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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 Related: Extinction Could Get Worse by 2200, Scientists Say 
 
 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. 
 
 Flash Interactive: See When Species Rose and Fell 
 
 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." 
 
 Related: Pope Francis Urges Revolution to Save Earth 
 
 A tally by the International Union for Conservation of Nature shows trends in the extinction of various groups of animal species. Stanford 
 
 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. ||||| Study reveals rate of extinction for species in the 20th century has been up to 100 times higher than would have been normal without human impact 
 
 The modern world is experiencing a “sixth great extinction” of animal species even when the lowest estimates of extinction rates are considered, scientists have warned. 
 
 The rate of extinction for species in the 20th century was up to 100 times higher than it would have been without man’s impact, they said. 
 
 
 
 Many conservationists have been warning for years that a mass extinction event akin to the one that wiped out the dinosaurs is occurring as humans degrade and destroy habitats. 
 
 But the authors of a study published on Friday said that even when they analysed the most conservative extinction rates, the rate at which vertebrates were being lost forever was far higher than in the last five mass extinctions. 
 
 The Earth stands on the brink of its sixth mass extinction and the fault is ours Read more 
 
 “We were very surprised to see how bad it is,” said Dr Gerardo Ceballos of the National Autonomous University of Mexico. “This is very depressing because we used the most conservative rates, and even then they are much higher than the normal extinction rate, really indicating we are having a massive loss of the species.” 
 
 Previous studies have warned that the impact of humans taking land for buildings, farming and timber has been to make species extinct at speeds unprecedented in Earth’s 4.5bn-year history. 
 
 Ceballos said that his study, co-authored by Paul R Ehrlich who famously warned of the impact of humanity’s “population bomb”, employed better knowledge of natural or so-called background extinction rates. He said it was conservative because it looked only at species that had been declared extinct, which due to stringent rules can sometimes take many years after a species has actually gone extinct. 
 
 Under a “natural” rate of extinction, the study said that two species go extinct per 10,000 species per 100 years, rather than the one species that previous work has assumed. 
 
 Modern rates of extinction were eight to 100 times higher , the authors found. For example, 477 vertebrates have gone extinct since 1900, rather than the nine that would be expected at natural rates. 
 
 “It’s really signalling we’ve entered a sixth extinction and it’s driven by man,” said Ceballos. 
 
 Facebook Twitter Pinterest Noah’s Ark and the General Deluge - a colour lithograph published in New York by Muller and Company, c 1870 by an unknown artist. Photograph: Corbis 
 
 However, Prof Henrique Miguel Pereira, the chair of the Group on Earth Observations Biodiversity Observation Network, said that the new paper did not add anything revolutionarily new. 
 
 “They argue that recent extinction rates are up to 100 times higher than in the past. I think it improves our documentation of the process but it does not yet confirm a sixth mass extinction. I tend to think we have a major biodiversity crisis, but it would take either a fast acceleration of current extinction rates or a couple of centuries at current extinction rates, for the current process to become a sixth mass extinction.” 
 
 The team behind the new analysis said “rapid, greatly intensified efforts” would be needed to stop or slow the extinctions currently underway. 
 
 Ceballos pointed to the Pope Francis’s encyclical on the environment, which was published on Thursday and lamented the loss of the world’s biodiversity, and interventions by Barack Obama, as signs of hope. “These important figures are starting to really grasp the problem,” he said. 
 
 On why people should be worried about the rate of extinctions, he said: “People say that’s really sad, but why does it affect me? There are many reasons we should care. We are the species that are causing the loss of all these other species.” 
 
 But the most important reason, he said, was that by losing species humanity was losing what enabled us to have a “good standard of living”. 
 
 The paper, Accelerated modern human–induced species losses: Entering the sixth mass extinction, was published in the journal Science Advances. ||||| Earth has entered its sixth mass extinction with animals now dying out at 100 times the normal rate, scientists have warned. 
 
 Humans have created a toxic mix of habitat loss, pollution and climate change, which has already led to the loss of at least 77 species of mammals, 140 types of bird since and 34 amphibians since 1500. 
 
