Climate Science Glossary

Term Lookup

Enter a term in the search box to find its definition.


Use the controls in the far right panel to increase or decrease the number of terms automatically displayed (or to completely turn that feature off).

Term Lookup


All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Home Arguments Software Resources Comments The Consensus Project Translations About Support

Bluesky Facebook LinkedIn Mastodon MeWe

Twitter YouTube RSS Posts RSS Comments Email Subscribe

Climate's changed before
It's the sun
It's not bad
There is no consensus
It's cooling
Models are unreliable
Temp record is unreliable
Animals and plants can adapt
It hasn't warmed since 1998
Antarctica is gaining ice
View All Arguments...

New? Register here
Forgot your password?

Latest Posts


The Earth's Sixth Mass Extinction May Be Underway

Posted on 9 March 2011 by dana1981

NOTE: A slightly simpler version of this article has also been published on Treehugger

A "mass extinction" event is characterized as a period during which at least 75% of the Earth's species die out in a geologically short interval of time.  In the past 540 million years, only five such mass extinction events have occurred, but according to a review by Barnosky et al. (2011) recently published in the journal Nature, there are signs that we may be entering a sixth such event.

Mass Extinction Events

The Earth's five previous mass extinction events occurred during the:

  • Ordovician (443 million years ago, 86% of species extinct);
  • Devonian (359 million years ago, 75% of species extinct);
  • Permian (251 million years ago, 96% of species extinct);
  • Triassic (200 million years ago, 80% of species extinct); and
  • Cretaceous (65 million years ago, 76% of species extinct)

These previous mass extinction events (also known as the "Big Five") are hypothesized to have been caused by key synergies such as unusual climate dynamics, atmospheric composition, and abnormally high-intensity ecological stressors (or in the case of the Cretaceous, an asteroid impact and subsequent effects).

Barnosky et al. note that scientists are increasingly recognizing modern extinctions of species due to various human influences, including some of the same effects which caused the Big Five:

"through co-opting resources, fragmenting habitats, introducing non-native species, spreading pathogens, killing species directly, and changing global climate"

Natural Extinctions

There are of course species extinctions which have nothing to do with human influences.  Scientists have identified a "background rate" of species extinctions from the fossil record, which allows for a comparison to the current extinction rate, thus allowing us to assess the human impact on the rate of species loss.  A widely-used metric is extinctions per million species-years (E/MSY), in which background rates are estimated from fossil extinctions that took place in million-year-or-more timeframes.  The authors note that it is difficult to compare the current rate of extinctions, which are occurring over periods of just decades to centuries, to this background rate determined from periods millions of years.

That being said, the average E/MSY over the fossil record is approximately 1.8 (meaning on average, fewer than 2 species go extinct every million species-years), and the most common E/MSY over periods less than 1,000 years is zero.  Bear in mind that these are species-years, and that there are an estimated 20 million species on Earth, so each year constitutes approximately 20 million species-years.

Current Extinction Rate

Branosky et al. find that over the past 1,000 years, the average extinction rate is 24 E/MSY (13x background).  Breaking the data into 1-year bins, the maximum extinction rate over that period is approximatley 693 E/MSY (385x background).  Clearly these values far exceed the background rate.  And the worst case scenario, if all currently threatened species go extinct, results in a clear divergence from the natural extinction rate:

"In the scenario where currently ‘threatened’ species would ultimately go extinct even in as much as a thousand years, the resulting rates would far exceed any reasonable estimation of the upper boundary for variation related to interval length"

The authors also find that the extinctions over the past 500 years are happening at least as fast as the species extinctions which triggered the Big Five:

"Current extinction rates for mammals, amphibians, birds, and reptiles, if calculated over the last 500 years (a conservatively slow rate) are faster than (birds, mammals, amphibians, which have 100% of species assessed) or as fast as (reptiles, uncertain because only 19% of species are assessed) all rates that would have produced the Big Five extinctions over hundreds of thousands or millions of years" 

Time to Worry?

