Life on Earth is hurtling towards extinction levels comparable to those following the dinosaur-erasing asteroid impact of 65 million years ago, propelled forward by human activities, say scientists.

This week, scientists from the University of California, Berkeley, announced that if current extinction rates continue unabated, and vulnerable species disappear, Earth could lose three-quarters of its species as soon as three centuries from now.

"That's a geological eyeblink," said Nicholas Matzke, a graduate student at UC Berkeley and author of a paper describing the doom-and-gloom scenario.

"Once you lose species, you don't get them back. It takes millions of years to rebound from a mass extinction event."

This means that not, too far in the future, backyards might not be buzzing with bees, bombarded by seagulls or shaded by redwood trees. And while that might seem far off, species are already disappearing on a global scale.

In recent history, we've lost the dodo bird and the passenger pigeon, the Javan tiger and the Japanese sea lion, and now, maybe the eastern cougar - declared extinct by the US Fish and Wildlife Service on Wednesday.

Amphibians, mammals, plants, fish - none are immune to going the way of the dinosaurs, courtesy of the human impact on fragile ecosystems.

Such enormous losses have occurred only five times in the past half-billion years, during events known as "mass extinctions".

The best-known of these events occurred 65 million years ago - a "really bad day," according to paleontologists - when an asteroid collided with Earth, sending fiery dust into the atmosphere and rapidly cooling the planet.

These "Big Five" events set the extinction bar high: to reach mass wipe-out status, 75 per cent of all species need to disappear within a geologically short time frame, meaning that Earth is currently on the brink of the sixth mass extinction.

To determine whether current losses could equal these mass extinction rates, scientists compared recent rates with species die-offs during the Big Five, taking into account presently endangered species.

They also looked at the number of species lost in recent history, and found that while rates are dramatically higher than expected, the percentage of vanishing species is not elevated - yet.

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We already are engaged in a seemingly inexorable march toward barren landscapes and empty seas, a procession fuelled by human population growth, resource consumption and climate change, scientists say.

"The good news is, we still have most of what we want to save," said Berkeley paleobiologist and lead study author Anthony Barnosky. "But things are clearly going extinct too fast today."

The paper, published in this week's issue of Nature, resulted from a graduate seminar Barnosky organised in autumn 2009.

Together, he and students used fossils to compare extinction rates with more modern data, wanting to answer whether we really are seeing the sixth mass extinction.

To make comparisons, scientists used information from well-preserved fossils and modern accounts of disappearing animals, focusing on our milk-bearing relatives: mammals.

Stanford University biologist Paul Ehrlich, who was not involved in the study, said evidence of the sixth extinction is all around.

For years, he studied the bay checkerspot butterfly on Stanford's campus - but then, the butterfly disappeared from the campus, more than a decade ago.

And, when Ehrlich journeyed to Morocco to sample a different checkerspot species, he found no butterflies, just "sheep droppings and not one blade of grass".

Scientists say habitat destruction, global climate change, introducing invasive species, and population growth are contributing to losses.

"Those four things working in concert are kind of a perfect storm that's setting up a recipe for disaster," Barnosky said. "But people are the ones who are driving this extinction, so we can fix it."

In addition to prioritising species preservation, Ehrlich suggested starting with caps on human population growth and limiting resource consumption.

"We could do something about it, but I don't see that we have the slightest inclination to," he said.