By Paul MacRae
In a popular textbook on writing creative non-fiction, the authors echo a familiar claim of global-warming alarmists: that thanks to our carbon emissions, we are creating a “sixth mass extinction” that will wipe out most of the planet’s animals and possibly humanity itself. The authors write:
Your [the reader’s] life has witnessed the eclipse of hundreds of thousands of species, even if they passed out of this world without your awareness. (The current rate of species extinction is matched only by that of the age of the dinosaurs’ demise.)[emphasis added]
This belief in a “current” mass extinction (usually blamed on climate change but also, much more plausibly, on habitat encroachment) is widely held and often cited by the environmental and anti-global-warming movements.
For example, eco-crusader and former U.S. vice-president Al Gore, in his 1992 book Earth in the Balance, contended that we are losing 100 species a day, or almost 40,000 species a year. Gore took this figure from a book by biologist Norman Myers; where Myers got his numbers is discussed below.
In his 2006 film and accompanying book, An Inconvenient Truth, Gore makes a similar although slightly vaguer claim:
Global warming, along with the cutting and burning of forests and other critical habitats, is causing the loss of living species at a level comparable to the extinction event that wiped out the dinosaurs 65 million years ago. That event was believed to have been caused by a giant asteroid. This time it is not an asteroid colliding with the Earth and wreaking havoc; it is us. [emphasis added]
In 2019, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), issued a news release warning that a million animal and plant species are at risk of extinction in the next few decades due to human activities, activities that include but are not limited to climate change.
As a story, the prospect of a sixth mass extinction is certainly highly dramatic and conveys what many feel is an “emotional” as well as factual truth—that many species are at threat due to human activities (as they are) and that we should be concerned (as we should).
But is Gore’s claim of 40,000 extinctions a year; or the textbook authors’ assertion of “hundreds of thousands” (or more) extinctions in a lifetime, as a “match” to the mass extinction of the dinosaurs; or the IPBES’s prediction of a million possible extinctions in the next few decades—are these claims based on actual empirical data? Or are they based on computer algorithms plus a lot of guesswork to create an alarming picture that doesn’t reflect reality?
Extreme claims need strong evidence
Thomas Huxley (1825-1895), also known as “Darwin’s bulldog,” was a pioneer of modern science. A basic principle of science, he wrote in 1896, is that
It is a canon of common sense, to say nothing of science, that the more improbable a supposed occurrence, the more cogent ought to be the evidence in its favour. [emphasis added]
More recently, astronomer Carl Sagan put the same idea this way:
Apocalyptic predictions require, to be taken seriously, higher standards of evidence than do assertions on other matters where the stakes are not as great.” [emphasis added]
(Huxley’s and Sagan’s view, by the way, is the opposite of the “precautionary principle,” which argues that if an apocalyptic disaster could occur, we must act as if it will occur, whether we have credible evidence or not.)
A sixth mass extinction certainly qualifies as “apocalyptic,” with high stakes for humanity and the natural world if true. But is there a “high standard” of evidence, as Sagan suggests, to support this claim?
The five ‘mass extinctions’
Mass extinctions in Earth’s history. Source: Our World in Data, “Extinctions”. Note that the extinction rate for present times would have to be many times current numbers to reach the same levels as the five earlier “mass” extinctions.
Let’s look at the five earlier mass extinctions to see how our human-caused so-called “sixth mass extinction” stacks up so far.
The graph clearly shows shows that the extinction rate for present times would have to be many times current numbers to reach the same levels as the five earlier “mass” extinctions. It also shows there have been many smaller extinction events interspersed among the five big ones (and we may well be causing one of these minor extinction events).
- The “fifth” mass extinction 65 million years ago, caused by an asteroid impact, killed almost 100 per cent of dinosaurs (except for a few that evolved into birds) and an estimated 75-85 per cent of all species, including all land mammals larger than 25 kilograms. Before the asteroid strike, massive volcanic activity (the Deccan Traps) may have already reduced the number of species.
- The “fourth” mass extinction, 200 million years ago, killed 70-75 per cent of all species; volcanic activity and some sort of asteroid strike may have been the cause.
- The “third” mass extinction, 252 million years ago, was the worst of the five and claimed 90-95 per cent of all species. It was probably triggered by massive volcanic eruptions (the Siberian traps) that reduced oxygen levels in the oceans (anoxia).
