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SIXTH MASS EXTINCTION

Posted on: February 18, 2021

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THIS POST IS A SUMMARY OF THE LITERATURE ON THE SIXTH MASS EXTINCTION WITH CRITICAL COMMENTARY

REFERENCE#1: Vertebrates on the brink as indicators of biological annihilation and the sixth mass extinction
Gerardo Ceballos, Paul R. Ehrlich, and Peter H. Raven, PNAS June 16, 2020 117 https://doi.org/10.1073/pnas.1922686117

Significance: The ongoing sixth mass extinction may be the most serious environmental threat to the persistence of civilization, because it is irreversible. Thousands of populations of critically endangered vertebrate animal species have been lost in a century, indicating that the sixth mass extinction is human caused and accelerating. The acceleration of the extinction crisis is certain because of the still fast growth in human numbers and consumption rates. In addition, species are links in ecosystems, and, as they fall out, the species they interact with are likely to go also. In the regions where disappearing species are concentrated, regional biodiversity collapses are likely occurring. Our results reemphasize the extreme urgency of taking massive global actions to save humanity’s crucial life-support systems. Abstract: The ongoing sixth mass species extinction is the result of the destruction of component populations leading to eventual extirpation of entire species. Populations and species extinctions have severe implications for society through the degradation of ecosystem services. Here we assess the extinction crisis from a different perspective. We examine 29,400 species of terrestrial vertebrates, and determine which are on the brink of extinction because they have fewer than 1,000 individuals. There are 515 species on the brink (1.7% of the evaluated vertebrates). Around 94% of the populations of 77 mammal and bird species on the brink have been lost in the last century. Assuming all species on the brink have similar trends, more than 237,000 populations of those species have vanished since 1900. We conclude the human-caused sixth mass extinction is likely accelerating for several reasons. First, many of the species that have been driven to the brink will likely become extinct soon. Second, the distribution of those species highly coincides with hundreds of other endangered species, surviving in regions with high human impacts, suggesting ongoing regional biodiversity collapses. Third, close ecological interactions of species on the brink tend to move other species toward annihilation when they disappear—extinction breeds extinctions. Finally, human pressures on the biosphere are growing rapidly, and a recent example is the current coronavirus disease 2019 (Covid-19) pandemic, linked to wildlife trade. Our results reemphasize the extreme urgency of taking much-expanded worldwide actions to save wild species and humanity’s crucial life-support systems from this existential threat.

REFERENCE#2: The misunderstood sixth mass extinction, Gerardo Ceballos1,*, Paul R. Ehrlich, Science 08 Jun 2018:
Vol. 360, Issue 6393, pp. 1080-1081, DOI: 10.1126/science.aau0191

SIXTH MASS EXTINCTION BIBLIOGRAPHY

(WAKE2008) Wake, David B., and Vance T. Vredenburg. “Are we in the midst of the sixth mass extinction? A view from the world of amphibians.” Proceedings of the National Academy of Sciences 105.Supplement 1 (2008): 11466-11473. Many scientists argue that we are either entering or in the midst of the sixth great mass extinction. Intense human pressure, both direct and indirect, is having profound effects on natural environments. The amphibians—frogs, salamanders, and caecilians—may be the only major group currently at risk globally. A detailed worldwide assessment and subsequent updates show that one-third or more of the 6,300 species are threatened with extinction. This trend is likely to accelerate because most amphibians occur in the tropics and have small geographic ranges that make them susceptible to extinction. The increasing pressure from habitat destruction and climate change is likely to have major impacts on narrowly adapted and distributed species. We show that salamanders on tropical mountains are particularly at risk. A new and significant threat to amphibians is a virulent, emerging infectious disease, chytridiomycosis, which appears to be globally distributed, and its effects may be exacerbated by global warming. This disease, which is caused by a fungal pathogen and implicated in serious declines and extinctions of >200 species of amphibians, poses the greatest threat to biodiversity of any known disease. Our data for frogs in the Sierra Nevada of California show that the fungus is having a devastating impact on native species, already weakened by the effects of pollution and introduced predators. A general message from amphibians is that we may have little time to stave off a potential mass extinction.

(CEBALLOS 2015): Ceballos, Gerardo, et al. “Accelerated modern human–induced species losses: Entering the sixth mass extinction.” Science advances 1.5 (2015): e1400253. 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.

