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The Sixth Extinction by Elizabeth Kolbert.
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In The Sixth Extinction, journalist Elizbeth Kolbert argues that humans are rapidly changing the shape of the earth and the composition of the atmosphere, unleashing a mass extinction of most living things, quite possibly including ourselves. Scientists have identified five previous mass extinction events over 500 million years and many believe a Sixth Extinction, set in motion by humans, is underway.

Humans began impacting the world from the start. As modern humans spread from East Africa around the globe, they found archaic humans similar to themselves and gigantic animals (megafauna)—both of which they wiped out.

They—we—started demolishing forests to grow food and spreading animals, plants, and other life forms to new continents, thus changing the face of the earth. With the discovery of energy sources underground, we began our greatest and most deadly transformation—of the composition of the atmosphere and the oceans. Some plants and animals have survived by migrating. But many, perhaps millions, are stranded where they are unable, or lack time, to adapt. Extinction rates are skyrocketing. No other species has so drastically changed life on earth.

The Big Five Mass Extinctions

Until the eighteenth century, scientists and naturalists had no concept of extinction. They believed life was a long, unbroken “chain of being”—that the animals and other life forms existing at the time were the only ones that had ever existed or would exist. Then bones of huge creatures such as mastodons and mammoths began turning up and naturalists puzzled over why they had disappeared.

Some theorized there had been “lost worlds” of fantastic species that were obliterated by catastrophes. Others believed that extinction only happened slowly as part of the process of evolution—animals with non-beneficial traits eventually died out. Eventually, with the discovery in the 1980s of the site of an asteroid strike on the Yucatan Peninsula, the idea of sudden mass extinctions gained adherents.

Today, scientists believe in both gradual and sudden extinctions. They’ve identified five mass extinction events of the distant past—the “Big Five”—plus a number of smaller extinctions. Each of the Big Five suddenly decimated the earth’s diversity of life.

(Shortform note: The Big Five extinctions were:

1) The End Ordovician period, 444 million years ago, 86% of species lost. The cause was a sudden cooling of the climate (carbon dioxide levels and temperatures dropped and things froze—glaciation) plus a huge drop in sea levels plus an ocean chemistry change resulting from the drop in CO2.

2) Late Devonian, 375 million years ago, 75% of species lost.

3) End Permian, 251 million years ago, 96% of species lost. This extinction seems to have been triggered by a sudden warming of the climate. For unknown reasons, an enormous amount of carbon was released into the air. Temperatures shot up and the seas heated up and became acidified. Oxygen levels in the water plummeted, probably suffocating nearly all life.

4) End Triassic, 200 million years ago, 80% of species lost.

5) End Cretaceous, 66 million years ago, 76% of all species lost when an asteroid traveling at 45,000 miles an hour crashed into the Yucatan Peninsula. A scorching cloud spread across North America, vaporizing everything. Dust blocked much of the sunlight, creating an “impact winter” or prolonged cooling.)

The Anthropocene Epoch

Each of the Big Five had its own unique causes, but in every case, species faced drastic changes for which they had no time to adapt.

Scientists believe we’ve entered a new epoch: the Anthropocene or human-dominated geological epoch, characterized by man-made, planet-altering changes.

Among the first signs that our actions are leading to catastrophe was the disappearance of amphibians beginning in the 1980s. Researchers in Panama first noticed that an iconic local species, the Golden Frog, was dying. Then they realized frogs were disappearing all over the globe.

In 2008, citing the precipitous drop in amphibian populations, an article in the Proceedings of the National Academy of Sciences, asked, “Are We in the Midst of the Sixth Mass Extinction?” The authors concluded that, based on the extinction rates among amphibians, a sixth catastrophic event is underway.

Besides amphibians, animals are in trouble everywhere. Among those suffering steep declines are reef-building corals, sharks, rays, fresh-water mollusks, reptiles, mammals, and birds. While different animals are disappearing for seemingly different immediate reasons, in every case you can ultimately trace the cause to humans.

Global Warming

Most importantly, we’ve changed the composition of the atmosphere by adding vast amounts of carbon dioxide—over...