- The “second” mass extinction, about 360 million years ago, took about 70 per cent of all species. Again, volcanoes may have been involved.
- The “first” mass extinction, about 450 million years ago, killed 85 per cent of all species, likely due to global cooling, with, again, low levels of oxygen.
All these mass extinctions were caused by enormous geological and/or cosmic forces. Does an extinction rate of 70-95 per cent of all species in present times, with no killer asteroid or massive volcanic eruptions in sight, seem probable? Or highly “improbable,” to use Huxley’s wording?
We don’t know how many species exist
The International Union for the Conservation of Nature and Natural Resources (IUCN) keeps track of known extinctions and threatened animals and publishes its results as a “Red List” online at https://www.iucnredlist.org.
And, for a start, while the IUCN is vitally concerned about vanishing species—it believes up to 40,000 species (not a million) are currently “at risk”—it also acknowledges that we don’t know how many species there are on the planet, surely a key piece of information if we’re going to claim a sixth “mass” extinction rivaling the previous five.
The IUCN estimates the number of possible species as anywhere from five million to 30 million; its best guess is 14-18 million. A 2011 paper on biodiversity estimates 8.7 million species, plus or minus 1.3 million, based on statistical analysis.
However, at the moment, the IUCN reports that only about 2.1 million species have actually been identified and named. The additional millions of species that underpin the sixth “mass extinction” claim may exist, or they may be imaginary. We don’t know.
Known extinctions = 900 species over 500 years
The IUCN’s estimate of the number of known extinctions since 1500 is about 900. That’s just under two known species extinctions a year over 500 years. Two extinctions a year over 500 years is regrettable and, given the quickened pace of human industrial and agricultural activity, it’s likely the recent extinction rate per year is higher than the overall average of two species a year. But how much higher? Enough to justify the label “sixth mass extinction”?
A 2012 academic paper reported that 129 bird species and 61 mammal species—the species we tend to care most about because they’re the most visible to us—were known to have gone extinct since 1500.
That’s fewer than one species per year (about 0.4 species per year to be more precise) but, of course, this figure does not include the many non-avian/non-mammalian species, such as insects, reptiles, amphibians, slugs, corals, etc., that are included in the 900 known extinctions.
However, the 2012 study also found that the vast majority of known extinctions—95 per cent—were on islands; only five per cent of known extinctions were on continents. On continents, the study reported, six bird and three mammal species are known to have gone extinct since 1500. On islands, where species at risk have no place to go and are therefore more vulnerable to environmental stress than continental species, the numbers were much higher—123 extinct bird species and 58 extinct mammal species.
These losses are, again, very unfortunate, but they are still far from a mass extinction given that the vast majority of species live on the continents and continents make up about 95 per cent of the world’s land surface.
Zeroing in on more recent research, a 2020 paper estimates that ten bird species and five mammal species have gone extinct since 1993. That’s 15 species in 27 years (1993-2020), or about half a species a year (at least for mammals and birds; again, other forms of life such as insects, reptiles, amphibians, corals, etc., aren’t included).
Many species are saved from extinction, too
By contrast, however, the article also estimates that the number of species of birds saved from extinction by conservation since 1993 was 9-18 species; the number of mammal species similarly saved is estimated at two to seven. (As anyone who watches the nature channels knows, humans are working very hard to save endangered species, with some success.)
Taking the worst-case scenario—that is, that the “saved” species were also allowed to go extinct—we would in total have lost 28 bird species and 12 mammal species, for a total of 40 species in 27 years. This is just over one and a half bird and mammal species a year.
Even if we include the much more numerous insects, frogs, reptiles, and other less visible forms of life that have also undoubtedly perished in these 27 years, there is no statistical gymnastics that could plausibly produce the 100 extinctions a day, or 40,000 extinctions a year, that climate alarmists like Gore, the IPBES, and many other alarmists predict as constituting a “mass” extinction.
Indeed, some scientists have even reported that new species have evolved within the last century to cope with the pressures of humanity. For this good news, see Chris D. Thomas, Inheritors of the Earth: How Nature Is Thriving in an Age of Extinction.
