The sixth mass extinction in Earth’s history is already underway, scientists said in a study published in the Proceedings of the National Academy of Sciences journal. The researchers believe that the “biological annihilation” of wildlife in recent decades was caused by human overpopulation and over-consumption.

“The situation has become so bad it would not be ethical not to use strong language,” said Professor Gerardo Ceballos, of the Universidad Nacional Autónoma de México, who led the study. Their sample comprised 27,600 terrestrial vertebrate species, nearly half of the known vertebrates. Of these, the populations of 32% are dwindling, the scientists said.

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The study uses the example of the lion to support its argument. “The lion was historically distributed over most of Africa, southern Europe, and the Middle East, all the way to northwestern India,” the report said. “[Now] the vast majority of lion populations are gone.”

Studies conducted before this one have shown species going extinct at a significantly faster rate millions of years ago, but extinctions have been relatively rare in recent times, The Guardian reported. While the study found that a third of the thousands of species with decreasing populations are not categorised as endangered yet, it also surmised that nearly 50% of individual animals have been lost in the past few decades. The research team has pegged the population loss of mammals, birds, reptiles and amphibians in billions.

“All signs point to ever more powerful assaults on biodiversity in the next two decades, painting a dismal picture of the future of life, including human life,” the report said. Ceballos said an international organisation was needed to sponsor wildlife conservation.

The last mass extinction took place 65 million years ago in the Cretaceous-Tertiary, The Guardian reported.