extinction-level catastrophes, focusing on prevention, response and resilience. Kemp et al. (Reference Kemp, Xu, Depledge, Ebi, Gibbins, Kohler, Rockström, Scheffer, Schellnhuber, Steffen and Lenton2022) outline a research agenda around climate-triggered catastrophes and societal fragility. Maher and Baum (Reference Maher and Baum2013), MacAskill (Reference MacAskill2022) and Belfield (Reference Belfield, Centeno, Callaghan, Larcey and Patterson2023) explore factors that might contribute to either extinction or recovery following a civilisational collapse. Drawing on a range of concepts in systemic risk, Arnscheidt et al. (Reference Arnscheidt, Beard, Hobson, Ingram, Kemp, Mani, Marcoci, Mbeva, hÉigeartaigh, Sandberg and Sundaram2024) argue that emergent properties of the global system make important contributions to the risk of global catastrophic outcomes, noting that a focus on hazards in early global catastrophic risk research critically neglects factors such as amplification and vulnerability. Further valuable insights are likely to come from collaboration between existential risk, systemic risk and collapse research.Footnote 12

Estimating the unprecedented

I have used the term “estimate” in this review for expert assessments of extinction risks posed by hazards ranging from novel bioweapons through to physics experiments. These assessments are provided by scholars with substantial expertise, but there is frequently limited empirical evidence to draw on. Research on human extinction invariably involves at least one of three methodological challenges that make the assignment of robust probabilities difficult (for an analysis of likelihood estimation methods for existential hazards, see Beard et al., Reference Beard, Rowe and Fox2020). A first category of risks involves extremely rare events, where there is limited evidence to go on. A second category relates to entirely unprecedented developments, for which we must often primarily draw on experts’ subjective judgements. A third challenge relates to scenarios involving multiple interacting risks or trends; the complexity of these scenarios makes them challenging to assign likelihood to. Plainly put, humans are quite different from other species. We have no similar extinctions to draw on and so must fumble in the dark with what we have.

In The Precipice, Toby Ord judges the overall likelihood of an existential catastrophe in the coming century at about one in six. This figure is itself informed by subjective judgements across a range of individual risks where certainty is not possible. Ord caveats the purpose of these judgements as showing “the right order of magnitude, rather than a more precise probability“; even so, it would be reasonable to regard them as incorporating some element of guesswork given the presently unknowable nature of many of the informing variables. The majority of Ord’s overall estimate is contributed by Ord’s estimate of a one in ten likelihood of extinction from unaligned AI, with engineered pandemics and other anthropogenic risks making up most of the remainder. Even with these latter categories, Ord’s estimates have been critiqued as being too high for climate- and engineered pandemic-caused extinction (Thorstad, Reference Thorstad2023a, Reference Thorstad2023b). However, Ord’s overall estimate is similar to that derived from a 2008 survey of global catastrophic risk experts, which provided an overall risk of extinction of 19% prior to 2,100 (Sandberg and Bostrom, Reference Sandberg and Bostrom2008). The majority of this survey’s overall risk estimate was also linked to emerging or future technological development. These are also subjective estimates, and the previous discussion applies.

Ethical and social dimensions of human extinction

The social, ethical and political implications of human extinction and its study are hotly debated. Some scholarship emphasises the importance of reducing human extinction risk drawing on frameworks such as longtermism, which places a high moral value on the lives of unborn future generations and future achievements of humanity (MacAskill, Reference MacAskill2022). Critics contend that a focus on extinction of humanity as a whole or on the value of hypothetical future lives ignores or may detract attention from ongoing catastrophic impacts on existing peoples, particularly marginalised and indigenous peoples (Torres, Reference Torres2023a,Reference Torresb; Mitchell and Chaudhury, Reference Mitchell and Chaudhury2020).

Bostrom (Reference Bostrom2013)’s commonly used definition of existential risk includes both extinction and destruction of Earth’s desirable future potential as outcomes to avoid. “Desirable future potential” is open to interpretation within the definition; however, Bostrom’s own articulations of desirable futures have been criticised as heavily influenced by transhumanism and unlikely to be shared as an ideal by a majority of current people worldwide (Cremer and Kemp, Reference Cremer, Kemp, Beard and Hobson2024). It has also been argued that human extinction is a dangerous motivator: such high stakes could be used to justify extreme measures such as pre-emptive military action or authoritarian governmental action (Bostrom, Reference Bostrom2019; Cremer and Kemp, Reference Cremer, Kemp, Beard and Hobson2024).

