1. Human Cause
Humans have become very strong to avoid natural hazards. But unfortunately he himself has developed a method for mass extermination. The most obvious is nuclear war. We have enough nuclear on the planet to destroy all life. But we can hope that one day we become safe enough to not use the weapon on a global scale.
2. Global Cooling
As depicted in Film "The Day After Tomorrow", for some time the earth would be too cold very quickly, causing the deaths of many polar and surrounding areas. In turn will lead to a massive migration to the equatorial region. If lucky, in some areas can still be found the remains of water because most of it into ice and snow.
3. Global Warming
It's basically the other side of Global Cooling. Melting of snow and sea ice will make deeper and possibly cause Anoxic event.
4. Clathrate Gun
It is the formation of clathrates in the Continental Shelf. Clathrate methane is water-covered. If it breaks and releases methane gas into the atmosphere, it will lead to accelerated global warming.
5. Anoxic Events
Anoxic events occur when the ocean has been empty for oxygen at the surface. Although anoxic not happened for millions of years, the geological record shows that there have been several times in the past. Anoxic can and have caused mass extinctions. Black line in the figure above is the trace of past anoxic event.
6. Hydrogen Sulfide Emissions
This can happen when a disruption of the balance between plankton and bacteria that reduce sulfate. Hydrogen sulfide emissions Imagine the sudden rise of the sea poison everything on land and sea. And if that was not enough, it can also destroy ozone.
7. Sea-Level Falls
Basically , if the sea level dropped drastically (either via Global Cooling or sunk in the middle of the ocean ridges), can reduce the area around the Continental Shelf and cause all the small fish, sharks, whales and almost all life in the oceans die. If that's not bad enough, the weather can change drastically leading to a mass extinction on the mainland.
8. Oceanic Overturn
Oceanic Overturn basically deeper water rising to the surface. Under water has less oxygen so that the mean sea surface will be deprived of oxygen too.
9. Gamma Ray Bursts
When a star has exploded, before we can realize it, we and everyone in the world has died. It is a Gamma Ray Burst. We see that the Gamma basically kills every cell in the body. But if he is weak only half of the planet who died instantly. However, ozone will be lost and eventually mass extinction will still happen.
10. Impact Events
Impact event is a big rock falling from space. Basically this sends big stone dust and aerosols to the atmosphere, causing not only dead plants, but also lowers the temperature. Large stones can cause mega-tsunamis and global wildfires.