By Niamh Hernon, Wymondham College
Hernon, N. (2022) Will climate change make a sixth mass extinction an inevitability?. Routes 3(1): 33-42
Five mass extinction events have occurred during Earth’s history. This article explores current debates encompassing the coming of a sixth such event by examining data surrounding current and background extinction rates, including uncertainties surrounding these figures. Direct and indirect human-generated causes of the sixth mass extinction, particularly climate change, are then explored. Evidence is presented for anthropogenic global warming becoming the most prevalent driver of species extinction in coming decades. Given systemic climate change represents the most difficult causal factor to control and is also likely to synergistically interact with existing environmental stressors to accelerate extinction rates, urgent and decisive actions are needed to address it.
Extinction is common and is part of life. Around 99% of all organisms that have existed since life first appeared on Earth are now extinct (Novacek, 2001). It becomes a problem, however, when the extinction rate rises significantly and continually above the rate at which species are diversifying, triggering a mass extinction event. With an estimated 1 million animal and plant species currently threatened with extinction and extinction rates accelerating (IPBES, 2019), arguably, this is what we are seeing today (Kolbert, 2014).
Unlike previous mass extinctions, scientists agree that the present loss of biodiversity and population decline being witnessed across many species, is directly or indirectly attributable to human activity. Human overconsumption is currently the dominant driver of species extinction on land and sea (Ceballos et al, 2017). However, with global average temperatures projected to rise between 1.5 ºC and 4ºC by 2101 (IPCC, 2021), anthropogenic climate change seems set to overtake overconsumption as the major threat to global biodiversity (IPBES, 2019), especially given many species will be impacted by climate driven sea-level rise and altered ocean chemistry (Maclean and Wilson, 2011). This review of the literature evaluates whether Earth is indeed entering a sixth mass extinction event and examines possible causes, including the specific threat posed by climate change and its synergistic effects.
2. Are we entering the sixth mass extinction?
Mass extinction events are defined as occurring when extinction rates outstrip speciation rates, resulting in over 75% of species dying out within a geologically ‘short’ period (Barnosky et al, 2011). Given life has existed on Earth for 3.5 billion years, a “short” geological period is generally understood as anything less than 2.8 million years. Earth has experienced five previous Mass Extinctions, known as the ‘Big Five’. The last of these, the Cretaceous Event, occurred 145 million years ago (Saltré and Bradshaw, 2019). All are thought to have been caused by a combination of natural climate oscillation, asteroid impacts, and catastrophic volcanic activity (see Figure 1).
Most scientists agree that evidence points to a sixth mass extinction being underway or fast approaching (Barnosky et al, 2011; Saltré and Bradshaw, 2019). For example, Ceballos et al (2015) found that at least 200 vertebrate species have become extinct over the last century. Recent estimates place 28% of all known species at risk of extinction, with some species at particular risk, including 40% of amphibians and a third of reef-forming corals and marine mammals (IPBES, 2019). Plants and animals with narrow geographical ranges, such as island flora and fauna (Breslin et al, 2020) and those at higher latitudes are especially susceptible to extinction due to climate change (Wan et al, 2019).
Uncertainty remains as to whether the current extinction rate is sufficiently above the background extinction rate to indicate extinction approaching a level that would precipitate a mass extinction (Briggs, 2017). Background extinction is the rate at which we might expect species to go extinct in between mass extinctions, without the presence of humans. It is calculated using the ‘fossil record, the overall diversification of species and the detailed pattern of how many species accumulate in a lineage over time elucidated from molecular phylogenies’ (De Vos et al, 2015, p. 453). It is estimated, by most researchers, to be between 0.1 and 1 E/MSY (extinctions per million species-years). Though some argue it is closer to 0.1 E/MSY (De Vos et al, 2015, others use a rate of 2 E/MSY to ensure robust analysis (Barnosky et al, 2011). However, Ceballos et al (2015) showed that using conservative extinction rates, the 200 species that have gone extinct in the last century, should have taken between 800 to 1000 years to disappear at the background extinction rate. This indicates we are experiencing elevated extinction rates and possibly a sixth mass extinction (Ceballos et al, 2015).
