Recently, a group of paleontologists and paleontology students met to discuss the “6th mass extinction.” I asked several questions of everyone. We drew on our collective knowledge and experience about mass extinctions, all of us having read the scientific literature and researched on extinctions for variable lengths of time.  Here, I provide the questions I asked and a summary of our answers:

Are we in the middle of a mass extinction? Or has it even started yet?

We all agreed that the “6th mass extinction” is real, but we disagreed on timing. Some preferred the idea that the extinction is underway, and so we were seeing the beginning of the mass extinction in terms of species losses. Species losses are presently outnumbering species originations (i.e., extinction of species was outpacing evolution of new species), and this is evidence that the Earth is in the beginning of a mass extinction event.

Others thought that we weren’t yet seeing the true start of the extinction because of lag time – the causes of the mass extinction, such as climate change, habitat loss, and exploitation of resources, haven’t caught up with the species yet. Many species that are being affected by extinction-causing factors are still present in our world, albeit eking by at low population levels. Those species are likely to go extinct in the future as the causes of their impending extinction catch up with them and their populations crash from environmental change. In other words, when something happens that is detrimental to a species’ existence, they don’t disappear immediately; there is a time period (lag time) where they hold on to existence but inevitably go extinct because their destroyed habitats never return to a survivable state.

So no, the extinction hasn’t fully occurred yet. The extinction is either just beginning or we’re in a lag period before we begin to see major species losses. But we all agree that the “6th mass extinction” is real.

What is causing the mass extinction?

We looked at a time in geological and paleontological history that is thought to be similar in trend to present day and future global warming: the Paleocene-Eocene Thermal Maximum (PETM) that occurred 55.5 million years ago. Global temperatures during the PETM are thought to have risen in a similar magnitude to what is projected from current conditions. Back then, emissions were caused by greenhouse gas release from volcanism rather than human activities. The effects of PETM warming not only involved climate change and ocean warming but also ocean acidification, which is a major problem for invertebrate animals that make shells and skeletons. The result of warming in the PETM biosphere was not a worldwide collapse of species diversity – a mass extinction – but instead, a turnover – species that couldn’t tolerate the conditions were replaced by others evolving that could tolerate warmer conditions. However, there were still major extinctions in some groups of animals, like microscopic foraminifera in the oceans.

Now here’s the caveat: we have the same type of warming today (albeit perhaps at a faster rate), but unlike during the PETM, we see extinctions of species without replacement by new species. No turnover. What’s different? Well, we are – our activities create stresses on Earth’s life in addition to global warming and its effects. When we considered the impact of the PETM on global life, we realized that species were only being hit by a single factor: warming-induced climate change. Although this warming caused other effects like ocean acidification, species were able to adapt. However, present day species can’t adapt to global warming and the associated problems like ocean acidification. Why? Because we have introduced additional stresses on species, not just global warming and related factors.

Past mass extinctions were usually initiated by multiple factors that either caused the extinction event or inhibited recovery. So a single-factor environmental stress may be survivable by life, but piling additional stresses on species may be too much: the proverbial straw on the camel’s back. What are the additional factors that are causing the present-day mass extinction?

We identified what we think are the two major stresses in addition to climate warming, and perhaps even more important than warming: permanent habitat loss and overharvesting/overfishing. In the first, we have created so much cropland, grazing areas, and urban areas that habitats have been permanently destroyed. Although we could restore those areas back to habitat for species needing protection, we don’t want to give up those areas that we have modified for our direct benefit. And as we use more land and ocean, we destroy more habitat, usually permanently.

In the second case, we regularly overfish. Many populations of fish that we eat are much, much smaller than what would naturally occur because the way we fish removes great numbers of individual fish from the population. Yes, there are fisheries that watch fish numbers, but many fisheries are not managed, or there are “loopholes” and “workarounds” for the fishing industry to continue to exploit dwindling fish populations. And we might have a size limit on fish that can be taken from a given population, but we also tend to reduce the size of the fish being taken over time so that we can continue to harvest great numbers of fish. Reducing the size can mean taking reproductive adults out of the population, and thus reducing the overall population through time to zero. My personal fear is that because we don’t really understand how fish populations “work” in the oceans (i.e., we don’t understand the oceans very well at all, and we rarely know how schools of fish exploited by the fisheries move around in the oceans, where they breed, where they get their food, etc.), someday the fishing industry will go out to harvest and find out that their target fish species has completely disappeared.