 They include creatures like the dodo, Steller’s Sea Cow, the Falkland Islands wolf, the quagga, the Formosan clouded leopard, the Atlas bear, the Caspian tiger and the Cape lion. 
 
 Scientists at Stanford University in the US claim it is the biggest loss of species since the Cretaceous-Tertiary mass extinction which wiped out the dinosaurs 66 million years ago. 
 
 “Without any significant doubt that we are now entering the sixth great mass extinction event," said Professor Paul Ehrlich, at the Stanford Woods Institute for the Environment. 
 
 “Species are disappearing up to about 100 times faster than the normal rate between mass extinctions, known as the background rate. 
 
 “Our calculations very likely underestimate the severity of the extinction crisis. There are examples of species all over the world that are essentially the walking dead.” 
 
 Humans were responsible for wiping out the dodo 
 
 Using fossil records and extinction counts from a range of sources, the researchers calculated the normal ‘background rate’ of extinctions and compared it with a conservative estimate of current extinctions. 
 
 Natural population changes in the wild usually lead to two species of mammals dying out every 10,000 years. But the current rate is 114 times that level. 
 
 And humans have been responsible for animal decline going much further back. In the islands of tropical Oceania, up to 1800 bird species are estimated to have gone extinct in the last 2,000 years. 
 
 It is likely that early humans were also responsible for wiping out the huge megafauna which used to live in Australia including a huge giant wombat a marsupial lion and a flesh-eating kangaroo. 
 
 Currently one in four mammals is at risk of going extinct and 41 per cent of amphibians. Many now only survive in captivity. 
 
 A giant wombat which used to live in Australia 
 
 "If it is allowed to continue, life would take many millions of years to recover, and our species itself would likely disappear early on," said lead author Gerardo Ceballos of the Universidad Autónoma de México. 
 
 Professor Ehrlich said that government must start working together to conserve threatened species. 
 
 Despite the gloomy outlook, there is a meaningful way forward, according to Ehrlich and his colleagues. 
 
 "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, over-exploitation for economic gain and climate change," the study's authors write. 
 
 Many types of wolves have become extinct including British and Falkland Island varieties 
 
 Earth has experienced five recognised mass extinctions. The Ordovician-Silurian mass extinction of 443 million years ago saw 83 per cent of sea life wiped out. In the Late Devonian mass extinction which followed 90 million years later three quarters of life on Earth became extinct . The Permian mass extinction of 248 million years ago was nicknamed The Great Dying as 96 per cent of species died out. Then 48 million years later half of Earth’s species were wiped out by climate change and asteroid impacts. The final one marked the end of the dinosaurs. 
 
 Dr Mike Barrett, WWF-UK’s Director of Science and Policy said, ‘These findings echo those of WWF’s Living Planet Report which highlighted a 52 per cent decline in vertebrate populations over the last 40 years. 
 
 "If this trend is not reversed it is easy to see how more extinctions could take place and it is further evidence that we clearly need to do more to protect wildlife and their habitats." 
 
 The research was published in the journal Science Advances. 
 
 10 most endangered species 
 
 South China tiger 
 
 Native to the southern Chinese provinces of Fujian, Guangdong, Hunan and Jiangxi the tiger had a population of 4,000 in the 1950s but it is no thought longer to exist in zoos. 
 
 Sumatran elephant 
 
 As more of Sumatran forest becomes converted for agricultural uses, the elephant has faced a critical loss of habitat. A 2007 study estimated there were less than 2,800 remaining in the wild. 
 
 Amur leopard 
 
 The world's rarest cat is believed to be making a comeback with at least 57 confirmed animals now confirmed Russia. Just 30 animals existed in 2007. But the animal is still vulnerable to hunters. 
 
 Atlantic goliath grouper 
 
 Despite the US issuing a moratorium on hunting the big fish in 1990, the animal remains critically endangered. 
 
 Gulf porpoise 
 
 The Gulf porpoise is now one of the rarest mammals in the world, with a global population estimated at under 100 in 2014. The last remaining porpoises live in North America's Gulf of California and experts expect it to become extinct by 2018. 
 
 Northern bald ibis 
 
 The bird’s natural habitat of North Africa, European and the Middle East has been plagued with war and civil unrest and now only one population exists in Morocco, with just a few hundred remaining. 
 