Many themes overlap with discussions of the Anthropocene,Footnote 13 and similar critiques have been raised in relation to both literatures. It is argued that a universalist narrative portraying humanity as a single, unified agent risks obfuscating inequalities in responsibility for and vulnerability to present crises such as climate change (Todd, Reference Todd, Davis and Turpin2015; Head, Reference Head2016), as well as responsibility for harms such as the legacies of colonialism (Whyte, Reference Whyte2017; Yusoff, Reference Yusoff2018). Extractive practices associated with Global North countries have made disproportionate contributions to a range of environmental problems; it has similarly been argued that risks posed by nuclear weapons arsenals (Biswas, Reference Biswas2014) and advanced AI (Abungu et al., Reference Abungu, Iradukunda, Cass-Beggs, Pauwels, Kovacs, Roberts, Araya, He, Governance, Fay and Fumega2024) are borne by countries worldwide, while responsibility for their production lies mainly with a subset of Global North nations. Debates over the role of capitalism feature in both literatures. Moore (Reference Moore2015) proposes the Capitalocene as an alternative framework to the Anthropocene, arguing that the ecological crisis stems from specific patterns associated with capitalism. Ware (Reference Ware2024) similarly centres capitalism and the pursuit of economic growth as the driver of a range of crises threatening humanity. In contrast, Aschenbrenner and Trammell (Reference Aschenbrenner and Trammell2020) argue that over a long time frame, continued growth is likely to contribute to a richer global society better able to allocate resources to its safety.

A detailed treatment of these complex debates is out of scope for this review. However, they are important considerations, especially as calls grow for extinction risk mitigation to be incorporated into global policy decisions (Boyd and Wilson, Reference Boyd and Wilson2020).

Conclusion

The literature suggests that the likelihood of human extinction from exogenous threats, where we have more robust data to draw on, is extremely low. For anthropogenic threats, there is far greater uncertainty. For many anthropogenic threats such as nuclear war, pandemics and environmental degradation, the likelihood of such events and the scale of their catastrophic consequences strongly justify prevention and mitigation efforts. However, it is unclear how plausible they are as causes of extinction of homo sapiens. Through the lens of civilisational resilience, however, it can be argued that a global civilisation that avoids or robustly addresses these catastrophic threats will be far better placed to respond to any extinction-level threats it encounters; in terms of available resources, ability to coordinate and foresight and governance tools to identify and address emerging threats.

This argument carries particular weight if, as many experts believe, the greatest risks are anthropogenic and lie ahead of us; stemming from a global civilisation with a greater capacity to alter or degrade its environment deliberately or unintentionally; and rapidly developing technologies with novel characteristics that are more powerful than those we have experience with. If so, having the foresight, governance and response capacities necessary at a scientific, political and civil society level will be crucial. Humans may be hard to wipe out, but that will not stop us giving it our best shot.

Open peer review

To view the open peer review materials for this article, please visit http://doi.org/10.1017/ext.2025.4.

Acknowledgements

I am grateful to Constantin Arnscheidt, Emma Bates, John Alroy, Barry Brook and three anonymous reviewers for helpful comments that greatly improved this article.

Competing interest

The author declares none.

Footnotes

1 For further discussion on existential risk definitions, see Torres (Reference Torres2023a).

2 Definitions typically vary by threshold of severity; for example, Bostrom and Cirkovic (Reference Bostrom and Cirkovic2011) propose that a disaster costing 10 million lives or 10 trillion dollars of damage would be a global catastrophe; the Global Challenges Foundation uses a threshold of 10% of the global population (https://globalchallenges.org/global-risks/).

3 For a history of extinction and existential risk studies, see Beard and Torres (Reference Beard, Torres, Beard and Hobson2024); for a recent bibliometric analysis of global catastrophic risk research, see Jehn et al. (Reference Jehn, Engler, Arnscheidt, Wache, Ilin, Cook, Sundaram, Hanusch and Kemp2024).

4 Ord (Reference Ord2020) concludes that it is possible humanity would survive a 10-km asteroid impact but that extinction risk is high; for 1- to 10-km asteroids, extinction risk is lower but cannot be ruled out.