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The Sixth Extinction Summary Introduction

Around two hundred thousand years ago, in eastern Africa, a new species emerged. While not especially strong, fast, or prolific, the newcomer—Homo sapiens—proved to be uniquely inventive.

In The Sixth Extinction: An Unnatural History, journalist Elizbeth Kolbert argues that our species today is rapidly changing the shape of the earth and the composition of the atmosphere, in the process unleashing a mass extinction of most living things, quite possibly including ourselves. Scientists have identified five previous mass extinction events (plus smaller disasters) over 500 million years and many believe a sixth extinction, set in motion by modern humans, is underway.

Humans began impacting the world from the start. As their population grew, modern humans spread from East Africa into new regions, undeterred by climate or geographic barriers like rivers and mountains. Humans adapted to the conditions and food supply wherever they landed—for instance, along coasts, they ate shellfish, while inland, they hunted mammals.

When they arrived in Europe, they found Neanderthals, archaic humans similar to themselves, and interbred with them before wiping them out. As modern...

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The Sixth Extinction Summary Chapter 1: The Sixth Extinction

Amphibians such as frogs and toads have been around longer than mammals, birds, and even dinosaurs—yet today they’re on the leading edge of another mass extinction. Amphibians’ ancestors emerged from the water 400 million years ago and early forms of today’s amphibian orders appeared 250 million years ago.

“Amphibian” means having a “double life”—they start their lives in water and live on both water and land. Some frogs lay their eggs in streams while others lay them in vernal ponds; some make nests or carry their eggs on their bodies. Amphibian eggs have to stay wet in order to develop because they lack shells.

Amphibians live in a variety of habitats on every continent except Antarctica. Of the seven thousand species we’ve identified, the largest number live in tropical forests—however, one lives in the desert of Australia (the sandhill frog) and one can live above the Arctic Circle. Spring peepers and other North American frogs can revive in the spring after being frozen solid.

Today, amphibians are the most endangered of the six main classes of animals. Researchers started realizing frogs were in trouble in the late 1980s.

Golden Frogs Disappear

Panamanian...

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The Sixth Extinction Summary Chapter 2: Tracing the Clues to Past Extinctions

Until the eighteenth century, scientists and naturalists had no concept of extinction. They believed life was a long, unbroken “chain of being”—that the animals and other life forms existing at the time were the only ones that had ever existed or would exist.

They believed this despite the existence of collections in London, Paris, and Berlin of the remains of creatures such as trilobites, belemnites, and ammonites. In addition, mammoth bones had been uncovered in Siberia, although they were thought to be from elephants.

Finally, in revolutionary France in the mid-1700s, a visionary naturalist, Georges Cuvier, began connecting the dots, starting with a giant molar found in New York state in 1705 and shipped to London, plus a cache of mastodon bones found in a sulfurous marsh along the Ohio River in 1739 by a French expedition. Today the site of the discovery is a state park in Kentucky called Big Bone Lick.

The mastodon bones, which included a three-and-a-half-foot thigh bone, a gigantic tusk, and several teeth, ended up at the Paris Museum of Natural History. A second shipment of bones from the site was sent to London. The bones seemed elephant-like, but naturalists were...

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The Sixth Extinction Summary Chapter 3: The Great Auk—an Observable Extinction

In the 19th century, those who believed in sudden mass extinctions were called catastrophists.

But geologist Charles Lyell, who greatly influenced Charles Darwin, believed in a slow process of extinction that accompanied evolution: the landscape changed gradually over millennia due to events and processes like volcanoes, erosion, and sedimentation. Some animals adapted to survive and others didn’t.

Those who believed only in gradual extinction were called uniformitarians. (Today, scientists believe the earth is subject to both gradual (background) extinction and sudden mass extinction.)

Darwin joined the five-year voyage of the Beagle in 1831, which headed for South America to survey the coast and improve existing maps. Through studies during and after the trip, he developed his theory of natural selection, which contends that life, like the physical environment, is always changing. Organisms better adapted to their environment produce more offspring, passing on the successful adaptations. This process of selecting and rejecting traits and variations is continuous and drives evolution.

Darwin believed that over vast amounts of...