The study also evaluated whether we are in (relatively) immediate danger of triggering an extinction event by evaluating a hypothetical scenario in which the Big Five extinctions occurred suddenly, over just 500 years rather than hundreds of thousands to millions of years.  In this case, the extinction rates during the Big Five would have had to exceed 1,000 E/MSY; a value which we have not yet reached.  However, Barnosky et al. note that if we consider a scenario where currently threatened species are inevitably extinct, the current extinction rate is almost as fast as the hypothetical 500-year Big Five extinction rates.  In other words, if we lose all currently threatened species, we will be on a course for a new mass extinction event in just over 500 years. 

In a similar hypothetical, examining how many more years it would take for current extinction rates to produce species losses equivalent to Big Five magnitudes, the authors arrive at a similar conclusion:

"if all ‘threatened’ species became extinct within a century, and that rate then continued unabated, terrestrial amphibian, bird and mammal extinction would reach Big Five magnitudes in ~240 to 540 years....This emphasizes that current extinction rates are higher than those that caused Big Five extinctions in geological time; they could be severe enough to carry extinction magnitudes to the Big Five benchmark in as little as three centuries."


The authors draw two main conclusions from these findings.  The first is that although we're clearly in dangerous territory in terms of extinction rates, we still have enough time to reverse course, although doing so will be a very difficult task.

"First, the recent loss of species is dramatic and serious but does not yet qualify as a mass extinction in the palaeontological sense of the Big Five. In historic times we have actually lost only a few per cent of assessed species...It is encouraging that there is still much of the world’s biodiversity left to save, but daunting that doing so will require the reversal of many dire and escalating threats"

The authors' second conclusion is that if we continue on our present course, we could be headed towards a mass extinction event within a timeframe of just a few centuries.  Therefore, it's very urgent that we steer away from our mass extinction course immediately.

"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."

It's also important to bear in mind that takes a very long time to recover the biodiversity loss from a significant extinction event:

"recovery of biodiversity will not occur on any timeframe meaningful to people: evolution of new species typically takes at least hundreds of thousands of years, and recovery from mass extinction episodes probably occurs on timescales encompassing millions of years."


In short, human influences, including our impacts on climate change, are causing extinctions at a rate faster than the average during a mass extinction event.  If we continue down our current path, we may face a sixth mass extinction event within the next few centuries.  However, we're still relatively early along in the process, so although it will be a difficult task, there is still time to change course and prevent a huge loss in biodiversity.  If we fail to do so, it may take millions of years to recover from the human-caused extinction event, and we're quickly running out of time to avoid this fate.

0 0

Printable Version  |  Link to this page


Prev  1  2  

Comments 51 to 64 out of 64:

  1. Hi Daniel, (yeah that's me at Neven's) Good to see you. In the absence of anybody else taking this up, I would like to query how much "further research and monitoring" is really needed... ...may or may not get you to the latest report on the research from Shakhova and Semilitov. (As you may be aware, my links rarely work.) I accept that they do not speculate about a catastrophically large release of the methane clathrate deposits from the East Siberian shelf. They speculate about a release of 1%. They do not speculate all about the release of gaseous methane buried below the disintegrating clathrate layer. I am speculating about a release of more than 1%, and I do think that this is bang on topic for a thread discussing "mass extinction events". As to possibility/probability, I would not really wish to suggest any actual betting odds, and any speculation from me would be entirely inconsequential, but... It does seem to me that discussion of, for example, sea-level rise by 2100 is perhaps less consequential and less pressing than the discussion of the odds of our current BAU activities triggering a global mass-extinction event in the next decade or so. It's a bet, quite a rash bet, and it looks to me like Dubya and Co put the farm on it. And, frankly, I don't think the farm is very safe. You pays your money, you takes your choice: the wisdom of the global petroligarchy versus the relentless moronic power of the already over-heated ocean. If I had the slightest chance of surviving to collect my winnings...
    0 0
    Moderator Response: [DB] You are very much "bang on topic". My caution was in regards to the rates of release and the timing. BAU for another 20 years or so moves a methane clathrate release from a possibility into the probability zone. And it will involve a sight more than 1%...and virtually guarantee an eventual loss of ice at both poles...
  2. Idunno – thank you. I didn’t want to sound alarmist when mentioning methane – other than point to it already poses a problem, which is not going to diminish. But since you mention its capacity to induce anoxia in the atmosphere, it should be remembered that it can produce those conditions in seawater. This has probably contributed to previous extinction events, particularly of marine life.
    0 0
  3. Anyone interested in the topic of mass extinctions - or at least if we're to be part of one - may be interested in the comment at the other end of this link. Not yet time for "Auld Lang Syne", but the time neareth. The Yooper
    0 0
  4. Hi The Yooper, I'm not sure I'm entirely comfortable for this to be even in the "possibility zone", and I'm not too sure that it really needs another 20 years to move it into the "probability zone". To recap my (limited and non-expert) understanding of the situation to date: 1. Global warming has been occurring at an increased rate in the Arctic region. Over the last decade the greatest SST anomaly in the world occurs in the extreme North of the Atlantic ocean. Gulf Stream water entering the Artic through the Fram Strait is now warmer than at any time in the last 2000 years. 2. Maslowski posited in 2006 that the imbalance in the heat budget of the Arctic would lead to a seasonally ice free Arctic by 2016 +/-3 years. This now looks IMO conservative. 3. While the surface ice remains, it cools the seawater, just as ice in a glass of 7up would cool your drink. When the ice melts, it will warm and "go flat" much quicker. 4. Under natural forcings the climate has only previously warmed much more slowly. Due to the extreme rapidity of AGW, sea level has hardly begun to rise yet. This means that we are well outside the range of any natural threat to the clathrate stability layer. (Under conditions of gradual change, the change in water temperature would be compensated by a rise in sea level, which would force dissolved methane back into solution due to increased pressure.) I realise I am banging on about this a lot, but I do think it is being overlooked. Trying a Google news search for "arctic sea ice", I get about 3,000 hits; if I try "arctic methane", I get 46 hits... For me, this is now the elephant in the room. Sea level rise of (worst case scenario) 10 metres by 2100 means loads of people have to move house. Methane-induced atmospheric anoxia at any time before then means that they won't be moving very fast... The effects of a very small release of methane causing oceanic anoxia in the Gulf of Mexico last year were well reported. The methane in the Arctic is not at sufficient depth to be absorbed by the seawater. It will vent into the atmosphere. The hazard will not be oceanic anoxia, but atmospheric anoxia. I wrestled with my own conscience before posting anything on this subject anywhere. I don't want to scream "Fire" in a crowded theatre. But I do think I can smell smoke, and it currently seems to me that I would be more irresponsible NOT to mention this. As I have now clearly brought it to your attention, and from all I have read, you are clearly much more informed than I, I might now make some effort to pipe down. If you want to point out any ways in which my position is entirely erroneous, I would actually welcome this, as I'm sure would anyone else reading this... Otherwise, love your work. Keep it up!
    0 0