(CAFARO 2015): Cafaro, Philip. “Three ways to think about the sixth mass extinction.” Biological Conservation 192 (2015): 387-393. A preponderance of evidence suggests humanity is causing a mass extinction event: the sixth mass extinction since the rise of complex life on Earth. This paper takes this empirical conclusion as given and asks a philosophical question: what is the meaning of the sixth mass extinction? How should we think about it, what should we do about it, and what does it tell us about humanity and our place in the world? Conservationists typically see mass extinction as an immense loss, as does most of the general public. But how best to characterize this loss is not immediately clear, and how we do so has important practical implications. This paper focuses on three common and plausible ways to think about the sixth mass extinction: as a loss of important resources (a mistake); as interspecies genocide (a crime); and as evidence that humanity is a cancer on the biosphere (as an inevitability). Considered together, these three approaches clarify the meaning of the sixth mass extinction and suggest how humanity ought to respond to it.

(CEBALLOS 2017): Ceballos, Gerardo, Paul R. Ehrlich, and Rodolfo Dirzo. “Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines.” Proceedings of the national academy of sciences 114.30 (2017): E6089-E6096. The population extinction pulse we describe here shows, from a quantitative viewpoint, that Earth’s sixth mass extinction is more severe than perceived when looking exclusively at species extinctions. Therefore, humanity needs to address anthropogenic population extirpation and decimation immediately. That conclusion is based on analyses of the numbers and degrees of range contraction (indicative of population shrinkage and/or population extinctions according to the International Union for Conservation of Nature) using a sample of 27,600 vertebrate species, and on a more detailed analysis documenting the population extinctions between 1900 and 2015 in 177 mammal species. We find that the rate of population loss in terrestrial vertebrates is extremely high—even in “species of low concern.” In our sample, comprising nearly half of known vertebrate species, 32% (8,851/27,600) are decreasing; that is, they have decreased in population size and range. In the 177 mammals for which we have detailed data, all have lost 30% or more of their geographic ranges and more than 40% of the species have experienced severe population declines (>80% range shrinkage). Our data indicate that beyond global species extinctions Earth is experiencing a huge episode of population declines and extirpations, which will have negative cascading consequences on ecosystem functioning and services vital to sustaining civilization. We describe this as a “biological annihilation” to highlight the current magnitude of Earth’s ongoing sixth major extinction event.

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THE ATLANTIC: THERE IS NO SIXTH MASS EXTINCTION: SCIENTISTS HAVE A RESPONSIBILITY TO MAKE ACCURATE COMPARISONS: https://www.theatlantic.com/science/archive/2017/06/the-ends-of-the-world/529545/
“As scientists we have a responsibility to be accurate about such comparisons.” PETER BRANNEN, JUNE 13, 2017 At the annual meeting of the Geological Society of America, Smithsonian paleontologist Doug Erwin took the podium to address a ballroom full of geologists on the dynamics of mass extinctions and power grid failures—which, he claimed, unfold in the same way. These are images from the NOAA website of the US blackout in 2003,” he said, pulling up a nighttime satellite picture of the glowing northeastern megalopolis, megawatts afire under the cold dark of space. This is 20 hours before the blackout. You can see Long Island and New York City. And this is seven hours into the blackout, he said, pulling up a new map, cloaked in darkness. New York City is almost dark. The blackout extended all the way up into Toronto, all the way out to Michigan and Ohio. It covered a huge section of both Canada and the United States. And it was largely due to a software bug in a control room in Ohio. Erwin is one of the world’s experts on the End-Permian mass extinction, an unthinkable volcanic nightmare that nearly ended life on earth 252 million years ago. He proposed that earth’s great mass extinctions might unfold like these power grid failures: most of the losses may come, not from the initial shock—software glitches in the case of power grid failures, and asteroids and volcanoes in the case of ancient mass extinctions—but from the secondary cascade of failures that follow. These are devastating chain reactions that no one understands. Erwin thinks that most mass extinctions in earth’s history—global die-offs that killed the majority of animal life on earth—ultimately resulted, not from external shocks, but from the internal dynamics of food webs that faltered and failed catastrophically in unexpected ways, just as the darkening eastern seaboard did in 2003.

