Existential Threats
Oxford University's Future of Humanity Institute identifies 12 major existential threats to humanity. While it's easy to see that the list is a good compilation of overall mishaps, the type and probability of each event is poorly understood - sometimes due to the nature of public discourse and other times because we simply don't have any prior data on complete unknowns.
Extreme Climate Change
Probability: certain within the next century
Impact: Poorer countries will bear the brunt of the impact, but there will also be global disruptions and sustained supply chain collapse. This is a threat to already stressed local and global economic systems. Systemic effects will result in mass casualties, further rise of extreme political and religious systems, a prolonged stagnation of scientific and intellectual output, a temporary halt to medical advances and reduced capability for medical intervention in general, and will likely delay an interplanetary expansion of our civilization by more than a century.
Threat range: narrow. While the systemic effects are not entirely predictable, the nature and scope of the threat is understood.
Result: developmental delay. While this threat will kill a lot of people both directly through its effects and indirectly through the inhibition of scientific and technological progress, it is not a likely threat to our civilization in the long term. The event is exceedingly unlikely to eliminate the human race or all life on the planet.
Ecological Catastrophe
Probability: multiple instances of varying certainty within the next century.
Impact: ecological catastrophes are a catch-all category for any severe change to the Earth's ecosystem or its parts. For example, climate change is certain to trigger several of these. The effects are expected to cover a wide range, from no discernable impact on human life, to mass casualty events.
Threat range: intermediate. Unforeseen scenarios are likely to happen. Advance warning windows range from months to centuries, with varying options for intervention.
Result: developmental delay. For this to eliminate all human life, the biosphere would have to be destroyed completely, and humanity would have to be unable to maintain isolated environments where life can still persist.
Supervolcano
Probability: improbable within the next century
Impact: a supervolcano eruption would trigger an immediate climate desaster. The eruption itself would be a mass casualty event, followed by a temporary collapse of global food production.
Threat range: narrow. The threat is well defined and understood. Advance warning will likely be measured in days or less. Options for intervention are extremely limited, but the scenario yields itself to reasonable impact mitigation.
Result: developmental delay. A supervolcano eruption will not eliminate our civilization or the human race.
Nuclear War
Probability: limited nuclear exchange likely within next centure, full arsenal exchange unlikely
Impact: If a full nuclear exchange were to occur, it would have a devastating impact on our civilization. The immediate effects would include widespread destruction and loss of life, as well as long-term effects such as radioactive contamination and the potential for a nuclear winter. In addition, the psychological effects on survivors and future generations would be profound.
Threat range: broad. Both the geopolitical dynamics driving a nuclear conflict, as well as the scope of the exchange are difficult to predict.
Result: political polarization, cultural stagnation, ultimately leading to developmental delay. A severe nuclear exchange could result in complete collapse of civilization.
Global System Collapse
Probability: low
Impact: If a global system collapse were to occur, the situation could evolve into a scenario that features the permanent collapse of our civilizational and technological systems. This could be due to political unrest, economic collapse, or other social factors. The consequences would be widespread and severe, including the loss of technological and societal advances.
Threat range: broad. The threat is compunded by the possibility that we may not notice it happening for quite some time. Complicating factors include: creeping incompetence, loss of local community knowledge and capability, population resistance to science and technological advancement, and a prolonged trend of obscured technological downgrades and loss of functionality.
Result: developmental delay or even eradication. Widespread loss of life and the potential for long-term societal and technological regression, possibly permanent if the new equilibrium at the end of this development leads to a stable society that rejects scientific inquiry and improvement.