An analysis of fine sediments at the bottom of an African lake has challenged the idea that the once-tropical-savanna-covered Sahara rapidly dried into what is now the largest desert on Earth. A report in tomorrow's issue of Science argues that the shift was gradual, taking place over 3000 years.
Summer monsoon rains that began some 12,500 years ago turned the Sahara from an uninhabitable desert to a green and humid paradise. But 7000 years ago, things changed back, when reduced solar radiation weakened the African monsoons. Just how fast the Sahara dried out, a process known as aridification, has remained a matter of debate. Modeling efforts suggest that the Sahara's aridification occurred over a span of a few hundred years, but they are based on limited data from Atlantic dust deposits.
Stefan Kröpelin, a geologist from the University of Cologne in Germany, wanted more direct evidence. He and colleagues hoped to find complete and detailed lake sediments of dust and vegetation remains from a site within the Sahara, but such sites are rare as most of the Sahara's lakes have dried over the past 7000 years. So Kröpelin and his team combed through war-torn areas of the desert until they eventually came upon a body of water that had endured: Lake Yoa in northern Chad.
From a small rubber boat platform, the scientists dug 9 meters into the lake's sediment and analyzed the record of animal and plant life, dust deposition, and the lake's saltiness over thousands of years. The data revealed a gradual change in the ecosystem that took place from 6000 to 2700 years ago, indicating that the Sahara didn't dry out as fast as many believe. Kröpelin says that one "couldn't dream of better samples," and he thinks that there are no other Saharan lakes with such a well-preserved and continuous record. If anyone identifies another one with Google Earth, they should let him know, he jokes.
Even though the Sahara appears to have dried out slowly, it fits the usual definitions of "abrupt climate change," meaning it was still difficult for animals, plants, and even people to adapt, says Richard Alley, a geoscientist at Pennsylvania State University in State College. "The questions that many people ask--'What will happen where I live to the water and the plants in my lake?--require additional advances before they can be answered, and the new data point to the challenge ... of providing those answers."