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What is… a mass extinction event?

The term ‘mass extinction event’ is often used in reference to our current biodiversity crisis, but is often misunderstood. This article clarifies the facts on mass extinction events throughout history. 

With human actions pushing thousands of species towards the brink of extinction, it becomes necessary to quantify the risk posed to the natural world. It is estimated that there are about eight million species of organisms, and of these, potentially 15000 are threatened with extinction. This may not seem like a lot, but it becomes a lot higher when we consider the extinction rates of previous centuries. 

The extinction rate is quantified as the number of extinctions which happen per 1 million species of organism, per year. Scientists determine this from fossil records, and although it is difficult to measure precisely, an estimated ‘background extinction rate’ - that is, extinction rate in a normal period of history - tends to sit at around 0.1 - 1 extinctions per 1 million species, per year. 

But there are periods where that extinction rate sits a lot higher than usual. This is either because a lot more are going extinct, or because they are going extinct at a much quicker rate. You might have heard the term mass extinction - this is where 75% of species go extinct within a short geological period, typically less than 2 million years. 

In the past 500 million years, there have been five mass extinction events as determined from the fossil record: the Late Ordovician, Late Devonian, End Permian, End Triassic and End Cretaceous extinction events. 

The earliest was the Late Ordovician extinction event, 445 million years ago. This was caused by natural climate change - a major ice age began in the southern hemisphere and global temperatures cooled, causing widespread extinction. Animals hadn’t colonised land yet, so the extinctions were restricted to the seas, where entire taxonomic families died out. A million years later, temperatures began to warm up again, causing a second pulse of extinctions. An estimated 85% of organisms disappeared during this time. 

The most recent was 66 million years ago: the End Cretaceous event, which is also probably the most famous - it is best known as the event which killed off the dinosaurs. The cause is hotly debated, but evidence points towards it largely being the consequence of an asteroid impact. Geologists also note two other factors around this period: a drop in global sea levels (known as the Maastrichtian regression) and a series of lava flows (known as the Deccan traps) which were also likely contributors to the sheer volume of extinction, amplified by the effects of the asteroid.

As well as wiping out all non-avian dinosaurs, the event also killed off many early mammals, marine reptiles, amphibians, birds and insects. The asteroid would have triggered a massive earthquake and unleashed hurricane-force winds across the Americas. Debris ejected by the asteroid would have fallen back to earth and warmed the atmosphere while doing so, triggering forest fires across the globe and creating a heat pulse which cooked anything unable to burrow underground or hide underwater

Whilst the mass extinctions of the past can be seen as tragedies attributable to uncontrollable forces of climate, the mass extinction we are currently facing is different. The next mass extinction will not be a natural phenomenon but one caused by humans, and mostly by a small but powerful subset of us, and many scientists believe that we are already in the midst of it. 

Since the year 1500, it is estimated that between 150000 and 260000 species on Earth have been lost - between 7.5 and 13%, and predictions of the current extinction rate is nearly one hundred times higher than the background extinction rate deduced from fossil records. These statistics mean that we are on what Robert Cowie describes as “a sad trajectory towards the Sixth Mass Extinction.”

Humans have altered the biosphere so much that in 2000, the term “anthropocene” was coined - referring to a new geological epoch marked by our presence. But the blame is not equal, and there is danger in falling into a generalised concept of ‘humanity.’ The University of Leeds found that the poorest 50% of humans globally consume less energy than the richest 5%, whilst another report found that just 100 fossil fuel companies accounted for 71% of global emissions from 1988 to 2015, and created more emissions in those years than they did total in the 237 years prior. 

Excessive fossil fuel production may characterise the unsustainable living patterns of the richest people in the Global North, but merely switching to renewable energy isn’t enough to stop those lifestyles from barrelling us towards the sixth mass extinction. Deforestation, water overuse, air pollution and overfishing are all amongst the activities driving biodiversity loss worldwide. 

We are staring down an event on par with the extinction of the non-avian dinosaurs. In order to stop it, we are going to need mass political action and radical change - otherwise we will witness catastrophic and irreversible losses. The good news is that there are thousands of grassroots movements spanning towns, countries and continents and more popping up every day, working to resist extractivism and nature exploitation and striving towards a future where we protect the planet’s biodiversity rather than harming it. It’s going to be a long and hard fight - but the consequences of giving up are immeasurable. 


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