The Sixth Extinction: An Unnatural History is a 2014 nonfiction book written by Elizabeth Kolbert and published by Henry Holt and Company.

She also describes specific species extinguished by humans, as well as the ecologies surrounding prehistoric and near-present extinction events.

The writing blends explanations of her treks to remote areas with interviews of scientists, researchers, and guides, without advocating a position, in pursuit of objectivity.

Hence, the sixth mass extinction theme is applied to flora and fauna existing in diverse habitats, such as the Panamanian rainforest, the Great Barrier Reef, the Andes, Bikini Atoll, city zoos, and the author's own backyard.

After researching the current mainstream view of the relevant peer-reviewed science, Kolbert estimates flora and fauna loss by the end of the 21st century to be between 20 and 50 percent "of all living species on earth".

[2][3][4][5][6][7] Kolbert equates current, general unawareness of this issue to previous widespread disbelief of it during the centuries preceding the late 1700s; at that time, it was believed that prehistoric mass extinctions had never occurred.

However, scientific studies have shown that human behavior disrupts Earth's balanced and interconnected systems, "putting our own survival in danger."

The human species contributes to this disruption—even without intending to—because of our innate capabilities to alter the planet at this stage of our cultural evolution; for instance, we now can harness energy from beneath the Earth's surface.

They are unable to migrate ahead of current rapid ecological changes or are hampered by artificial barriers such as roadways, cityscapes, and suburban sprawl, which increase discontinuity between viable habitats throughout the world.

[9][10][11][12][13] Kolbert's decision to write this book was influenced by a 2008 Proceedings of the National Academy of Sciences paper, titled, "Are we in the midst of the sixth mass extinction?

Subsequently, Kolbert wrote an article for The New Yorker, titled (similarly to her eventual book), "The Sixth Extinction?

Kolbert states that studies by the National Zoological Park in Washington, DC, and a mycologist at the University of Maine, have identified the reason for the increased mortality of Panamanian frogs as a type of Chytrid fungus.

Kolbert explains how fossils of the American mastodon (Mammut americanum) shaped Georges Cuvier's views on catastrophism.

However, the settlers found the auks to be “very good and nourishing meat.” They also used their oily bodies for fuel and fish bait, and their feathers for stuffing mattresses.

Kolbert uses the great auk as a symbol of how human overexploitation of resources is another important mechanism of extinction.

Kolbert explains that, even though ammonites were 'fit' for their current environment, a single moment can completely change which traits are advantageous and which are lethal.

We have dammed most of the major rivers of the world, increased levels of nitrogen higher than can be fixed naturally by terrestrial ecosystems, used more than half of the world's readily accessible freshwater run-off, removed more than one-third of the primary producers of the oceans' coastal waters and changed the composition of the atmosphere by deforestation and fossil fuel combustion.

Kolbert uses the drastic decline in life forms around the Castello Aragonese as a warning sign of what is to come if we continue to increase carbon dioxide levels in the atmosphere.

[26] This extra energy that will eventually be expended on calcification is currently vital to corals, as they use it to recover from being eaten away by marine species and battered by waves.

[27] Kolbert points out that the poles are not the only places affected by global warming, and that other areas have much higher latitudinal diversity gradients.

[30][31][32] Kolbert points out that there is an evolutionary arms race, in which each species must be equipped to defend against their potential predators and need to be more fit than their competition.

[34] Kolbert then explains that global trade and travel are creating a virtual "Pangaea", in which species of all kinds are being redistributed beyond historical geographic barriers.

A captive breeding program was widely regarded as a failure and resulted in the deaths of several rhinos, and it was decades before a single baby was born.

Some sources for the book include The Song of the Dodo by David Quammen, The Ghost With Trembling Wings by Scott Weidensaul, and reports from Edward O. Wilson, a biologist.

Also included are excerpts from interviews with a forest ecologist, atmospheric scientist Ken Caldeira, wildlife and conservation experts, a modern-day geologist, and fungus researchers in New England and New York State.

[40] On the May/June 2014 issue of Bookmarks, the book received a (4.0 out of 5) with the critical summary saying, "By mixing reporting trips around the world with interviews with scientists, Kolbert offers a compelling take on how we've altered our environment--from hunting to extinction large mammals to introducing invasive species into delicate ecosystems, damming major rivers, and more".