 Attempts are currently underway in Austria, Spain and Italy to breed the animals for reintroduction into the wild. 
 
 Hawksbill turtle 
 
 The 20,000 strong female population is under severe threat by hunters seeking their brown and gold shells. 
 
 Black rhinoceros 
 
 The black rhino has suffered the most drastic decline in total numbers of all rhino species and was officially declared extinct in the wild in 2011. However a major conservation effort has seen numbers swell to 5,000 and now the animals are kept under armed guard. 
 
 Pygmy three-toed sloth 
 
 Found on Panama's uninhabited Escudo de Veraguas island, a 2012 study found fewer than 80 sloths were still living because of habitat loss by loggers. 
 
 Chinese pangolin 
 
 Used extensively as a food, and for Chinese medicines, the pangolin has declined by 94 per cent since the 1960s. Exact numbers have been difficult to estimate because the creature is solitary and nocturnal. ||||| Abstract The oft-repeated claim that Earth’s biota is entering a sixth “mass extinction” depends on clearly demonstrating that current extinction rates are far above the “background” rates prevailing between the five previous mass extinctions. Earlier estimates of extinction rates have been criticized for using assumptions that might overestimate the severity of the extinction crisis. We assess, using extremely conservative assumptions, whether human activities are causing a mass extinction. First, we use a recent estimate of a background rate of 2 mammal extinctions per 10,000 species per 100 years (that is, 2 E/MSY), which is twice as high as widely used previous estimates. We then compare this rate with the current rate of mammal and vertebrate extinctions. The latter is conservatively low because listing a species as extinct requires meeting stringent criteria. Even under our assumptions, which would tend to minimize evidence of an incipient mass extinction, the average rate of vertebrate species loss over the last century is up to 100 times higher than the background rate. Under the 2 E/MSY background rate, the number of species that have gone extinct in the last century would have taken, depending on the vertebrate taxon, between 800 and 10,000 years to disappear. These estimates reveal an exceptionally rapid loss of biodiversity over the last few centuries, indicating that a sixth mass extinction is already under way. Averting a dramatic decay of biodiversity and the subsequent loss of ecosystem services is still possible through intensified conservation efforts, but that window of opportunity is rapidly closing. 
 
