A Sixth Mass Extinction?

A variety of global changes are driving rates of species extinction that greatly outpace background rates in the fossil record. If these trends continue, projections suggest that within 240 years Earth may face the sixth mass extinction.

- Sala, O. E. et al., Barnosky, A. D. et al.

There have been 5 major mass extinction events in known history, known in the field as “The Big Five”:

Figure 1a: Extinction event diagram (Source: 6th ExtinctionDoc)

Figure 1: Mass Extinction Timeline (Source: Izilwane)


First major extinction near the end of the Ordovician Period 

  • c. 440 million years ago
  • Climate change – relatively severe and sudden global cooling  – caused dramatic change in marine life (there was little or no life existed on land at that time)
  • 25% of families lost (a family may consist of a few to thousands of species)

Second major extinction  near the end of the Devonian Period 

  • c. 370 million years ago
  • May or may not have been the result of global climate change. 19% of families lost

Third major Extinction at the end of the Permian Period

  • c. 245 million years ago
  • Perhaps the greatest mass extinction event of all
  • explained by a complex combination of climate change, plate tectonics movements and more recently theories of a bolide impact similar to the end-Cretaceous event
  • 54% of families lost

Fourth major extinction at the end of the Triassic Period 

  • c. 210 million years ago
  • Shortly after dinosaurs and mammals had first evolved
  • Precise cause not yet known
  • 23% of families lost

Fifth major extinction at the end of the Cretaceious

  • c. 65 million years ago
  • Wiped out the remaining terrestrial dinosaurs and marine ammonites
  • Wiped out many other species across the phylogenetic spectrum, in all habitats sampled from the fossil record
  • Scientific consensus that it was caused by one  and possibly multiple collisions between Earth and an extraterrestrial bolide, probably cometary
  • Some geologists also point to the great volcanic event that produced the Deccan traps of India as part of the chain of physical events that disrupted ecosystems so severely that many species on land and sea rapidly succumbed to extinction
  • 17% of families lost

Sixth major extinction during the Antropocene

Now, there is talk amongst scientists about a sixth extinction event  becoming increasingly likely – only this one would not be “natural”, but man-made during the age of the Anthropocene.

Dr. Anthony D. Barnosky

Figure 2: Years until Extinction (Source: Anthony Barnoksy, Ashlyn Kong)


Anthony D. Barnosky et al published a paper in Nature 471, 51–57 (03 March 2011) entitled Has the Earth’s sixth mass extinction already arrived? 


Palaeontologists characterize mass extinctions as times when the Earth loses more than three-quarters of its species in a geologically short interval, as has happened only five times in the past 540 million years or so. Biologists now suggest that a sixth mass extinction may be under way, given the known species losses over the past few centuries and millennia. Here we review how differences between fossil and modern data and the addition of recently available palaeontological information influence our understanding of the current extinction crisis. Our results confirm that current extinction rates are higher than would be expected from the fossil record, highlighting the need for effective conservation measures.

Barnosky et al concluded that if conservation efforts are not vigilent, a mass extinction could occur in just 300 years or less

“The finding is both encouraging and discouraging,” said principal author Anthony Barnosky. “We clearly are in a major extinction process, but we haven’t come so far down the road that we can’t do something about it. It’s really a call to action to make sure that we keep the conservation efforts we have in place and actually add more.”

While extinctions are normal, they are usually balanced by the creation of new species – a drop in biodiversity results from more extinction than origination. But in the past 500 years, the rate of extinction has accelerated and is currently three to 80 times higher than what is considered normal over the course of the Earth’s history, according to Emily Lindsey, an author on the paper and graduate student in the department of integrative biology. Humans are the main cause through activities such as:

  • co-opting resources
  • fragmenting habitats
  • introducing non-native species
  • spreading pathogens and toxins
  • killing species directly
  • changing global climate

The paper cites other research which indicates that biodiversity recovery will not occur on any timeframe meaningful to people because evolution of new species occurs on the scale of hundreds of thousands of years while the recovery from mass extinction episodes occurs on the scale of millions of years.