Extinctions: inflating the numbers
So, how do alarmist environmentalists come up with these wildly inflated extinction figures amounting to a “sixth mass extinction” comparable to that of the dinosaurs’?
For a start, again, the actual number of species isn’t known. But let’s say there are 8.7 million species, the vast majority still not identified. Using the IUCN yardstick (900 known lost species for 2.1 million known species) gives us an extinction rate of .04% of all (known) species over 500 years.
If we apply this 0.04% extinction rate to 8.7 million species (three-quarters of them unknown and therefore hypothetical) we would expect 3,480 extinctions in 500 years (of which, to repeat, only 900 are actually known). That’s about seven extinctions a year since 1500, which is far, far from the 40,000 extinctions a year estimated by Gore, Myers, and other alarmists.
Let’s consider the highest number of possible species estimated: 30 million—maybe that will give us the tens of thousands of extinctions a year that Gore and others would have us believe. No such luck.
For 30 million species, pro-rated at 0.04%, the total extinctions over 500 years would be 12,000, or 24 extinctions a year. This is bad, but, again, far from the “100 extinctions a day,” or “40,000 extinctions a year” that alarmists like Gore and others would have us believe. That said, those 12,000 hypothesized species extinctions in 500 years are based on an unrealistically high estimate of species numbers (30 million) that may or may not (and almost certainly don’t) exist.
But whatever species number we use—2.1 million, 8.7 million, even 30 million—by no stretch of the imagination does a plausible extinction rate in our time compare to the mass die-off of 65 million years ago nor the four mass extinctions before that.
In the last major extinction, huge regions of the planet were destroyed in an asteroid-strike disaster that has been compared to the results of nuclear war (including a years-long “nuclear winter”). If we were approaching this level of extinctions in our own time, wouldn’t we notice? Especially in an era of mass communications in which humans are vitally concerned with, for example, the numbers of condors and spotted owls?
So, again, where do these wildly exaggerated estimates of modern-day extinctions, the “sixth mass extinction,” come from?
Extinction ‘estimate’ taken as fact
Ecologist Dr. Norman Myers was one of the first to warn about what he called a “human-caused biotic holocaust.” In his 1979 book The Sinking Ark, Myers wrote:
“Let us suppose that, as a consequence of this manhandling of the natural environments, the final one-quarter of this century witnesses the elimination of one million species, a far from unlikely prospect. This would work out, during the course of 25 years, at an average rate of 40,000 species per year, or rather over 100 species per day.” [emphasis added]
As it might, if there were any empirical evidence to support this claim. However, as Myers himself later acknowledged: “The estimate of 40,000 extinctions per year was strictly a first-cut assessment, preliminary and exploratory, and advanced primarily to get the issue of extinction onto scientific and political agendas.” [emphasis added] In other words, Myers was using scare tactics rather than scientific facts to make his case.
That is, the “40,000 extinctions a year” figure is a propaganda tool, an “emotional truth,” with no actual evidence whatsoever behind it. This “emotional truth” is then taken up and passed on to the public as factual truth by environmentalist campaigners like Gore.
In a 2006 talk in Australia, Myers stuck to his rhetorical guns, claiming that 50 per cent of the earth’s 10 million species (Myers’s estimate) may be lost if fossil-fuel use continues.
It’s worth remembering that Myers’s 2006 estimate of massive extinctions to come is still, like his 1979 estimate, pure speculation, based on no empirical evidence. (In the same talk, Myers urged Australians to abandon fossil fuels and nuclear power in favor of “renewable” energy sources, which would be a great way of causing the extinction of hundreds of millions of people who depend on modern technological civilization.)
With these facts in mind, shouldn’t we all be asking ourselves: Does this claim of a “sixth” mass extinction seem reasonable, or even plausible?
‘Sixth mass extinction’ exists only in computer models
If we do some actual investigation we discover that, quite apart from well-meant but fanciful propaganda efforts like Gore’s and Myers’s, other claims of hundreds of thousands of current and future extinctions are in part based on computer models that purport to estimate the number of extinctions for a certain area. Of these models, science writer Fred Pearce asks:
Can we really be losing thousands of species for every loss that is documented? Some ecologists believe the high estimates are inflated by basic misapprehensions about what drives species to extinction. So where do these big estimates come from? Mostly, they go back to the 1980s, when forest biologists proposed that extinctions were driven by the “species-area relationship.”
