Natural disasters, disease and the failure to adapt to a changing or changed environment are the three major causes for a mass extinction event. In any event, one or all of the three could cause the complete loss of life on any part of our Earth or our entire Earth. Maybe some life within our seas will survive, but for how long will such life continue to survive there is no true answer.
A natural disaster, could strike a large area of our Earth at any time. As a matter of fact, earthquakes, tidal waves, fires, floods and various kinds of sever weather has recently killed hundreds of thousands of people, together with other life forms within many places on our Earth. Will our bad luck continue in the form of global warming that could cause the deaths of tens of millions of people?
Incurable diseases can also cause the deaths of every person who lives within such a place that is struck by one or more of those incurable viruses. As of now, millions have died from the effects of the A.I.D.S. Virus and millions more are infected with that virus World-Wide. The Ebola Virus is no less deadly and it kills its host within a matter of days. Who knows what other deadly virus is hiding and waiting to be dug up so that it might kill all who come in contact with it.
In the meantime, it is quite clear to me that global warming and the depletion of Earth's protective ozone layer is raising our Earth's average temperature and rapidly melting the ice at the poles and other places where ice exists year around. In the future millions of people will be displaced from their homes if that trend continues. The cause for global warming has been proved to be an increasing use and burning of fossil fuels, and yet more people are driving gas powered vehicles and the World's people have failed to develop and use green friendly forms of energy production.
Millions more people now have skin cancer as a result of their exposure to all types of UV rays that were once shielded by a five mile thick ozone layer that once surrounded our Earth. Most of our Earth is now protected by an ozone layer that is less than a mile thick and you now see the result of that. And yet, ozone layer destroying chemicals are still being manufactured and spewed into the atmosphere.
You can also add stupidity, as a fourth cause for mass extinctions. When there are better ways to produce energy and restore our once people friendly environment and nothing meaningful is accomplished toward that goal, Mankind is truly inviting its doom at some time within the future. For that problem you can blame our World leaders and the owners and the controllers of our World who want nothing more than to rule all of the World's people and themselves become filthy rich as they do it.