The paper took various approaches to looking at when a mass extinction would occur. One estimated the rates of extinction during the Big Five and found that if currently threatened species become extinct in the next century and this elevated rate continues, a mass extinction could occur in a few hundred years.

Another method examined what would happen if it takes 500 years for threatened species to become extinct. In this case, the sixth mass extinction would be reached in a couple thousand years – still “a blink of an eye” in geologic time, said H. Richard Lane, program officer in the Sedimentary Geology and Paleobiology Program at the National Science Foundation.

“Life is interrelated – everything depends on everything else,” Lane said. “It’s hard to predict exactly where we are headed, only to say that there’s a tremendous uncertainty. We have a major extinction event beginning and under way, and just where that will leave us and how humans will survive is hard to say.”

The growing human population and increased development has greatly caused the destruction of natural environments and depletion of resources and is thus blamed for organism population decreases, researchers said.

“Perhaps we’re like the viruses which create their own toxins that eventually kill themselves off,” Lane said. “Maybe it’s the same kind of cycle taking place in higher life.”

According to the International Union for Conservation of Nature, 33 percent of all species they have studied are threatened, meaning they have experienced at least a 50 percent reduction in population over the past 10 years. To prevent a fate like that of the dinosaurs, Barnosky said more rigorous efforts must be taken to preserve habitats of threatened species.

“It’s really important that we don’t let those endangered and threatened species really go extinct,” he said. “It’s going to require conservation efforts at the local and the national level … It’s a new ball game.”

The conclusion of the paper underscores the urgency of immediate action:

There are clear indications that losing species now in the ‘critically endangered’ category would propel the world to a state of mass extinction that has previously been seen only five times in about 540 million years. Additional losses of species in the ‘endangered’ and ‘vulnerable’ categories could accomplish the sixth mass extinction in just a few centuries. It may be of particular concern that this extinction trajectory would play out under conditions that resemble the ‘perfect storm’ that coincided with past mass extinctions: multiple, atypical high-intensity ecological stressors, including rapid, unusual climate change and highly elevated atmospheric CO2. The huge difference between where we are now, and where we could easily be within a few generations, reveals the urgency of relieving the pressures that are pushing today’s species towards extinction.

(Source: The Daily Californian)

Dr. Niles Eldredge

Paleontologist Dr. Niles Eldredge has been writing about the sixth extinction event since 2001. He is the Curator-in-Chief of the permanent exhibition “Hall of Biodiversity” at the American Museum of Natural History, adjunct professor at the City University of New York and contributor to the actionbioscience website. He writes:

There is little doubt left in the minds of professional biologists that Earth is currently faced with a mounting loss of species that threatens to rival the five great mass extinctions of the geological past. As long ago as 1993, Harvard biologist E.O. Wilson estimated that Earth is currently losing something on the order of 30,000 species per year — which breaks down to the even more daunting statistic of some three species per hour. Some biologists have begun to feel that this biodiversity crisis — this “Sixth Extinction” — is even more severe, and more imminent, than Wilson had supposed.

Dr. Eldredge goes on to describe the characteristics of this Sixth extinction event and how it is peculiarly human-caused:

How is the Sixth Extinction different from previous events?
The current mass extinction is caused by humans.

At first glance, the physically caused extinction events of the past might seem to have little or nothing to tell us about the current Sixth Extinction, which is a patently human-caused event. For there is little doubt that humans are the direct cause of ecosystem stress and species destruction in the modern world through such activities as:

  • transformation of the landscape
  • overexploitation of species
  • pollution
  • the introduction of alien species

And because Homo sapiens is clearly a species of animal (however behaviorally and ecologically peculiar an animal), the Sixth Extinction would seem to be the first recorded global extinction event that has a biotic, rather than a physical, cause.

We are bringing about massive changes in the environment.

Yet, upon further reflection, human impact on the planet is a direct analogue of the Cretaceous cometary collision. Sixty-five million years ago that extraterrestrial impact — through its sheer explosive power, followed immediately by its injections of so much debris into the upper reaches of the atmosphere that global temperatures plummeted and, most critically, photosynthesis was severely inhibited — wreaked havoc on the living systems of Earth. That is precisely what human beings are doing to the planet right now: humans are causing vast physical changes on the planet.

