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“We 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.”

So write Gerardo Ceballos, a professor at the National Autonomous University of Mexico’s Institute of Ecology, and his co-authors in a blunt and frightening paper published on Friday in the journal Science Advances. They compared contemporary rates of extinction to the baseline rate before human activity began affecting the environment, and found that species today are disappearing at rates far above the baseline.

Under their most conservative estimates, mammal species have gone extinct at 28 times the baseline rate since 1900, and amphibian species have gone extinct at 22 times the baseline. A less conservative estimate puts those numbers at 55 and 100. If baseline rates had held, the number of species lost in the last century alone would have taken 800 to 10,000 years to go extinct.

Previous studies have also found big increases in extinction rates, but they’ve used less conservative measures and attracted some skepticism. Dr. Ceballos and his team wanted to use extremely conservative estimates of the current extinction rate — if they found a large increase even using those numbers, they’d have especially convincing evidence that we are indeed in a period of mass extinction.

That mass extinction, said Dr. Ceballos, is caused by habitat destruction, pollution, trade in products made from endangered species and climate change.

And unless we make big changes, we face an immeasurable loss. “Imagine a world without lions, without rhinos, without elephants, without birds,” said Dr. Ceballos. “It wouldn’t be the same.”

The forces that are causing animal extinctions threaten us too. In the next few decades, habitat destruction and climate change could cause ecosystems around the world to collapse, Dr. Ceballos said, taking with them the benefits they provide, like maintaining the balance of gases in the atmosphere. “Every time we lose a species,” he said, we draw closer to “a complete collapse of civilization as we understand it.”

He and his team tried to make their paper extremely straightforward to drive the risk home to a general audience. For those disturbed by their conclusions, he offered a few recommendations for stemming the tide of extinctions: Drive an electric car or a hybrid if you can afford to, eat less meat (again, if you can afford to) and avoid products that contribute to habitat destruction (some containing palm oil, for example). And, he added, we need to put pressure on governments to enforce endangered species protections. “What is at stake,” he said, “is the survival of mankind.”