  5. @ idunno (54) Thanks for the kind words. Points: 1. Agreed. To it I would add the extra warmth flowing through the Bering Strait as well as the thinning of the mixing layer immediately below the Arctic Ocean sea ice. Since most of the ice loss occurs through melting from below, this layer thinning means even warmer water is brought into play nearer the ice - and it is occurring year-round. Once the melt-season cranks up, if we get even a moderate Arctic Dipole, the daily losses will be breathtaking. Thinner ice means even more turbidity & "sloshing" around of the top layers of the Arctic Ocean, with even more thinning of the mixing layer (a temperature anomaly gradient would resemble a quadratic curve if graphed)... 2. Agreed. Logicman's March 2011 Arctic Ice post (update here) contained a strong prediction of an ice-free pole this summer. If not, 2012 will be a lock. I posted on Tamino's blog (Open Mind) last fall that 2011 had but a slim chance of seeing an open-pole, with 2012 being 50/50. I have since revised that opinion in light of the sum of evidence available. The models are simply unable to match what we see visually. Simple extrapolation, as you and others have performed, shows Maslowski to now be on the conservative side. Logicman's September 2011 prediction, for those who missed it: 3. Agreed. But also consider the historical role of Arctic Ocean sea ice: to inhibit surface turbidity & insulate the surface from the warmer waters found below (essentially the ice acts as a 2-way insulator preventing both heat loss to the atmosphere and a warming/cooling of the surface layer). Some models show that a loss of summer ice initiates a phase change, wherein the system then proceeds to a no-ice (year-round) phase in a decadal timescale (essentially the model says the system supports only a fear-round ice OR a no-ice solution as having long-term stability). 4. Agreement, completely. I would add that a warming Arctic Ocean with no summer ice to retard mixing will then warm even more, increasing the forcing applied to the clathrate layers, causing even more melting than now currently being observed by Shahkova in the East Siberian Sea Shelf area. Clathrate destabilization has already been observed in other areas as well, even down to several hundred meters depths. Not being an expert I can't speak to atmospheric anoxia, but any sizable clathrate melting would likely raise the spectre for local-to-regional scale oceanic anoxic events, resulting in outbreaks of red tide in areas that have never seen them. Please do not "pipe down". Clathrate melting is perhaps the biggest under-researched and under-reported danger to our climate there is. I would encourage anyone to read as much as is available on the subject. One recent paper, summarized here, provides evidence for the "clathrate gun" hypothesis. An open copy is available here. The only other advice I would offer is on the "presentation" of what you say (in regards to the atmospheric anoxia), not the message itself. Some may find you a bit alarmist & dismiss you out of hand (indeed, some consider me to be alarmist already). In sales, it is not the steak that sells, but the "sizzle" of the steak. Best, The Yooper
    0 0
  6. #27
    Firstly, current temperatures have already equaled those of the HCO, AKA the Holocene Altithermal (Hansen, 2011).
    The context was the Mesopotamian era which took place near thirty degrees north. The Holocene optimum was due to orbital variation and we should recall that it was a period of more extreme climate in this regard - the northern hemisphere summers were hotter and longer AND the winters were colder but shorter. Thus the range of extremes was greater and the transition experienced a greater rate of change. Global averages do not reveals this behavior. And of course, the Southern hemisphere experience a nearly opposite cycle. In any event the Mesopotamians certainly experienced a higher than average temperature during their advancement of civilization. There are other factors to note about the reference, though. This is not a peer reviewed publication for one. Further, the temperature series is at odds with both the borehole proxy data ( which was a global land date set): It is also at odds with the Greenland ice core data:
    Secondly, emissions are trending at the IPCC "High" emissions level.
    Look again - I was pointing out that the temperature trends are all below even the IPCC "Low" scenario. If the emissions are in fact increasing at a greater than modeled rate, , while temperatures are rising at less than the "low" scenario, then this is an indicator of even less forcing, or sensitivity, or both.
    Lastly, it was exactly the climate stability of the HCO that allowed the development of agriculture. Given what we know of the orbital variation, Mesopotamia encountered more sunshine for a longer period from spring through fall, but less sunshine than today during winter. For Mesopotamia, we may then surmise that warmer overall temperatures and longer summers aided agriculture, even though winters were colder and weather more extreme.
    and the ensuing desertification to come as a result
    I know of no basis for the claim of desertification above. Do you have a reference to one? The greatest example of desertification that I know of is the Sahara: which according to fossil records enjoyed a wet period during the Holocene Climatic Optimum - corresponding with the local wamrth! The cause of the Wet Sahara was likely more due to dynamic factors (shifting of the inter tropical convergence zone northward) rather than being due to thermal conditions, but even so, the desertification took place when hemispheric temperatures fell. I have not seen any compelling model that would indicate desertification. That is as it should be. Precipitation occurs largely where the dynamics of surface convergence can foster lifting, not something in the domain of climate models.