What is the Sixth Extinction?

We can divide the Sixth Extinction into two discrete phases:

  • Phase One began when the first modern humans began to disperse to different parts of the world about 100,000 years ago.
  • Phase Two began about 10,000 years ago when humans turned to agriculture.
Phase 1
Humans began disrupting the environment as soon as they appeared on Earth. The first phase began shortly after Homo sapiens evolved in Africa and the anatomically modern humans began migrating out of Africa and spreading throughout the world. Humans reached the middle east 90,000 years ago. They were in Europe starting around 40,000 years ago. Neanderthals, who had long lived in Europe, survived our arrival for less than 10,000 years, but then abruptly disappeared — victims, according to many paleoanthropologists, of our arrival through outright warfare or the more subtle, though potentially no less devastating effects, of being on the losing side of ecological competition.

Everywhere, shortly after modern humans arrived, many (especially, though by no means exclusively, the larger) native species typically became extinct. Humans were like bulls in a China shop:

  • They disrupted ecosystems by overhunting game species, which never experienced contact with humans before.
  • And perhaps they spread microbial disease-causing organisms as well.

The fossil record attests to human destruction of ecosystems:

Wherever early humans migrated, other species became extinct.
  • Humans arrived in large numbers in North America roughly 12,500 years ago-and sites revealing the butchering of mammoths, mastodons and extinct buffalo are well documented throughout the continent. The demise of the bulk of the La Brea tar pit Pleistocene fauna coincided with our arrival.
  • The Caribbean lost several of its larger species when humans arrived some 8000 years ago.
  • Extinction struck elements of the Australian megafauna much earlier-when humans arrived some 40,000 years ago. Madagascar-something of an anomaly, as humans only arrived there two thousand years ago-also fits the pattern well: the larger species (elephant birds, a species of hippo, plus larger lemurs) rapidly disappeared soon after humans arrived.

Indeed only in places where earlier hominid species had lived (Africa, of course, but also most of Europe and Asia) did the fauna, already adapted to hominid presence, survive the first wave of the Sixth Extinction pretty much intact. The rest of the world’s species, which had never before encountered hominids in their local ecosystems, were as naively unwary as all but the most recently arrived species (such as Vermilion Flycatchers) of the Galapagos Islands remain to this day.

Phase 2
The invention of agriculture accelerated the pace of the Sixth Extinction. Phase two of the Sixth Extinction began around 10,000 years ago with the invention of agriculture-perhaps first in the Natufian culture of the Middle East. Agriculture appears to have been invented several different times in various different places, and has, in the intervening years, spread around the entire globe.

Agriculture represents the single most profound ecological change in the entire 3.5 billion-year history of life. With its invention:

  • humans did not have to interact with other species for survival, and so could manipulate other species for their own use
  • humans did not have to adhere to the ecosystem’s carrying capacity, and so could overpopulate
Humans do not live with nature but outside it.

Homo sapiens became the first species to stop living inside local ecosystems. All other species, including our ancestral hominid ancestors, all pre-agricultural humans, and remnant hunter-gatherer societies still extant exist as semi-isolated populations playing specific roles (i.e., have “niches”) in local ecosystems. This is not so with post-agricultural revolution humans, who in effect have stepped outside local ecosystems. Indeed, to develop agriculture is essentially to declare war on ecosystems – converting land to produce one or two food crops, with all other native plant species all now classified as unwanted “weeds” — and all but a few domesticated species of animals now considered as pests.

The total number of organisms within a species is limited by many factors-most crucial of which is the “carrying capacity” of the local ecosystem: given the energetic needs and energy-procuring adaptations of a given species, there are only so many squirrels, oak trees and hawks that can inhabit a given stretch of habitat. Agriculture had the effect of removing the natural local-ecosystem upper limit of the size of human populations. Though crops still fail regularly, and famine and disease still stalk the land, there is no doubt that agriculture in the main has had an enormous impact on human population size:

Earth can’t sustain the trend in human population growth. It is reaching its limit in carrying capacity.
  • Estimates vary, but range between 1 and 10 million people on earth 10,000 years ago.
  • There are now over 6 billion people.
  • The numbers continue to increase logarithmically — so that there will be 8 billion by 2020.
  • There is presumably an upper limit to the carrying capacity of humans on earth — of the numbers that agriculture can support — and that number is usually estimated at between 13-15 billion, though some people think the ultimate numbers might be much higher.