Species-area relationship studies, Pearce explains, assume that an area of a habitat, such as a tropical rain forest, holds a certain number of species, many or most of them unknown to us, so the species number in that area is estimated by a computer algorithm.
If a portion of the pristine habitat is logged, burned, developed or otherwise lost, then the algorithm predicts that a similar proportion of species will also be lost (go extinct)—perhaps dozens or even hundreds a day, assuming these postulated species actually exist in the numbers the computer models predict.
Using this mathematical calculus of many hypothesized (but actually unknown and possibly imaginary) species, it’s possible to predict thousands of extinctions a year from habitat loss and climate change, even though these lost creatures are never actually seen by anyone because they exist only in the computer models.
For example, biologist E.O. Wilson estimated that the rain forests (e.g., the Amazon) contain about 10 million species (again, nobody knows the actual number).
If the destruction of rain forests is more than one per cent a year, then using the species-area formula, Wilson calculated that 77,000 species would be lost every year (which is even more than Gore’s and Myers’s more “conservative” estimates). This really would constitute a “biotic holocaust”—770,000 species lost in a decade, 7.7 million lost in a century! We’d lose about 70 per cent of all species in a few lifetimes! That would definitely be a “mass extinction.”
But let’s assume Wilson is right in his calculations. How much would the physical existence of humanity be affected by the loss of these “77,000 species” a year in sections of destroyed rain forest, given that no one has ever seen 90 per cent of these species and that they exist only in computer models?
And the realistic answer is: we would survive. While we might feel emotionally devastated, the fact is we didn’t know these thousands of species were in the rain forest before and we would therefore not be aware of their loss (unless the planet’s biosphere collapsed, which is unlikely—see death of the megafauna in the Americas, below).
Under the species-area formula, wrote Australian researcher Nigel Stork in a 2009 paper cited by Pearce, the planet would have lost up to half its species in the last 40 years, which is patently not the case. Instead, Stork concluded: “There are almost no empirical data to support estimates of current extinctions of 100, or even one, species a day.”
But these vastly inflated numbers are widely accepted and cited not because they are based in actual, like, you know, scientific evidence, but because they are “dramatic” and convey an “emotional truth” that advances the alarmist environmental agenda.
The Americas: Death of the megafauna
We know that humans have had and are having a major impact on the animal and plant world. Indeed, one of the largest human impacts occurred when aboriginal people from Siberia migrated to North and South America 11,000 years ago (or earlier) and, in the view of many researchers, exterminated all of the megafauna then existing on those continents: mammoths and mastodons, giant sloths, giant wolves and bears, huge predatory cats including the sabre-tooth cat (Smilodon), even giant beavers and armadillos.
If ever there was a “human-caused biotic holocaust” this was it, and the same biotic holocaust of large animals and birds is believed to have occurred in Australia, New Zealand, Madagascar, and elsewhere (mostly islands) after the arrival of humans.
Of course this view of human “overkill” contradicts the politically correct view that aboriginal peoples are and always have been the “conservators” of their lands. Therefore Ross D.E. McPhee, in his 2019 book End of the Megafauna, argues that many researchers are reluctant to place the blame on aboriginal hunters, and believe that climate change and other environmental factors were more likely the main cause of the North and South American megafauna extinctions.
A book from a slightly earlier (1997), less politically correct era is unequivocal after examining the evidence for and against the human “overkill” hypothesis:
“We know ‘whodunit.’ We did it. Our species, our kind—humanity—armed only with stone-tipped spears, caused the extinction of the great mammoths and mastodons and perhaps that of many other large mega-mammal species. We did it simply by killing off about 2% of the population per year, year after year.”
E.O. Wilson is also solidly on the side of humans, not climate, as “the planetary killer” in his 2002 book The Future of Life. In fact, Chapter 4 of his book is entitled “The Planetary Killer” and notes, “The noble savage never existed.”
But political correctness aside, how can we be sure humans almost certainly “dunnit”, in the Americas and elsewhere? Because these same megafauna survived more than one previous interglacial warm period like our own (we are currently in the latest interglacial of a two-million-year-old ice age). Only in the current interglacial, with the growing human presence, were the megafauna of the Americas, Australia and elsewhere exterminated.