 Keywords 
 
 Sixth mass extinction 
 
 vertebrate extinctions 
 
 rates of extinction 
 
 background extinction 
 
 modern vertebrate losses 
 
 INTRODUCTION The loss of biodiversity is one of the most critical current environmental problems, threatening valuable ecosystem services and human well-being (1–7). A growing body of evidence indicates that current species extinction rates are higher than the pre-human background rate (8–15), with hundreds of anthropogenic vertebrate extinctions documented in prehistoric and historic times (16–23). For example, in the islands of tropical Oceania, up to 1800 bird species (most described in the last few decades from subfossil remains) are estimated to have gone extinct in the ~2000 years since human colonization (24). Written records of extinctions of large mammals, birds, and reptiles date back to the 1600s and include species such as the dodo (Raphus cucullatus, extinguished in the 17th century), Steller’s sea cow (Hydrodamalis gigas, extinguished in the 18th century), and the Rodrigues giant tortoise (Cylindraspis peltastes, extinguished in the 19th century). More species extinction records date from the 19th century and include numerous species of mammals and birds. Records of extinction for reptiles, amphibians, freshwater fishes, and other organisms have mainly been documented since the beginning of the 20th century (14, 17). Moreover, even in species that are not currently threatened, the extirpation of populations is frequent and widespread, with losses that far outstrip species-level extinctions (18, 25). Population-level extinction directly threatens ecosystem services and is the prelude to species-level extinction (18). Here, we analyze the modern rates of vertebrate species extinction and compare them with a recently computed background rate for mammals (7). We specifically addressed the following questions: (i) Are modern rates of mammal and vertebrate extinctions higher than the highest empirically derived background rates? (ii) How have modern extinction rates in mammals and vertebrates changed through time? (iii) How many years would it have taken for species that went extinct in modern times to have been lost if the background rate had prevailed? These are important issues because the uncertainties about estimates of species loss have led skeptics to question the magnitude of anthropogenic extinctions (26) and because understanding the magnitude of the extinction crisis is relevant for conservation, maintenance of ecosystem services, and public policy. Until recently, most studies of modern extinction rates have been based on indirect estimates derived, for example, on the rates of deforestation and on species-area relationships (11, 14). Problems related to estimating extinction since 1500 AD (that is, modern extinctions) have been widely discussed, and the literature reflects broad agreement among environmental scientists that biases lead to underestimating the number of species that have gone extinct in the past few centuries—the period during which Homo sapiens truly became a major force on the biosphere (1–4, 6–8, 14, 15). However, direct evaluation is complicated by uncertainties in estimating the incidence of extinction in historical time and by methodological difficulties in comparing contemporary extinctions with past ones. Less discussed are assumptions underlying the estimation of background extinction rates. The lower these estimates, the more dramatic current extinction rates will appear by comparison. In nearly all comparisons of modern versus background extinction rates, the background rate has been assumed to be somewhere between 0.1 and 1 species extinction per 10,000 species per 100 years (equal to 0.1 to 1 species extinction per million species per year, a widely used metric known as E/MSY). Those estimates reflect the state of knowledge available from the fossil record in the 1990s (7, 9–13). In a recent analysis, which charted the stratigraphic ranges of thousands of mammal species, extinction rates were measured over intervals ranging from single years to millions of years, and the mean extinction rate and variance were computed for each span of time (7). In this way, the background extinction rate estimated for mammals was estimated at 1.8 E/MSY, here rounded upward conservatively to 2 E/MSY (that is, 2 extinctions per 100 years per 10,000 species). This is double the highest previous rough estimate. Those previously estimated background rates were primarily derived from marine invertebrate fossils, which are likely to have greater species longevity than vertebrates (10, 15). Data deficiencies make it impossible to conduct empirical analyses (as was done for mammals) for non-mammal terrestrial vertebrates; therefore, we assume the background rates of other vertebrates to be similar to those of mammals. This supposition leads to a more conservative assessment of differences between current and past extinction rates for the vertebrates as a whole, compared with using the very low background extinction rate derived from marine invertebrates. The analysis we present here avoids using assumptions such as loss of species predicted from species-area relationships, which can suggest very high extinction rates, and which have raised the possibility that scientists are “alarmists” seeking to exaggerate the impact of humans on the biosphere (26). Here, we ascertain whether even the lowest estimates of the difference between background and contemporary extinction rates still justify the conclusion that people are precipitating a global spasm of biodiversity loss. 
 
