The Sixth Extinction: Chapter 7 Summary & Analysis

At the southernmost tip of the Great Barrier Reef in Australia, there’s a place called One Tree Island (which actually has many trees). On the island, there’s a small research station affiliated with the University of Sydney. In the station, teams of scientists from around the world study the chemical composition of the nearby coral reefs.

The first Europeans to see the Great Barrier Reef were Captain James Cook and his crew in 1770. Cook had never seen a coral reef before, and he didn’t understand how it was formed. More than half a century later, Charles Darwin visited a coral reef in Tahiti, and concluded that it was really a large atoll (i.e., a ring-shaped coral structure) that had become submerged in water due to rising sea levels. Scientists now know that coral reefs are “part animal, part vegetable, and part mineral.” Coral reefs contain many calcifying creatures—indeed, a coral reef itself is largely composed of a calcium-rich structure that contains many different plants, animals, and minerals. Like a human city, coral reefs grow over time, but the crucial difference is that they grow by adding new life forms to their composition, rather than annihilating them. A series of recent scientific papers have argued that, by the year 2050, the Great Barrier Reef will have eroded into a “rubble bank.”

During her time on One Tree Island, Kolbert meets a scientist named Ken Caldeira. Caldeira’s research focuses on the impact of carbon dioxide on ocean pH (i.e., acidity), but he has also studied the chemical composition of forests and the recent changes in global temperature. Caldeira takes Kolbert on a diving expedition to photograph octopi, and Kolbert feels some of the same mystification that Cook must have felt when he first saw the Great Barrier Reef more than two centuries years ago.

The first evidence that carbon dioxide could destroy coral reefs came in the late 1980s with the Biosphere Project in Arizona, which was a huge, glass structure designed to be a self-sustaining ecosystem. While the earliest version of Biosphere was a failure—the people who tried to live there got altitude sickness because the carbon dioxide levels were too high—it yielded some interesting conclusions. The high carbon dioxide levels eroded the composition of coral inside the Biosphere, challenging the then-common notion that coral reefs are immune to changes in carbon dioxide levels. In recent years, scientific studies have shown that coral reefs erode significantly when carbon dioxide levels rise in the surrounding water, meaning that, at the current rate of carbon dioxide emission, reefs are expected to “dissolve” in the next half-century.

Kolbert goes snorkeling with the scientists stationed on One Tree Island and sees beautiful sea life—sting rays, turtles, sharks, fish, and, above all, coral. The diversity of coral reefs is astounding, especially considering that tropical waters are usually low in nitrogen and phosphorus. The nitrogen and phosphorous levels mean that, theoretically, the waters surrounding Australia should be barren. This is a puzzle that Charles Darwin was the first to point out, one which scientists have yet to solve

Coral reefs have come and gone throughout the planet’s history—in the Triassic Era, for example, nearly all the world’s reefs dissolved. It’s likely that coral reefs will disappear faster than they’ve disappeared at any point in the past, since they face the combined threats of carbon dioxide emissions, overfishing, and pollution. The rising temperature of ocean water also poses a significant threat to coral reefs. Warm water threatens coral reefs by increasing the amount of algae and plankton in the ecosystem, which in turn decreases the food available for larger coral species.

During her final days on One Tree Island, which coincides with “mating season” in the natural world, Kolbert observes the mating of the birds. As she explores the coastal area, she looks up at the stars and feels a profound sense of her own smallness and insignificance. Later, she notes the irony of her feeling—even though she’d come to One Tree Island to write about the enormous influence of human beings on the environment, she herself felt utterly insignificant when contemplating the environment.

Once per year, the corals of the Great Barrier Reef participate in a “mass spawning,” an event during which corals reproduce asexually, creating millions of tiny egg-sperm bundles. One of the most common kinds of coral in the reef, Acropora millepora, produces “bundles” containing many dozens of eggs and thousands of sperm. Scientists have studied samples of Acropora millepora in order to test the impact of acidification on coral. So far, their findings have been discouraging—acidification can wipe out coral egg-sperm bundles. Late at night, however, Kolbert witnesses the spawning of coral in the Great Barrier Reef—from a human perspective, the corals seem to release a sudden stream of vivid pink bubbles.