As recently as 5,000 years ago, the Sahara—today a vast desert in northern Africa, spanning more than 3.5 million square miles—was a verdant landscape, with sprawling vegetation and numerous lakes. Ancient cave paintings in the region depict hippos in watering holes, and roving herds of elephants and giraffes—a vibrant contrast with today's barren, inhospitable terrain.
The Sahara's "green" era, known as the African Humid Period, likely lasted from 11,000 to 5,000 years ago, and is thought to have ended abruptly, with the region drying back into desert within a span of one to two centuries. Now researchers at MIT, Columbia University and elsewhere have found that this abrupt climate change occurred nearly simultaneously across North Africa. The team traced the region's wet and dry periods over the past 30,000 years by analyzing sediment samples off the coast of Africa. Such sediments are composed, in part, of dust blown from the continent over thousands of years: The more dust that accumulated in a given period, the drier the continent may have been.
From their measurements, the researchers found that the Sahara emitted five times less dust during the African Humid Period than the region does today. Their results, which suggest a far greater change in Africa's climate than previously estimated, will be published in Earth and Planetary Science Letters. David McGee, an assistant professor in MIT's Department of Earth, Atmospheric and Planetary Sciences, says the quantitative results of the study will help scientists determine the influence of dust emissions on both past and present climate change.
This study, McGee says, is the first in which researchers have combined the two techniques—endmember modeling and thorium-230 normalization—a pairing that produced very precise measurements of dust emissions through tens of thousands of years. In the end, the team found that during some dry periods North Africa emitted more than twice the dust generated today. Through their samples, the researchers found the African Humid Period began and ended very abruptly, consistent with previous findings. However, they found that 6,000 years ago, toward the end of this period, dust emissions were one-fifth today's levels, and far less dusty than previous estimates. McGee says these new measurements may give scientists a better understanding of how dust fluxes relate to climate by providing inputs for climate models. Natalie Mahowald, a professor of earth and atmospheric sciences at Cornell University, says the group's combination of techniques yielded more robust estimates of dust than previous studies. "Dust is one of the most important aerosols for climate and biogeochemistry," Mahowald says. "This study suggests very large fluctuations due to climate over the last 10,000 years, which has enormous implications for human-derived climate change.