April 6, 2013
Sahara’s Lush Greens Turned To Desert 5,000 Years Ago
Lee Rannals for redOrbit.com - Your Universe Online
Researchers say that there was an abrupt and widespread climate change that took place in the Sahara Desert 5,000 years ago.
The Sahara's "green" era lasted from 11,000 to 5,000 years ago, but came to an abrupt ending when the region dried back into a desert in the span of one to two centuries. The researchers say this abrupt change occurred simultaneously across North Africa.
Researchers from several universities traced the region's wet and dry episodes over the past 30,000 years by analyzing sediment samples off the coast of Africa. These sediments are composed 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.
The team wrote in the journal Earth and Planetary Science Letters that they found that the Sahara emitted five times less dust during the African Humid Period than the region does today.
“Our results point to surprisingly large changes in how much dust is coming out of Africa,” said David McGee, an assistant professor in MIT´s Department of Earth, Atmospheric and Planetary Sciences. “This gives us a baseline for looking further back in time, to interpret how big past climate swings were. This [period] was the most recent climate swing in Africa. What was it like before?”
McGee analyzed sediment samples collected in 2007 by researchers from Columbia and Woods Hole Oceanographic Institution. The team sampled from sites off the northwest coast of Africa for a distance of more than 550 miles, collecting core samples at each site.
According to McGee, their core samples represent about 30,000 years of sediments deposited, layer by layer, in the ocean. Researchers used a combination of techniques to determine how fast sediments accumulated over time, then subtracted out the accumulation of marine sediments and biological remnants.
The study is the first in which researchers have combined two different techniques, helping to produce very precise measurements of dust emission through tens of thousands of years. Ultimately, the team found that during some dry periods North Africa emitted more than twice the dust generated today. They also found that the African Humid Period began and ended abruptly.
McGee says the team's new measurements could give scientists a better understanding of how dust fluxes relate to climate by providing inputs for climate models.
“Dust is one of the most important aerosols for climate and biogeochemistry,” said Natalie Mahowald, a professor of earth and atmospheric sciences at Cornell University. “This study suggests very large fluctuations due to climate over the last 10,000 years, which has enormous implications for human-derived climate change.”
McGee said the next step is to test whether these new measurements may help to resolve the inability of climate models to reproduce the magnitude of wet conditions in North Africa 6,000 years ago. Models may eventually be able to replicate North Africa back then.