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The Sixth Extinction Summary Chapter 4: Evidence of an Asteroid Strike

In the late nineteen-seventies in Italy, American geologist Walter Alvarez discovered traces of the asteroid that ended the Cretaceous period, causing the fifth mass extinction, which wiped out 75% of all species.

He was studying rock layers in the Gola Del Bottaccione gorge outside the town of Gubbio, Italy, which is north of Rome. The region once lay at the bottom of the sea. The remains of marine animals built up through millennia, eventually creating the Apennine Mountains and elevating limestone cliffs.

Between the diagonal bands of limestone reflecting different time periods, Alvarez saw a thin layer of clay that contained none of the marine lifeforms seen in the limestone layers below or above it. Something had wiped out the foraminifera—tiny creatures with calcite shells that fossilize—below the clay layer; when forams appeared later in the limestone layer above the clay, they were different species and much smaller. Alvarez determined that the larger forams seen below the clay layer had vanished at the time dinosaurs were known to have died off (the End Cretaceous period).

Further, Alvarez and his father Luis, a physicist at UC Berkeley, tested the samples from...

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The Sixth Extinction Summary Chapter 5: A New Science of Extinction

Today’s science of extinction developed from a series of paradigm shifts.

Sociologist Thomas Kuhn came up with the paradigm shift idea in 1962 to explain how a fundamental change in the basic concepts of scientific discipline leads to a totally new way of thinking. Kuhn showed through studies that when people receive “disruptive” information, which goes against their beliefs, they first try to fit it into their current thinking or framework. They disregard aspects that don’t fit for as long as possible.

When the inconsistencies between the new information and their old way of thinking become too great to ignore, they reach a crisis point. They have what psychologists call a “My God!” reaction and begin to assimilate the new reality.

Both individuals and entire fields of study experience this process. In science, new data that doesn’t fit accepted assumptions and principles is dismissed or rationalized. As the contradictory information grows, the explanations become crazier until people finally acknowledge the new reality. The old framework collapses and the paradigm shifts, opening the way for new insights.

The science of extinction evolved in this way. There wasn’t any...

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Shortform Exercise: Your Carbon Footprint

Humans have changed the composition of the atmosphere by adding vast amounts of carbon dioxide—over two hundred years, the level of carbon dioxide in the air has risen by 40%. This is heating up the climate and acidifying the oceans, possibly setting in motion a sixth mass extinction.


Use this Carbon Footprint Calculator to determine your contribution to global warming. What are some ways you could decrease your carbon footprint? What could you do immediately?

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The Sixth Extinction Summary Chapter 6: Impacts—Ocean Acidification

Oceans, covering 70% of the earth’s surface, absorb a lot of the carbon we’re pumping into the air—two-and-a-half-billion tons a year when this book was written in 2014—which is changing ocean chemistry.

In the past, there was a fairly even exchange of gases: the ocean absorbed gases from the atmosphere and also released dissolved gases back into the atmosphere. At this point, however, more CO2 is entering the oceans than they can release, resulting in acidification. (Carbon dioxide dissolves in water and forms carbonic acid.)

As a result, the pH of the oceans’ surface water has decreased, making them 30% more acidic than they were in 1800. The pH is on track to fall to 7.8 (from today’s average of 8.1) by the end of this century, making the oceans 150 percent more acidic than before the industrial revolution.

Ocean acidification may have played a major role in the most recent mass extinction event—the End Cretaceous. It’s believed to have been a factor in two more of the Big Five—the End Permian and End Triassic and possibly in two lesser extinction events.

Why Acidification is Dangerous

**In terms of destructive effects, ocean acidification has been called...

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The Sixth Extinction Summary Chapter 7: Impacts—The End of Coral Reefs

Corals have endured for many geologic epochs, but researchers believe they won’t survive the Anthropocene. Instead, they’re on a course to be the first major ecological system to go extinct. The driving forces are acidification and climate change.

Some scientists project they’ll last out the century; others don’t give them even that long.

One paper in Nature predicted that visitors to the Great Barrier Reef in 2050 will find it rapidly disintegrating.

