Sea-level falls
Basically, if the sea level drops drastically (whether through the Global Cooling or sink in the middle of the ocean ridges), can reduce the area around the Continental Shelf cause all the small fish, sharks, whales and almost all life on the large blue die. If that's not bad enough, the weather can change dramatically leading to mass extinctions on earth (that means us).
Impact Events
Impact event was the collapse of large stones from outer space. Basically it sends a big stone dust and aerosols into the atmosphere, causing not only the plants die, but also lower the temperature. Large stones can lead to mega-tsunamis and global forest fires.
Global Cooling
As depicted in "The Day after Tomorrow", for some reason the earth will be too cold too fast cause of death for many of the poles and were animals. This in turn will lead to mass migration to the equator. Lucky to find water because most of the ice and snow.
Global Warming
This is basically the other side of Global Cooling. Withdrawal of snow and sea ice will create deeper and possibly cause Anoxic event
Clathrate Gun
This is the formation of clathrates in the Continental Shelf. Clathrate is a methane-covered water. If this rupture releasing methane gas into the atmosphere causes the acceleration of Global Warming.
Anoxic Events
Anoxic Events occur when the ocean has been empty for oxygen on the surface. Although anoxic did not happen for millions of years, the geological record shows that there have been several times in the past. Anoxic may have caused mass extinctions. Black lines in the picture above is from aptian anoxic event.
Hydrogen Sulfide Emissions
This can occur when the disturbance of the balance between plankton and bacteria that reduce sulfate. Imagine the hydrogen sulfide emissions suddenly rise from the sea poisoning everything on land and sea. And if that's not enough, it can also destroy ozone.
Oceanic Overturn
Oceanic Overturn basically a deeper water rising to the surface. Water below have little oxygen so that the mean sea surface to be deprived of oxygen too.
Gamma Ray Bursts
When the star has exploded. Before you can say "oh shit!" You and everyone else in the world has died. It is a Gamma Ray Burst. You see that Gamma basically kill every cell in your body. But if he is weak only half of the planet, who died instantly. However, ozone will be lost and eventually will die also
Human Cause
Humans have become too powerful to avoid natural hazards. But unfortunately he himself has developed a method for mass extermination. The most obvious one is nuclear war. We have enough nuclear on the planet to destroy all life - something that seems to close in the past (think of the Cuban missile crisis). But we can hope that one day we become secure enough not to use the weapons on a global scale.
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