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Because it was not clear how to manage that collapse—although after the fact it was clear that it should have been easily contained—it cascaded into failure of grids across the northeastern United States … It turns out that, from a mathematical point of view, the problem of understanding these food webs is exactly the same as understanding the nature of the power grid. There’s a very rapid collapse of the ecosystem during these mass extinctions. I had written to Erwin to get his take on the contemporary idea that there is currently a sixth mass extinction under way on our planet on par with the so-called Big Five mass extinctions in the history of animal life. Many popular science articles take this as a given, and indeed, there’s something emotionally satisfying about the idea that humans’ hubris and shortsightedness are so profound that we’re bringing down the whole planet with us. Given how severely humans have damaged the natural world over the millennia, it was an idea I found attractive, and it’s one even shared by many geologists and paleontologists. Our destruction is so familiar—so synonymous with civilization—in fact, that we tend to overlook how strange the world that we’ve made has become. For instance, it stands to reason that, until very recently, all vertebrate life on the planet was wildlife. But astoundingly, today wildlife accounts for only 3 percent of earth’s land animals; human beings, our livestock, and our pets take up the remaining 97 percent of the biomass. This Frankenstein biosphere is due both to the explosion of industrial agriculture and to a hollowing out of wildlife itself, which has decreased in abundance by as much as 50 percent since 1970. This cull is from both direct hunting and global-scale habitat destruction: almost half of the earth’s land has been converted to farmland. The oceans have endured a similar transformation in only the past few decades as the industrial might developed during World War II has been trained on the seas. Each year fishing trawlers plow an area of seafloor twice the size of the continental United States, obliterating the benthos. Gardens of corals and sponges hosting colorful sea life are reduced to furrowed, lifeless plains. What these trawlers have to show for all this destruction is the removal of up to 90 percent of all large ocean predators since 1950, including familiar staples of the dinner plate like cod, halibut, grouper, tuna, swordfish, marlin, and sharks. As just one slice of that devastation, 270,000 sharks are killed every single day, mostly for their tasteless fins, which end up as status symbol garnishes in the bowls of Chinese corporate power lunches. And today, even as fishing pressure is escalating, even as the number of fishing boats increases, even as industrial trawlers abandon their exhausted traditional fishing grounds to chase down ever more remote fish stocks with ever more sophisticated fish-finding technology, global fish catch is flatlining. Closer to shore, coral reefs, the wellspring of the ocean’s biodiversity, have declined in extent by as much as a third since only the 1980s. These paradises are plagued by overfishing, pollution, and invaders, but 500 million people, many of them poor and living in the developing world, rely on them for food, storm protection, and jobs. As in the handful of reef collapses of the geological past, modern reefs are expected to collapse from warming and ocean acidification by the end of the century, and possibly much sooner. During record-breaking temperatures in 1997–1998, 15 percent of the world’s reefs died. In the past few months a similar wave of death struck the Great Barrier Reef. It won’t be the last. So things don’t look so good, no matter where we look. Yes, the victims in the animal world include scary apex predators that pose obvious threats to humans, like lions, whose numbers have dropped from 1 million at the time of Jesus to 450,000 in the 1940s to 20,000 today—a decline of 98 percent. But also included have been unexpected victims, like butterflies and moths, which have declined in abundance by 35 percent since the 1970s. Like all extinction events, so far this one has been phased and complex, spanning tens of thousands of years and starting when our kind left Africa. Other mass extinctions buried deep in earth’s history have similarly played out over tens of thousands, even hundreds of thousands of years. To future geologists, then, the huge wave of extinctions a few thousand years ago as First Peoples spread out into new continents and remote archipelagoes will be all but indistinguishable from the current wave of destruction loosed by modernity and its growing appetites. Surely we’ve earned our place in the pantheon next to the greatest ecological catastrophes of all time: the so-called Big Five mass extinctions of earth history. Surely our Anthropocene extinction can confidently take its place next to the juggernauts of deep time—the Ordovician, Devonian, Permian, Triassic and Cretaceous extinctions. Erwin says no. He thinks it’s junk science. “Many of those making facile comparisons between the current situation and past mass extinctions don’t have a clue about the difference in the nature of the data, much less how truly awful the mass extinctions recorded in the marine fossil record actually were,” he wrote me in an email. “It is absolutely critical to recognize that I am NOT claiming that humans haven’t done great damage to marine and terrestrial [ecosystems], nor that many extinctions have not occurred and more will certainly occur in the near future. But I do think that as scientists we have a responsibility to be accurate about such comparisons.” I had a chance to sit down with Erwin after his talk at the annual geology conference. My first question—about a rumor I had heard from one of his colleagues that Erwin had served as a sort of mass extinction consultant to Cormac McCarthy while the notoriously secretive author was constructing the post-apocalyptic world of The Road—Erwin coyly evaded. But on the speculative