To be consistent with alarmist “sixth mass extinction” claims of impending planetary doom, the deaths of all the megafauna in North and South America should have precipitated a biological and ecological collapse (the “biotic holocaust”), a cascade of extinctions (like pulling the thread on a sweater that unravels the whole sweater) that included the Stone Age aboriginal hunters who caused them.
Again, no such luck—nature is much more resilient than eco-alarmists believe. Somehow the biosphere of the two American continents and their remaining animals, including the humans, survived and managed to thrive, as did the surviving animals and people of Australia, New Zealand, and other regions. In other words, the loss of these megafauna was a tragedy, but it did not pose an existential threat to humans or other animals on any of the continents.
Yet today many environmental groups claim that their cause is aimed at preventing an existential crisis for humanity (we’re all gonna die!). For example, a British Columbia old-growth-forest campaigner argues: “It’s literally our future, the future of humanity is dependent on protecting these carbon sinks and lowering carbon emissions.”
The reality is that even if a “sixth mass extinction” that didn’t include a planet-killer asteroid or massive volcanic eruptions did occur (highly unlikely, but if), humanity and its civilizations would almost certainly survive. The megafauna extinctions in the Americas and elsewhere show that, by any rational standard, these “biotic holocaust” claims are pure rhetorical “overkill.”
The ‘background’ extinction rate myth
One final alarmist claim needs to be mentioned: that the current extinction rate is a “hundred” to a “thousand” (or even “ten thousand”) times higher than the “background” rate of extinctions (that is, the extinctions that would occur without human interference).
This sounds very scary and the strong implication is that extinctions a “hundred” or a “thousand” times above the “background” level are equivalent to the “sixth mass extinction” (40,000 extinctions a year) claimed by Gore, Myers, Wilson and others.  As we’ll see, this impression is seriously misleading.
E.O. Wilson sets the “background” extinction rate at one species per million species per year. This fits well with the known extinction rate: two million known species, two known species extinctions a year since 1500. If there are eight million species, then we’d expect eight species extinctions a year as “background.” In other words, the known extinction rate appears to be about the same as the estimated “background” extinction rate.
But, for Wilson, the human-caused rate is actually a hundred to a thousand to ten thousand times higher than the background rate thanks to the “species-area” hypothesis. This gives us extinction figures for eight million species of 800 species a year (if a hundred times) to 8,000 extinctions a year (if a thousand times) to 80,000 extinctions a year (if ten thousand times), which is more or less Wilson’s estimate of 77,000 extinctions a year.
Only if we accept the most extreme estimate—ten thousand times the “background” extinction rate—do we approach and then almost double Gore’s and Myers’s 40,000 lost species a year. But a death rate of 80,000 species a year seems highly “improbable,” in Thomas Huxley’s words, and lacks “cogent” empirical proof, unless computer models are considered empirical proof.
A 2015 paper estimates that the natural, “background” extinction rate (i.e., without humans) for vertebrates—mammals, birds, reptiles, amphibians and fish—was nine vertebrate species since 1900. That’s about one-tenth of a vertebrate species a year in 114 years (up to 2015), which is quite a bit lower than Wilson’s estimate for all species, vertebrate and non-vertebrate, of two species a year (assuming two million known species).
The likely extinctions, according to this paper, are 468 vertebrate species in 114 years, or about four species a year.
Doing the math: If the natural “background” extinction rate for vertebrates is nine species in 114 years, or about one-tenth of a species a year, and the observed rate is 468 species over that time, or about four vertebrate species a year, then we have an extinction rate for vertebrates that is 52 times the “background” rate. This is half the “hundred” times the background rate, and nowhere near a “thousand” or “ten thousand” times the natural rate.
Four extinct vertebrate species a year is four too many and humans need to do better. And, of course, this calculation doesn’t include non-vertebrates, which might bring the extinction number closer to the claimed “hundred” times the background rate, or about ten species a year over 114 years (again recognizing that these are largely computer-generated numbers, not actual known species).