Here in my summary of our discussion, I am reminded of two things that I think are relevant to mention.

1. Almost no, and perhaps no, place on Earth is untouched by humans. Trash has been found in the deepest portions of the oceans and on Antarctica. Dust from pollutants and radioactive materials are in the ice sheets. The weight from cities presses down on the crust, causing an isostatic adjustment of the crust so that the crust underneath the city rides a little deeper into the mantle.

2. Some studies have suggested that in the presence of stress, if there are ameliorating conditions, organisms can handle the stress. Vermeij1 suggested that sufficient nutrients could help mitigate stressful conditions when species survived through extinctions in refugia (more on refugia in future posts). Graham and others2 showed that kelp migrate to deeper water and thus, lower levels of light, to escape warm water. Quite a few studies suggest a possible survivorship for corals into slightly deeper water (e.g., deeper than 25 m) in areas of extreme warming, although this idea is contentious. My own research synthesizing a half-century of studies3 suggests that conditions in protected areas don’t need to be pristine, but they need to be better than external, hostile conditions, and should have one or more ameliorating conditions as suggested by the previous studies, like cooler water or extra nutrients.

I think this is why the PETM was a non-event with regards to mass extinction compared to the current mass extinction underway or about to begin. The PETM had one major factor (that created several, greenhouse gas-related stressors, but still ultimately one cause). Perhaps there were other, ameliorating conditions present that allowed biodiversity to continue and new species to evolve, replacing those that were lost. Now, we have global environmental deterioration: greenhouse gas emissions (along with the effects of greenhouse gas emissions) plus extreme habitat loss, overharvesting, pollution, etc. Thus, the additive stresses, plus how fast the stresses are affecting species, are just too great; species that disappear aren’t being replaced by new species to fill those roles in ecosystems. Any ameliorating conditions are insufficient as all of these stresses add up. The result: biodiversity is crashing down around us.

Is the extinction avoidable?

We basically agreed that, in order to avoid a mass extinction by reversing the lag period or to stop the extinction at its current state, ceasing our detrimental activities (destroying habitat, polluting, overharvesting, greenhouse emissions, etc.) is not enough. Here’s the crux: we need to not only stop our detrimental activities, but we need to reverse the damage done: we also need to fix habitats that have been affected by destruction, pollution, climate change, etc. Stopping the damage in its tracks is not enough. Stopping damage plus fixing the damage that has been done can stop the mass extinction in its tracks.

That means that in addition to ceasing our activities that cause habitat loss, we need to also restore more habitat that was already lost. We need to have a positive gain of habitat restored for species, instead of a zero-change state of maintaining the habitats we already have preserved. Similarly, we need to not only cease greenhouse gas emissions, we also need to lower what we have already emitted. Our suggestions to reduce emissions in the atmosphere were things like planting trees or carbon capture and storage activities.

Do we like the name “6th mass extinction?” Or “mass extinction” at all?

For this discussion, we also read several magazine and online articles. One was rather scathing4, and we agreed that the wording was a bit incendiary and verbose, but one of the points that the author makes is that he preferred “extermination” to “extinction.” Many of us agreed that there needed to be some wording that indicated an anthropogenic, rather than non-human, cause for the extinction. Some liked “extermination” or “annihilation,” but the point was brought up that the wording could be too extreme: if people thought that the extinction was too huge, too horrible, more might say “what’s the point of trying to fix it if everything is going to hell anyway?” Still, some preferred “6th mass extinction.”

I think I was the only one who really didn’t like “6th mass extinction” for the connotations meant by “6th.” The use of “6th” was meant to point out that this is the “6th” time that life on Earth has faced a major mass extinction. Guess what? It’s not the “6th” time. It’s not even the “10th” time. Species are destined to become extinct as conditions change, whether disappearing or evolving into new species. As regions changed in the fossil record, large areas faced regional extinctions; this is a normal state of the fossil record, given tectonic plate movements, environmental changes, and climate shifts. More importantly, life as a whole is fragile and prone to mass extinctions as global changes and stressors add up.