sixth mass extinction, he was more forthcoming. “If we’re really in a mass extinction—if we’re in the [End- Permian mass extinction 252 million years ago]—go get a case of scotch,” he said. If his power-grid analogy is correct, then trying to stop a mass extinction after it’s started would be a little like calling for a building’s preservation while it’s imploding. “People who claim we’re in the sixth mass extinction don’t understand enough about mass extinctions to understand the logical flaw in their argument,” he said. “To a certain extent they’re claiming it as a way of frightening people into action, when in fact, if it’s actually true we’re in a sixth mass extinction, then there’s no point in conservation biology.” This is because by the time a mass extinction starts, the world would already be over. “So if we really are in the middle of a mass extinction,” I started, “it wouldn’t be a matter of saving tigers and elephants—” “Right, you probably have to worry about saving coyotes and rats. “It’s a network collapse problem,” he said. “Just like power grids. Network dynamics research has been getting a ton of money from DARPA [Defense Advanced Research Projects Agency]. They’re all physicists studying it, who don’t care about power grids or ecosystems, they care about math. So the secret about power grids is that nobody actually knows how they work. And it’s exactly the same problem you have in ecosystems. “I think that if we keep things up long enough, we’ll get to a mass extinction, but we’re not in a mass extinction yet, and I think that’s an optimistic discovery because that means we actually have time to avoid Armageddon,” he said. Erwin’s other point, that the magnitude of the Big Five mass extinctions in earth’s past dwarfs humanity’s destruction thus far, is a subtle one. He’s not trying to downplay the tremendous destruction wrought by humans, but reminding us that claims about mass extinctions are inevitably claims about paleontology and the fossil record. “So there are estimates of what the standing crop of passenger pigeons was in the 19th century,” said Erwin. “It’s like 5 billion. They would black out the sky.” Passenger pigeons all but serve as the mascot of the “sixth mass extinction,” their extirpation an ecological tragedy on a massive scale, and proof that humans are a geologically destructive force to be reckoned with. “So then you ask: in a non-archaeological context, how many fossil passenger pigeons are there? How many records are there of fossil passenger pigeons?” “Not many?” I offered. “Two,” he said.“So here’s an incredibly abundant bird that we wiped out. But if you look in the fossil record, you wouldn’t even know that they were there.” Erwin likes to recall a talk he once went to by an ecologist who had documented the troubling losses he had seen over his career in high-altitude rainforests. “He was using this as an example of the destruction of plants in these cloud forests in Venezuela, all of which could be completely true,” Erwin said. “The problem is, the probability of finding one of those cloud forests in the fossil record is zero.” The fossil record is incredibly incomplete. One rough estimate holds that we’ve only ever found a tantalizing 0.01 percent of all the species that have ever existed. Most of the animals in the fossil record are marine invertebrates, like brachiopods and bivalves, of the sort that are both geologically widespread and durably skeletonized. In fact, though this book (for narrative purposes) has mostly focused on the charismatic animals taken out by mass extinctions, the only reason we know about mass extinctions in the first place is from the record of this incredibly abundant, durable, and diverse world of marine invertebrates, not the big, charismatic, and rare stuff like dinosaurs. “So you can ask, ‘Okay, well, how many geographically widespread, abundant, durably skeletonized marine taxa have gone extinct thus far?’ And the answer is, pretty close to zero,” Erwin pointed out. In fact, of the best-assessed groups of modern animals—like stony corals, amphibians, birds and mammals—somewhere between 0 and 1 percent of species have gone extinct in recent human history. By comparison, the hellscape of End-Permian mass extinction claimed upwards of 90 percent of all species on earth. When mass extinctions hit, they don’t just take out big charismatic megafauna, like elephants, or niche ecosystems, like cloud forests. They take out hardy and ubiquitous organisms as well—things like clams and plants and insects. This is incredibly hard to do. But once you go over the edge and flip into mass extinction mode, nothing is safe. Mass extinctions kill almost everything on the planet. While Erwin’s argument that a mass extinction is not yet under way might seem to get humanity off the hook—an invitation to plunder the earth further, since it can seemingly take the beating (the planet has certainly seen worse)—it’s actually a subtler and possibly far scarier argument. This is where the ecosystem’s nonlinear responses, or tipping points, come in. Inching up to mass extinction might be a little like inching up to the event horizon of a black hole—once you go over a certain line, a line that perhaps doesn’t even appear all that remarkable, all is lost. “So,” I said, “it might be that we sort of bump along where everything seems okay and then . . .” “Yeah, everything’s fine until it’s not,” said Erwin. “And then everything goes to hell.” Or put another way, mass extinctions may unfold the same way that a dissolute character in Hemingway’s The Sun Also Rises explains that bankruptcies do: “Two ways. Gradually and then suddenly.” “The only hope we have in the future,” Erwin said, “is if we’re not in a mass extinction event.” This article has been adapted from Peter Brannen’s new book, The Ends of the World. PETER BRANNEN is a science writer based in Boulder, Colorado. His work has appeared in The New York Times, The Washington Post, and Wired. He is the author of The Ends of the World: Volcanic Apocalypses, Lethal Oceans, and Our Quest to Understand Earth’s Past Mass Extinctions.

