Scientists have documented five major mass extinction events in Earth's history, during which at least three-quarters of life went extinct. But with humans clearing habitats, exterminating species and changing the climate, are we now in a sixth mass extinction?
Many researchers claim the sixth mass extinction is underway, with one team describing "biological annihilation" and "mutilation of the tree of life" in their scientific studies. However, others argue that the mass extinction hasn't begun yet.
Robert Cowie, a research professor at the University of Hawaii, told Live Science that, strictly speaking, you can't declare a mass extinction until it's actually happened — once 75% of species are gone.
A 2022 study led by Cowie and published in the journal Biological Reviews estimated that up to 13% of known species have gone extinct since 1500 — well below the 75% mass extinction threshold.
"It hasn't happened yet," he said.
Some researchers have estimated that we'll reach the 75% threshold within 10,000 years, while other studies have concluded that we could be at this grim milestone in just a few centuries — with the potential for an even shorter time frame if things get worse.
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Mass extinctions occur within a short geological time period of less than 2.8 million years, according to the Natural History Museum in London. The centuries to millennia it could take to reach the mass extinction threshold is well within that time frame. So, if you take those estimates as predictive, researchers can argue that the event has already started.
"We are witnessing the sixth mass extinction in real time," Anthony Barnosky, a professor emeritus of integrative biology at the University of California, Berkeley, told Live Science in an email.
Studies have estimated that species are currently going extinct between 100 and 1,000 times faster than the normal background rate of extinction, calculated based on when species evolve and go extinct in the fossil record. "I think the rate is going to increase as we destroy more of the planet," Cowie said.
Barnosky noted that the species extinction rate may mask the rapid decline in wildlife populations because we don't count species as extinct until the last individual is gone. Species are often declared extinct decades after they are last seen in the wild, while others persist with conservation measures when most of their population is dead.
"We have killed almost 70% of the planet's wild animals since I was born," Barnosky said. "Obviously that can't go on too much longer without making the sixth mass extinction an actuality."
A 2022 WWF report found that monitored vertebrate populations of mammals, birds, amphibians, reptiles and fish declined by an average of 69% between 1970 and 2018. That figure is a global average; Latin America had the highest regional decline, with 94%. Plus, that number doesn't include the more numerous invertebrate species.
Data on invertebrate decline is lacking, but some groups have suffered staggering losses. For instance, a 2015 study co-authored by Cowie and published in the journal Conservation Biology highlighted the decline of Hawaii's Amastridae snails due to invasive species and habitat loss. Of the 282 species that historically inhabited Hawaii, the researchers could only confirm that 15 were still alive. "That's a mass extinction," Cowie said.
Barnosky described the decimation of biodiversity and burgeoning mass extinction as "the bad news." But he said it's not too late to save most species heading for extinction and thus prevent us from reaching the sixth mass extinction threshold.
"Although we're wiping out populations and species astoundingly fast, we haven't completed the job yet," Barnosky said. "We still have a chance to turn things around, but the window of opportunity for that is slamming shut fast."