"Species are disappearing up to about 100 times faster than the normal rate between mass extinctions, known as the background rate.
"Our calculations very likely underestimate the severity of the extinction crisis. There are examples of species all over the world that are essentially the walking dead."
Using fossil records and extinction counts from a range of sources, the researchers calculated the normal "background rate" of extinctions and compared it with a conservative estimate of present extinctions. Natural population changes in the wild usually lead to two species of mammals dying out every 10,000 years. But the rate is 114 times that level.
Humans have been responsible for animal decline going much further back. In the islands of tropical Oceania, up to 1800 bird species are estimated to have gone extinct in the last 2000 years. It is likely that early humans were also responsible for wiping out the huge megafauna which used to live in Australia including a giant wombat, a marsupial lion, and a flesh-eating kangaroo.
One in four mammals is at risk of going extinct and 41 per cent of amphibians. Many now only survive in captivity.
"If it is allowed to continue, life would take many millions of years to recover, and our species itself would likely disappear early on," said lead author Gerardo Ceballos, of the Universidad Autonoma de Mexico.
Ehrlich said governments must start working together to conserve threatened species. Despite the gloomy outlook, there is a meaningful way forward, according to Ehrlich and his colleagues.
"Avoiding a true sixth mass extinction will require rapid, greatly intensified efforts to conserve already threatened species, and to alleviate pressures on their populations - notably habitat loss, over-exploitation for economic gain and climate change," write the authors of the study, which was published in the journal Science Advances.
10 of the planet's most endangered species
1 South China tiger
Native to the southern Chinese provinces of Fujian, Guangdong, Hunan and Jiangxi the tiger had a population of 4000 in the 1950s but is now thought to only exist in zoos.
2 Sumatran elephant
As more of Sumatra's forest becomes converted for agriculture, the elephant has faced a critical loss of habitat. A 2007 study estimated there were less than 2800 remaining in the wild.
3 Amur leopard
The world's rarest cat is believed to be making a comeback with at least 57 confirmed animals in Russia. Just 30 animals existed in 2007. But the animal is still vulnerable to hunters.
4 Atlantic goliath grouper
Despite the US issuing a moratorium on hunting the big fish in 1990, the animal remains critically endangered.
5 Gulf porpoise
The Gulf porpoise is now one of the rarest mammals in the world, with a global population estimated at under 100 in 2014. The last remaining porpoises live in North America's Gulf of California and experts expect it to become extinct by 2018.
6 Northern bald ibis
The bird's natural habitat of North Africa, European and the Middle East has been plagued with war and civil unrest and now only one population exists in Morocco, with just a few hundred remaining. Attempts are under way in Austria, Spain and Italy to breed the animals for reintroduction into the wild.
7 Hawksbill turtle
The 20,000 strong female population is under threat by hunters seeking their brown and gold shells.
8 Black rhinoceros
The black rhino has suffered the most drastic decline in total numbers of all rhino species and was officially declared extinct in the wild in 2011. However, a major conservation effort has seen numbers swell to 5000 and now the animals are kept under armed guard.
9 Pygmy three-toed sloth
Found on Panama's uninhabited Escudo de Veraguas island, a 2012 study found fewer than 80 sloths were still living because of habitat loss by loggers.
10 Chinese pangolin
Used extensively as a food, and for Chinese medicines, the pangolin has declined by 94 per cent since the 1960s. Numbers have been hard to estimate as the creature is nocturnal and solitary.
The 5 mass extinctions
•83 per cent of sea life wiped out in Ordovician-Silurian mass extinction 443 million years ago.
•In the Late Devonian mass extinction, which followed 90 million years later, three quarters of life on Earth became extinct.
•The Permian mass extinction of 248 million years ago was nicknamed The Great Dying as 96 per cent of species died out.
•48 million years later, half of all species were wiped out by climate change and asteroid impacts.
•The final one marked the end of the dinosaurs
