Remember how I said in the last entry that climate change could have devastating impacts on life as a whole? Read that entry first for the necessary background and the politics of climate change. As I was saying, politics will become completely irrelevant at around 4 degrees Celsius above pre-industrial average temperatures. This is because the risk of a mass extinction of Earth’s biodiversity becomes significant at this stage. To me, that is the ultimate concern about climate change on this planet, even more so than humans and our way of life. After all, in the span of over 500 million years that Earth has had abundant complex life, there have only been five of these truly apocalyptic events. In fact, the four worst of them have each killed over 75% of all then-living species on Earth, with the fifth killing at least half of all life. The best way to understand what’s in store for us at this level of devastation is to look back in time, and the best way to do that is through the fossil record. Through fossils, the rocks beneath our feet contain five stark warnings about where life on Earth would be headed for at this rate of increase, and analyzing these records more publicly may be the best way to change minds about climate change. Therefore, I will go over all five of these mass extinctions in devastating detail.

The Ordovician extinction 440 million years ago was the first of these, caused by a rapid shift in climate from decently warm to ice-cold. That was devastating enough, but then the ice melted at an incredibly rapid rate, resulting in low oxygen levels in the oceans. That was the real killer, because life on land barely existed back then, and the event wiped out up to 85% of all species. This event happened so long ago that even fish had barely begun to evolve from the first vertebrates, but our story could’ve ended there, before it even really began. The first reference fossil I will use to demonstrate the effects are of trilobites, and while they took a bit of a blow, they recovered quickly from this. Their predators weren’t so lucky, and the apex predators of the time in particular would give way to new beasts.

The second mass extinction event began 375 million years ago, in the Devonian period. This time of great instability lasted 15 million years, and sent much of the world at the time into a tailspin. Around 80% of all species on land and sea perished, and the causes were different for each of the two waves. The first wave was possibly due to the colonization of land by plant and animal life alike, as plant life adapted more and more to land. This not only sapped oxygen out of the oceans again, but it also removed carbon dioxide from the atmosphere. This paradigm shift in regards to plant life killed the ocean life first, with the iconic trilobites being hit incredibly hard. By this time, their predators, which were armored fish, sea scorpions, and the first sharks, were catching up to them. This mass extinction would send trilobites to the brink of extinction, along with the sea scorpions, but the armored fish completely died out. On land, life took a hit, too, but the next mass extinction would dwarf everything that came before.

That extinction occurred around 252 million years ago, and it was the biggest mass extinction of the five that took place. 96% of all life was wiped out both on land and in the sea. In fact, if all life on Earth could be completely killed off, that would only barely exceed the devastation wrought at the end of the Permian. All life today is descended from a select few species of the 4% of life at that time that pulled through. That’s how close our world’s story came to ending in a volcanic blaze. This extinction marked the final demise of the remaining trilobites and sea scorpions, and even land vertebrates were decimated by the “Great Dying.” The causes of this apocalypse are numerous, hence the staggering toll. First, massive volcanic activity released greenhouse gases and burning-hot lava to a point that choked and buried just about everything. After this, the details get fuzzy, but it is believed methane was also entering the atmosphere at alarming rates at this time, which warmed the planet almost to its death.

Early reptiles were hit hard, and the many distant relatives of modern mammals at this time had an even more severe bottleneck. In fact, one genus of  “mammal-like reptiles” known as Lystrosaurus represented up to 95% of bones in some Early Triassic fossil beds. That level of dominance was unreplicated in natural history, and only humans and cyanobacteria have even possibly exceeded that level of ecological influence. Even then, no species before or since has been that abundant in numbers, especially after a mass extinction.

After that came the least apocalyptic “Big Five” mass extinction, and that was at the end of the Triassic, which by the way, began with the previous “Great Dying.” This one is a little more cut and dry, in that the main cause was again massive volcanic activity, specifically flood basalts. However, the oceans were also dealing with acidity issues at this time, likely due to CO2 coming from the volcanic floods. Amazingly, “only” somewhat more than half of all species died during this series of disasters, probably because it happened over many millions of years as the continents began to drift apart. The only real victors here, if there are any winners of mass dieings, were the dinosaurs, who would dominate the Jurassic and Cretaceous landscapes.

The last mass extinction so far occurred 66 million years ago, and anyone with a decent education should know this one. This is the one where that massive asteroid ruined the rule of the dinosaurs and wiped out 75% of species. This ended up leaving mammals to finally take control of much of the world, much like their earliest distant cousins of the Permian.

So where does all this leave humanity now? Honestly, the outlook isn’t great without a major, nearly immediate rebuild of our entire global economy. Therefore, I will try to hypothesize to what degree a human-induced mass extinction would measure up to the five mentioned earlier, and try to prepare for whatever may come. First, let’s see what similarities we can draw between past mass extinctions and the two present threats of climate change and degraded biodiversity. 

First, let’s set a status quo, that being the end of the last Ice Age. The world still has ice far beyond the polar regions, but that is about to give way to the climate we know today. Regardless, humans and most other present lifeforms evolved into their present diversity at this time, which to me suggests an ecological adaptation for cooler climates. Well, with the exception of tropical rainforests. That sets up a second status quo, one more relevant to present climate science, that being the pre-industrial climate. It is this norm of sorts that climate science, and the previous entry, are comparing all other scenarios to, and clarifying that feels highly important.             This scenario of an ice age leading to sudden warming is most reminiscent of the Ordovician extinction, but the Ice Age was the result of gradual glaciation over many millions of years. In contrast, the Ordovician glaciation was probably a lot more sudden than that. Therefore, I can only say that a little less than half of that level of devastation would be likely to occur now. However, there are two factors that remind me more of the Permian extinction. These two factors are the sheer amount of greenhouse gas emitted into the atmosphere and well as the incredible rates at which said emissions are happening. That’s extremely troubling, because the Permian and Ordovician die-offs are the two worst mass extinctions of complex life so far. 

Making matters worse, the math I did researching my hypothesis raises even more of an alarm bell than I thought it would. Assuming that the ten degree Celsius warming at the time of the Permian extinction occurred over around 5000 years, simple division would indicate a rate of change around .002 degrees Celsius a year. That extinction killed up to 96% of Earth’s species at that time. As for the present rate according to this probably oversimplified model, if the world warms “only” four degrees Celsius from 1800 to 2100, the rate of change would be .013 degrees Celsius per year. That’s six and a half times greater than the rate of change during the darkest hour in the history of complex life. In fact, to get an equal, proportional value to the present rate, the Permian warming would have had to occur in only 750 years. Yikes. Granted, I oversimplified the climate models beyond nearly all scientific use, and four degrees of warming is about half of a degree above most projections for 2100 right now. However, I chose this number because climate change has a flair for being even more dire than any scientific expectations. 

We cannot underestimate the severity of the problem, and my hypothesis is that at least 80% of all species on Earth, including humans, are at risk of extinction due to present climate change. This is little more than an educated guess, if I’m being honest, but the outlook looks exceptionally bleak regardless of how you look at it. However, there are solutions to this existential crisis, and they may have been right under our noses this whole time. I’m not talking about fracking, but I’m talking about wind and solar energy, a greater focus on sustainable consumption, and a systemic retooling of global society away from capitalism and socialism alike. Whether this happens remains to be seen, but if natural history is to teach us anything, it better be this.