It’s now unequivocal: the sixth great spasm of species extinctions has begun.   We – homo sapiens – are its cause. And only we can slow it down.

Over the last century, the average rate of loss of vertebrate species — fish, amphibians, reptiles, birds and mammals – has been up to 100 times higher than the background extinction rate, according to a new study published last week in the journal Science Advances.

In order to help settle the question of whether a sixth extinction episode has indeed begun, the scientific team chose assumptions that would tend to minimize evidence that it has.  As a result, their calculations almost certainly underestimate the severity of the extinction crisis under way.

“[W]e can confidently conclude that modern extinction rates are exceptionally high, that they are increasing, and that they suggest a mass extinction under way – the sixth of its kind in Earth’s 4.5 billion years of history,” the researchers write.

The study team included scientists from Princeton, Stanford, the University of California-Berkeley, the Universidad Nacional Autónoma de México, and the University of Florida.

The last episode of mass extinction occurred about 65 million years ago, when the dinosaurs and about half of all species living on Earth at the time were wiped out.

A huge crater off Mexico’s Yucatán Peninsula dated to the time of this event suggests an extraterrestrial impact as a leading cause.

But, in a first, the current mass extinction is driven by human activities – deforestation, dam-building, over-harvesting, wetland-draining, pollution and the myriad other ways we destroy the lives and homes of the rich diversity of animals with which we share the planet.

And this will not end in some Darwinian-style victory for we humans.

Rather, this die-off is a direct threat to our own well-being and survival, because so many of the “ecosystem services” the diverse web of life delivers are so crucial to us. Imagine the security of our food system with no natural pollination, or the cleanliness of our waters with no natural purification.

“If [the loss of species] 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, in a synopsis released by Stanford News.

Under the team’s conservative assumptions about the background extinction rate (the loss of species occurring between the mass extinction events), if no mass extinction episode was now happening, it would have taken between 800 and 10,000 years for the number of vertebrate species that actually went extinct since 1900 to go extinct.

Although it presents an existential crisis of the first order – Stanford University biologist and study co-author Paul Ehrlich likens it to “sawing off the limb we are sitting on” – the loss of global biodiversity has never risen to the top of our economic or even environmental concerns.

Stemming the tide of species loss would take vastly stepped up efforts to conserve habitats, reduce the exploitation of fishes and other species, and slow the pace of climatic change.

While here and there, gains are being made, the scale of our global response pales next to that of the crisis.

And, as the research team concludes, “the window of opportunity is rapidly closing.”

Originally posted at National Geographic Newswatch.