What caused Earth’s biggest extinction?

Extinction is a scary word and a scary topic — but it’s one that needs talking about.

Why? Because it seems to be happening now. Scientists believe Earth is on the verge of its sixth mass extinction, an event that could devastate ecosystems all over the globe.

Habitat destruction and overexploitation are some of the major reasons so many plants and animals are disappearing around the world, and most scientists agree that human-caused climate change is soon to be the number one threat.

The topic has inspired fascination and dismay from scientists and science communicators alike. Earlier this year, science writer Elizabeth Kolbert published her book “The Sixth Extinction: An Unnatural History,” and the Smithsonian Channel recently released a new documentary called “Mass Extinction: Life at the Brink.” Both works describe some of Earth’s previous mass extinctions and explore what they can teach us about the impending one.

The Great Dying

Out of the previous five extinctions, the most famous is the Permian extinction — the worst of them all. It happened about 250 million years ago, and scientists believe it killed off about 90 percent of all life on Earth, earning it the nickname “The Great Dying.” Clues from the fossil record give us information about what species went extinct and when, but how the extinction happened in the first place has been a puzzling question for scientists.

One of the biggest theories involves a series of volcanic eruptions, which took place about 250 million years ago in Siberia. According to Smithsonian’s “Mass Extinction,” these eruptions spewed enough lava to bury the continental US under a layer 1,000 feet deep. In fact, there’s an area of about 2 million square kilometers in Siberia that’s still covered by volcanic rock today — scientists call it the “Siberian traps.”

But it’s not the lava that would have caused the extinction: The eruptions would have also released massive amounts of climate-changing carbon dioxide into the air — enough to cause the kind of disturbance that might kill off nearly every lifeform on Earth.

Carbon dioxide is a heat-trapping gas. With trillions of tons of CO2 being released into the atmosphere during the eruptions, the Earth would have grown warmer over time. Many land animals would have been unable to adapt to the rising temperatures.

In addition, the ocean would have absorbed much of the excess carbon dioxide in the air. When CO2 mixes with water, a chemical reaction occurs that causes the sea to become more acidic. This phenomenon, called ocean acidification, can be devastating for certain kinds of marine animals, like corals, because it prevents them from completing the chemical processes that allow them to build their protective shells. Many of these lifeforms would have gone extinct as a result.

What we don’t know

There are still some mysteries to be solved. Research published last March in the journal Proceedings of the National Academy of Sciences suggests that the Permian extinction took place over an extremely rapid timescale: only about 60 million years, or “a geological blink of an eye,” according to The New York Times. The Times article also reports that most scientists believe the Siberian eruptions lasted 1–2 million years, so nobody is quite sure yet why the extinction itself happened so fast.

There are also other theories besides the Siberian eruptions, although they tend to circle back to the idea of massive amounts of greenhouse gases changing Earth’s atmosphere. For instance, some researchers believe that a microbe called Methanosarcina belched tremendous quantities of methane into the atmosphere, changing the climate and causing many of the same issues the Siberian eruptions theory describes.

For now, it’s impossible to know the truth for sure.

These are issues worth studying, though. With Earth at the cusp of yet another mass extinction, scientists are eager to understand the processes that led to such events in the past, especially if they’re linked to carbon dioxide in the atmosphere. With human-caused climate change at the forefront of scientific discussions these days, information on past carbon-related events could give scientists insight into how we can deal with, or even prevent, such catastrophes in the future.

It could also help us understand and deal with the impacts of human-caused climate change that are coming.

This article is published in collaboration with Business Insider. Publication does not imply endorsement of views by the World Economic Forum.

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Author: Chelsea Harvey is a science reporter for Business Insider. She is based in New York City.

Image: Lava explodes from a volcano next to the village of Portela on Fogo Island November 28, 2014. REUTERS.