Coral reefs stretch around the middle of the globe. The largest is the Great Barrier Reef, which extends, with breaks, for more than fifteen hundred miles; in some places, it’s five hundred feet thick. The next largest is off the coast of Belize. There also are sizeable reefs in the Pacific, the Indian Ocean, and the Red Sea, plus smaller reefs in the Caribbean.

Reefs are enchanting in their beauty—Darwin described them as “amongst the wonderful objects of the world.” Biologically, they are even more distinctive. Yet they are under dire threat.

How Reef-Building Corals Work

Coral reefs are strong enough to destroy ships, yet they’re built by generations of tiny marine invertebrates called polyps that work together...

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The Sixth Extinction Summary Chapter 8: Impacts—Rainforests and Biodiversity

Most people think of global warming primarily as a threat to cold-climate species such as polar bears, penguins, and seals. Their worlds are changing dramatically as polar sea ice declines and gets thinner. And as the ice declines, the larger areas of open water absorb more heat, which melts more ice.

Half of the Arctic’s perennial sea ice has disappeared in the last thirty years and the rest may be gone in thirty more. Perennial sea ice, also known as multiyear ice, is thicker ice that survives the summer season. As the earth warms, the outlook for species that rely on the ice is grim.

But according to researchers, global warming will have an even greater impact in the tropics because that’s where the most species live. In Canada’s boreal forest of nearly a billion acres, there are only about twenty species of trees. In the U.S., eastern deciduous forests contain fifty to two hundred species. In contrast, Belize, in Central America, has some 700 native tree species. Manu National Park, in the Andes of Peru, is a forest reserve where researchers have counted more than a thousand species of trees.

The same pattern—fewer species in cold climates and many more in warmer...

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The Sixth Extinction Summary Chapter 9: Impacts—Fragmentation of Habitat

As humans have reshaped the earth’s landmass, we’ve constrained the ability of other species to survive the life-altering effects of global warming.

Currently, the earth contains about fifty million square miles of land not covered by ice. We’ve transformed more than half—27 million square miles—into cities, highways and shopping centers, cropland and pasture, logging and mining operations, and manufacturing plants. That leaves 23 million square miles, which are mostly (three-fifths) forest; the other two-fifths are high mountains, tundra, and desert.

Another way of looking at how we use the planet is by dividing the landmass into “anthromes,” or human-altered zones, such as urban, irrigated cropland, and populated forest. Researchers have identified eighteen anthromes covering 39 million square miles. That leaves 11 million square miles of wildland, including parts of the Amazon, Siberia, northern Canada, and the deserts.

However, there’s virtually no area left that’s truly untouched—roads, logging, and mining have sliced up and cut off every wild area to some extent. There are two results: **1) species lose the ability to move or flee, and 2) their ability to reproduce...

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The Sixth Extinction Summary Chapter 10: Impacts—Dispersal of Species

In the past, the range of many species was limited by geographic barriers such as oceans, rivers, and mountains. Today, however, species are being dispersed widely by humans, with disastrous consequences.

Darwin believed each species originated in one place. It spread by dispersing seed via the wind or it moved under its own power. With a lot of time, any organism could eventually spread widely. However, geographic features like oceans, mountains, and deserts set limits, which explained why flora and fauna on one continent could be different from those on another—they’d evolved separately.

However, Darwin struggled to answer the question of how the original colonizers got started. Also, his theory didn’t explain why fossils of the same types of reptiles and plants were found on different continents. In later years, scientists wondered whether land bridges had once spanned oceans, allowing travel, or whether the continents had once been larger and then separated and shifted. The latter theory suggested there was originally one giant continent, Pangaea.

**In the Anthropocene, humans are, in a sense, reuniting the continents into a New Pangaea by dispersing species all around...

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Shortform Exercise: Invasive Species

Humans have spread invasive species around the world through various means of travel, shipping, and trade. Often, the invasives wipe out local species by predation, damaging crops, or spreading new diseases.


Visit the following website to identify the common invasives in your state. Which ones have you encountered near you or even in your backyard? How can you help get rid of them?

