Even conservative calculations show the world is in the midst of a sixth mass extinction that's being caused by our species — and is likely to lead to humanity's demise if unchecked, scientists reported Friday.
The scientists' analysis, published in the open-access journal Science Advances, follows up on more than a decade's worth of warnings about a rapid loss of global biodiversity. Many experts say the loss has risen to the scale seen during five previous global extinction events — the most recent of which occurred 65 million years ago when the dinosaurs died off.
The claims provide the theme of Elizabeth Kolbert's Pulitzer Prize-winning book, "The Sixth Extinction." But such claims have drawn skeptical responses as well. The skeptics say it's difficult to judge the "background rate" of extinctions, as well as the current rate of species extinction.
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The team behind the new study, led by Gerardo Ceballos of the National Autonomous University of Mexico, tried to address such concerns by doubling the commonly cited background rate — going from one species extinction per million species per year to two extinctions. And for the current rate, they turned to a list maintained by the International Union for Conservation of Nature — which they characterized as a conservative accounting.
Based on those figures, the extinction rate since 1900 has been eight to 100 times higher than the expected background rate, the researchers said. "The particularly high losses in the last several decades accentuate the increasing severity of the modern extinction crisis," they reported.
"There are examples of species all over the world that are essentially the walking dead," one of the study's co-authors, Paul Ehrlich of Stanford University, said in a news release.
The researchers noted that amphibians, which account for 7,300 of the species documented by the IUCN, have been particularly hard-hit. "Only 34 extinctions have been documented with a high level of certainty since 1500, yet [more than] 100 species have likely disappeared since 1980," they said.
Many of the causes for biodiversity loss have been traced to industrialization, deforestation and other, less obvious human factors. For example, in the case of the amphibians, the wave of extinctions have been traced to the rapid spread of a fatal fungus through global shipping.
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Ceballos warned that the human species itself could eventually fall victim to the sixth extinction.
"If it is allowed to continue, life would take millions of years to recover, and our species itself would disappear early on," he said.
Although the report paints a depressing picture, Ceballos and his colleagues hold out a slender ray of hope.
"Avoiding a true sixth mass extinction will require rapid, greatly intensified efforts to conserve already threatened species, and to alleviate pressures on their populations — notably habitat loss, overexploitation for economic gain and climate change," they write. "All of these are related to human population size and growth, which increases consumption (especially among the rich), and economic inequity. However, the window of opportunity is rapidly closing."
Update for 10 p.m. ET June 19: In the past, Danish environmentalist Bjorn Lomborg has stirred up controversy by arguing that extinction rates won't be as dire as they've been made out to be. In an email to NBC News, Lomborg said the newly published paper appeared to be "well in line with current assumptions ... namely that humanity is definitely causing more extinctions, likely hundreds of times more than the natural background rate."
Lomborg said the authors' claim that current extinction rate could be as much as 114 times the background rate "definitely sounds plausible," but he took issue with their assessment of the implications for the future.
"These extinction rates are not likely to persist for many centuries, if even one century, as richer countries can afford to and do reforest, while they also can afford to embark on ever more serious conservation," Lomborg wrote.
He said that if you run the numbers, the paper suggests 1,508 vertebrate species could go extinct over the course of the next century. (Assuming that there are 66,178 vertebrate species, the formula is 2*114*66178/10000=1508.).
"This is definitely a problem, but it is not the eradication of the natural world," Lomborg wrote. "It equates to 2.3 percent of all vertebrates over the next 100 years — after which it is likely the problem will decline rapidly because most countries will be rich and care much for the natural world."
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In addition to Ceballos and Ehrlich, the authors of "Accelerated Modern Human–Induced Species Losses: Entering the Sixth Mass Extinction" include Anthony Barnosky, Andres Garcia, Robert Pringle and Todd Palmer.