Such conclusions are not universally accepted, however. Some scientists argue that current extinction rates are greatly exaggerated, and are balanced out by speciation (Briggs, 2017). However, given speciation, the evolutionary process by which populations evolve to become distinct species, takes significantly longer than current extinction rates to occur, this seems implausible.
Rates of population decline, not just extinction rates, are becoming increasingly recognised as important precursors to mass extinction (Briggs, 2017). Ceballos et al (2017) found evidence of population decline in 32% of vertebrate species studied, even those considered at low risk of extinction, with 40% of species experiencing severe population decline. Populations of African Lion, for example, fell by 43% since 1993. Two studies similarly identified large-bodied marine and land-based mammals as at heightened risk of population decline (Payne et al, 2016; Wan et al, 2019). Given the relative importance of large-bodied animals within ecosystems, this potentially signals greater ecological system disruption than that generated by similar levels of species loss in previous extinction events (Payne et al, 2016).
When evaluating evidence for a sixth mass extinction, it is important to acknowledge that much of what we know about comparative extinction rates across geological periods comes from the fossil record, which is subject to certain limitations (De Vos et al, 2015) Firstly, examination of the fossil record relies on uniformitarianism, that the same natural laws and processes that operate in our present-day scientific observations have always operated in the past and apply everywhere in the universe. Secondly, calculation of the fossil record is based on the law of superposition (that increasing depth equates to increasing age). Both assumptions affect the accuracy and reliability of the fossil record for calculating the extinction rate. However, it is impossible to determine how much margin of error each introduces (Barnosky et al, 2011).
The fossil record is also biased, since only a small number of species are able to become fossilised, favouring extinction patterns among certain types of organisms, while those among other unfossilised organisms remain unknown (Ceballos et al, 2017). These combined factors make it difficult to compare extinction rates calculated from fossil records with contemporary data and calculate the extent of contemporary extinction rate rise. A further limitation is that only approximately 2.7% of all named species have been formally assessed for extinction risk by the International Union for Conservation of Nature, whose Red List is used to compare extinction rates against the fossil record (Barnosky et al, 2011). Neither do we know how many species may be going extinct without being named (Wake and Vredenburg, 2008). Notwithstanding these limitations, scientists remain reasonably confident that extinction rates for mammals, amphibians, birds, and reptiles, when calculated over the past 500 years, are at least equal to, if not faster than extinction rates during the previous ‘Big Five’ mass extinctions (Ceballos et al, 2015).
Despite uncertainties and data disparities, scientists have extrapolated two main hypotheses from available data on threatened and endangered species. Both conclude that the extinction of 28% of all threatened species (IUCN, 2021) would result in a mass extinction event. One hypothesis is that this would occur immediately if all were to go extinct suddenly. The alternative hypothesis is that mass extinction will occur across the next 240 to 540 years, if current extinction rates continue (Barnosky et al, 2011). However, neither hypothesis considers how ecological stressors might increase in line with projected global warming between 1.5ºC and 4ºC by 2101.
3. Impact of climate change versus other anthropogenic drivers of extinction
When examining evidence for a sixth mass extinction event, it is important to explore the conditions which have precipitated previous such events. Whilst two of the ‘Big Five’ involved an asteroid colliding with Earth, others were preceded by periods of global cooling followed by global warming, accompanied by varied sea levels, changes in carbon dioxide concentration, geological activity, volcanic activity, and ocean acidification (Wake and Vredenburg, 2008; Saltré and Bradshaw, 2019).
