| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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HORIZONS Stone Age Climate Change
Humans are old pros at changing global climate. With our factories, power plants, and automobiles, we’ve been turning up planetary temperatures since the Industrial Revolution. But modern warming might be only the latest twist in an age-old story. According to a provocative new theory, humans first began tampering with global temperatures when stone tools were still the rage. Armed with crude axes, digging sticks, and plows, humans began clearing vast swathes of forest in Asia and Europe and transforming them into irrigated agricultural plots over the next 8,000 years. Logging and farming may have released enough greenhouse gases to have turned back an incipient ice age, reported climate scientist William Ruddiman of the University of Virginia at a December meeting of the American Geophysical Union in San Francisco. “A continued explanation of what humans were doing during this time can explain the increase in greenhouse gases all the way to 1700. Natural explanations do not work,” Ruddiman says. To tell this story, Ruddiman has stitched together clues from all over the academic map. His data points include the remains of Neolithic rice terraces; the conquest of the New World; even the writings of Plato. Together, they place humanity’s sooty fingerprints at the scene of a global climate shift born alongside civilization. The incriminating evidence has been stored thousands of meters beneath the perennial snows of Antarctica. By thawing air bubbles frozen in the ice, scientists have been able to measure levels of two greenhouse gases, methane and carbon dioxide, back to prehistoric times. For nearly 400,000 years, concentrations of these gases were determined by the intensity of sunlight reaching Earth, as the planet’s not-quite-circular orbit brought it closer to the sun and then farther away again. During periods of strong solar radiance, summer heat forced large masses of equatorial surface air upward, allowing cooler, moisture-laden air to rush in. These tropical monsoon rains filled marshes and swamps, accelerating the decay of vegetation and the activity of methane-producing bacteria. Then, says Ruddiman, “something goes wrong. Methane should’ve followed the solar radiation down 5,000 years ago, but it went up instead. That happened when people started to irrigate rice.” The domestication of wild strains of rice began almost 7,500 years ago, in eastern Asia. To maintain the productivity of the thin tropical soils, people flooded their lowland fields to bring in more nutrients. As land grew scarce, farmers terraced hillsides, further expanding these artificial wetlands. Every year, in addition to rice, these early farmers produced a second crop—methane. About 5,000 years ago, when a drying climate should have reduced wetlands and methane production, thousands of acres of rice paddies from China to India were more than able to compensate for the natural decrease. These archeological records are backed up by historical accounts. In the third century BCE, Plato witnessed the retreat of forests up Grecian hillsides, and William the Conqueror’s Domesday Survey chronicled an England that was about 80 percent deforested by 1086. The ancient carbon dioxide cycle seems to derail at the same time. At 10,500 years ago, CO2 levels began dropping in a cycle that has been repeating for thousands of years. Then suddenly, 2,000 years later and far ahead of schedule, they began climbing back up again. The timing of this change, Ruddiman says, is no coincidence. People seeking land to grow their crops, he says, “were clearing forests at enormous rates.” New farmers burned or clearcut forests from Asia to Europe, leaving the atmosphere awash in carbon. Native tree stands were leveled in northern China as early as 10,000 years ago. By 6,000 bce, pollen records from Mesopotamia to the Hungarian plain show a shift from native grasses to barley, wheat, lentils, and peas. An increase in charcoal in older soil layers suggests the use of slash and burn agriculture from the Americas to Eurasia. By 5,700 years ago, the plow had spread farming across Europe. By the time of Christ, much of the original forests of the Amazon, North Africa, India, and sub-Saharan Africa had been replaced by crop furrows and grain fields, meadows and pastures. Where climate and soil did not permit farming, people tended ever larger herds of cattle, goats, and sheep. To encourage forage growth, shepherds burned lowland scrub and steep forests. The digestive bacteria within these grazers added even more methane to the atmosphere. “If there was no human-caused warming, there would have been an incipient glaciation,” Ruddiman says. “Nature was no longer in control.” This growing human influence on climate seems to have extended even to the ends of the earth. The advance and retreat of the polar ice sheets closely followed the amount of solar radiation reaching northern latitudes for nearly three million years. About 5,000 years ago, solar radiation was on the decline—normally a trigger for the ice sheets to creep south—yet the borders of the glaciers never budged. Ruddiman estimates that early human activities released enough carbon and methane into the atmosphere to heat the entire planet by about 0.8 C. In the Arctic, reflective ice and snow pushed temperatures upward by about 2 C—more than enough to halt the ice sheets advancing on Canada’s Baffin Island in their tracks. But the theory didn’t seem to accommodate every trend in the ice core records. After thousands of years of increases, the frozen bubbles indicate that CO2 levels tumbled at least three times since 200 CE. The downswings mystified Ruddiman. What could have pulled thousands of tons of CO2 out of the atmosphere within a century, then returned it all soon afterward? His explanation is more than a little controversial. If human behaviors were behind the carbon dioxide rise, Ruddiman reasoned, then something must have decimated human populations to allow forests to regrow and sequester more carbon. “The only thing capable of killing that many people so quickly was disease,” Ruddiman says. “People abandoned their farms by the millions” after pandemics of plague and smallpox. Indeed, plagues chronicled by the likes of Galen, Daniel Defoe, and the Spanish conquistadors line up remarkably well with the carbon dioxide wobbles. Roman records show that outbreaks of bubonic plague began afflicting the empire in the year 290 CE, culminating in a pandemic that took the lives of a quarter of the population of Europe and North Africa. At the same time, a disease thought to be smallpox swept through China between 265 and 313, killing an estimated 10 million people. Then, between 1347 and 1377, the largest plague pandemic in history killed an estimated 40 million people throughout Eurasia. The third and most severe CO2 drop appears between 1500 and 1750—the years of the conquest of the New World by Europeans. Disease preceded the armies of Pizarro and Cortez, burning through populations almost overnight. Between 60 and 90 percent of Native Americans died of smallpox and other unfamiliar scourges, leaving forests to regrow and store carbon until human populations recovered. The possibility of Neolithic farmers turning back an ice age is far from fantasy, says paleoecology expert I.G. Simmons of Britain's Durham University. “I’m convinced that there were big ecosystem changes due to hunter-gatherers. Hunter-gatherers burned about the whole planet when they had the chance,” he says. However, Simmons and other scientists are cautious about accepting Ruddiman’s theory without more data. To prove that agriculture, deforestation, and global warming are truly linked, “You have to tally things up and say how much carbon was in that forest and calculate what the CO2 releases might be. You have to examine historical records in China, estimate how many hectares of rice would have been planted, and calculate methane emissions,” says Eric Davidson of The Woods Hole Research Center in Massachusetts, who studies the effects of land use on carbon outputs. “He’s got more work to do.” Kathleen M. Wong is Senior Editor of California Wild. |
