By Michael Hannah
There are solutions to the ecological problems facing the planet and we have some time to implement them – we had better use that time wisely.
Towards the end of the Permian period, a little (geologically speaking) over 250 million years ago, the Earth’s climate was distinctly unpleasant. Modelling suggests that the level of carbon dioxide in the atmosphere then was exceptionally high, which in turn produced extreme temperatures across the planet. Living under these conditions must have been difficult, but things were about to get much worse. In the area we now call Siberia, a massive volcanic eruption was taking place. Over a period of about one million years, this eruption may have produced five million cubic kilometres of lava covering an area of seven million square kilometres. It pushed an additional 170,000 gigatons of carbon dioxide into the atmosphere along with a vast amount of halogen gases (mainly hydrochloric acid). The consequences for life on Earth were devastating (the data I’m using here comes from Wignall, 2015). The additional carbon dioxide added to the atmosphere increased global temperatures even further and resulted in a staggering reduction in the level of oxygen in the oceans and a sharp rise in their acidity. The overheated ocean in turn caused gas hydrates, today found in cold, deep marine settings, to melt and release methane, another powerful greenhouse gas, which continued the pattern of rising temperature. In the atmosphere, the halogen gases destroyed the ozone layer allowing deadly UV rays to reach the surface of the planet.
Life barely survived this cataclysm. In the ocean, up to 96% of all species went extinct. Reefs, which had flourished in the late Permian, disappeared entirely from the face of the planet and didn’t re-appear for over 20 million years. Life on land didn’t fare much better, with massive extinctions across most of the biota. This mass extinction at the end of the Permian is the largest recorded in the fossil record. It is one of the so-called “Big Five” mass extinctions. The other four mass extinctions, which occurred at the end of the Ordovician, Devonian, Triassic and Cretaceous periods, had marine species extinction rates ranging from around 75 to 85%. All but one (the end Ordovician event) are closely linked in time with a massive volcanic eruption, like the one in Siberia that caused the end Permian mass extinction. The most famous of these events is probably the Cretaceous one, as it includes the extinction of the charismatic dinosaurs and has the added complication of being associated with the impact of a meteorite.
The five mass extinction events in the history of life was first documented in detail in a paper by David Raup and Jack Sepkoski published in 1982. In their study, they recognized the five sharp increases in the level of extinction above what would be considered a normal background level—mass extinctions. Today most researchers define a mass extinction as:
“An increase in the level of extinction over a very short interval of geological time of more than one geographically widespread higher biological group resulting in a drop in biological diversity”
With this definition and equipped with improved datasets and vastly increased computer power, palaeontologists now recognize many more than five mass extinction events in the fossil record. Although the exact number is still being debated, some researchers have suggested that over the past 600 million years there have been 19 diversity crashes worthy of being labeled as mass extinctions. So, in a trivial sense I’ve answered the question posed in the title of this piece. Rather than living in the sixth mass extinction, maybe we’re part of the twentieth. And perhaps we are asking the wrong question. Instead, perhaps we should forget about which number extinction event and simply ask: Are we living in a mass extinction? The answer to that question is not all that straightforward.
What is happening to the planet’s environment today is frighteningly similar to what was happening at the end of the Permian. Global temperatures are rising as the carbon dioxide levels in the atmosphere increase. Oxygen levels in many parts of our oceans are falling, and the oceans are becoming increasingly acidic. Some scientists are deeply concerned that the gas hydrates currently being stored in the deeper parts of the oceans may melt and release huge amounts of methane. The biggest single difference between what happened at the end of the Permian and what is happening now is the source of the carbon dioxide. Two hundred and fifty million years ago, it was the gigantic eruption in Siberia, today it’s humans burning fossil fuel. So environmentally, the stage seems to be set up for a mass extinction, but how does today’s biotic crisis measure up to the definition of a mass extinction I gave above?
Firstly, is the rate at which extinctions are occurring rising? While it’s hard to quantify, Ceballos and others (2015) have estimated that the current extinction rate for all vertebrate animals is currently 53 times the level it was prior to the evolution of modern humans. For Amphibians, they suggest that the extinction rate may be 100 times higher. This increase is fast. Humans have not been around for very long – geologically speaking, barely the blink of a geological eye. So, the rate of extinction is rising – but has the number of extinctions reached the level of any of the big five mass extinctions? The 75 to 96% level.
To answer that, we can consult the Red List – an amazing resource maintained by the International Union for Conservation of Nature – you can find it at http://www.iucnredlist.org/. The Red List attempts to assess the threat of extinction for all known species of animals and plants and place each into appropriate categories, which include extinct, extinct in the wild (that is species that are only found in zoos or sanctuaries), threatened, which has an important subset – critically endangered (species under extreme threat of extinction), all the way down to under no threat at all. When we look at the numbers of species that are included in the extinct and extinct in the wild categories – they are surprisingly low. For example, of the 5,899 species of mammals that have been assessed only 1.46% are extinct or extinct in the wild. 7,833 species of reptiles have been assessed and about 0.42% are extinct or extinct in the wild. 6,892 species of amphibians have been assessed with 0.54% in the extinction categories. I collected these data in December 2020, and I don’t believe there have been any dramatic movements since. These extinction numbers from the Red List suggest that we are not close to the level of extinction documented during the Big Five mass extinctions. However, focusing entirely on the extinction numbers prevents us from seeing the wider picture and from appreciating the full scale of the current biotic crisis. It could lull us into a false sense of security because our current extinctions reflect only a part of the problem.