This explosion of human population, especially in the post-Industrial Revolution years of the past two centuries, coupled with the unequal distribution and consumption of wealth on the planet, is the underlying cause of the Sixth Extinction. There is a vicious cycle:

Overpopulation, invasive species, and overexploitation are fueling the extinction.
  • More lands are cleared and more efficient production techniques (most recently engendered largely through genetic engineering) to feed the growing number of humans — and in response, the human population continues to expand.
  • Higher fossil energy use is helping agriculture spread, further modifying the environment.
  • Humans continue to fish (12 of the 13 major fisheries on the planet are now considered severely depleted) and harvest timber for building materials and just plain fuel, pollution, and soil erosion from agriculture creates dead zones in fisheries (as in the Gulf of Mexico)
  • While the human Diaspora has meant the spread, as well, of alien species that more often than not thrive at the detriment of native species. For example, invasive species have contributed to 42% of all threatened and endangered species in the U.S.


Peter Ward’s Medea Hypothesis: an Alternative Theory for the Sixth Mass Extinction 

Paleontologist Peter Ward puts for a theory that we are headed for a global mass extinction event far sooner than anyone ever imagined – as early as a century away.. Ward  proposes a radically different theory on life’s relationship with the Earth’s biosphere–one that has implications for our immediate  future.

Ward’s book, The Medea Hypothesis is a critical response to James Lovelock’s Gaia concept, which contends that homeostatic physical and chemical interactions work to maintain Earth’s habitability. Ward argues that the opposite is in fact true–living organisms decrease Earth’s habitability, hastening its end by perhaps a billion years. The mechanism for all this to happen? – something relatively unknown until his theory became popular – Aneorobic microbes and H2S. 

Paleontologist Peter Ward talks about the Medea Hypothesis on Radio Ecoshock

Peter Ward on TED talking about the Medea Hypothesis

Using the latest discoveries from the geological record, Ward argues that life might just be its own worst enemy. This is in stark contrast to James Lovelock’s Gaia hypothesis–the idea that life sustains habitable conditions on Earth. In fact, Ward entitled his book The Medea Hypothesis in response to Lovelock’s “Gaia” hypothesis. Ward chose to name his theory after Medea,  the mythical wife of Jason the Argonaut, who swiped the Golden Fleece. In a fit of rage against her husband, Medea killed her own children. According to the Medea hypothesis, life evolves to become its own worse enemy.  Using his aneorobic microbe / H2S theory, Ward demonstrates that all but one of the mass extinctions that have struck Earth were caused by life itself, not by meteors or volcanos.

Ward interprets the alarming decline of diversity and biomass as one that is naturally brought about by  life’s own “bio-suicidal” tendencies. The Medea hypothesis applies not just to our planet, but extends to all potential life in the universe. While life on Earth doesn’t have to become extinct, Ward warns that our time window to prevent extinction is quickly running out.

Here is how Ward explains the mass extinction event we are faced with if we do not act quickly:

  1. About  2.4 billion years ago, microscopic cyanobacteria emerged newly equipped with photosynthesis and the massive amounts of oxygen produced as the product of photosynthesis triggered the Great Oxygenation Event
  2.  Subsequently, aerobic organisms thrived and anaerobic organisms, the dominant life form until then began dying out in massive numbers
  3. The sudden release of oxygen is also likely what set off the Huronian Glaciation, a deadly “snowball earth” that kept the planet locked in ice for 300 million years
  4. As atmospheric carbon rose above 1,000 parts per million (ppm), a super-charged greenhouse effect dramatically weakens the temperature differential between the poles and the tropics
  5. Without pronounced temperature gradients to drive ocean mixing, only the top layer of the sea remains oxygenated
  6. Anaerobic bacteria populations began to explode because they thrive below this zone, producing massive quantities of hydrogen sulfide gas to poison the entire planet
  7. This poisoning may be to blame for the End-Permian event, “the mother of all extinctions“, when 96% of all marine organisms disappeared


