Nuclear weapons have the explosive capability to ingite large parts of the landscape, particularly materials present in cities and the urban environment, and send large amounts of particulate into the atmosphere. If enough nuclear detonations are made, and a large amount of this matter stays in the atmosphere for a long period, it affects sunlight reaching the surface, cutting off plant’s main energy source and creating a nuclear “winter”.
Run Away or Delayed Effect Climatic Change
Since gaining access to abundant fossil fuel energy sources and modern chemistry, humanity has caused significant alteration to the atmosphere by adding new gases and by altering the ratio of existing ones (most prominently GHGs and CFCs). As a gas may stay in the atmosphere for many years, the consequences are often far removed from the cause of the emission. However, there have been several threats pointed to by scientists involving altering the atmosphere, either directly or through feedback effects, that are extreme enough to make it hostile to large numbers of existing species.
Accidental virus outbreak threatening stability of civilization
Conditions present in impoverished urban environments provide breeding ground for “super viruses”. Laboratory techniques also run a risk of accidentally creating a virus threat great enough to threaten civilization’s stability.
Deliberate viral outbreak threatening stability of civilization, possibly targeted gene pools or human species
Deliberate attempts by rogue governments, terrorist groups and cults to create biological weapons, already having killed many people, runs the risk of becoming easier and severe enough to threaten large portions of the human population as technologies proliferate.
This scenario exists where an escalation of military capabilities leads to more sophisticated and automated artificial intelligence being given control of military duties. Given certain design or programming flaws, such entities may designate humanity or part of it as a threat to its existence or one of its programmed objectives, and strike against humanity with a force and sophistication that humans are unable to respond to.
AI/AL units outcompeting humanity and other species for resources
Due to increasing reliance on artificial intelligence for military and economic objectives, artificially intelligent mechanic systems are assigned a greater and greater allocation of available resources. Eventually, humans no longer have access to enough resources to sustain a viable civilization or population.
Machine self-replication (including ‘grey goo’ and other more likey scenarios)
The ability of machines to self-replicate given access to resources leads to out-of-control expansion that destroys life or the resources required for life.
Poorly Planned Singularity
The ability to automate the production of ideas and new technology leads to an exponential curve in technological changes, with unknown consequences.
Destruction of viable human population by tyranny
A form of tyranical sociopathic human leadership, seeing no intrisic value in fellow humans, and with machines providing greater economic and military utility, allows or creates a scenario in which a viable human population is destroyed.
Genetic alteration of humanity
Due to either a dsygenic evolution, perverse outcomes of trivial fashions in high-tech child selection, or direct manipulation of humanity’s DNA, humanity is genetically altered to the point where the species is ended.
Genetic alteration of wildlife leading to widespread genetic instability in ecosystems
Genetic manipulation for agricultural or recreational purposes leads to new species that destroy many existing species, possibly leading to a largely homongenous biosphere.
Widespread Habitat Destruction
Widespread human land use leads to destruction of remaining natural eco-systems. Currently habitat destruction is the greatest factor in species extinctions, and will remain so for the short-term future.
Human loss of will to reproduce
Due to cultural change, or embedding in virtual reality systems causing a loss of emotional connectivity to real world neccesities, the overwhelming majority of humans decide not to reproduce.
Off-planet (including lesser/low probability threats)
A large asteroid impact can be equivalent to many nuclear weapons. If such an impact occurred, the resulting particulates thrown into the atmosphere could alter the climate enough to cause widespread extinction and chaos.
Encounter with extra-terrestrial life – Novel viruses/microscopic threats
An encounter with new viruses or microbes that earth organisms have no evolved resistance to could create a pandemic. This might occur due to space exploration exposing humans to an undetected virus, failure in quarantining of samples brought to Earth, or an external object bringing new viruses/life to Earth.
Encounter with extra-terrestrial life – Intelligent species
Given the emerging information about the number of planets within the galaxy, it seems possible that our biosphere might eventually come into contact with life from other planets. It also seems at least possible that an encounter might result in conflict of some kind.
Encounter with extra-terrestrial life – Mechanical creations of intelligent species
One theory stipulates that the reason humans have not detected other sentient life because sentient life only lasts for a short period before destroying itself, one way or another. In such a scenario, it is more probable that we would encounter some kind of automated mechanical system left behind by a civilization. Especially if this system was military in nature, there is a chance that an encounter with it could present a serious risk to our well-being.
Encounter with extra-terrestrial life – Interstellar communications virus (low probability)
This threat will tweak some people’s absurdity filter pretty strongly, but it’s worthwhile to think about even as a thought-experiment. This idea proposed by unconventional minds of astronomy is that the SETI programs of civilizations that emerge throughout space constitute an environment in which an interstellar communications virus could exist. Each civilization picks up the virus, becomes infected, then retransmits it as a new signal to be picked up by other civilizations. This might take the form of instructions for a computer/mechanical/biological system designed to capture communications facilities and retransmit itself (a slight variation on the movie “Species”), or possibly a type of supremely adapted computer virus not yet thought of by humans. This possibility is proposed based on the assumption of very large numbers of viable targets for such a virus to exist, transmit and evolve, with a virus within a highly advanced alien civilization being the original source. The virus would adapt from an intra-civilization virus (like a bad computer virus), into one that can traverse the technology or biology of multiple civlizations.