hypernova-011 Hyper-nova
If a stellar hyper-nova explosion occurred close to our Solar System, the ensuing blast of high-energy gamma radiation would tear apart the DNA coding of every living organism within several miles of the surface of our planet. All that would be left are the simplest of bacteria which feed on inorganic substances deep within the Earths crust. Our planet would be left as it was two billion years ago.

2 Supervolcanic eruption: If a VEI (Volcanic Explosivity Index) 8 volcano erupted anywhere on the planet then the entire world would be affected.The eruption itself would pulverize anything within tens or hundreds of miles of the vent. This was spectacularly demonstrated at Yellowstone park several hundred thousand years ago. The volcano obliterated a ninety kilometre stretch of mountain range at a distance of seventy kilometres. Over a thousand cubic kilometres of ash,lava, gas and dust would permeate into the atmosphere and cover the ground. Global temperatures would fall as the ash in the atmosphere reflected sunlight back into space. This would trigger an Ice Age like phenomenon known as volcanic winter which could last for centuries.Humans would run out of food and fresh water…resulting in massive reduction in populations of all large life forms worldwide, including humans.

3 Nuclear Holocaust
With our current nuclear arsenal we could theoretically kill up to half of the humans alive on the planet. After a nuclear conflict the affected area(s) would be riddled with “dead zones” where the radiation concentration is too high for human habitation for years afterward. The clouds of nuclear particles could cause a nuclear winter. This is similar to the volcanic equivalent but has the added problem of the radiation produced by the decay of unstable isotopes of uranium and plutonium.

4 Global sea level rise
This is a little less violent that the other options but a lot more likely.
If the entire Northern ice cap melted, no one would care (except polar bears) as the ice is already floating on, therefore displacing, water. So if the whole lot melts then no sea level rise will occur. This is a bit like your vodka martini not getting bigger as the ice melts.
However if Greenland’s ice cap or the Southern sheet were to melt then the result would be catastrophic. Huge swathes of land would be wiped out including most major cities and population centres.

5 Asteroid Strike
This would have been top but it is just so overused.
An asteroid one hundred metres across would destroy a city sized area with approximately the force of a sizable nuclear weapon.The reason it has so much power is its speed. Even when the atmosphere shrinks and slows it using friction,it still slams into the Earth with resounding force at thousands of kilometres an hour. If the asteroid is four to six thousand metres across then it has mass extinction potential. An asteroid of approximately this size dusted up the dinosaurs and it could do the same to us. A body ninety thousand kilometres across would vaporise our oceans and sterilise the planet. If the asteroid (or planetoid) was less than about a hundred thousand times less massive than Earth,it would break open the planet and the heat energy would return it to its state at the beginning of the Solar System.

To be continued.

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