Yes, you’ve guessed it: a new series – and one we think is (even) more important than the previous climate series on Bits of Science, like our short (ongoing) understanding sea level rise series, and the more elaborate series about climate-temperature inertia (the ‘real’ global temperature trend).

This time we want to dig deep into the science of climate-ecology interaction – to try to develop a sense, based on facts, figures and research, of the Holocene Mass Extinction, and the role anthropogenic climate change plays as one of the main drivers of this escalating loss of global biodiversity.

Major extinctions of marine species in Earth’s 540 million year history of complex life, based on a publication in Nature from 2005, called ‘Cycles in fossil diversity’ – the authors of which note that major extinction events seem to show a pattern of recurrence, being placed on average 62 million years apart. Science usually refers to marine biodiversity to assess and compare the extent of past extinction events, as the fossil record of ocean life is much better preserved than that of terrestrial, land-based life forms. Modern data confirms the existence of 5 major extinction events standing out, as first suggested in a Science publication in 1982 that actually stated (based on the marine fossil record) the end-Ordovician, end-Permian, end-Triassic, and end-Cretaceous (that’s 4, not 5!) are very clear cut, whereas the ‘late Devonian’ is a bit messier, more a grouping of lesser events. In that sense the late Cambrian period was also relatively hostile to complex life [‘dynamic’ is probably a better word, let’s not forget it’s also the same Cambrian period during which complex life quite suddenly erupted – the Cambrian Explosion]. Overall, life took hundreds of millions of years to settle and help create more stable conditions, as is indicated by the seeming long-term slow decline of background extinction rates.

The Holocene extinction event is, from a global, biological and geological perspective, more than any other modern development, the most defining thing that’s happening around us – defining because it actually changes the fossil record of our shared planet, drawing another line in the sand on Earth’s millions of years timescale, and possibly a deep line, listing it as the ‘Sixth’ of the Earth’s Big Extinction Events (a ‘mass extinction’ being defined as a loss of 75 percent or more of species within a class). We’ll get to that list, and the difficulties with it in a bit…

Now what is an extinction event?

An extinction event, or rather an extinction period, is any time in the history of life on Earth during which the process of dying out of species (which is natural) happens at a faster pace than the evolutionary process of species formation (called speciation) – due to, obviously, deteriorating conditions for life, in general.

As we have pointed out in previous articles on Bits of Science indeed the current rate of species extinctions is far greater than that of natural evolution. As we will find out in this series, it’s very hard to come up with absolute numbers (as researching the current well-being of all of Earth’s biodiversity, and relating that to all of Earth’s evolutionary history is… challenging). Therefore, we’ll work with probability ranges, indicative figures and simply try to create an overview of the available science.

Extrapolating the current trend leads to mass extinction ‘within centuries’

Current extinction rates outpace evolutionary speciation anywhere between ‘at least 300 times for birds’ to 80,000 times for mammals (that generally have more difficulty flying away, in search of an new suited habitat). The latter figure comes from a 2011 study in Nature that compared the current mammal extinction rate to ‘natural background extinction rate’, which on very large timescales ought to be more or less in balance with evolution speed. (The natural extinction rate for mammals lies at around 1 species dying out every half a million years; in the last 500 years at least 80 mammal species became extinct – out of a documented total of 5,570 mammal species.)

The same study notes that current extinction rates are also much higher now than during the past five mass extinctions – and that under current trends a mass extinction (defined as 75 percent of species lost) could be reached for birds in 537 years, for amphibians in 242 years and for mammals in about 334 years.

Big question: is it right to extrapolate like that? After all, environmental stressors, the driver of the current extinction event, are all increasing, not decreasing – and tend to work synergistically, leading to threshold-bound collapses. So it seems right to interpret the above as the absolute minimum projection, apart perhaps from a Utopian scenario, the one in which extinction drivers are actively diminished, and become smaller than they have been on average over the last 500 years.

Figure shows percentage of species per class (in black) that are currently considered to be threatened with extinction – compared to the 75 percent extinction line that defines the 5 historical mass extinction events. The ancient class of Cycadopsida (that includes palm trees and ferns) is relatively the most endangered, with 64 percent of all species ‘threatened’. Among terrestrial animals the class of amphibians is most endangered – with a percentage of threatened species that is possibly as high as 43 percent. Image and figures from the above-mentioned Nature study.

Difficulties properly naming the modern extinction event

Trends show Earth’s current extinction event is on track to define as an actual mass extinction. Properly naming this event proves difficult though. As hinted above listing it as the ‘Sixth Mass Extinction’ is problematic, as one could argue Earth has experienced not 5, but 4 (exluding Devonian Extinction) or 6-8 (including Cambrian extinctions and the also Permian end-Capithian extinction) such events.

Also with listing comes the risk of unnecessary categorisation. Earth’s past mass extinction events do have interesting similarities (climate change being a driver or accelerating feedback to name one) but are also all very different. Ecology was different, geology was different, and as the above Nature study indicates, even the magnitude was different – with current extinction rates possibly higher than ever before in Earth’s fossil history (possibly excluding the (onset of the) Cretaceous-Tertiary Extinction, no one can compete with a direct hit by an asteroid – you can dispute its existence though).

It’s also worthwile to separate the Permian-Triassic Extinction (sometimes called ‘the Great Dying’) as being bigger than all others, showing the escalating potential of any mass extinction – something that’s intrinsically unpredictable, as such escalation depends much more on feedbacks than on initial drivers.

When exactly did we kill the last woolly mammoth?

Another approach is to link the modern extinction to the geological record, leading to various possibilities, including end-Holocene Extinction or Holocene-Anthropocene Extinction, taking into account the fact that drivers are indeed anthropogenic – and linking to the proposed new epoch of the Anthropocene. One additional complicating factor is that human-induced extinction did not start in the Holocene, but around the Pleistocene-Holocene boundary, with overhunting of ice age megafauna as the first iconic example. In that sense alternative names could be end-Quaternary extinction event, or simply the Anthropocene Extinction.

We think these naming complexities make for an ideal bar fight topic for librarians. We will refer to it as the Holocene-Anthropocene Mass Extinction from here, and in the next chapters of this series we’ll focus our energy on something else: trying to understand the drivers – and assessing the specific role of climate change…

© Rolf Schuttenhelm | www.bitsofscience.org