These factors, Barnosky et al (2011) argue, interacted with existing ecological stressors, creating the ‘perfect storm’ of conditions implicated in previous extinction events. What is concerning is how many of these ecological pressures we are seeing today, but as a result of human rather than geological, asteroid, or glacial activity. Rising carbon dioxide levels in the atmosphere due to human industry, especially deforestation and burning of fossil fuels over the past 200 years, have precipitated a global rise in average temperature of 1.1ºC compared to 1850-1900 (IPCC, 2021), and have contributed to ocean acidification and sea level rise. Several studies have also identified climate change as a driver of extinction risk and population decline (Breslin et al, 2020; Wan et al, 2019).
Direct human causes of extinction, including habitat fragmentation through deforestation and urbanisation, and overhunting of larger animals are causing particular concern and are currently considered the main drivers of extinction over climate change (Payne et al, 2016). Human population growth also represents a significant threat to global biodiversity, as it results in increased consumption (Ceballos et al, 2015) which can further exacerbate existing issues such as habitat loss and climate change. Anthropogenic impacts are felt particularly among amphibians who are particularly vulnerable to diseases, habitat loss and climate change, as they have a small geographical range, with 44% of amphibians currently at risk of extinction (IUCN, 2021). This is noted as especially concerning, given amphibians have survived most previous mass extinctions (Wake and Vredenburg, 2008). Although unlikely to cause a mass extinction event in isolation, together these factors can seriously undermine species’ resilience to extinction in response to further ecological stress (Briggs, 2017).
Climate change, arguably an indirect human cause of species loss and extinction, is also the stressor most implicated in causing previous mass extinctions, but this time precipitated by humans on an unprecedented scale (Kolbert, 2014). Whilst not currently regarded by many as the most important factor in contemporary species loss, climate change is predicted in future to surpass “the impact of land and sea use change and other drivers” (IPBES, 2019). Climate change also synergistically interacts with other anthropogenic factors contributing to species loss, including habitat fragmentation, which causing geographical range contraction (Wan et al, 2019). This is particularly worrying given many species, as a result of climate change, will have a far smaller area which they can inhabit. Climate change’s impact on range contraction on is especially dramatic in the tropics and on small oceanic islands (Briggs, 2017; Breslin et al, 2020), where many species have narrow geographic ranges that restrict their potential for adaptation.
What makes climate change especially worrying is that its impacts on extinction risk and ecosystem function are projected to accelerate over time (UN, 2019). These impacts are likely to become increasingly dramatic unless prompt action is taken and may become irreversible if certain thresholds are breached (Ceballos et al, 2017).
Controversy remains about the coming of a sixth mass extinction event, with some commentators maintaining that we are simply experiencing population decline rather than mass extinction (Briggs, 2017). Data disparities, difficulties estimating the background extinction rate and the incomplete list of endangered species, also make it hard to determine whether we are truly witnessing signs of a mass extinction. However, since today’s extinction estimates of are likely to be severe underestimates, we are also faced with the possibility of reaching the next mass extinction event within 240 to 540 years, if current extinction rates continue and threatened species become extinct (Barnosky et al, 2011).
From the evidence presented, it appears highly probable that the sixth mass extinction is approaching or already upon us. This is especially given the presence of key synergies, including unusual climate dynamics, atmospheric composition and abnormally high-intensity ecological stressors that preceded previous mass extinction events (Barnosky et al, 2011). As this sixth mass extinction is attributable to human activity, this does increase the possibility of us exerting some control over it. However, as this window is likely very small, possibly 2 or 3 decades (Ceballos, et al, 2017) our strategy to address climate change must be immediate and include reducing the risk of species extinction as an integral part of the plan. Ongoing research and a commitment to long term policy change is essential to prevent accelerating species extinctions associated with climate change from “eroding the very foundations of our economies, livelihoods, food security, health and quality of life worldwide” (IPBES, 2019).
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Breslin, P.B., Wojciechowski, M.F. and Albuquerque, F., 2020. Projected climate change threatens significant range contraction of Cochemiea halei (Cactaceae), an island endemic, serpentine‐adapted plant species at risk of extinction. Ecology and Evolution, 10(23), pp.13211-13224.
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