Are we headed for a sixth mass extinction?


Rothman, Daniel H. “Thresholds of Catastrophe in the Earth System.” Science Advances, vol. 3, no. 9, 2017, doi:10.1126/sciadv.1700906.

A mass extinction is an event in which the world very rapidly loses a large number of its living species.  You’ve probably heard of the mass extinction that occurred sixty-five million years ago, when an asteroid crashed near Mexico and led to the extinction of the dinosaurs.  There have been four other mass extinctions in the last 500 million years, and each has resulted in the loss of at least 60% of living species.  In a recent study, Professor Daniel Rothman at the Massachusetts Institute of Technology argues that human activities – specifically our inundating the atmosphere with carbon – may result in a sixth mass extinction.


Why do mass extinctions occur?

The theory of evolution tells us that organisms evolve in ways that promote success in their environment.  However, if a species’ environment changes too quickly, evolution can’t keep up.  For example, when dodos were discovered and subsequently hunted by humans, they were suddenly confronted with a new predator and could not adapt in time.  The last dodo was seen in 1662.

Sketches of dodos from 1601. Image courtesy of Wikipedia.

In order to trigger a mass extinction, a sudden change has to occur for not just one, but most living species.  This requires a global catastrophe, like the asteroid that killed the dinosaurs or the volcanic eruptions that created the Siberian Traps and likely resulted in the end-Permian extinction 252 million years ago.  The end-Permian extinction is also known as the “Great Dying” because 90-95% of all species were lost.  All five mass extinctions that have occurred to this date are believed to have been associated with huge, rapid changes in the amount of greenhouse gas in the atmosphere.  Such changes can occur due to a variety of reasons, including volcanism, increased plant activity, or an asteroid impact.


Why do greenhouse gases matter?

In a recent study on mass extinctions, Daniel Rothman suggests that the ocean-atmosphere system is a bit like the dodo: if you change the system slowly, it can adjust and maintain equilibrium, but if you change it too quickly, the whole system spins out of control.  In the context of Rothman’s study, this “change” refers to a substantial shift in atmospheric CO2, which can have a profound effect on our climate.

Seltzer, which contains dissolved CO2, is also slightly acidic.  Fortunately, the CO2 in the ocean has much lower pressure, so the ocean is unlikely to fizz. Image courtesy of Getty Images.

If atmospheric CO2 increases too rapidly, it becomes difficult for the ocean to adjust, and the earth’s climate can become unstable.  Since CO2 is a greenhouse gas, it warms our climate.  Oceans mitigate this effect by absorbing roughly a quarter of our CO2 emissions.  The CO2 gets dissolved in the oceans, kind of like how CO2 is dissolved in seltzer.  Generally speaking, if you put more CO2 in the atmosphere, then the oceans will also absorb more CO2, which protects our climate from changing too rapidly.  Unfortunately, the oceans also become more acidic when they absorb CO2.

A sea butterfly. The shells of these snails have been dissolving as a result of ocean acidification. Photograph by Steve Ringman, NOAA; image courtesy of National Geographic.


Ocean acidification is dangerous for two reasons.  Firstly, acidic oceans are lethal to marine wildlife like snails and coral, and secondly, the ocean might eventually become so acidic that it can no longer absorb atmospheric CO2, which means that it can no longer mitigate the effects of greenhouse warming.  The ocean’s pH actually can return back to normal, but this takes thousands of years.  If atmospheric CO2 increases faster than that, then the ocean will be unable to adjust in time, putting the earth at risk of ocean acidification and rapid climate change.  Both of these can have catastrophic consequences for the earth’s inhabitants, especially for those species which can’t adapt to their new climate quickly enough.


How much carbon is too much?

Rothman’s study indicates that a rapid change in in atmospheric CO2 is a key component of mass extinctions in the last 500 million years.  Using carbon isotope ratios of ocean-floor sediments, Rothman examined 31 different disruptions to the carbon cycle.  Only those that occurred exceptionally rapidly led to mass extinctions.  Rothman also used these results to deduce the amount of carbon that the oceans would have to absorb in the present-day in order to match the rapid changes of past mass extinctions.  His results suggest that, relative to 1850 (the Industrial Revolution), the oceans would have to pick up an additional 310 gigatons of carbon in order to set the stage for a new mass extinction.

According to current estimates, our fossil fuel emissions have already added about 150 gigatons of carbon to the ocean since 1850.  Scientists can estimate how much more carbon the oceans will absorb by first estimating how much CO2 we’ll add to the atmosphere in the next century.  When Rothman examined the IPCC carbon emissions projections for the year 2100, he found that we will stay under the 310 gigaton limit only if we are exceptionally proactive about reducing our fossil fuel use over the next century.


Are we making any other rapid changes?

Rothman’s analysis suggests that, by 2100, we may have emitted so much CO2 that a global mass extinction will be imminent.  Unfortunately, this is not the only rapid change we’ve caused.  In The Sixth Extinction, Elizabeth Kolbert reviews several other changes we’ve made, and a few of these are described below.

A brown tree snake, which is an invasive species in Guam. Image courtesy of the USDA.

1) Globalization.  When we travel the earth, we bring animals, plants, and pathogens with us.  This results in a disruption of native species’ biomes, which are unprepared to deal with “invaders” like the brown tree snake pictured here.

2) Fragmentation.  When we build roads through forests, each resulting section is smaller and less likely to recover from random disasters.

3) Overpopulation.  As of 2016, there are 7.4 billion of us, and space for humans comes at the cost of habitats for other species.

These changes result in a loss of biodiversity, which means that the total number of species is dropping.  We’re experiencing a loss, not just in the number of species, but also in the number of individual animals: according the World Wildlife Fund, the number of animals has dropped by 50% in the last 40 years.  At the time of that report, the cause was not yet climate change, but other human activities such as over-predation and habitat destruction.


Has the sixth extinction already started?

Earlier this year, a group of scientists warned that the sixth extinction may indeed have already started.  They noted that even animals which are not extinct have experienced losses in population and, worryingly, range.  All 177 mammals in their study have experienced losses in their ranges, with varying degrees of severity.  For example, the map below shows just how much lion habitats have shrunk.

Historic versus present ranges of African and Asiatic Lion habitats. Image courtesy of Tommyknocker [Public domain], via Wikimedia Commons.
In conclusion, humans have made rapid and profound changes to global ecosystems.  Firstly, our CO2 emissions are acidifying the oceans and increasing greenhouse gas concentrations in the atmosphere.  Daniel Rothman’s study suggests that, if we do not act immediately and drastically to reduce CO2 emissions, a mass extinction may be imminent by 2100.  Secondly, as discussed by Elizabeth Kolbert, the impact of human activities is not limited to climate change.  Everyday aspects of our lives, including traveling, living in previously undeveloped areas, and driving on roads that fragment natural habitats, also have serious consequences for global biodiversity.  In order to prevent another mass extinction, we’ll have to reduce our dependence on fossil fuels, and we may also need to reconsider how we interact with our environment in the first place.

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Rohini Shivamoggi

I'm a PhD student studying atmospheric sciences at MIT. I study the formation of secondary eyewalls in hurricanes, which hopefully will help us improve our forecasts of hurricane intensity! Before I got to MIT, I grew up in Florida and studied Chemistry and Physics at Harvard University. My other interests include weather forecasting, photography, and encouraging diversity in STEM! You can find me on Twitter @RShivamoggi.

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