Deep in the Sahara desert, ancient rock paintings depict a verdant world full of elephants, cattle, giraffe, hippos, and antelope. The people who created these images lived in North Africa when the now hyperarid Sahara was a very different place. About 11,000 years ago, the desert turned green. Grass grew on the dunes, lakes filled dry depressions, grassland animals moved in, and people followed. It was the most recent African Humid Period, or Green Sahara.
During the last Green Sahara, the region received 10 times the rain that falls in the desert now, according to a 2017 study published in Science Advances, led by Jessica Tierney, a paleoclimatologist at the University of Arizona. The study also found evidence of a 1,000-year pause in the Green Sahara conditions 8,000 years ago, during a time when people abandoned permanent settlements in the area.
Tierney studies Earth’s past climate through mud, laid down on the ocean floor over the millennia and pulled up in long columns from the depths with big shipborne drills. These mud columns stretch backward in time as they extend deeper underground, preserving chemical and biological signatures of the climate at the time the mud became mud.
Mud cores taken off the coast of West Africa indicate the Sahara’s wet period ended abruptly about 5,000 years ago. Within a few hundred years, the landscape dried drastically. As the sands took back the Sahara, people congregated along the Nile Valley, and the Egyptian civilizations arose.
Green Saharas recur because of a wobble in Earth’s axis of spin, called axial precession, combined with Earth’s less than perfectly circular orbit around the Sun. The axis of Earth’s spin tilts about 23° from perpendicular with respect to Earth’s orbit around the Sun. This tilt creates seasons, bringing summer to the Northern Hemisphere when the North Pole leans toward the Sun and winter when it angles away. But the axis also rotates slowly, like an unbalanced spinning top. This rotation takes about 26,000 years.
When precession brings Northern Hemisphere summer into alignment with the closest point of Earth’s orbit to the Sun, the extra warmth juices the African monsoon and brings water to the desert. So the Green Sahara returns about every 20,000 years, unless other climatic patterns, like large-scale glaciation, intervene.
Tierney is interested in how past climate changes, like the Green Sahara, may have motivated human migrations. Even small climatic shifts can have wide-ranging consequences for ecosystems and the people who depend on them. A run of wet years 800 years ago may have aided Genghis Khan in his conquest of Southeast Asia. Drought in Mesoamerica 1,000 years ago may have precipitated the collapse of the Mayan civilization. Humans have lived with change throughout our time on Earth, from small droughts to the advance and retreat of the great ice age glaciers. Dry and cold conditions may even have motivated the major diaspora out of Africa 60,000 years ago, Tierney suggests in a 2017 study published in Geology.
In this Centennial episode of Third Pod from the Sun, she reveals the secrets of the mud, how humans may have weathered climate swings of the past, and what the past can tell us about our warming world.
—Liza Lester (@lizalester), Contributing Writer