What is a 'mass extinction' and are we in one now ?

Mass extinction refers to a substantial increase in degree of extinction or when Earth loses more than three-quarters (75%) of its species in a geologically short period of time.

So far there have been five mass extinctions, in last 450 million years, that have led to destruction of 70-95 % of species of plants, animals and microorganisms that existed earlier.

These were caused by events such as massive volcanic eruptions, depletion of oceanic oxygen or collision with an asteroid.

After each of these extinctions, it took millions of years to regain species.

Ongoing sixth mass extinction may be one of the most serious environmental threats to persistence of civilization as loss of species will be permanent.

400 vertebrate species went extinct in last century - Sixth mass extinction is human-caused (Anthropocene extinction) and is more immediate than climate destruction.

There will be more pandemics if we continue destroying habitats and trading wildlife.

First Mass Extinction: The Ordovician mass extinction that occurred about 445 million years ago killed about 85% of all species.

Second Mass Extinction: The Devonian mass extinction (about 375 million years ago) wiped out about 75% of the world?s species.

Third Mass Extinction: The Permian mass extinction (about 250 million years ago) also known as the Great Dying caused the extinction of over 95% of all species.

Fourth Mass Extinction: The Triassic mass extinction (about 200 million years ago) eliminated about 80% of Earth?s species, including some dinosaurs.

Fifth Mass Extinction: This Cretaceous mass extinction (about 65 million years ago) is known for wiping out non-avian dinosaurs.