MASS EXTINCTIONS OF THE PAST

(WHITESIDE 2016) Whiteside, Jessica H., and Kliti Grice. “Biomarker records associated with mass extinction events.” Annual Review of Earth and Planetary Sciences 44 (2016): 581-612. The history of life on Earth is punctuated by a series of mass extinction episodes that vary widely in their magnitude, duration, and cause. Biomarkers are a powerful tool for the reconstruction of historical environmental conditions and can therefore provide insights into the cause and responses to ancient extinction events. In examining the five largest mass extinctions in the geological record, investigators have used biomarkers to elucidate key processes such as eutrophy, euxinia, ocean acidification, changes in hydrological balance, and changes in atmospheric CO2. By using these molecular fossils to understand how Earth and its ecosystems have responded to unusual environmental activity during these extinctions, models can be made to predict how Earth will respond to future changes in its climate.

(MCELWAIN 2007): McElwain, Jennifer C., and Surangi W. Punyasena. “Mass extinction events and the plant fossil record.” Trends in ecology & evolution 22.10 (2007): 548-557. Five mass extinction events have punctuated the geological record of marine invertebrate life. They are characterized by faunal extinction rates and magnitudes that far exceed those observed elsewhere in the geological record. Despite compelling evidence that these extinction events were probably driven by dramatic global environmental change, they were originally thought to have little macroecological or evolutionary consequence for terrestrial plants. New high-resolution regional palaeoecological studies are beginning to challenge this orthodoxy, providing evidence for extensive ecological upheaval, high species-level turnover and recovery intervals lasting millions of years. The challenge ahead is to establish the geographical extent of the ecological upheaval, because reconstructing the vegetation dynamics associated with these events will elucidate the role of floral change in faunal mass extinction and provide a better understanding of how plants have historically responded to global environmental change similar to that anticipated for our future.

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SUMMARY OF THE FIVE GREAT EXTINCTION EVENTS PROVIDED BY SAMNOBLE MUSEUM: LINK: https://samnoblemuseum.ou.edu/understanding-extinction/mass-extinctions/end-permian-extinction/

