- Today's Sahara desert has been dominated by grasslands and trees more times in the recent geological past than earlier assumed; at least three times during the last 120,000 years. Too little is still known about the Sahara's climatic history, and scientists are unsure whether the desert may be greening right now.
"Reconstructing the climate of the past is an important tool for scientists to better understand and predict future climate changes that are the result of the present-day global warming," emphasises Rik Tjallingii of the University of Kiel, Germany. "Although there is still little known about the Earth's tropical and subtropical regions, these regions are thought to play an important role in both the evolution of prehistoric man and global climate changes," he adds.
However, new North African climate reconstructions made by the northern German university reveal three "green Sahara" episodes during which the present-day Sahara Desert was almost completely covered with extensive grasslands, lakes and ponds over the course of the last 120,000 years. The findings of Dr Tjallingii, Professor Martin Claussen and their colleagues are published in the October issue of 'Nature Geoscience'.
German scientists studied a marine sediment core off the coast of Northwest Africa to find out how the vegetation cover and hydrological cycle of the Sahara and Sahel region changed. The scientists were able to reconstruct the vegetation cover of the last 120,000 years by studying changes in the ratio of wind and river-transported particles found in the core.
"We found three distinct periods with almost only river-transported particles and hardly any wind dust particles, which is remarkable because today the Sahara Desert is the world’s largest dust-bowl," says Mr Tjallingii.
The scientists explain these periods by an increase of the precipitation that resulted in a much larger vegetation cover resulting in less wind dust and stronger river activity in the Sahara region. The "green Sahara" episodes correspond with the changing direction of the earth's rotational axis that regulates the solar energy in the tropical Atlantic Ocean. Periods of maximum solar energy increased the moisture production while pushing the African monsoon further north and increasing precipitation in the Sahara.
They therefore conclude differently from several other studies, which have linked a green Sahara with periods where the global climate was hotter than presently, and some of which even indicated that global warming could lead to the foresting of the desert. Mr Tjallingii's research however does not indicate that there is such a link.
To validate their interpretations, the scientist had compared their geological reconstruction with a computer model simulation of the Sahara vegetation cover, performed by the research group of Mr Claussen. The computer model simulation had shown "three periods with an almost completely vegetated Sahara at the same time as seen in the geological record."
"This supports the interpretation of geologists and, in turn, demonstrates the value of computer model results," the scientists hold. Additionally, the computer model had indicated that "only a small increase in precipitation is sufficient to develop a vegetation cover in the Sahara."
Computer model simulations for the future suggest an expansion of the vegetation cover in the Sahara Desert if human-driven climate change leads to aggressive global warming. "However, it is difficult to conclude that the Sahara will actually become greener than it is today, as the simulations do not account for the influence of human activity in this area," the study concludes.
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