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 poison gas. Glaciers subsume the land and lock up the seas 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..

Still, scientists say, it’s possible to avert their gloomy predictions. They give us about a generation.

“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.”