To really understand, we need to examine the threatened categories, which changes the outlook dramatically. With mammals the combined total of extinct and under threat of extinction rises to 23.5%. In reptiles the combined total is about 18%, and for amphibians, 33.5%. If we allow these threatened species to go extinct, we will be on the way to similar levels of extinction as the previous mass extinctions. To avoid being blinded by a focus on only the level of extinction, some biologists prefer to use the term defaunation to describe our current situation. Despite the term’s shortcomings, it more properly reflects our biota’s situation, because it includes both species that are extinct and those that are threatened with extinction.
Vast areas of the planet are being cleared to make room for our megacities, and intensive agriculture has transformed much of the planet’s surface, reducing the available habitat for many species. In addition, we are changing the planet’s climate through our continued burning of fossil fuels – placing even more stress on its ecosystems. But how close are we to a mass extinction such as the Big Five? How long do we have before we reach the level of say the Cretaceous mass extinction of about a 75% species loss? In 2011 a study by Barnosky and others tried to put some numbers on that.
Barnosky’s study used data collected from the Red List and focused on vertebrate species only. They first calculated the rate of extinction that would result if we allowed all the vertebrate species threatened with extinction to go extinct over the next 100 years – essentially business as usual. Once they had calculated that rate, they extrapolated it into the future. Their results suggested that at this rate of extinction we have between 240 and 540 years before we reach a 75% loss of vertebrates. Not a very cheery number. But things improve dramatically if we do something to protect the threatened species. They calculated a second rate, one based on us protecting some threatened species, losing only those that are in the critically endangered category over the next 100 years. Because critically endangered is a subcategory of threatened therefore this second rate is lower than the first and therefore will give us more time. Using this second rate, Barnosky’s team suggest that this change would extend the time it takes to reach the 75% extinction rate to between 890 and 2270 years.
There are, of course all sorts of issues with these estimates. For example, there are limitations with the Red List Data – it can only categorize species that biologists have identified and described, and we haven’t managed to document all the species on the planet. Also, the rates that were calculated are based on vertebrate species only and may not be applicable across the entire biota. However, they do highlight the situation we are facing, and, despite the sobering estimates of time before we get to a 75% level of extinction, these estimates do offer some positive potential scenarios: Firstly, we have time to change our behaviours and avoid sliding into a mass extinction, which in turn, risks our own existence. Not a lot of time, particularly if we take the business as usual path – but some. Secondly, small changes in our behaviour can make a big impact. Look at how much time we gain by protecting most of our threatened species. And there is more we can do. Recent modelling by Strassburg and others (2020) suggests that preserving the current natural ecosystems and restoring 30% of the land that we have damaged will save many species. They further suggest that regenerating the ecosystems would absorb a significant proportion of the excess carbon dioxide in the atmosphere – surely a win/win situation. There are solutions to the ecological problems facing the planet and we have some time to implement them – we had better use that time wisely.
Barnosky, A. D., Matzke, N., Tomiya, S., Wogan, G. O. U., Swartz, B., Quental, T. B., . . . Ferrer, E. A. (2011). Has the Earth’s sixth mass extinction already arrived? Nature, 471, 51.
Ceballos, G., Ehrlich, P. R., Barnosky, A. D., García, A., Pringle, R. M., & Palmer, T. M. (2015). Accelerated modern human–induced species losses: Entering the sixth mass extinction. Science Advances, 1(5).
Raup, D. M., & Sepkoski, J. J. (1982). Mass Extinctions in the Marine Fossil Record. Science, 215(4539), 1501-1503.
Strassburg, B. B. N., Iribarrem, A., Beyer, H. L., Cordeiro, C. L., Crouzeilles, R., Jakovac, C. C., . . . Visconti, P. (2020). Global priority areas for ecosystem restoration. Nature
Wignall. P. B. (2015). The Worst of Times. Princeton, New Jersey: Princeton University Press. (Book)
Michael Hannah is an Associate Professor in the School of Geography, Environment and Earth Sciences at Victoria University of Wellington.
He explore the ideas outlined above (and more) in more detail in an upcoming book: Extinctions, Living and Dying in the Margin of Error. Published by Cambridge University Press – it will be released on September 16. You can read an extract here: http://ow.ly/ZjH050FnkHG.
Disclaimer: The ideas expressed in this article reflect the author’s views and not necessarily the views of The Big Q.
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