Mass Extinctions: Are We in One?
The Earth is about four billion years old, and it has sustained life on its surface for over two billion years (Jacobsen, 2003). Although it is still incredibly young in comparison to other planets, the Earth is old enough to have let evolution run its course and come up with "intelligent life". Currently, the Earth finds itself in a period in which the global balance is being distorted. Incredible amounts of toxic gases have been added to the atmosphere, causing a global rise in temperature. This has dire consequences, including a vast decrease in ice caps at the North pole, acidification of global water, and major climate change around the globe. History shows that climate change may sometimes lead to a phenomenon known as a mass extinction event, in which a large portion of all the species on Earth dies. Before evaluating whether humanity finds itself in one right now, it is worth discussing the causes of past mass extinction events.
In 1982, Raup and Sepkoski characterised five events in the history of the Earth that resulted in such widespread extinction of species. They termed these the “Big Five Extinction Events” (Raup & Sepkoski, 1982). In this research, the authors calculated the extinction rate of species on Earth over time. By studying fossils, Raup and Sepkoski were able to determine the pace at which species went extinct. If a certain species stops appearing at a certain point in the fossil record, it is evident that the species were going extinct at that time. Generally speaking, the fact that organisms go extinct is unalarming. Climate change is a continuous process, and so is evolution. These two factors cause some organisms to be less well-adapted to the environment. As a result, by selection of the fittest, these species go extinct over time. Interestingly, Raup and Sepkoski found that, with the exception of five time points, the extinction rates on Earth all form a tight cluster, as shown by the relatively straight line in Figure 1 below. At five points in history, however, the extinction rates were found to be significantly higher than usual, and they were assumed to be indications of mass extinction events. These events took place in the Ordovician, Permian, Devonian, Triassic, and Cretaceous periods respectively (Raup & Sepkoski, 1982).
Global climates change constantly as a result of many different factors. One aspect that is always involved is the greenhouse effect. Carbon dioxide, as well as other greenhouse gases including methane and nitrogen dioxide, are present in the atmosphere at all times as they work to reflect sunlight towards the surface of the Earth (Driga & Drigas, 2019). When sunlight comes into the atmosphere and warms up the surface of the Earth, part of it is reflected away from the surface by, for example, the icecaps at the North and South poles. This reflected light hits the atmosphere again, and a portion of it is trapped by the greenhouse gases instead of penetrating through and reentering space. Light that is trapped within the Earth's atmosphere in turn raises the surface temperature. In other words, the more such greenhouse gases are present, the more warmth is reflected on the Earth. This is known as the greenhouse effect, which is actually essential for the Earth's climate and for life to develop naturally. Scientists have estimated that without any form of the greenhouse effect, i.e., no greenhouse gas in the atmosphere, the average temperature on Earth would be as low as -18 degrees Celcius (0°F), which is significantly lower than the 14 degrees Celcius (57°F) it is today (Driga & Drigas, 2019).
This greenhouse effect serves as an underlying principle when it comes to mass extinction events because of its direct correlation to global warming (Bond & Grasby, 2017). The timing of these events in history has been described in detail. When geologists analyse fossils from these times, they find that there was a significantly higher concentration of greenhouse gases in comparison to fossils from any other time. Additionally, researchers have found that volcanic activity increased immensely just before these mass extinction events. It is therefore not surprising that experts have associated volcanic activities with rising greenhouse gas concentrations and subsequent mass extinctions.
In the short term, volcanism causes global cooling (Harpp, 2005). The ash that is released in the air may be so abundant that the amount of sunlight reaching the Earth heavily decreases. As a result, the main energy source of the Earth, sunlight, cannot warm the former’s surface. However, volcanic eruptions release massive amounts of carbon dioxide and other greenhouse gases. As soon as the ash has fallen down to the planet, and sunlight is once again able to penetrate the atmosphere, the now increased concentration of greenhouse gases reflect more sunlight back to the surface than they used to, resulting in a sharp increase in global temperature (Harpp, 2005; UCAR, n.d.). This hypothesis has been shown to be the primary cause of most mass extinction events in history.
