Sixth Mass Extinction Event: Definition, Causes, Facts & Evidence

Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

There have been five recorded mass extinctions in Earth's history, so where does the sixth one come in? In this lesson, we'll explore a theoretical sixth mass extinction, and see what it means for Earth's history.

The Holocene Extinction

There have been several mass extinctions in Earth's history. None of them were particularly pleasant, but the one that has received the most scientific attention lately is the Holocene Extinction, named for the Holocene epoch of geologic history. Wait a minute. The Holocene? That's now.

The Holocene is still very young in geologic time
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The Holocene is a geologic time period that began with the end of the Ice Age about 11,700 years ago. Since then, the world has been changing very rapidly. In fact, it may be changing too rapidly for many species to keep up. There are five mass extinctions in Earth's history, in which more than 50% of species died out, and many scientists believe that we are entering the sixth. Well, that sounds like a problem.

Evidence for the Sixth Mass Extinction

Extinction is a perfectly normal phenomenon. Species either die out or evolve into something new over time. From the fossil record, we can tell that the average rate of extinction per year throughout Earth's history is around 1 species per million species. That means that we can expect anywhere between 10 and 100 species to go extinct every year, depending on the point in geologic history we're examining. We call this the background extinction rate.

If plants and animals go extinct all the time, why are we suddenly becoming so concerned by it? It's natural, right? Unfortunately, extinctions in the modern world are occurring on a much higher scale than the expected background extinction rate. Take mammals, for example. From the geologic record, we know that the average lifespan of a mammalian species is about 1 million years before it goes extinct or evolves. In the last 400 years, however, 89 species of mammals have gone extinct. That's almost 45 times the background extinction rate.

Mammals aren't alone in this trend. When we look at loss of genetic diversity across vertebrates, invertebrates, plants, fungi, and other living things, the extinction rate compounds somewhere between 1,000 and 10,000 times the normal rate. A wide-ranging 2017 study looked at almost half of all known vertebrate species and found that roughly a third are decreasing in population size and range. Another study found that 3,000 different species had all lost half their populations since 1970. If this keeps up, 30-50% of all species in the world could be in danger of extinction by the middle of the century. The only other times in Earth's history that we see these sorts of statistics are during mass extinction events.

Likely Causes

So, what's happening? Did fidget spinners distract us so much that we somehow missed a giant asteroid striking the planet? Unlike other mass extinctions in history, this one isn't being caused by an asteroid or massive volcanoes. The most likely cause is us.

Humans have increased in population and distribution faster and more efficiently than practically any other living thing in the history of the planet. In fact, some scholars even think that we need a new designation for the period in Earth history impacted by humans. They call it the Anthropocene, and this the Anthropocene Extinction because it's being caused by our actions. In fact, scientists estimate that 99% of currently endangered species are at risk due to human impacts on the environment.

Deforestation is a huge factor in modern extinction rates. These are countries with at-risk forests, with brown and red indicating severest risks
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As humans spread, we require more space for our physical dwellings and to produce our food. That's billions of acres no longer being used by other species. Studies have found that in mammals alone, about half of all species lost 80% or more of their distribution range between 1900 and 2015. The more space we use, the less there is for other living things.

Beyond that, humans have been pumping carbon dioxide into the air at exponentially increasing rates since the Industrial Revolution. The vast majority of studies show a clear link between rates of carbon emissions and changes in global temperatures. This, in turn, has severely impacted many habitats. Amphibians, which are highly susceptible to changes in climate, pollution, and loss of habitat may be the most affected with an extinction rate up to 45,000 times the normal background rate.

Changes in global temperatures from 1880s (top) to 1980s (bottom), with redder colors indicating higher temperatures
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The Debate

Now, obviously this theory is not accepted by everyone. There are a number of points where this is debated, but let's look at the big three.

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