The sixth great mass extinction event is here "without any significant doubt" and under conservative estimates, a Stanford University scientist has said.

Paul Ehrlich, from the Stanford Woods Institute for the Environment, and colleagues found that species are disappearing around 100 times faster than the normal rates expected between mass extinctions – known as the background rate.

Published in the journal Science Advances, the team used extremely conservative estimates from fossil records and extinction counts from a range of records. They also used a background rate twice as high as those used in previous analysis.

By doing this, they brought the current extinction rate and average background as close to one another as possible.

"[The study] shows without any significant doubt that we are now entering the sixth great mass extinction event," Ehrlich said.

Current consensus says extinction rates have reached their highest levels since the dinosaurs went extinct 66 million years ago. However, some have said the crisis has been overestimated.

Lead author Gerardo Ceballos said: "If it is allowed to continue, life would take many millions of years to recover, and our species itself would likely disappear early on."

earth from space
Mass extinction upon us Nasa Earth Observatory

Researchers also noted that their calculations "very likely" underestimate the severity of the crisis as their aim was to place a "realistic lower bound" on mankind's impact on biodiversity.

The list of impacts include land clearing from farming, logging and settlement, invasive species, carbon emissions and toxins. "There are examples of species all over the world that are essentially the walking dead," Ehrlich said, noting that as species disappear, so do crucial ecosystems.

At the current rate of loss, humans will lose many biodiversity benefits within three generations. "We are sawing off the limb that we are sitting on," Ehrlich warned.

Concluding, the authors wrote: "Estimates reveal an exceptionally rapid loss of biodiversity over the last few centuries, indicating that a sixth mass extinction is already under way. Averting a dramatic decay of biodiversity and the subsequent loss of ecosystem services is still possible through intensified conservation efforts, but that window of opportunity is rapidly closing.

"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, overexploitation for economic gain and climate change."