(#1: END PERMIAN) When did it happen?: There were two significant extinction events in the Permian Period. The smaller, at the end of a time interval called the Capitanian, occurred about 260 million years ago. The event at the end of the Permian Period (at the end of a time interval called the Changshanian) was much larger and may have eliminated more than three-quarters of species of marine animals. It happened about 252 million years ago and geological evidence shows that it may have taken no more than 200,000 years. In terms of geological time the extinction occurred quickly. Who became extinct? Important groups of marine animals disappeared at the end-Permian extinctions. Trilobites, which had lived in the oceans for more than 250 million years, were lost, along with tabulate and rugose corals. Reef building in shallow seas stopped for about 14 million years until the middle of the following Triassic Period. At that time, an entirely new group of corals, the stony or scleractinian corals, appeared in the oceans. Although they did not become entirely extinct, rhynchonelliform brachiopods, crinoids, shelled cephalopods and snails also suffered significant losses. On land, primitive synapsids (relatives of mammals) disappeared. Some estimates suggest that up to 70 percent of vertebrate genera were lost. Below are some groups of marine animals that became extinct at the end-Permian event. trilobites, Tabulate corals, Rugose corals, goniatitic cephalopods, Productid brachiopods, cladid crinoids. What caused the extinction? Warming of the Earth’s climate and associated changes to oceans were the most likely causes of the extinctions. At the end of the Permian Period volcanic activity on a massive scale in what is now Siberia led to a huge outpouring of lava. The eruptions also produced carbon dioxide, a greenhouse gas that helps warm the planet. The lava flows erupted onto carbon rich rocks like coal and as they were heated by the hot lava, greenhouse gases, including methane, were also produced. The global warming that followed may have increased average ocean water temperatures by as much as 14.5°F (8°C). Much of the carbon dioxide released by the eruptions would have been absorbed by the oceans. High levels of dissolved carbon dioxide in seawater are toxic to many marine invertebrates. Also, the dissolved carbon dioxide would have produced changes in seawater chemistry that may have made it difficult for some marine invertebrates, such as corals, to grow shells or skeletons. If that wasn’t bad enough, there is also geological evidence that the amount of oxygen dissolved in sea water (which invertebrates and fishes breath with their gills) was reduced, probably as a result of changes in ocean circulation.

(#2: END TRIASSIC): When did it happen?: The extinction occurred near the end of the Triassic Period, about 201 million years ago. Who became extinct?: All major groups of marine invertebrates survived the extinction, although most suffered losses. Brachiopods, shelled cephalopods, sponges and corals were particularly hard hit. On land, casualties included the phytosaurs, a group of crocodile-like animals. What caused the extinction?: At the end of the Triassic, the supercontinent of Pangea, which combined all of the modern continents into a single landmass, began to break (rift) apart. As North America separated from Africa and the Atlantic Ocean began to form, volcanic activity on a massive scale introduced carbon dioxide into the atmosphere. This led to global warming and changes to the oceans that were similar to (although not as large) those that occurred at the end-Permian extinction. Reconstruction of Late Triassic global geography: All of today’s continents were combined into the supercontinent of Pangea. Pangea was beginning to break apart. As North America and Africa began to separate there was a vast outpouring of lava. The area of volcanic rocks that formed at this time is shown in yellow. Gases, including carbon dioxide, produced during the eruptions led to global climate change.

(#3: LATE DEVONIAN): When did they happen?: The end-Frasnian extinction happened about 375 million years ago. The oldest of the three extinctions, towards the end of a time interval called the Givetian, occurred about 10 million years before the Frasnian event. The youngest extinction happened near the end of the Devonian period, about 365 million years ago, during a time interval called the Famennian. Who became extinct?: The end-Frasnian extinction was most pronounced in tropical environments, particularly in the reefs of the shallow seas. Reef building sponges called stromatoporoids and corals suffered losses and stromatoporoids finally disappeared in the third extinction near the end of the Devonian. Brachiopods associated with reefs also became extinct. Groups of trilobites disappeared at each of the three extinctions and very few survived into the following Carboniferous Period. Examples of groups of brachiopods and trilobites that became extinct are: Odontopleurid trilobites, Dalmanitid trilobites, Phacopid trilobites, Atrypid brachiopods, Pentamerid brachiopods. How did they happen?: As extinctions were mostly of tropical groups climate change may have been involved, and there is geological evidence for cooling of the global climate at the end-Frasnian event and near the end of the Devonian Period. Cooling may have been caused by a drop in the amount of carbon dioxide in the atmosphere. Carbon dioxide is a greenhouse gas that helps warm the planet, so if levels fall, cooling will follow. In the Late Devonian, large trees evolved and formed the first forests. As plant life expanded, they used up more carbon dioxide in photosynthesis. When dead plant material decays, carbon dioxide is returned to the atmosphere, but some plant material (e.g., leaves) will be buried in swamps, lakes and rivers. This buried plant material removes carbon permanently from the atmosphere and often forms coal. When we mine coal and burn it we return carbon dioxide to the atmosphere and warm the planet.