But even with a fudge factor for unknown species, four or ten or even twenty extinct species a year—numbers that might actually approach a “hundred” times the postulated “background” rate—are a far, far cry from Gore’s or Myers’s “100 species a day” or “40,000 species a year.” Ten or twenty species a year is bad, but it is not a “sixth mass extinction,” even if it is a scary-sounding “one hundred times” the background rate. The numbers just don’t add up and this argument is just more misleading alarmist propaganda.
For alarmists, however, even ten extinctions a year can be seen as equivalent to a “mass extinction” since this rate of extinction is considerably higher than the “background” rate and, at this rate over many years, extinctions could eventually reach the 70-plus percentage of extinct species that defines a “mass” extinction (assuming humans did nothing to stop or slow down the slaughter, which we would).
Taking a reasonable worst-case-scenario of ten extinct species a year, and assuming ten million species, a “sixth mass extinction” of 70 per cent of species (seven million) would take 700,000 years. In that very long time (anatomically modern humans have only existed for about 200,000 years), humanity would undoubtedly take steps to prevent this level of species loss, assuming humans still exist in 700,000 years.
In other words, claims of a modern-day “sixth mass” extinction may be “rate-based” rather than “numbers-based,” even if the actual number of extinct species was nowhere near 70 per cent or higher. It’s another example of torturing the data until it says what the alarmist researchers want it to.
Again, this doesn’t mean extinctions aren’t occurring and in greater numbers than previously. It just means that we aren’t facing a “sixth” mass extinction, with losses ranging from 70 to 95 per cent of all major species, as occurred during the previous five “mass” extinctions.
Why do scientists exaggerate?
Which raises the question, at least for reasonable people: why are scientists, who should be the keepers of the flame of Truth, continuing to support this absurd “sixth mass extinction” scare story?
The answer, or part of the answer, is that working almost entirely with computer models, rather than empirical data, many scientists sincerely believe that global warming will be a disaster, although to do this they have to ignore or downplay evidence that Earth’s climate has been much warmer in the geological past without destroying the planet and its creatures.
But by exaggerating in this blatant way, alarmist scientists and environmentalists don’t realize that they are, in fact, harming their cause with the public, or at least the informed public. Most of us are vitally concerned with preventing extinctions: we care deeply about endangered gorillas and elephants and rhinos, and even the spotted owl. But, as Stalin put it, “A single death is a tragedy; a million deaths is a statistic.”
When we read threats of millions of species dying, we have three choices: we can blindly believe these extreme claims (like Gore and the authors of the writing textbook mentioned at the start of this article); or we can tune out (“a million deaths is a statistic”); or, as I’ve tried to do here, we might actually investigate whether a claim like a current “sixth mass extinction” is plausible.
Once the human-caused “sixth mass extinction” is revealed as wildly improbable, then climate alarmism’s other, possibly more valid assertions also come under suspicion. Scientists must be honest; truth is a scientist’s highest calling. When we learn that scientists are deliberately embellishing the facts, or lying, or misleading the public, as they are with the “sixth mass extinction” claim, science itself is cast into doubt.
To be clear: There is no question that humans are cutting a swathe through the planet’s animal life—we may well be creating one of the many minor “extinction events” that our planet has regularly seen since the “first” great extinction. We should do everything we can to avoid creating extinctions—and we are trying, with some success, to reduce our impact through conservation, species protection, and so on.
That said, the idea of an impending human-caused massive “sixth extinction” and the “end of nature” (as Bill McKibben calls it) that threatens humanity’s very existence is a product, like global-warming alarmism itself, of computer models, not empirical evidence.
This manufactured fear is pure propaganda aimed at stampeding public opinion toward the alarmist global-warming ideology. It’s our job, as rational citizens in a democracy, to examine the so-called “evidence,” look at the actual numbers as I’ve tried to do here, and draw our own, much less alarming, conclusions.
Paul MacRae is the author of False Alarm: Global Warming Facts Versus Fears and publishes his blog False Alarm at paulmacrae.com. He is also a contributor to the website of Climate Realists of Victoria BC, climaterealists.ca.
 Brenda Miller and Suzanne Paola, Telling It Slant: Writing and Shaping Creative Non-Fiction. Toronto: McGraw-Hill, 2005, p. 35.
 Al Gore, Earth in the Balance: Ecology and the Human Spirit. Toronto: Penguin Books, 1993 (1992), p. 28.
 Al Gore, An Inconvenient Truth. Emmaus, PA: Rodale Press, 2006, p. 10.