 RESULTS Modern and background rates of vertebrate extinctions Modern rates of vertebrate extinction were much higher than a background extinction rate of 2 E/MSY. Among the vertebrate taxa evaluated by the International Union of Conservation of Nature (IUCN), 338 extinctions have been documented since 1500 [“extinct” (EX), Table 1]. An additional 279 species have become either “extinct in the wild” (EW) or listed as “possibly extinct” (PE), totaling 617 vertebrate species summed over the three categories. Most extinctions have occurred in the last 114 years (that is, since 1900; Table 1). Our estimated “highly conservative” (that is, using data for EX species only) and “conservative” (that is, by including EX, EW, and PE) modern extinction rates for vertebrates varied from 8 to 100 times higher than the background rate (Table 2). This means, for example, that under the 2 E/MSY background rate, 9 vertebrate extinctions would have been expected since 1900; however, under the conservative rate, 468 more vertebrates have gone extinct than would have if the background rate had persisted across all vertebrates under that period. Specifically, these 468 species include 69 mammal species, 80 bird species, 24 reptiles, 146 amphibians, and 158 fish. Table 1 Numbers of species used in the Table 2 calculations of “highly conservative” and “conservative” modern extinction rates based on the IUCN Red List ( 17 ). For the highly conservative rates, only species verified as “extinct” (EX) were included; for the conservative extinction rates, species in the categories “extinct in the wild” (EW) and “possibly extinct” (PE) were also included. View this table: Table 2 Elevation of “highly conservative” and “conservative” modern vertebrate extinction rates above background rate of 2 E/MSY (see table S2 for calculations). For each assessment category, two periods are shown: extinction rates computed from 1500 to the present, and from 1900 to the present. View this table: Variation in modern extinction rates through time Modern extinction rates have increased sharply over the past 200 years (corresponding to the rise of industrial society) and are considerably higher than background rates (Fig. 1). Rates of modern extinctions vary among vertebrate groups (Fig. 1). For example, amphibians, comprising of ~7300 species, show an accelerating rate of extinction: only 34 extinctions have been documented with a high level of certainty since 1500, yet >100 species have likely disappeared since 1980 (17, 23). This may not only reflect real trends but also a shortage of data for groups for which most species are not yet evaluated, such as reptiles and fish (21, 22). Fig. 1 Cumulative vertebrate species recorded as extinct or extinct in the wild by the IUCN (2012). Graphs show the percentage of the number of species evaluated among mammals (5513; 100% of those described), birds (10,425; 100%), reptiles (4414; 44%), amphibians (6414; 88%), fishes (12,457; 38%), and all vertebrates combined (39,223; 59%). Dashed black curve represents the number of extinctions expected under a constant standard background rate of 2 E/MSY. (A) Highly conservative estimate. (B) Conservative estimate. Modern extinctions if background rate had prevailed Our results indicate that modern vertebrate extinctions that occurred since 1500 and 1900 AD would have taken several millennia to occur if the background rate had prevailed. The total number of vertebrate species that went extinct in the last century would have taken about 800 to 10,000 years to disappear under the background rate of 2 E/MSY (Fig. 2). The particularly high losses in the last several decades accentuate the increasing severity of the modern extinction crisis. Fig. 2 Number of years that would have been required for the observed vertebrate species extinctions in the last 114 years to occur under a background rate of 2 E/MSY. Red markers, highly conservative scenario; blue markers, conservative scenario. Note that for all vertebrates, the observed extinctions would have taken between 800 to 10,000 years to disappear, assuming 2 E/MSY. Different classes of vertebrates all show qualitatively similar trends. 
 
 DISCUSSION Arguably the most serious aspect of the environmental crisis is the loss of biodiversity—the other living things with which we share Earth. This affects human well-being by interfering with crucial ecosystem services such as crop pollination and water purification and by destroying humanity’s beautiful, fascinating, and culturally important living companions (4, 5, 15, 27–30). Our analysis shows that current extinction rates vastly exceed natural average background rates, even when (i) the background rate is considered to be double previous estimates and when (ii) data on modern vertebrate extinctions are treated in the most conservative plausible way. We emphasize that our calculations very likely underestimate the severity of the extinction crisis because our aim was to place a realistic “lower bound” on humanity’s impact on biodiversity. Therefore, although biologists cannot say precisely how many species there are, or exactly how many have gone extinct in any time interval, we can confidently conclude that modern extinction rates are exceptionally high, that they are increasing, and that they suggest a mass extinction under way—the sixth of its kind in Earth’s 4.5 billion years of history. A final important point is that we focus exclusively on species, ignoring the extirpation of populations—the units relevant to ecological functioning and the delivery of ecosystem services (4, 5, 29). Population extinction cannot be reliably assessed from the fossil record, precluding any analysis along the lines of that presented here. Also, although it is clear that there are high rates of population extinction (18), existing data are much less reliable and far harder to obtain than those for species, which will remain true for the foreseeable future. Likewise, we have not considered animals other than vertebrates because of data deficiencies. The evidence is incontrovertible that recent extinction rates are unprecedented in human history and highly unusual in Earth’s history. Our analysis emphasizes that our global society has started to destroy species of other organisms at an accelerating rate, initiating a mass extinction episode unparalleled for 65 million years. If the currently elevated extinction pace is allowed to continue, humans will soon (in as little as three human lifetimes) be deprived of many biodiversity benefits. On human time scales, this loss would be effectively permanent because in the aftermath of past mass extinctions, the living world took hundreds of thousands to millions of years to rediversify. 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 (31–33). All of these are related to human population size and growth, which increases consumption (especially among the rich), and economic inequity (6). However, the window of opportunity is rapidly closing. 
 