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The Sixth Extinction Summary Chapter 11: Last of the Megafauna

A great variety of supersized animals—megafauna—once stalked the earth. Near the end of the Cretaceous period, there were many groups of huge dinosaurs besides Tyrannosaurus. Members of the Saltasaurus group weighed around seven tons. A member of the Therizinosaurus group was thirty feet long,

Near the end of the last ice age, there were enormous animals all over the world. In Europe, the roster included woolly rhinos, cave bears, giant elk, and hyenas. North America had mastodons, mammoths, giant camels, grizzly-size beavers, saber-toothed cats, and a giant ground sloth. South America had glyptodonts, which resembled armadillos the size of small cars. Australia’s even weirder animals included diprotodons, a group of huge marsupials called rhinoceros wombats; a marsupial lion, and a ten-foot-tall kangaroo. New Zealand had giant birds—the South Island giant moa was twelve feet tall.

Being very big was an evolutionary advantage—large animals had no predators. So the question of why the megafauna died out has puzzled scientists dating back to Cuvier’s day, when the fossils of huge unknown creatures began turning up.

Scientists have debated whether megafauna...

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The Sixth Extinction Summary Chapter 12: Pruning the Family Tree

From the day that modern humans migrated from Africa to the Middle East a hundred twenty thousand years ago, something in humans’ genes set them apart from all other species, driving them to cross oceans, explore, and take over new areas, by killing off the locals and by altering the environment as they chose.

It might be called a restlessness or insanity gene, in that we’ve been pressing the envelope ever since, exploring the outer reaches of space but also altering our own world in ways that are driving other species—and maybe eventually ourselves—extinct.

Pre-Modern Humans Discovered

We share DNA with an archaic human, the Neanderthal, who didn’t survive contact with us. The first bones of these distant relatives were found in 1856 in the Neander Valley in Germany is north of Cologne. The valley was lined with limestone cliffs, which were being quarried when workers discovered the bones in a cave.

They tossed the bones aside and they might have been lost, if the quarry’s owner hadn’t heard about them. Believing they were from a cave bear, the owner passed them on to a fossilist, who recognized them as resembling human bones and called them “a primitive member of...

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The Sixth Extinction Summary Chapter 13: Saving Species, Saving Ourselves

At the San Diego Zoo’s Institute for Conservation Research, researchers are maintaining cell cultures of critically endangered and extinct species preserved in vials in tanks of nitrogen. One of them is the black-faced honeycreeper from Maui, believed to have gone extinct in the early 2000s.

sixth-extinction-bird.jpg

In its Frozen Zoo, the institute has saved cell lines of about a thousand species, most of which still exist. The Cincinnati Zoo is doing something similar, as is England’s University of Nottingham. (Shortform note: According to Wikipedia, genetic material can be stored in nitrogen indefinitely for possible use for artificial insemination, in vitro fertilization, embryo transfer, and cloning.)

While humans as a species have been destructive and shortsighted, researchers and conservationists also have joined together in heroic efforts, like the Frozen Zoo and the El Valle Amphibian Conservation Center in Panama, to save threatened species. Examples include:

  • The British Act for the Preservation of Sea Birds (to stop their wholesale slaughter by sailors and hunters in...

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Shortform Exercise: Your Representative’s Record

The U.S. Congress has voted on 257 environment-related bills this session.


Track your Congress member’s votes on environmental issues here. What is his or her overall record? How would you like to see it change?

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Table of Contents

  • 1-Page Summary
  • Introduction
  • Chapter 1: The Sixth Extinction
  • Chapter 2: Tracing the Clues to Past Extinctions
  • Chapter 3: The Great Auk—an Observable Extinction
  • Chapter 4: Evidence of an Asteroid Strike
  • Chapter 5: A New Science of Extinction
  • Exercise: Your Carbon Footprint
  • Chapter 6: Impacts—Ocean Acidification
  • Chapter 7: Impacts—The End of Coral Reefs
  • Chapter 8: Impacts—Rainforests and Biodiversity
  • Chapter 9: Impacts—Fragmentation of Habitat
  • Chapter 10: Impacts—Dispersal of Species
  • Exercise: Invasive Species
  • Chapter 11: Last of the Megafauna
  • Chapter 12: Pruning the Family Tree
  • Chapter 13: Saving Species, Saving Ourselves
  • Exercise: Your Representative’s Record