In the past half billion years, five great extinctions have transformed life on Earth, whether brought about by a catastrophic meteor impact that killed off the dinosaurs or prolonged periods of intense volcanic activity, which wiped out as much as 97 percent of all species.
Now, say scientists, Earth may be experiencing a sixth wave of mass extinction. But this time, humans are the cause.
Forty-one percent of amphibians, 26 percent of mammals, and 13 percent of birds face the threat of extinction, according to an analysis conducted by the journal Nature. Over hunting, habitat destruction, and climate change are propelling the die-offs, as well as the spread of invasive species and disease.
"Scientists are seeing very high extinction rates right now," Elizabeth Kolbert, New Yorker staff writer and author of The Sixth Extinction: An Unnatural History, told VICE News. "Some people would say 100, some people would say 1,000, and some people are saying 10,000 times what we should be seeing."
The Nature report estimates somewhere between 500 and 36,000 species a year are going extinct. Scientists dispute the precise figure because estimates of the number of distinct plant and animal species range widely, from less than two million to more than ten million.
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"We're losing species at just an incredible rate and I think most scientists think that rate is increasing," Noah Greenwald of the Center for Biological Diversity told VICE News.
The worst-case scenario in the Nature analysis predicts that 75 percent of life on Earth may become extinct by 2200. In that scenario, the paper assumes there are 5 million species on Earth and extinction occurs at a steady rate of 0.72 percent of species a year. There's a great amount of uncertainty in projecting extinction rates and the number of species that might be impacted, however, because of the widely varying estimates on the number of existing types of plants and animals.
Other studies support Nature's findings yet highlight the high level of uncertainty in trying to nail down the scale of species extinction. A 2010 paper in the journal Science says that between 23 and 36 percent of all mammals, birds, and amphibians consumed by humans are threatened with extinction. The International Union for Conservation of Nature (IUCN) warns that more than 20,000 of 75,000 species they studied are at risk.
The causes driving the current wave of extinction are varied — yet each comes back in some way to human activity.
"One driver of extinction is simply overhunting or overharvesting," Kolbert said. "Rhinos are in terrible trouble, because they're being killed and they can't reproduce fast enough to save themselves. So one driver is simply, we're killing them."
Habitat destruction is another major factor driving extinction. As global human population doubled over the last forty years, more than fifty percent of the Earth's surface was degraded or destroyed due to human intervention. During that time, half of the world's wildlife population was killed.
Invasive species and disease present acute challenges for some plants and animals. Humans have proven to be very adept at introducing critters into environments where they can cause great havoc. Think of the Asian Carp that is advancing northward along the Illinois and Mississippi rivers and threatens to populate the Great Lakes. Kolbert noted that amphibians are dying in droves because of a particularly lethal fungus that piggybacks on humans.
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Perhaps the most troubling factor driving the sixth great extinction is all the greenhouse gas emissions humans are spewing into the atmosphere and oceans.
"Climate change and ocean acidification are predicted to become, and I think it's hard to really avoid the idea that they're going to become, drivers of extinction, as we move outside historical tolerances," Kolbert told VICE News.
Even before massive swaths of species become extinct, scientists warn these drivers will also immediately alter fragile ecosystems.
"Even though I wrote a book called the Sixth Extinction and it was about extinction — there's too much emphasis actually on extinction, on the last animal," Kolbert said. "So say we're down to the last 100 and we save them and that animal is not technically extinct — but it is functionally extinct in the ecosystem. I think people are more and more trying to get away from extinction being the problem; the problem is declines in population, to the point that the animal might as well be extinct."
Kolbert used cod as an example. Overfishing decimated cod populations in the Atlantic. Governments placed bans on cod fishing hoping populations might rebound. But so far, that hasn't happened. In the absence of a healthy cod population, the species it preyed upon have flourished at the same time that predators that competed with cod have thrived. Although some scientists are hopeful the ecosystem may recover, they don't know if cod will play the same role in the marine habitat as it did before overfishing caused its population to plummet.
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The sixth great extinction shows the extent to which humans might be a plague upon the land, whether driving the bulldozers that transform prairie lands, burning the coal that warms the atmosphere, or poisoning lakes and rivers with phosphate-enriched fertilizer that boost the volume of our vegetables. But in a plot seemingly from a Greek tragidy, humans are the only ones that can put a stop to what we have set in motion.
"One couldn't see a way to un-crash an asteroid," Jon Hoekstra, Vice President and Chief Scientist of the World Wildlife Fund (WWF), told VICE News. "At a fundamental level, this mass extinction is so different because not only is it caused by people, but it can be averted by people."
Nearly 200 nations have ratified the Convention on Biological Diversity, a 1993 treaty aimed at protecting the world's biodiversity and scaling back extinction rates. Under the treaty's provisions, nations coordinate efforts at stopping the spread of invasive species, establish large-scale conservation areas, and pledge to mitigate the impacts of climate change. The United States has not ratified the treaty.
"This is really the most serious problem that humanity faces, because you can't undo extinction," Greenwald told VICE News. "You're essentially creating irreparable harm, but it's not discussed in the halls of Congress. And President Obama's not talking about the extinction crisis."
Follow Shelby Kinney-Lang on Twitter: @ShelbKL