A tree in a deforested area of the Amazon in 2014. Biologists say that deforestation and other human activity has contributed to the extinction of hundreds of species in the past 114 years. (Raphael Alves/AFP via Getty Images)

A vast chunk of space rock crashes into the Yucatan Peninsula, darkening the sky with debris and condemning three-quarters of Earth’s species to extinction. A convergence of continents disrupts the circulation of the oceans, rendering them stagnant and toxic to everything that lives there. Vast volcanic plateaus erupt, filling the air with poisonous gas. Glaciers subsume the land and lock up the oceans in acres of ice.

Five times in the past, the Earth has been struck by these kinds of cataclysmic events, ones so severe and swift (in geological terms) they obliterated most kinds of living things before they ever had a chance to adapt.

Now, scientists say, the Earth is on the brink of a sixth such “mass extinction event.” Only this time, the culprit isn’t a massive asteroid impact or volcanic explosions or the inexorable drifting of continents. It’s us.

“We are now moving into another one of these events that could easily, easily ruin the lives of everybody on the planet,” Stanford biologist Paul Ehrlich said in a video created by the school.

In a study published Friday in the journal Science Advances, biologists found that the Earth is losing mammal species 20 to 100 times the rate of the past. Extinctions are happening so fast, they could rival the event that killed the dinosaurs in as little as 250 years. Given the timing, the unprecedented speed of the losses and decades of research on the effects of pollution, hunting and habitat loss, they assert that human activity is responsible.

“The smoking gun in these extinctions is very obvious, and it’s in our hands,” co-author Todd Palmer, a biologist at the University of Florida, wrote in an e-mail to The Washington Post.

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Since 1900 alone, 69 mammal species are believed to have gone extinct, along with about 400 other types of vertebrates. Evidence for species lost among nonvertebrate animals and other kinds of living things is much more difficult to come by, the researchers say, but there’s little reason to believe that the rest of life on Earth is faring any better.

This rapid species loss is alarming enough, according to the study’s authors, but it could be just the beginning.

“We can confidently conclude that modern extinction rates are exceptionally high, that they are increasing, and that they suggest a mass extinction under way,” they write. “If the currently elevated extinction pace is allowed to continue, humans will soon (in as little as three human lifetimes) be deprived of many biodiversity benefits.”

Stanford researcher Paul Ehrlich discusses mass extinction research. Ehrlich and his co-authors call for fast action to conserve threatened species, populations and habitat, but warn that the window of opportunity is rapidly closing. (Stanford University)

The Science Advances study is not the first to propose that the die-offs caused by human activity are now on par with the fatal cataclysms of millennia past. In 1998, an American Museum of Natural History poll of 400 biology experts found that 70 percent believe the Earth is in the midst of one of its fastest mass extinctions, one that threatens the existence of humans as well as the millions of species we rely on. In his 2003 book “The Future of Life,” noted Harvard biologist E.O. Wilson calculated that Earth would lose half its higher life forms by 2100 if the current rate of human disruption continued. Scores of scientific studies have sought to bolster that claim, offering evidence of current die-offs and predicting future ones. And many more have contributed to the International Union for the Conservation of Nature’s Red List, which keeps a bleak accounting of the extinction risk for tens of thousands of species.

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It’s true that throughout history, extinctions have happened for comparatively mundane reasons. Even without asteroid impacts or human disruption, species are always dying out — the “unfit” in Darwin’s terminology — and being replaced. Scientists estimate that 99 percent of the species that ever existed no longer do. It’s a routine part of life on Earth.

What’s happening now, the researchers say, is not routine.

To prove how extraordinary the losses of the past 114 years have been, the authors of the new study used data from the IUCN Red List to calculate modern extinction rates and compared that number to the “background,” or routine, rate of extinctions. To counter claims that their research might be exaggerated or alarmist, the authors of the Science Advances study assumed a fairly high background rate: 2 extinctions per 10,000 vertebrate species each century, or 2 species per million each year (a metric known as E/MSY), based on the fossil record. Most commonly used estimates are much lower — typically between 0.1 and 1 MSY.

Under normal conditions, this assumed background rate means that Earth should have seen 9 vertebrate extinctions since 1900, the study says. (The researchers focused on vertebrates and mammals in particular because those species have been the subject of the most thorough conservation status assessments.)

But species these days are not living under normal conditions, the biologists say. Forests are vanishing. Animals are hunted for their tusks and teeth and fur. Toxins are leaching into streams and lakes and the ground beneath us. The global climate is changing, and habitats around the world are changing with it.

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And, as in past mass extinctions, even the “fit” have been unable to adapt.

Based on the IUCN list of species that have been declared extinct, extinct in the wild, and possibly extinct (species that haven’t been seen in the wild for years but whose loss hasn’t been confirmed), 468 more vertebrates have died out since 1900 than should have. That translates to an extinction rate 53 times the rate of baseline levels at the “high” background extinction rate and more than 100 times the rate most other biologists use. Even using a highly conservative calculation that includes only the 199 vertebrate species definitively declared extinct, the rate of vertebrate species loss is 22 times higher than the 2 MSY baseline.

Though these extinctions are happening much faster than usual, they’re not yet comparable to the “Big Five” mass extinctions commonly recognized as the worst in Earth’s history. The losses of the past century account for only about 1 percent of the roughly 40,000 known vertebrate species — a statistic that pales in comparison to the level of destruction seen during previous mass extinction events. Even in the least of them, between 60 and 70 percent of species were killed off. During the end-Permian event about 250 million years ago, known as “the Great Dying,” that number was more than 90 percent.

But the loss of biodiversity we’re seeing now could trigger even more catastrophic species loss within a few years.

“Ecological communities are composed of many interacting parts, and there are potential ‘tipping points’ in these communities where if you lose too many species, or lose species that are particularly important, the ecosystem may rapidly degrade or change states,” Palmer wrote.

If die-offs continue at current rates, the current extinction event could reach “Big Five” magnitudes in 240 to 540 years, he said — an unprecedented speed for this kind of ecological change.

Past mass extinctions unfolded in geological time over the course of thousands of years. The calamitous “Great Dying” at the end of the Permian Period took about 6,000 centuries, as the super-continent called Pangaea coalesced, disrupting ocean currents and raising global temperatures, and lava oozed out of a vast volcanic region called the Siberian Traps, poisoning the air and seas with clouds of toxic gases. For a mass extinction to happen fast enough to be perceived in a human lifetime is unheard of.

“In terms of scale, we are now living through one of those brief, rare episodes in Earth history when the biological framework of life is dismantled,” paleobiologist Jan Zalasiewizc, who was not involved in the study, wrote in an analysis for the Guardian. He went on to note that none of the “familiar horsemen” of planetary change — “massive volcanic outbursts to choke the atmosphere and poison the seas, the mayhem caused by major asteroid impact and the wrenching effects of rapid climate change” — have factored into the current crisis (the effects of current climate change are still in their early stages, he wrote, and can’t yet be blamed for species loss). Instead, the deaths we see now are all due to pollution, predation and habitat change from one species: humans.

Still, scientists say, it’s possible to avert their gloomy predictions. They give us about a generation to make the changes needed to slow the rate of species loss.

“We have the potential of initiating a mass extinction episode which has been unparalleled for 65 million years,” co-author Gerardo Ceballos told CNN. “But 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.”