Global Catastrophe Scenarios
The shorter (than Wikipedia article) of it
Humans face two types of foreseeable existential threats (+unforeseeable): Natural (non-modern human) and Anthropogenic (modern human). Everyone of high school age or older should know which is by far the greater threat
.
Black Swan Events
Unforeseeable; your guess is as good as anyone’s (and unthinkable, e.g. overshoot).
Natural
Species extinction
About one per year baseline is the norm. [Current estimates of 8,000 extinctions/year rapidly (this century) to exceed 10k times baseline. This is the Anthropocene mass extinction event, currently underway.]
Asteroid impact
A 1 km diameter asteroid have impacted Earth on average once every 500,000 years, but will not cause widespread extinctions, unlike the Chicxulub asteroid (about ten kilometers in diameter) likely caused the extinction of all non-avian dinosaurs.
Planetary or interstellar collision
Rogue planets/moons not in orbit can enter the solar system. A collision is not required as a close pass by a large object could cause massive tidal forces, liquification of the Earth’s crust to Earth being torn apart, becoming a disrupted planet. Incoming stars and black holes are easier to detect from a longer distance.
Physics hazards
Strangelets, if they exist, might naturally be produced by strange stars, and in the case of a collision, might escape and hit the Earth. Likewise, a false vacuum collapse could be triggered elsewhere in the universe.
Gamma-ray burst
An interstellar threat is a gamma-ray burst, typically produced by a supernova when a star collapses inward on itself and then “bounces” outward in a massive explosion could, under certain circumstances, produce massive bursts of gamma radiation that could significantly affect the Earth’s atmosphere and pose an existential threat to all life.
The Sun
A powerful solar flare, solar superstorm or a solar micronova could have severe consequences for life on Earth. The Sun will become a red giant within the next seven billion years, making Earth uninhabitable.
Uninhabitable universe
The ultimate fate of the universe is uncertain, but is likely to eventually become uninhabitable, either suddenly or gradually. If it does not collapse into the Big Crunch, over very long time scales the heat death of the universe may render life impossible.
Extraterrestrial invasion
Intelligent extraterrestrial life, if it exists, could decide to spray Earth with humanocide or consume what is left that we haven’t (including us).
Natural pandemic
The Black Death killed up to 50% of the European population between 1346 and 1353 and up to 90% of the indigenous people of the Americas died in colonial times due to a lack of immunity to European diseases.
Natural climate change
The climate has ranged from ice ages to warmer periods when palm trees grew in Antarctica. The rate of climate change can be rapid, e.g. the Younger Dryas cooling of 10°C (18°F) in just a few years in some regions (e.g. Greenland).
Volcanism
Yellowstone sits on top of four overlapping calderas, and is one supervolcano which can cause regional catastrophe and via a volcanic winter, global climate change and crop failures for several years.
Anthropogenic
Experts at the Future of Humanity Institute at the University of Oxford and the Centre for the Study of Existential Risk at the University of Cambridge prioritize anthropogenic over natural risks due to their much greater estimated likelihood.
Artificial intelligence
A survey of AI experts estimated that the chance of human-level machine learning having an “extremely bad (e.g., human extinction)” long-term effect on humanity is 5% by 2100.
Biotechnology
Biological warfare/Bioterrorism/Biotechnology can pose a global catastrophic risk in the form of intentional (or not) bioengineered organisms (viruses, bacteria, fungi, invasive plants/animals).
Chemical weapons
By contrast with nuclear and biological weapons, chemical warfare, while able to create multiple local catastrophes, is unlikely to create a global one.
Financial collapse
Hyper-inflation followed by chronic deflation. Economic growth ends.
Having too few children
The world population may decline due to fewer children. Modern humans, living in overdensity population, like the rats and mice of John B. Calhoun’s experiments, may have the same outcome — in all experiments, Calhoun’s rats and mice lost functional behaviors (a behavioral sink effect) and experienced a decline in fertility, as are modern humans, that ended in extinction.
Climate change
One mouse species endemic to an atoll flooded by rising sea levels has gone extinct due to climate change, a symptom of anthropogenic overshoot.
Agriculture
Climate change will be an increasing cause of species extinction, but by far the number one driver of species extinction is the turning of ecosystems into industrially farmed land, e.g. most of the Amazon and Indonesia in recent decades and North America in the last two centuries.
Cyberattack
Cyberattacks have the potential to destroy everything from personal data to electric grids to destroying modern techno-industrial monetary culture, but pose little risk to life on Earth apart from human mutualists (e.g. crops, livestock, pets).
Environmental disaster
An environmental or ecological disaster is being induced by the present trends of overpopulation, economic development, agriculture, and sprawl. Most environmental scenarios involve one or more of the following: scarcity of fresh water that could lead to approximately half the Earth’s population of humans being without safe drinking water, pollinator decline, overfishing, massive deforestation, desertification, climate change, massive water pollution episodes…, persistence of metastatic modernity.
Evolution
Humans could use genetic engineering or technological modifications to split into normal humans and a new species — posthumans/transhumans who will displace all other humans.
Experimental accident
An experiment in nuclear and high-energy physics could create unusual conditions with catastrophic consequences.
Mineral resource exhaustion
Earth’s finite stock of mineral/metal/energy resources is presently being extracted and the world economy as a whole is heading towards inevitable contraction. with universal economic decline.
Nanotechnology
When nanofactories gain the ability to produce other nanofactories, production of weapons and stuff would be limited only by input materials, energy and software.
Nuclear war
Some fear a hypothetical World War III could cause the annihilation of humankind. On the plus side, the increased mutation rates in the lifeforms that persist will reduce the time needed to respeciate the planet after the Anthropocene ends.
World population
The world socioeconomic-political system is heavily dependent on petroleum energy, e.g. on turning fossil fuels into food. When (not if) industrial food production, processing transportation, storage ends, expect an initial 77% to 99% depopulation of humans, livestock, and pets.