Washington, Jul 11 (IANS): Many scientists say it is "abundantly" clear that Earth is entering its sixth mass-extinction event, meaning three-quarters of all species could disappear in the coming centuries, the media reported.

According to a study published on Monday in the Proceedings of the National Academy of Sciences, people are inflicting "biological annihilation" on the natural world, reports CNN.

Gerardo Ceballos, an ecology professor at the Universidad Nacional Autonoma de Mexico, and his co-authors, including well-known Stanford University biologist Paul Ehrlich, cite striking new evidence that populations of species we thought were common are suffering in unseen ways.

"What is at stake is really the state of humanity," Ceballos told CNN.

Their key findings: Nearly one-third of the 27,600 land-based mammal, bird, amphibian and reptile species studied are shrinking in terms of their numbers and territorial range.

The researchers called that an "extremely high degree of population decay".

The scientists also looked at a well-studied group of 177 mammal species and found that all of them had lost at least 30 per cent of their territory between 1900 and 2015; more than 40 per cent of those species "experienced severe population declines", meaning they lost at least 80 per cent of their geographic range during that time, CNN reported.

Looking at the extinction crisis not only in terms of species that are on the brink but also those whose populations and ranges are shrinking helps show that "Earth's sixth mass extinction is more severe" than previously thought, the authors wrote.

They say a major extinction event is "ongoing".