Mass extinction is defined as the loss of at least 75% of earths species in a relatively short geological time, resulting from a catastrophin global even, natural disaster, or abrubt change to the environment (Biology-online.org)Edit
There have been five recorded mass extinctions in the last 540 million years, occuring at the end of the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods.
Over the last few centuries humans have depleted natural resources, fragmented habitats, introduced invasive species, spread disease , exploited and killed off various species, and have changed the environment through global warming (Bansky, Matzke, etc. 2011). All these contributions are what scientists are saying are causing the sixth mass extinction.
Mass Extinction is defined by both rate and magnitude, requiring a specific fraction of species to have gone extinct per unit time. Observations of rates of extinctions in the last century has led scientists to believe that if all 'threatened' species became extinct in the next 100 years, with a constant rate, the time until 75% of the Earth's species was lost would occur in ~240 -540 years (Bansky, Matzke,etc, 2011).
With a sixth mass extinction so close in our future, some believe that it is inevitable under the best conditions. Combined with rapid climate change we now face the urgency to take action and relieve these pressures on threatened species are at an all time high.
Barnosky, A. D., Matzke, N., Tomiya, S., Wogan, G. O., Swartz, B., Quental, T. B., ... & Ferrer, E. A. (2011). Has the Earth/'s sixth mass extinction already arrived?. Nature, 471(7336), 51-57.