The relationship between greenhouse effect and mass extinction is very complex. While it involves a combination of many factors, the main mechanism is believed to begin from the sea, and it works as follows. As more greenhouse gases reflect sunlight back to Earth, the ice caps melt, and large amounts of fresh water are added to seawater. This leads to a decrease in the oxygen concentration of seawater, as icecaps do not hold oxygen (Bond & Grasby, 2020). Less oxygen in turn results in much of the plankton dying because of their need for oxygen to survive. And since phytoplanktons make up the bottom of the marine food chain, their inability to survive causes a complete disruption of the food chain, resulting in the gradual extinction of many species on Earth (Bond & Grasby, 2020).
The above is how all mass extinctions but one were caused (Bond & Grasby, 2020; Chen et al., 2022; Rakociński et al., 2020; Shen et al., 2022). The only exception was the most recent one which occurred approximately 66 million years ago. It was responsible for killing the dinosaurs (with the exception of the ancestor of the chicken), and it was a direct consequence of an enormous asteroid hitting the Earth (Chiarenza et al., 2020). This catastrophic event caused so much dust to be added to the atmosphere that plants and phytoplanktons became deprived of sunlight. The organisms that had not died immediately after the asteroid impact witnessed a complete disturbance of the food chain, as the bottom of it suddenly could not survive anymore (Chiarenza et al., 2020).
In the history of the Earth, there have been five devastating events that led to a significant decrease in the abundance of life. Not taking into account the most recent mass extinction, all these events have one thing in common: global warming. Although the cause of global warming was very different at the time of these catastrophic events, current global warming portrays striking similarities with the process that predates these events. A key difference is, however, that the pace at which species nowadays are pushed towards extinction is significantly higher. While previous extinction events took place over millions of years, recent studies find that if species continue to die at the current rate, within merely three centuries 75% of all animal species may be wiped out, making the sixth extinction a reality (Gibbons, 2011). That said, it must be noted that there exists a heavy debate on whether the present biosphere finds itself in a sixth mass extinction. Like many aspects of biology, the network by which things influence one another can be incredibly complex to comprehend. In this case, how global warming, in combination with pollution and deforestation, affects animals on a global level is an example of such a complex network. This complexity feeds debate, as the science that is available does not paint the full picture of reality.
One important factor that needs to be taken into account when evaluating the current situation is that the vast majority, in fact well over 90%, of all biomass on the planet is livestock, over which humans have full control (Brannen, n.d.). Such control has not been the case in past mass extinction events. Rapid industrialisation and advancements in technology have enabled humans to freely manipulate and change the global food chain, making it very difficult to predict how a sixth mass extinction would play out. Because of this reason, some experts are very hesitant to talk about the current situation as a mass extinction. In a fascinating interview with The Atlantic (Brannen, n.d.), paleontologist Doug Erwin said that he would not call it as such: "People who claim we are in the sixth mass extinction do not understand enough about mass extinctions to understand the logical flaw in their argument. To a certain extent, they are claiming it as a way of frightening people into action, when in fact, if it is actually true we are in a sixth mass extinction, then there is no point in conservation biology." Erwin's point is that given the complexity of the food chain in nature, it is likely that if this network is put out of balance to the level of a mass extinction event, the hope of humanity being able to restore that balance is naive. As a renowned geologist with expertise in mass extinctions, Erwin stated that naming the current situation as a mass extinction would do more harm than good, but he remained ambiguous in reasoning why it cannot be classified as such (Brannen, n.d.).
In conclusion, although the number of species facing extinction as a consequence of human actions is not that high yet, humanity finds itself at the brink of what could be the sixth mass extinction in the history of the Earth. Through human behaviour influencing climate change, pollution, deforestation, and much more, an alarming portion of the Earth's biosphere is on the verge of going extinct. Instead of focusing on saving the obvious large animals from extinction, a societal change should take place in which humans learn to understand and appreciate the balance of biology on the planet. At the very least, this should come from consideration of how human actions impact the climate and the Earth as a whole. If anything, the history of the blue planet teaches humans one important lesson. However dire the changes taking place on the planet, Earth will eventually survive and restore its balance. After all, ensuing every mass extinction in the past, life has bloomed all over again as if it had a fresh start. The important question lies in whether humans can sustain themselves on such a changing planet.
Bibliographical References
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