A Short History of Nearly Everything: Chapter 27 Summary & Analysis

In 1815, the largest volcanic explosion in 10,000 years takes place when an Indonesian mountain named Tambora explodes with the force of 60,000 atom bombs. The ash it expels diffuses worldwide, cooling the atmosphere slightly. Earth’s climate is cooler in the 19th century than it is now, so it's hard for scientists of the time to imagine that their climate is much warmer than what Earth has witnessed in its past. In the 1700s, James Hutton hypothesizes that Earth might have had glaciers covering it in the past—but since Hutton is such an unclear writer, most of his ideas are overlooked by the scientific community.

Although local folk knowledge already holds that unusual rock formations were transported from distant locations by glaciers, scientists are still slow to accept this hypothesis. A botanist named Karl Schimpher coins the term “ice age” in 1837, though Swiss naturalist Loius Agassiz formalizes the theory. Unfortunately, Agassiz’s ideas are poorly-received. They only start to catch on after scientists make an expedition to Greenland for the first time and learn that parts of the world are covered in ice sheets. Curiously, James Croll—a university janitor who taught himself science from books in the university library—writes the most famous paper on the topic in 1864. He speculates that changes in Earth’s orbit and its ice ages are related.

Croll calculates that Earth’s most recent ice age was 80,000 years ago, but geological evidence suggests a more recent ice age since then. In 1900, a Serbian mechanical engineer named Milutin Milankovitch realizes that Croll forgot to factor a variable into his calculations: namely, Earth wobbling on its axis. It takes Milankovitch years to do the calculations, but he eventually publishes a book in 1930, correctly arguing that Earth’s wobbling on its axis accounts for its ice ages. Then, a meteorologist named Wladimir Köppen realizes that when summers are too cool to melt ice quickly, sunlight is bounced back by ice’s reflective sheen, exacerbating the cooling effect.

In the 1970s, scientists calculate that if North America, Eurasia, and Greenland were just 300 miles north of where they are, they’d experience permanent ice ages, meaning that “we’re very lucky.” Bryson says that technically, we’re actually in an ice age right now. 20,000 years ago, 30 percent of Earth’s land surface was covered with ice. Now, 10 percent still is. Earth’s longer history, however, shows that the planet is usually hotter and doesn’t have permanent ice at all. In geological terms, the current ice age started 40 million years ago. Scientists think that the formation of the Himalayas is responsible, as this altered wind patterns. Consequently, Africa dried up, causing apes to climb down from trees and start living on the ground.  

Earlier ice ages tended to be more dramatic: a “super ice age” happened about 2.2 billion years ago that might have caused the entire surface of the planet to freeze solid. Icy planets tend to stay frozen over, and it would have been that way for Earth had volcanic activity not disrupted the ice age—and as a result, triggered the Cambrian explosion. The thawing triggers violent weather that might be the closest Earth has come to wiping out life altogether. Ice cores from Greenland also show that Earth’s historical climate is much more volatile than scientists think, though they have no idea why. Bryson says that there’s “no reason to suppose that this stretch of climatic stability should last much longer.”

Scientists even predict that, paradoxically, global warming might trigger more cloud cover, which would cause snowfall to linger and trigger another deep freeze—though it’s hard to do more than speculate. Bryson says “much is simply beyond us.” On the other hand, if humans end up melting ice through climate change, sea levels would rise by 200 feet, flooding every coastal city in the world. Curiously, we don’t know which way the future will go. Bryson concludes that “Only one thing is certain: we live on a knife edge.”