 MATERIALS AND METHODS To estimate modern extinction rates, we compiled data on the total number of described species and the number of extinct and possibly extinct vertebrate species from the 2014 IUCN Red List (17). In the IUCN’s list, extinct species can be viewed as the minimum number of actual extinctions during recent human history (that is, since 1500) because it lists species known to be extinct (EX), extinct in the wild (EW), and possibly extinct (PE, a subcategory within “critically endangered” reserved for species thought to be extinct, but not confirmed) (17) (table S1). We used the IUCN data to calculate modern extinction rates in two ways: (i) we estimate a “highly conservative modern extinction rate” by using the data exclusively on species listed as EX, and (ii) we calculate a “conservative extinction rate” by including also both EW and PE species (table S2). Including these latter two categories recognizes that there is only a slim chance that most of the species in those categories can reestablish viable populations in their native habitats. In terms of biological impact and the provision of ecosystem services, we consider EW and PE species to be functionally equivalent to EX species: even if some individuals still exist, their abundances are not sufficient to have a substantial influence on ecological function and processes. The IUCN’s list is considered the authoritative, albeit likely conservative, assessment of the conservation status of plant and animal species. About 1.8 million species have been described since 1758 (when the current nomenclature system was developed), of which 1.3 million are animals (3, 17). Of these animal species, about 39,223 (of the currently counted 66,178) vertebrate species have been formally assessed and reported in the 2014 IUCN Red List (17). In the IUCN sample, mammals, birds, and amphibians have had between 88 and 100% of their known species evaluated, whereas only 44% of reptiles and 38% of fish species have been assessed (Table 1). We focus our comparisons on vertebrates because they are the group for which the most reliable data exist, both fossil and modern. To produce conservative comparisons with modern extinctions, we assumed a background extinction rate of 2 E/MSY as the highest likely baseline average background extinction rate (7); that is, we should expect 2 extinctions per 10,000 vertebrate species per 100 years. That background extinction rate was empirically determined using the exceptionally good fossil records of mammals, combining extinction counts from paleontological databases and published literature on the fossil, subfossil, and historical records (7). Using the resulting high background extinction rate provides a stringent test for assessing whether current modern extinction rates indicate that a mass extinction event is under way. Previous estimates of background extinction rates for other taxa are invariably lower than the mammal-derived estimate of 2 E/MSY used here. 
 
 SUPPLEMENTARY MATERIALS Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/5/e1400253/DC1 Table S1. Definitions of IUCN categories (17) used to assess modern extinction rates. Table S2. Estimation of modern extinction rates since 1500 and 1900. 
 
 This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited. 
 
 Acknowledgments: We would like to thank B. Young for helping us with the data on “possibly extinct species” published by IUCN. J. Soberon, C. Mendenhall, and J. Pacheco gave valuable suggestions on the manuscript. Funding: This work has been supported by the Programa de apoyo a proyectos de investigación e innovación tecnológica from UNAM. Competing interests: The authors declare that they have no competing interests.
– Feeling upbeat? Enough of that: Planet Earth is undergoing a sixth mass extinction that will likely annihilate the human race unless we can curb the trend, scientists say. According to a new paper, the current rate of species extinction outpaces the natural rate far more than anyone knew, NBC News reports. The analysis "employed better knowledge" of Earth's natural extinction rate, the Guardian reports, upping the number from one extinction per million species annually to two extinctions. It then compared that to a current extinction rate that study authors consider conservative. Presto, they got a rate since 1900 that's eight to 100 times worse than the natural one. "We were very surprised to see how bad it is," says study leader Gerardo Ceballos. "This is very depressing." "If it is allowed to continue, life would take millions of years to recover, and our species itself would disappear early on," says Ceballos. But nations can reverse the crisis—which has been linked to deforestation, industrialization, and other human factors—by conserving threatened species and lowering pressures on their populations, the paper says. So far critics seem to buy the paper's basic math but doubt that species extinction will continue at this high rate for centuries. "It is likely the problem will decline rapidly because most countries will be rich and care much for the natural world," writes one environmentalist. Earth has seen five other mass extinctions, including one 443 million years ago that killed off 83% of sea life, and the most recent, which wiped out the dinosaurs 66 million years ago, the Telegraph notes.