Earth is entering a mass extinction that threatens humanity's existence, researchers have declared.

A team of American scientists claim that their study shows 'without any significant doubt' that we are entering the sixth great mass extinction on Earth.

The study says that species are disappearing at a rate 100 times faster than would normally be expected - and that is a conservative estimate.

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This chart shows the enormous uptick in species loss over the last century. Since 1500, more than 320 terrestrial vertebrates have become extinct.

320 EXTINCTIONS SINCE 1500 Since 1500, more than 320 terrestrial vertebrates have become extinct. Populations of the remaining species show a 25 percent average decline in abundance, and the situation is similarly dire for invertebrate animal life. Across vertebrates, 16 to 33 percent of all species are estimated to be globally threatened or endangered. Large animals – described as megafauna and including elephants, rhinoceroses, polar bears and countless other species worldwide – face the highest rate of decline, a trend that matches previous extinction events. Advertisement

And such a catastrophic loss of animal species presents a real threat to human existence, the experts warn, as crucial ecosystem 'services' such as crop pollination by insects and water purification in wetlands is also put at risk.

At the current rate of species loss, humans will lose many biodiversity benefits within three generations, according to Paul Ehrlich, the Bing Professor of Population Studies in biology and a senior fellow at the Stanford Woods Institute for the Environment, who led the research.

'We are sawing off the limb that we are sitting on,' Prof Ehrlich said.

The study warns that humans are are precipitating 'a global spasm of biodiversity loss' - and that the window for conserving threatened species is rapidly closing.

Prof Ehrlich said: 'The study shows without any significant doubt that we are now entering the sixth great mass extinction event.'

The study, published in the journal Science Advances, shows that even with extremely conservative estimates, species are currently disappearing up to about 100 times faster than the normal rate between mass extinctions, known as the background rate.

'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 Autónoma de México.

The authors fear that 75 per cent of the species on Earth today could be lost in just two generations' time.

HOW HUMANITY HAS RUINED THE EARTH The human population growing in numbers, per capita consumption and economic inequity has altered or destroyed natural habitats, the researchers say. They said the main impacts were: Land clearing for farming, logging and settlement

Introduction of invasive species

Carbon emissions that drive climate change and ocean acidification

Toxins that alter and poison ecosystems Advertisement

'We emphasize that our calculations very likely underestimate the severity of the extinction crisis, because our aim was to place a realistic lower bound on humanity's impact on biodiversity,' the researchers write.

To history's steady drumbeat, a human population growing in numbers, per capita consumption and economic inequity has altered or destroyed natural habitats.

Large animals – described as megafauna and including elephants, rhinoceroses, polar bears and countless other species worldwide – face the highest rate of decline, a trend that matches previous extinction events

Now, the specter of extinction hangs over about 41 percent of all amphibian species and 26 percent of all mammals, according to the International Union for Conservation of Nature, which maintains an authoritative list of threatened and extinct species.

'There are examples of species all over the world that are essentially the walking dead,' Ehrlich said.

THE FIVE GREAT EXTINCTION EVENTS Five times, a vast majority of the world's life has been snuffed out in what have been called mass extinctions, often associated with giant meteor strikes. End-Ordovician mass extinction

The first of the traditional big five extinction events, around 440 million years ago, was probably the second most severe. Virtually all life was in the sea at the time and around 85% of these species vanished. Late Devonian mass extinction About 375-359 million years ago, major environmental changes caused a drawn-out extinction event that wiped out major fish groups and stopped new coral reefs forming for 100 million years. End-Permian mass extinction (the Great Dying)

The largest extinction event and the one that affected the Earth’s ecology most profoundly took place 252 million years ago. As much as 97% of species that leave a fossil record disappeared forever. End-Triassic mass extinction

Dinosaurs first appeared in the Early Triassic, but large amphibians and mammal-like reptiles were the dominant land animals. The rapid mass extinction that occurred 201 million years ago changed that. End-Cretaceous mass extinction An asteroid slammed down on Earth 66 million years ago, and is often blamed for ending the reign of the dinosaurs. Advertisement

A broken Earth: Researchers say a new study shows 'without any significant doubt' that we are entering the sixth great mass extinction on earth.

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,' the study's authors write.

Ceballos added: 'I'm optimistic in the sense that humans react - in the past we have made quantum leaps when we worked together to solve our problems.'