A Devastating Desert Expansion
Starting around 6,000 years ago, the Sahara Desert started an abrupt and devastating expansion. In a geological blink of an eye, the boundary between hard-core desert and grassland shifted by about 500 miles (800 km), which increased the size of the desert by nearly 50 percent. Moreover, the summer monsoons that had sustained the grass-harvesters with the quartz-sandstone tools failed. Areas of the desert that used to get 8–10 inches (200–254 mm) of rain in a year dried out so that they received only 3–4 inches (76–101 mm). Reliable rainfall in several different seasons gave way to a new climate in which sometimes no rain fell at all for years at a time in some areas. Suddenly, grasslands that once harbored elephants, giraffes, and other large animals could shelter only camels. Areas that once blossomed after summer rains withered and subsided into sandy desert silence.
Climate researchers have worked hard to understand the abrupt change, especially since the Sahara Desert has been expanding again in recent decades. Several studies suggest that the change some 6,000 years ago was caused by a small shift in the Earth's orbit and axis of rotation, compounded by a feedback effect caused by the response of plants and dust storms.
The root cause was connected to a change in the amount of solar radiation reaching Earth. The Earth's orbit around the sun varies slightly from year to year. As a result, during certain periods the Earth does not get quite as close to the sun at the near point of its annual orbit. The timing of that close approach also shifts, causing a realignment of the seasons. Moreover, the Earth wobbles just slightly as it spins on its axis, like a top. Some 9,000 years ago, Earth's tilt was 24.14 degrees, compared to the current 23.45 degrees. In addition, the Earth's closest approach to the sun came some six months sooner than it does now. These small changes in Earth's spin and orbit gradually destabilized the planet's climate. For instance, the shift in seasons resulted in the Northern Hemisphere receiving more summer sunlight, which increased the power of the African and Indian summer monsoons. That shift in the timing and strength of the monsoons in turn dramatically affected rainfall patterns in the Sahara.
Ironically, the abrupt expansion of the Sahara Desert may have played a key role in spurring the rise of Western civilization. The great irrigationbased civilizations of the Middle East that arose along the Nile, Tigris, and Euphrates rivers all coincided with this period of climate change and desert expansion. Perhaps the climate shift forced the nomadic desert dwellers that had lived for tens of thousands of years off the bounty of the grasslands and the summer monsoons into the fertile river valleys. That concentration of population and the need to develop irrigation systems to compensate for the fickle rainfall may have given rise to the increasingly complex civilizations of the Middle East. The descendents of Sahara grasslands dwellers living along these river valleys went on to build the pyramids, invent writing, and lay the groundwork for many of the world's religions.