The Sixth Mass Extinction

Now it is simple matter to explain how humans may be the trigger for the Sixth Mass Extinction. Humans are also an element of the earth’s self-destructive tendency. If our Co2 emissions go above the tipping point of 1,000 ppm, as predicted by some upper-end IPCC estimates for 2100, we may trip an event identical to the one that wiped out the trilobites. Half the oxygen in the planet is produced by the ocean. By going above 1,000 ppm’s, we may reduce the temperature gradient between the Poles and the Equator. While temperatures at the equator cannot increase any further, at the poles they certainly can and in fact, are the regions currently experiencing the highest increase in temperature on the planet. As the average temperature at the poles increase, this will have the effect of slowing down the Ocean’s current. When the current reaches zero, no more oxygen will be produced and aneorobic microbes will create massive quantities of H2S which will kill all aerobic lifeforms, including us, of course.

Ironically, while CO2 may be the threat which can cause mass extinction in the short term, in the long run, it’s a carbon shortage that may spell the end of life, long before the sun vaporizes the oceans. It is predicted that in 500 million years, life’s insatiable need for carbon – the basic building block of every organism – could mean that atmospheric carbon might eventually drop below 10 ppm, the amount needed to sustain grasses. This is because as organisms die, they deposit their carbon rich bodies in layers and are covered over, making that carbon inaccessible to the atmosphere. Hence, concentrations of CO2 will continually diminish until it reaches a lethal 10 ppm. After grasses have all died out, then all animal life forms will soon follow. In terms of total biomass, evidence indicates that earth is already in its “old age”.

Life bubbled up from a happy coincidence of molecules. According to Ward, it will stick around for about 4 billion years, and extinguish itself after that. Hence, the universe’s great experiment with life will be finished for another cycle.


The Medea Hypothesis has been presented in the form of a popular book instead of a peer reviewed paper. There does not appear to be much research dedicated to either supporting or refuting the theory.

Dr. Andrew Glikson’s critique

Dr. Andrew Glikson, a visiting fellow of the Australia National University critiqued it in The Journal of Cosmology ,Oct 18,  2009, Vol 2, pages 230-234. in an article entitled: Mass Extinction of Species: The Role of External Forcing, Comment on “The Cronus hypothesis: Extinction as a necessary and dynamic balance to evolutionary diversification.”


he terrestrial biosphere, constituting a less than 20 km-thick zone constrained by the crustal depth at which bacteria occur and the atmospheric level to which birds can fly, has been repeatedly affected by external forcing, including deep Earth-derived volcanic events, asteroid and comet impacts, solar insolation and orbital forcing cycles, and likely supernovae. Intrinsic biological evolution and diversification through natural selection and adaptation has been repeatedly overprinted, and in some instances almost obliterated, by these events. Both, the ‘Medea hypothesis’ and the ‘Cronus hypothesis’, hinging on metaphors focused on biological self-destruction, appear to underestimate the role of externally forced destruction. The search for unifying principles and for a ‘dynamic balance’ in biological evolution must not overlook the unique origin and consequences of each of the mass extinction events.

Glikson believes that both the Medea Hypothesis and the Cronus Hypothesis (both propose a dominant role of biological self-destruction in mass extinction events) underestimate the role of externally forced destruction. The conclusions of his comment are:

1. Inherent in metaphors such as ‘Medea hypothesis’ or ‘Cronus hypothesis’ is a focus on intra-biosphere self-destructive processes, which overlook the major to critical role of extra-biosphere forcing events, including asteroid/comet impacts and volcanic eruptions.

2. Attempts at arriving at common intrinsic principles with regard to the respective roles of gradual evolutionary processes vis-a-vis mass extinction events must not overlook the unique nature and consequences of each extra-biosphere forcing event and related mass extinction.