5 Drawing on research by Halstead (Reference Halstead2022).

6 Global Mortality estimates range from 17.4 million to 100 million.

7 Natural pandemics are included within endogenous threats, as their spread is substantially mediated by human-environment and human-human interaction.

8 Aum Shinrikyo’s ambition involved wiping out all of humanity except the several thousand members of the cult itself.

9 https://www.safe.ai/statement-on-ai-risk accessed 29 December 2023.

10 For example, see the Science Media Centre’s expert reactions to the AI extinction risk statement https://www.sciencemediacentre.org/expert-reaction-to-a-statement-on-the-existential-threat-of-ai-published-on-the-centre-for-ai-safety-website/ (accessed 3 January 2024).

11 A strangelet is a hypothetical strange matter particle; it is speculated that strangelets could be extremely stable and convert normal matter to strange matter on contact, which could have catastrophic consequences if created in a heavy ion collider (Jaffe et al., Reference Jaffe, Busza, Wilczek and Sandweiss2000).

12 See How Worlds Collapse (eds. Centeno et al., Reference Centeno, Callahan, Larcey and Patterson2023) for a recent edited volume combining scholarship across these fields.

13 The validity of the Anthropocene concept has been heavily contested and was recently rejected by the International Union of Geological Scientist’s Anthropocene Working Group (Witze, Reference Witze2024).

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Author comment: Extinction of the human species: What could cause it and how likely is it to occur? — R0/PR1

Published online by Cambridge University Press:  07 March 2025
DOI: https://doi.org/10.1017/ext.2025.4.pr1 [Opens in a new window]
Sean ÓhÉigeartaigh [Opens in a new window] University of Cambridge, United Kingdom of Great Britain and Northern Ireland
Revision round: 0
Role: author

Comments

No accompanying comment.

Review: Extinction of the human species: What could cause it and how likely is it to occur? — R0/PR2

Published online by Cambridge University Press:  07 March 2025
DOI: https://doi.org/10.1017/ext.2025.4.pr2 [Opens in a new window]
Reviewer_1
Date of review:  11 February 2024
Revision round: 0
Role: reviewer
Recommendation/decision: accept

Conflict of interest statement

Reviewer declares none.

Comments

This is a competent, albeit quite brief, overview of the current literature on the potential causes of human extinction. The author is clearly familiar with the relevant literture, and does a nice job of summarizing key ideas within that literature. My only critique is that I am not sure exactly what this adds to the field: there are already a number of synoptic literature reviews out there, some of which are quite recent (e.g., “The Era of Global Risk,” by Rees et al.). Despite this hesitation, I am opting for “accept” with no revisions.

Review: Extinction of the human species: What could cause it and how likely is it to occur? — R0/PR3

Published online by Cambridge University Press:  07 March 2025
DOI: https://doi.org/10.1017/ext.2025.4.pr3 [Opens in a new window]
David Higgins [Opens in a new window] University of Leeds, United Kingdom of Great Britain and Northern Ireland
Date of review:  12 March 2024
Revision round: 0
Role: reviewer
Recommendation/decision: minor-revision

Conflict of interest statement

Reviewer declares none.

Comments

This is an excellent review article: lucid, knowledgeable, and judicious. The author takes an admirably balanced approach to this complex and controversial topic, offering a valuable synthesis of the field without making any tendentious claims. The essay is well structured and covers the literature on both exogenous and endogenous threats thoroughly and economically. I appreciated the author’s acknowledgement of the limitations of the discussion, including the potential for moral hazard in focusing on future extinction rather than present-day suffering (4). I sometimes find that work in this field (e.g. Ord 2020) unhelpfully describes the human species as if it was a single unified agent, but generally this article avoids that problem.

I certainly recommend the article for publication, but have a few minor queries.

I think it’s generous to describe the guesses made about the likelihood of human extinction as ‘estimates’. Given the numerous factors and uncertainties involved, I find it hard to take Ord’s ‘one in six’ seriously. Perhaps I’m quibbling, but at what point does a ‘subjective estimate’ (10.9) become a guess?

While I could not see any obvious gaps in the works cited, the author might in future want to look into recent work on human extinction as an idea e.g. E Torres 2024, Ware 2024.

1.16 There is a hyphen before ‘unlikely’. I think a dash is required.

1.17 What evidence is there that ‘decision-makers’ are likely to factor in concerns about human extinction as compared to more directly pressing issues?

