I. A Supernova
The explosion of a star
In a galaxy the same size as the Milky Way, there is a supernova, on average, around every 50 years. A common type of supernova, in short, is the sudden collapse then explosion resulting from it’s inability to withstand its own gravitational force. This will happen during the last evolutionary stages of a star, when some of its own mass flows into its core. If a supernova where to happen any closer than 10 parsecs (33 light-years), it would effectively destroy at least 60% of Earth’s ozone layer. The Gamma rays produced from this explosion would would hit our outer atmosphere, causing its molecular nitrogen and oxygen to convert to nitrogen oxides, depleting the ozone layer and exposing us to high levels of cosmic and solar radiation. This alone would cause catastrophic damage to any terrestrial planet such as earth, and all of its delicate ecological systems, and subsequently any remaining level of humanity would quickly self corrupt and ultimately collapse. Of course any supernova occurring even closer would also possess the high possibility of simply deep frying us instantly.
II. Nuclear Warfare
Military strategy in which nuclear weaponry is used to inflict damage on the enemy
I would say that Nuclear annihilation, unfortunately, is one of the more likely disasters on this list. Although you would think that the people in charge would have enough common sense to know that detonating a series of extremely destructive atomic weapons over multiple countries containing some of the most powerful military forces on the planet may be a bad idea, that simply may not be the case. If a situation such as this were to occur on a grand scale, multiple things would end up happening, none of which are considered in the best interest of the people. Aside from the instant destruction of entire cities and regions and forceful pressure shock waves, there would be a considerable amount of other effects that would take place afterwards. Almost immediately after the initial blasts a residual and very radioactive substance known as “Nuclear Fallout” would shoot up into the upper atmosphere. The amount and expansion of this substance is dependant on the size of the nuclear weapon. The Nuclear fallout produced from the bomb dropped on Hiroshima in 1945 “fell out” of the sky within 30-40 minutes of the blast. This caused terrible radioactive sickness and intense changes to the local climate that, due to advances in technology, could quite possibly be even worse today. However, multiple atomic bombs of the magnitude in which they are currently constructed, detonated at or above Fallout-Free altitude causing the dispersion of particles of 10 nm to 20 μm in diameter into the stratosphere, could take several years to settle. The fallout in the stratosphere would block direct sunlight, causing a cooling effect (definitely not the type of cooling effect we actually need) and stunting all of the environmental processes that require consistent sunlight. In addition to the fallout is the air there would also be a drastic increase in radioactivity on the ground, based on the bombing zones, and altitude (radioactive particles have a tendency to reside in lower areas due to weight). This would of course, be in affect with the large fire storms. Firestorms occur when several excessively fueled fires condense and create an independant updraft that blows inward from all directions, substantially increasing the intensity of the fires and allowing the, now firestorm, to sustain itself. One more side effect of nuclear weaponry is the loss of technology from electromagnetic pulses, those being short bursts of electromagnetic energy most likely originating from the bombing sights. While this phenomenon does happen naturally, the intensity of the EMPs caused by these radioactive fields would probably have a much more detrimental effect on electrical technology.
A solid piece of space debris that was able to sustain itself during its passage through the atmosphere, hitting the planet’s surface
In order to instantly kill all life, earth would have to be struck by a meteorite slightly smaller than the city of London, but the deadliness of a meteorite can’t be summed up by the impact alone. For example, a foriegn object around the size of mount everest hitting the earth would most likely do the trick. Not only would it leave a crater over 300 miles in diameter also completely vaporise anything within remote proxy of it, but the impact would cause high magnitude earthquakes, tsunamis, and drastic changes in heat and climate. A meteorite 1 mile across would be equal to around 1 million megatons on earth, this mass hitting the earth at about 30,000 mph would be roughly 10 million times the energy of the bomb dropped on hiroshima in 1945. This could destroy New York and half of massachusetts, also cause tremendous global disasters, but would most likely not eradicate all life. Now, multiply that by about 9 or ten, and we have our mount everest meteorite, which would most likely do it.