3. The anthropogenic “Sixth mass extinction” appears to be unique in terrestrial history. The question remains subject to philosophical notions, for example in terms of the vulnerability of extreme complexity in nature to self-destruction and the “price” in terms of information entropy of the achievement of deep insights into nature. Homo sapiens may never know the answer to the deepest questions.

Dragon’s Tale (DT) Critique

The Dragon’s Tale website also contains a detailed critique of Ward’s hypothesis. The website author is a computer modeller working with scientists on a variety of computer models so is in an insider position to critique the finer points of Ward’s hypothesis. He breaks his critique into 5 parts:


Misuse of Models – DT’s Summary

  • Ward relies on Franck et al’s model for carbon cycling over multiple billions of years
  • Franck’s model is very simple: the model itself describes how carbon is cycled and sequestered in the earth system
  • Franck’s model takes into account continental growth, some biological activity, and even weathering of kerogen, Biomass is represented as a function of carbon dioxide and temperature
  • Franck’s model is a nice, simple, useful tool, but it has limitations:
  • Some examples of limitations of Franck’s model:
    • it cannot take into account the Snowball Earth episodes; nowhere can the model fit these important planet wide episodes in their model’s data
    • it cannot account for the temperature fluctuations of the Pharenozoic much at all: the ice ages that have come and gone are not represented almost at all
    • The temperature spikes (during Permian, Eocene) cannot be included
    • The increase in vulcanism attributed to supercontinental fragmentation is not included
    • Mass extinction,  important to biodiversity and biomass cannot be included or represented in this data
  • Franck et al. acknowledge these limitations.  Unfortunately, Ward uses this simple model and extrapolates results that are beyond its capability. DT claims that the model is far too simple and Ward, being familiar with the observational/paleontological data should know better.
  • When a model cannot take into account some very fundamental and important events as they pertain to the question of what you are trying to answer, it should not be used.
  • When a model contradicts key observational data, it should not be used.

Misleading Statements

DT raises some serious issues with at least 3 major claims made by Ward:

The Permian Extinction: The Great Dying

The Permian Extinction is considered the greatest of all mass extinctions. In the marine realm, potentially as much as 95% of everything died. and on land,  as high as 75% species may have died. This extinction event is very well researched and the mechanisms and causes are fairly well understood.

In DT’s opinion, Ward distorts this to fit the Medea Hypothesis. He claims the main kill mechanism of the mass extinction was  the excretions of anaerobic bacteria, hydrogen sulfide (H2S), which is highly toxic to most aerobic life.

DT’s claim is that this is highly distorted because there were many other equally likely kill mechanisms proven to coexist at that time:

  • The Siberian Traps were erupting; the single biggest eruption in the whole of the past 650 million years and possibly as much as the past billion years
  • Because of these volcanic eruptions, the world radically warmed: well over 10 C in very short time periods
  • The warming caused the oceans to turn very anoxic and subsequently, the methane calthrates erupted from the oceans
  • The ozone layer was destroyed: unblocked UV scorched the land and even the surface of the sea
  • The vast majority of land desertified
  • Hypersaline lakes released lethal halogen gases
  • The atmospheric oxygen levels crashed
  • Precipitation was probably extremely seasonal and possibly even a megamonsoon model
  • Anaerobic bacteria released hydrogen sulfide

DT points out that Ward’s bacterial menace was but one of many killing mechanisms. While anaerobic bacteria may have played an important role, it was the vast eruptions that took place in Siberia which was the main driver. DT feels Ward has greatly misled the reader by distorting the importance of the anaerobic bacteria above all the other killing mechanisms.

The Snowball Earths

The Snowball Earth episodes are time periods in Deep Time where the world is suspected to have virtually iced over. There is very sketchy data available on this period, certainly not enough to substantiate Wards claim that life caused them. Ward asserts that life, bacteria and more advanced photosynthetic life drew down the carbon dioxide so much that it caused the earth to lose its greenhouse effect and ice over.

The Pleistocene Glaciations

Ward again asserts that life caused the carbon dioxide to drop sufficiently for glaciations to take place. He presents no evidence to backup his claim and in fact,  the  CO2 data was actually higher than it is now. The actual data does not match his constant downward trending plots.