3.25-30 The word ‘advances’ is used three times in quick succession. This is a little inelegant; also, the term ‘developments’ might be preferable as less value-laden in the context of science.

3.40 ‘Desirable’ for whom?

6-7 I think a bit of rephrasing is needed in the ‘Climate change, environmental and ecological degradation’ section. In particular, it needs to be clear that biodiversity loss is not only a consequence of climate change but also caused and accelerated by separate factors, such as changing land use, pollution, etc. The importance of defaunation might also be acknowledged (see e.g. Dirzo et al. 2014).

8.14 ‘Strangelets’ may need glossing (the word was new to me).

9.32 I was unaware of the Baum & Handoh 2014 article combining ‘concepts from global catastrophic risk and the planetary boundaries framework’. Given the increasingly profile of the latter, I wondered if anyone else had developed this potentially valuable combined approach.

12 Missing line space between the references to Dasgupta and Ehrlich.

Throughout, superscript numbers indicating footnotes are placed before punctuation marks when they should be placed after.

Review: Extinction of the human species: What could cause it and how likely is it to occur? — R0/PR4

Published online by Cambridge University Press:  07 March 2025
DOI: https://doi.org/10.1017/ext.2025.4.pr4 [Opens in a new window]
Reviewer_2
Date of review:  14 March 2024
Revision round: 0
Role: reviewer
Recommendation/decision: major-revision

Conflict of interest statement

Reviewer declares none.

Comments

Thank you for the opportunity to review this manuscript - it is ambitious in aim and scope, which is great to see. There are areas to work on in a revision that will enable this manuscript to better achieve its goals.

First, the conceptual work on the Anthropocene is suggested but not made explicit in the manuscript. It is included as a keyword and is clearly implied in the topic of the paper, but the vast body of research on extinction in the Anthropocene is barely engaged with and the import of this concept (that has recently been voted down by the scientists trying to evaluate its value as a label for new epoch).

Second, while the Anthropocene is not rigorously engaged with as a scientific concept or term that is still being debated, the critical literature trying to tease out the implications of this universalist thinking is also not. For example, this reference to how Indigenous people experienced massive loss of life as a result of colonisation is a demonstration of a lack of critical engagement with the social scientific literature on the Anthropocene: ‘Koch et al. (2019) estimate that epidemics killed 90% of the indigenous population of the Americas over the century following the European arrival in 1492.’ The epidemics arrived with colonisers: they were a result of settler colonial practices. If Zoe Todd’s or Kyle Powys Whyte’s insightful contributions to critiques of the Anthropocene had been grappled with properly here, then the absolving of responsibility and the glossing over the impacts of colonisation would not have happened.

Third, the opening premise setting the scene in this manuscript draws on a claim about how modern human activity has affected species across the world: ‘Modern human activity is estimated to have accelerated species extinction rates across the Earth’s biosphere to between 100 and 1,000 times higher than their background rate over the past tens of millions of years (Dasgupta, 2021)’. To present the current biodiversity extinction wave as a result of all modern human activity absolves responsibility for the uneven extractive practices of the Global North and the inequitable distribution of the burdens of environmental harms in one universalist claim. Again, if a critical engagement with scholarship on what the Anthropocene is (or is not) had grounded this paper, such a claim would not be possible. For instance, Lesley Head and Chris Gibson have written extensively on the impacts of ‘the moderns’ and offer critical readings of universalising Anthropocene thinking - unpacking it as a contingent lived reality. Further, Jessica McLean does this work too.

The paper is well written and engaging but framed in such a way that it misses major key arguments and overlooks vast literatures on the very thing it is trying to examine. I hope the author/s take the opportunity to engage with this work in a revision as these are important topics that we need to read more about.

Recommendation: Extinction of the human species: What could cause it and how likely is it to occur? — R0/PR5

Published online by Cambridge University Press:  07 March 2025
DOI: https://doi.org/10.1017/ext.2025.4.pr5 [Opens in a new window]
Barry Brook [Opens in a new window] School of Natural Sciences, University of Tasmania, Australia
Date of review:  14 March 2024
Revision round: 0
Role: Handling Editor
Recommendation/decision: major-revision

Comments

The manuscript “Extinction of the Human Species: What Could Cause It and How Likely Is It to Occur?” presents a timely overview of the literature on potential pathways to human extinction, covering both exogenous and anthropogenic threats. The author synthesises a broad spectrum of research, providing valuable insights into the likelihood of various extinction scenarios and acknowledging the limitations inherent in such speculative analyses. For a reader new to the topic, this paper presents an excellent knowledge grounding and primer.

