The End of the World (As We Know It)
Scientific Scenarios:
1) Asteroid impact
There is little doubt that a comet or asteroid will hit the Earth in the future. In 1908, a 200-foot-wide comet fragment exploded over a region of Siberia with nearly 1,000 times the energy of the Hiroshima atomic bomb. More recently, comets and asteroids have passed close by the Earth, watched with trepidation by astronomers and governments alike. A ten-mile asteroid is thought to have done for the dinosaurs; without taking the problem seriously, the human race could be next.
2) Ecological collapse due to human activity
The constraints of the biosphere are fixed. The bottleneck through which we are passing is real. The overall loss of biodiversity could therefore be disastrous for Earth if it continues unabated, especially when one considers that humans are directly responsible for 30,000 species becoming extinct every year. The subtle interlinking of organisms will be something that the human race continues to overlook at its own peril.
3) Global epidemic
As old diseases grow resistant to current antibiotics, and international travel allows diseases to spread much more quickly, another global epidemic might not be to far away. A new, unfamiliar strain, possibly encouraged by a generally warmer climate and our interference in ecological matters, could spread at such a rate as to defeat all preventative measures. Our worst enemies could once again prove to be the organisms with which we co-exist.
4) Global high-tech warfare
Although the US and Russia are on relatively good terms today, the threat of war remains very real. There are now eight nuclear nations in the world, with over 20,000 active nuclear weapons, and politics change notoriously quickly. Added to that is the possibility of an accidental nuclear exchange, with current interception technology unable to prevent a significant number of missiles. Bioweapons could be an even more realistic threat, because they are cheaper, easier to produce, and simpler to conceal. This could prove attractive to hostile, no-nuclear nations wishing to wreak havoc. And as genetic engineering technology continues to progress, genetic warfare, in which particular ethnic groups are targeted, might not be far behind.
5) Global warming
Global warming is a well-documented problem now, but that does not lessen the potential for disaster which it brings. A warmer planet could encourage disease, possibly coupling with climate change to cause famine. Temperatures could be increased by evaporating water and carbon dioxide being driven from rocks.
6) Solar superflare
Bradley Schaefer of the Univ. of Texas has discovered that solar flares which bombard the Earth, and which the Earth's atmosphere effectively defends against, may have cousin superflares which are potentially destructive. Such a superflare, or giant solar flare, emitted by the sun could fry the Earth, resulting in the complete destruction of the ozone layer.
7) Gamma ray burst
Gamma-ray bursts, which are thought to be caused by the merging of two collapsed stars, could completely destroy the ozone layer of the Earth, destroying all electronics, and leaving all surface life exposed to intense, deadly UV radiation. .
8) Rogue black hole
There are estimated to be about ten million black holes in the Milky Way, all of which are by definition invisible, though not undetectable. If a black hole were to pass through our solar system, it would distort the other planets' orbits. This, in turn, could mean the Earth becoming drawn into an elliptical path that would result in massive climate swings or, more catastrophically, the Earth could even be ejected from the solar system altogether and sent hurtling in to space.
9) Solar overheating
The sun is gradually getting hotter as it converts hydrogen to helium. Astronomers estimate that in about 1 billion its temperature will have risen high enough to boil the seas off the earth. After that, things will only get worse: as the sun swells to become a red giant its surface will brush up against the earth.
10) Universal instability
Long-range forecasts suggest that all galaxies will become isolated from one another, and that each will eventually collapse into a black hole. Before this happens, it may be that atomic nuclei themselves will decompose into quarks. The black holes themselves will eventually wither away, leaving only scattered subatomic particles.
Sources:
Corey S. Powell, "Twenty Ways the World Could End Suddenly," Discover Magazine, Oct. 2000.
Edward O. Wilson, The Future of Life. (New York: Alfred A. Knopf, 2002)