Humans have caused so many changes to our planet that some experts say we're on par with mass extinctions of eons past. We are equivalent to the asteroid that crushed most of the dinosaurs or the Great Dying that wiped out nearly all life on Earth. Some have proposed calling our current geological age the "Anthropocene," derived from the Greek word "anthropo" for "human."
While it may sound like an accomplishment being able to wield so much influence we can alter our home planet, human activity has done so in dangerously unsustainable ways. Indeed, according to a recent study published in the journal Proceedings of the National Academy of Sciences, humans have caused so many extinctions in the last 500 years that it would have taken 18,000 years for those same genera to have naturally vanished if we had never existed. After all, animals go extinct regularly on geological timescales, but this mass extinction is largely human-driven.
And that is just the first sobering statistic from the study. Authors Gerardo Ceballosa and Paul R. Ehrlich from Stanford University and Mexico's Universidad Nacional Autonoma de Mexico examined the classification statuses of 5,400 vertebrate genera (excluding fishes), a task that included 34,600 species. After they had finished, the scientists determined that 73 genera of animals had become extinct since 1500 AD.
"Beyond any doubt, the human-driven sixth mass extinction is more severe than previously assessed and is rapidly accelerating," the authors explained. This means animals are vanishing at a rate 35 times higher than those which prevailed over the past million years — that is, before the advent of the Anthropocene. While this news might have a silver lining if humanity was at least arresting the trends driving these mass extinctions, the exact opposite is true.
"Current generic extinction rates will likely greatly accelerate in the next few decades due to drivers accompanying the growth and consumption of the human enterprise such as habitat destruction, illegal trade and climate disruption," Ceballosa and Ehrlich write. "If all now-endangered genera were to vanish by 2100, extinction rates would be 354 (average) or 511 (for mammals) times higher than background rates, meaning that genera lost in three centuries would have taken 106,000 and 153,000 [years] to become [extinct] in the absence of humans."
They concluded, "Such mutilation of the tree of life and the resulting loss of ecosystem services provided by biodiversity to humanity is a serious threat to the stability of civilization."
To illustrate the magnitude of the mass extinction epidemic, the authors mention that "there were around 10,000,000 African elephants at the beginning of the 20th century, and now there are only about 450,000 remaining," a drop over 95%. Yet overall the researchers found that "most recorded extinctions have occurred in birds, followed by amphibians, mammals, and then reptiles."
In terms of the numerous bird species lost, the most prevalent extinct orders include giant birds like the elephant birds (Aepyornithiformes) of Madagascar and the moas (Dinornithiformes) of New Zealand. The researchers also added that, while there is "scanty" data, it seems most of the genera lost have only vanished over the past two centuries.
The authors point to some species, like the Steller sea cow, which went extinct around 1768, but many more species — including passenger pigeons, Tasmanian tigers and the Yellow river dolphin — have "departed since modern science began to pay attention."
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The scientists consider it far from a good thing that so many extinctions have occurred at a time when humans have had the technology to record them. At this rate of extinctions, we are in for a very unpleasant shock in the near future.
"In other words, projected losses of genera over three centuries (1800 to 2100) would have taken 106,000 [years] for all vertebrates and up to 153,000 [years] for mammals to become [extinct] under the normal, background rates," the authors write. They don't mince words, descibing a "biological holocaust" and comparing us to rogue landscapers hacking away at every branch of life. "Mutilating the tree of life is changing the systems in which human beings and all other living organisms have evolved," they warn.
"During past mass extinctions there was no species with the power or interest to stop extinctions, and no conscious stake in maintaining biodiversity," they add. "Today there is a species that should know it is not able to wait millions of years for its life-support systems to be restored after a mass extinction."
This is not the first recent study to underscore Earth's increasingly compounding ecological crises. According to a recent study in the journal Science Advances, humanity is at existential risk for six of its nine planetary boundaries, or the framework that establishes how safely humanity can operate within Earth's biological and physical limitations before undermining our own ability to survive. These include maintaining a stable climate, land system change, freshwater change, novel entities (like plastics, pesticides and industrial chemicals) and the flows of biological and geological chemicals.
"We live by using the Earth's resources and we throw our waste into the open environment," the study's lead author Dr. Katherine Richardson, professor in Biological Oceanography at the University of Copenhagen's Sustainability Science Centre, told Salon in an email. "The Earth's resources are limited and our demand exceeds their supply. You can party even when your bank account balance is declining — but you cannot party forever and that is the situation humanity has brought itself into."