 IPBES, “Nature’s Dangerous Decline: ‘Unprecedented’ Species Extinction Rates ‘Accelerating’.” 2019. Available at https://ipbes.net/news/Media-Release-Global-Assessment
 Thomas H. Huxley, “An Episcopal Trilogy,” Science and the Christian Tradition. New York: D. Appleton, 1896, p. 135.
 Carl Sagan, “Nuclear War and Climatic Catastrophe: Some Policy Implications,” Foreign Affairs, Winter 1983/84, pp. 257-258.
 See Wikipedia, “List of extinction events.” The five previous major extinctions are shown in blue highlight; the list includes the many other less deadly extinction events as well. See also Wikipedia “Extinction event” for a more detailed listing of these events and what caused them. The website Our World in Data also offers an excellent overview of what we know about extinctions now and in the past.
 Wikipedia, “Cretaceous–Paleogene extinction event.” The Encyclopedia Britannica’s entry sets the fifth mass extinction rate at 80 per cent for animals. See “K-T extinction,” available at https://www.britannica.com/science/K-T-extinction.
 IUCN Red List, “Species Extinction—the Facts.” PDF, 2007. Available at https://www.iucn.org/sites/dev/files/import/downloads/species_extinction_05_2007.pdf.
 C. Mora et al., “How Many Species Are There on Earth and in the Ocean?” PLOS Biology, Aug. 23, 2011. Available at http://www.unep-wcmc.org/medialibrary/2011/08/24/ef86d88a/journal_pbio1001127_1_.pdf.
 IUCN: “Species Extinction—The Facts”. Available online. See also Our World in Data, “How many species have gone extinct?” which quotes the IUCN. Available at https://ourworldindata.org/extinctions#how-many-species-have-gone-extinct.
 Craig Loehle and Willis Eschenbach, “Historical bird and terrestrial mammal extinction rates and causes.” Diversity and Distributions, January 2012 (18,1), pp. 84-91.
 Frederick C. Bolam, et al., “How many bird and mammal extinctions has recent conservation action prevented?” Conservation Letters, Society for Conservation Biology, August 23, 2020.
 Published in New York by Hachette Book Group Public Affairs, 2019.
 Norman Myers, The Sinking Ark. Oxford: Pergamon Press, 1979, p. 5.
 Myers, “Specious: On Bjorn Lomborg and species diversity.” Grist, Dec. 12, 2001.
 Nassim Khadem, “Earth faces mass extinction.” The Age, March 16, 2006. Available online.
 Fred Pearce, “Global Extinction Rates: Why Do Estimates Vary So Wildly?” Yale Environment 360, August 17, 2015. Available online
 Stephen H. Schneider, Laboratory Earth: The Planetary Gamble We Can’t Afford to Lose. New York: Basic Books, 1997, p. 104.
 Nigel Stork, “Re-assessing current extinction rates.” Biodiversity and Conservation, February 2010, pages 357-371.
 Ross D.E. McPhee, End of the Megafauna: The Fate of the World’s Hugest, Fiercest and Strangest Animals. New York: W.W. Norton, 2019, p. 178
 Peter D. Ward, The Call of Distant Mammoths: Why the Ice Age Mammals Disappeared. New York: Copernicus, 1997, p. 222.
 E.O. Wilson, The Future of Life. New York: Borzoi Books, 2002, Chapter 4, “The Planetary Killer,” pp. 79-102.
 Brenna Owen, “Blockades over old-growth logging aimed at forcing a dialogue: activists.” Victoria Times Colonist, May 4, 2022.
 For more details on the “background” rate, see Kate Anderson, “What’s Normal: How Scientists Calculate Background Extinction Rate.” Population Education, Dec. 11, 2018. Available online.
 Wilson, The Future of Life, p. 99.
 Ceballos, G., et al., “Accelerated modern human-induced species losses: Entering the sixth mass extinction.” Science Advances, Vol. 1, Issue 5, June 19, 2015. Available at https://www.science.org/doi/10.1126/sciadv.1400253.
 See Our World in Data, “Extinctions/How do we know if we’re heading for a sixth mass extinction?”, for an explanation of this rate-based, rather than numbers-based, definition of a “mass” extinction.