sophisticated tribal organization. Then, from about fifty thousand to about ten thousand years ago, Upper Paleolithic peoples experienced a veritable cultural explosion. The artifacts they left show technologies more advanced in almost every way than those of their predecessors, more varied, more elaborate, more refined, and more beautiful, with some tools even seemingly designed to be purely aesthetic in function—which is to say, works of art.33
About fifteen thousand years before Hurricane Katrina, the planet started to warm up again, entering another interglacial period, with the most intense and rapid warming happening around 11,000 BCE. Humans began developing villages, basic animal husbandry, and more deliberate systems of gathering. A brief, localized cold snap called the Younger Dryas, caused by glacial meltwater spilling into the Atlantic Ocean and shutting off the Gulf Stream, brought frigid temperatures to Europe and drought to southwest Asia for a thousand years.34 According to archaeologist Brian Fagan, it was almost certainly this drought that drove the people in a small village on the Euphrates, who had until then depended for their sustenance on hunting desert gazelles, harvesting nuts, and gathering wild grasses, to begin deliberately cultivating rye, lentils, and a grain called einkorn.35 By 9000 BCE, after the Younger Dryas had ended and the Gulf Stream had switched back on, the agricultural revolution had begun. Neolithic humans hunted, herded, gathered, and farmed from Europe to South America, thriving in the warm and mild climate of what is now called the Holocene.
A few thousand years later, the Laurentide ice sheet in northern Canada collapsed, causing a rapid rise in sea levels, and, as had happened with the Younger Dryas, shutting off the Atlantic Gulf Stream. Cold, dry conditions descended on Europe and southwest Asia. This cold drought lasted four centuries before the Gulf Stream switched on again. In the marshy confluence of the Tigris and Euphrates rivers, farmers began constructing irrigation canals to control flooding and increase crop yields. Villages grew into towns, and as the people living in these new towns struggled to deal with the complex, difficult group effort required to construct and maintain levee systems, keep records of floods and farm yields, and bring in the harvest, they began to develop refined divisions of labor, hierarchical political structures, sophisticated religions, and writing. Temples and marketplaces were built, traders carried goods from one town to another, and priests accumulated power as they hoarded knowledge of weather patterns and seed growth. The settlements of Ur, Eridu, and Uruk grew from clusters of villages to immense cities of tens of thousands. Through a complex interplay of droughts, population growth, and the increasing centralization of agriculture, human society based in and around cities—what we call “civilization”—emerged in the fertile crescent of land arcing from present-day Lebanon through Syria and Iraq to the Persian Gulf. From approximately 3100 to approximately 2200 BCE, Sumerian and then Akkadian kings ruled a vibrant collective form of life that stretched across Mesopotamia. But when a three-hundred-year-long drought hit the region, that vibrant empire fell apart.36 Mighty Uruk stood desolated.
From ancient Uruk to modern-day Iraq we span about five thousand years, including nearly all of recorded human history—the Greeks, the Romans, the Tang dynasty, the Mongolian khanate, World War II, the invention of the cellphone, and all seven seasons of Mad Men. If human existence on Earth were a day, our approximately five millennia of recorded history would take up the last half hour before midnight. Throughout 99.9 percent of humanity’s two hundred thousand years on Earth, the average planetary temperature never rose above 61 degrees Fahrenheit and carbon dioxide concentrations never went above 300 parts per million (ppm). Nearly all of our energy came from photosynthetic processes: most of our fuel was plant food, for ourselves directly, through the animals we used and preyed on, or through biomass like wood, with some use of water and air power through technologies like mills and sails, and negligible use of coal and oil.
Then, in 1781, James Watt invented the continuous-rotation steam engine. Suddenly power was portable, independent of living beings or natural forces, and able to run continuously. The steam turbine offered a vast improvement in energy production over wind, water, and animal labor, but it needed dense, hot-burning fuel for maximum output. Luckily for Watt, there happened to be loads of the stuff all over England: fossilized carbon in the form of coal.