In a 1982 paper, Raup and Sepkoski5 identified five mass extinctions. These were the five greatest mass extinctions that ever happened in terms of biodiversity loss, now known as “the Big 5.” Another reason I don’t like the phrase “6th mass extinction” is that, scientifically, it implies that the present extinction occurring or about to occur will be 6th largest after the Big 5. We don’t yet know how big it will be. That depends on us. If we stop our habitat-deteriorating and species-exploiting ways plus restore good conditions for species survival, the present extinction could wind up being merely a blip in the history of life – one of the “lesser” mass extinctions that don’t make it into the Big 5. But if we do nothing, and continue our globally-destructive habits, some scientists think that this present mass extinction will rival the biggest ever: the mass extinction that happened at the Permo-Triassic boundary about 250 million years ago, when as much as 95% of all life on Earth6 went extinct.

So what word should we use? We didn’t come to an agreement and none of the words we used were adequate. What do I suggest? I don’t have a good solution for this one. “Mass extinction” is relevant because it refers to a coupled global environment-biological process that drastically reduces biodiversity, and thus is biologically meaningful. However, “mass extinction” doesn’t quite capture the unique human cause of the current extinction. “Annihilation” and “extermination” are good words that refer to things like wars, genocides, and holocausts, which is also comparable to what we are doing to the Earth in terms of species loss and environmental change, but lacks the biological connotation of “extinction.”

1 Vermeij, G.J. 1986: Survival during biotic rises: the properties and evolutionary significance of refuges. In Elliott, D.K. (ed.): Dynamics of Extinction, 231-246, Wiley, New York, N.Y.
Vermeij reviewed our theories and knowledge of refugia to 1986, and included several examples. This has long been a seminal paper on extinction survivorship in refugia.

2 Graham, M.H., Kinlan, B.P., Druehl, L.D., Garske, L.E., and Banks, S., 2007. Deep-water kelp refugia as potential hotspots of tropical marine diversity and productivity. Proceedings of the National Academy of Sciences 104, 16576–16580.
In this study, Graham and colleagues modelled where they would predict tropical kelp to survive warming ocean water, and then applying that model, discovered a species of tropical kelp in the Galapagos Islands that had expanded the richer portion of the kelp forest into deeper water.

3 Schneider, Chris L., 2018. Refugia past, present, and future: lessons from ancient geologic crises for modern marine ecosystem conservation. In Conservation Paleobiology, Tyler, Carrie L. and Schneider, Chris L., eds., p. 163-208.
I reviewed many papers on marine refugia through mass extinctions and considered what information is relevant to making successful marine protected areas.
Refugia are those places where habitats are sufficiently low-stress so that species can survive through times of high stress elsewhere in the world. Species might migrate to those refugia, they might shift their geographic range or change their habitat, or they might just happen to be where refugial conditions form.

4 McBrien, Justin, This is not the 6th extinction, it’s the first extermination event. Originally printed at Accessed at Oct 11, 2019

5 Raup, D. & Sepkoski, J. (1982). “Mass extinctions in the marine fossil record”. Science 215: 1501–1503.
            In this paper, the scientists compiled all fossil families for analysis: the result? Four major extinctions that were distinct from “background extinction” (the “normal” level of extinction that happens through time: species go extinct with changing environments, species evolve to become new species and so that original species is extinct, etc.). A fifth (the Late Devonian) stood out as an extinction event, although wasn’t statistically different from background extinction rates.

6 An estimate in a paper by Sepkoski published in 1982 found that the Permo-Triassic mass extinction of approximately 252 million years ago took out 82% of genera. In 2006, Erwin’s book gave an estimate of 95% of species on Earth went extinct.
Sepkoski, J.J. Jr., 1995. Patterns of Phanerozoic extinction: a perspective from global databases. In: Walliser OH (ed) Global events and event stratigraphy. Springer-Verlag, Berlin, pp 35–51.
Erwin, D.H., 2006. Extinction: how life on earth nearly ended 250 million years ago. Princeton University Press, Princeton