Reviewer 1 recognises the author’s familiarity with the relevant literature and the concise summary provided but questions the manuscript’s novelty. Reviewer 2 commends the balanced approach and thorough coverage but suggests clarifying the speculative nature of extinction likelihood estimates and advises minor stylistic improvements and updates to the literature. Reviewer 3 urges a more explicit engagement with the Anthropocene and its social and ethical dimensions, recommending a critical examination of universalist claims and the inclusion of diverse perspectives on environmental responsibility.

In light of these reviews, I recommend a major revision to address the following core requirements:

1. Differentiate this review from existing literature by clearly articulating its unique contributions to the field.

2. Clarify the speculative nature of likelihood estimates for human extinction and consider the semantic distinction between ‘estimates’ and ‘guesses’.

3. Engage more explicitly with the concept of the Anthropocene, incorporating critical perspectives on its implications and the role of human activity in biodiversity loss.

4. Expand the discussion on the social and ethical dimensions of human extinction, integrating insights from indigenous studies and critical social science to challenge universalist narratives.

5. Make minor stylistic and formatting corrections as suggested by Reviewer 2, including the use of dashes, repetition of terms, and footnote placement.

These revisions will strengthen the manuscript’s contribution to the field, providing a more nuanced and comprehensive review that acknowledges the complexity and ethical considerations surrounding the topic of human extinction.

Barry Brook (acting as Handling Editor)

Decision: Extinction of the human species: What could cause it and how likely is it to occur? — R0/PR6

Published online by Cambridge University Press:  07 March 2025
DOI: https://doi.org/10.1017/ext.2025.4.pr6 [Opens in a new window]
John Alroy [Opens in a new window] School of Natural Sciences, Macquarie University, Australia
Revision round: 0
Role: Editor in Chief
Recommendation/decision: major-revision

Comments

No accompanying comment.

Author comment: Extinction of the human species: What could cause it and how likely is it to occur? — R1/PR7

Published online by Cambridge University Press:  07 March 2025
DOI: https://doi.org/10.1017/ext.2025.4.pr7 [Opens in a new window]
Sean ÓhÉigeartaigh [Opens in a new window] University of Cambridge, United Kingdom of Great Britain and Northern Ireland
Revision round: 1
Role: author

Comments

No accompanying comment.

Recommendation: Extinction of the human species: What could cause it and how likely is it to occur? — R1/PR8

Published online by Cambridge University Press:  07 March 2025
DOI: https://doi.org/10.1017/ext.2025.4.pr8 [Opens in a new window]
Barry Brook [Opens in a new window] School of Natural Sciences, University of Tasmania, Australia
Date of review:  14 January 2025
Revision round: 1
Role: Handling Editor
Recommendation/decision: accept

Comments

Following the thorough revisions undertaken by the author, I have re-examined the manuscript entitled “Extinction of the Human Species: What Could Cause It and How Likely Is It to Occur?” and cross-checked the response against the reviewer feedback.

In my view as Handling Editor, the updated version nicely addresses all substantive critiques, including expanding discussions on multiplicative stresses and tipping points, refining the scope and relevance of cited literature (such as clarifications around the ‘Big Five’ mass extinctions), and adding a dedicated section on ethical, social, and political dimensions of human extinction. The author has also satisfactorily responded to smaller editorial concerns, such as minor stylistic points and clarifications of assumptions around extinction likelihood estimates.

Given these updates, I am confident the manuscript now meets the journal’s standards and provides a clear, incisive, and balanced assessment of critical issues around human extinction. It should stand as an important, accessible summary of the human-extinction literature for the general reader.

Each reviewer’s central concerns have been carefully addressed, and no further technical or conceptual revisions appear necessary. I therefore recommend the manuscript be accepted for publication without requiring additional rounds of review.

Decision: Extinction of the human species: What could cause it and how likely is it to occur? — R1/PR9

Published online by Cambridge University Press:  07 March 2025
DOI: https://doi.org/10.1017/ext.2025.4.pr9 [Opens in a new window]
John Alroy [Opens in a new window] School of Natural Sciences, Macquarie University, Australia
Revision round: 1
Role: Editor in Chief
Recommendation/decision: accept

Comments

No accompanying comment.