    0 0
  7. 28, Honeycutt: ClimateWatcher @ 10... Do you honestly not understand the difference between weather and climate? It's really hard to take you seriously on anything when you don't even understand such elemental aspects of climate. You must have missed: "adaptation/migration are slow processes. What matters is rate of change." to which I responded with examples of rate of change. One cannot claim that 'rate of change' is what matters to life forms then ignore the most intense rates of change. My point is this - species that were extremely sensitive to temperature change would have been selected out of the gene pool long ago, given the hourly, daily, seasonal, decadal, century, and millenial changes.
    0 0
  8. 30 Honeycutt: ClimateWatcher @ 27... Your arguments here are essentially the same as to say, "Well, humanitarian atrocities are a normal process that humans go through once in a while, so what-the-heck." No, my point is that there's no atrocity from a couple of degrees of warming and in fact humans flourished in such an environment. They might also have flourished in a cooler or warmer still environment, and that's even more to the insignificance of small variations. Regarding keeping temperatures below the HCO, that's not the concern here. We are currently about 1C below the HCO. If we could hold global temperatures steady at 2C over preindustrial, that's is essentially right about the danger point. But our civilization emerged in a warmer environment. It was not danger at all!
    0 0
  9. Firstly it is the movement of climatic zones through the latitudes, including precipitation, not just temperature. What basis do you have for believing precipitation zones will change significantly? Then of those significant changes, what evidence do you have of the portion of changes that would be not just adverse, but more adverse than the benefits that such a change would convey? Secondly, it is the rate of such transitions that is absolutely critical i.e. if the rate of change outpaces adaptation, then decimation of ecosystems occurs. But the warming at sunrise is a great rate of change. The difference between winter and summer is a great extent of change. Life exposed to the environment experiences much greater extent and rate of change already than 1.6 degrees over a century.
    0 0
  10. ClimateWatcher... No, our civilization emerged in a climate able equal to where were are today, possibly 1C higher than today at the peak. Human civilization has very definitely NOT seen global temperatures that are already baked into the system. What we are all concerned about is not the 2C. We are concerned about business as usual. We are concerned about humanity driving temperatures well past the 2C mark, into the territory of 3-4C over pre-industrial levels. That, my friend, is a world of pain.
    0 0
  11. ClimateWatcher: No, my point is that there's no atrocity from a couple of degrees of warming and in fact humans flourished in such an environment. They might also have flourished in a cooler or warmer still environment, and that's even more to the insignificance of small variations. But our civilization emerged in a warmer environment. It was not danger at all! CW, the world population was a tiny fraction of what it is now and they were hunter-gathers and nomadic then. There are very few places for today's population and agricultural areas to move, especially with rising sea level.
    0 0
  12. ClimateWatcher - There was a very interesting article in last month's Scientific American, A Shifting Band of Rain. Sorry that this is just the abstract, I haven't found a full free-access copy. The band of tropical rain just north of the equator is shifting further north, now 3°N to 10°N, the furthest in that direction in at least 1200 years. Another 5°N movement is possible by 2100, drying out farmland from Central America to the southwestern US with multi-year droughts. "What basis do you have for believing precipitation zones will change significantly? Then of those significant changes, what evidence do you have of the portion of changes that would be not just adverse, but more adverse than the benefits that such a change would convey?" First - based on observations, and second, how is this beneficial?
    0 0
    Moderator Response:

    [DB] KR, an openly available copy of the Sachs & Myhrvold 2011 study can be found here.

  13. RickG @ 61... We could all note that the fertile crescent (where agriculture first started) is no longer so fertile.
    0 0
  14. I promised a while back to comment further on this story about mass extinctions. Anything I might have to say now pales into insignificance compared with Oliver Knevitt's article on this topic: beware_metastudies_example_whale_evolution ------ Daniel: thank you for linking to my Arctic ice article. A further update is nearing completion. idunno: I mentioned in my blog that I agree with what Daniel Bailey has to say in reply to your comments above. I repeat that agreement here for the benefit of Skeptical Science readers. This is an area of Arctic -related science that warrants urgent study.
    0 0
    Moderator Response:

    [DB] You're welcome & look forward to the next update. In the meantime, interested readers might like to see this:

    Global Distribution of Methane Hydrates

Prev  1  2  

You need to be logged in to post a comment. Login via the left margin or if you're new, register here.

The Consensus Project Website


(free to republish)

© Copyright 2024 John Cook
Home | Translations | About Us | Privacy | Contact Us