Industrial coal changed everything. For the last two hundred years, just about one tenth of one percent of human existence, most of our energy has come not from direct photosynthesis but from stored carbon energy in fossil fuels. Switching from a photosynthetic-based energy economy to a carbon-based energy economy increased human wealth beyond what anyone could have possibly imagined, raising the overall standard of living across the world through such technologies as diesel-fueled tractors, Haber-process nitrogen-fixed fertilizer, Bessemer steel, railroads, steamships, airplanes, electric power plants, plastics, the internal combustion engine, and the automobile. It also began a massive transformation of the physical systems regulating life on Earth. By transferring millions of tons of carbon from the ground into the air, we have wrought profound changes in the Earth’s climate, biosphere, and geology. Average atmospheric CO2 levels have rocketed from 290 ppm to over 400 ppm, a level the planet hasn’t seen in more than two million years. At the same time, methane (CH4) levels have increased from 770 parts per billion to more than 1,800 parts per billion, the highest concentration of atmospheric methane in at least eight hundred thousand years.37 These changes have disrupted the climate patterns regulated by the Earth’s orbit around the Sun and will continue to disrupt them for thousands of years.
Our planet can sustain life because when energy from the Sun strikes the Earth, some of it is trapped in the atmosphere as heat. There are several greenhouse gases that help this happen, including carbon dioxide, methane, water vapor, and nitrous oxide. These gases are integral to the planet’s complex geophysical homeostasis. That homeostasis has shifted, over millions of years, back and forth between “greenhouse” and “icehouse” states. During the last major greenhouse state, in the Eocene, the planet was ice-free, tropical from pole to pole, up to 20 degrees Fahrenheit warmer than it is now, and had CO2 concentrations up to ten times those of today. The oceans were 300 feet higher. Large reptiles and dwarf mammals ranged through lush forests: a forty-foot-long snake that weighed more than a ton, a tiny horse the size of a dog, sleek feline predators, and lemur-like primates. Crocodiles and palm trees thrived along the Arctic Circle. That was fifty million years ago. Since then, the Earth has cooled, and it has been in an “icehouse” state for more than two and a half million years. We have very likely brought that state to a premature end.
For the first sixty thousand years of Homo sapiens’ life on Earth, global temperatures 5 to 9 degrees Fahrenheit colder meant an ice sheet covering what is now Chicago and New York. That same amount—5 to 9 degrees Fahrenheit—is about as much as the planet is expected to warm up over the next few generations, and it doesn’t sound like a lot. After all, the temperature changes more than that every day, and frequently much more, depending on season and locale. But when you’re talking about planetary averages, those differences are enormous. A future 5 to 9 degrees Fahrenheit warmer will mean the Arctic Ocean will be ice free in the summer. Mountain glaciers will all but disappear, and with them, skiing, snowpack and a great many freshwater streams. Freak weather will play havoc with agricultural systems, plant and animal habitats, and human infrastructure. We’ll have to contend with more extreme temperature fluctuations, more humidity, more and more intense rainfall, stronger and longer-lasting storms, severe droughts, and unpredictable changes in formerly reliable climate dynamics, such as the jet streams, the El Niño southern oscillation, and the Gulf Stream. Coral reefs will go extinct, along with countless other species caught in ecosystems changing too swiftly for them to adapt to or migrate out of.
More important, warming of 5 to 9 degrees Fahrenheit will eventually lead to sea levels 90 to 200 feet higher. No one is sure how quickly that will happen: If the ice melts slowly, we might only see a few feet of sea level rise by 2100. If the ice melts quickly and the Greenland ice sheet collapses, we could witness seas 20 to 30 feet higher within decades. Ice sheets are already melting faster than models have predicted, there is evidence that they have broken apart very quickly in the past, and ice melt is a feedback phenomenon, meaning that the more ice that melts, the faster the remaining