(#4: END-ORDOVICIAN): When did it happen?: There were two distinct extinctions roughly a million years apart. The first of these began about 443 million years ago. Together, these extinctions may have removed about 85 percent of species of marine animals. Who became extinct?: All of the major animal groups of the Ordovician oceans survived, including trilobites, brachiopods, corals, crinoids and graptolites, but each lost important members. Widespread families of trilobites disappeared and graptolites came close to total extinction. Examples of fossil groups that became extinct at the end-Ordovician extinction: Trilobite family Trinucleidae, Trilobite family Bathyuridae, Brachiopod genus Thaerodonta, Brachiopod genus Plaesiomys, Graptolite family Climacograptidae, Graptolite family Diplograptidae. What caused the extinction? The evidence indicates that climate change caused the extinctions. A major ice age is known to have occurred in the southern hemisphere and climates cooled world-wide. The first wave of extinctions happened as the climate became colder and a second pulse occurred as climates warmed at the end of the ice age. Reconstruction of Late Ordovician global geography (southern hemisphere), showing the south polar icecap (white). The Ordovician continent of Laurentia corresponds to most of present day North America; Baltica included part of modern western Europe. Gondwana was a super-continent composed of most of the major modern continents.

(#5: END CRETACEOUS): When did it happen?: The extinction occurred at the end of the Cretaceous Period, about 65.5 million years ago. Who became extinct?: In addition to the non-avian dinosaurs, vertebrates that were lost at the end of the Cretaceous include the flying pterosaurs, and the mosasaurs, plesiosaurs and ichthyosaurs of the oceans. Important marine invertebrates also disappeared, including ammonites, groups of cephalopods and some bivalves, such as the reef-building rudists and some relatives of modern oysters. Examples of invertebrate groups that became extinct at the end_Cretaceous event. tes that disappeared at the end-Cretaceous extinction. Ammonite (Cephalopod), gryphaeid oyster (Bivalv), Inoceramid (Bivalve). What caused the extinction?: Several lines of geological evidence indicate that an asteroid that was as much as 10 kilometers (6 miles) in diameter hit the Earth at the end of the Cretaceous Period. This evidence includes an ancient impact crater in the Yucatan Peninsula of Mexico (now filled in by younger rocks) that dates to the time of the extinction. The impact would have produced an enormous dust cloud that would have risen up into the atmosphere and encircled the planet. The dust cloud greatly reduced the amount of sunlight reaching the Earth’s surface and prevented photosynthesis by plants on land and plankton in the oceans. As plants and plankton died, extinctions expanded up the food chain, eliminating herbivores and carnivores. If that was not bad enough, dust and debris falling back to Earth was hot and may have triggered widespread wildfires. There is some debate over whether the asteroid was the sole cause of the extinction or whether other factors were also involved. Towards the end of the Cretaceous, volcanic activity in India produced lava flows over a vast area. Some paleontologists and geologists have suggested that gases (e.g., sulfur dioxide; carbon dioxide) released by the volcanoes might have altered the climate. Others point to geological evidence for a fall in sea level that would have reduced the area of shallow seas and, possibly, coastal plains.

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SUMMARY AND CONCLUSIONS:

The characteristics of the five mass extinctions described above show that the two essential things that they all have in common are signficant global geological upheaval involving enormous amounts of energy and time scales of millions of years. Humans appeared on earth 66 million years ago and we could have been a creation of the End Cretaceous Extinction event but we surely could not have caused it. Human civilization did not appear until this Holocene interglacial about 8,000 years ago and the industrialization of human civilization did not occur until 100 years ago. The time scale needed for mass extinction events excludes any role for humans and the geological nature of these events also excludes any significant role for humans. That humans are causing a sixth mass extinction in the sequence of the five described above is a fanciful idea of extreme environmentalism but not credible.

It should also be noted that published research papers have two things in common – that they are published by the same journal (PNAS) and include a common co-author (Paul Ehrlich). In terms of Ehrlich’s failed population bomb hypothesis and his obsession with human population and climate change, his interest in the theory of a human caused sixth mass extinction can be understood in that context. It is possible that the sixth mass extinction idea derives from a need in climate science to inject additional fear into the fear based activism against fossil fuels that is not going well for them. The need for climate change science to include a sixth mass extinction in its fear porfolio is also seen in the way the 5 great mass extinctions are now described. These descriptions include the idea that the planetary scale geological holocaust had increased or decreased atmospheric CO2 and had thereby caused global warming or global cooling – in other words a role for AGW-like climate change is inserted.

We therefore agree with the presentation at the annual meeting of the Geological Society of America by Professor Douglas Erwin that the sixth mass extinction is a fanciful extension of the climate change discussion with imagination but without substance.

Image result for Smithsonian paleontologist Doug Erwin

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