Sahara may be twice as old as previously thought

Nature : Based on geological evidence, the Sahara desert in northern Africa has been thought to be between 2 million and 3 million years old. However, studies of dune deposits in northern Chad and of dust and pollen in sediments from sea floors off northern Africa now indicate that the region may have experienced extended dry spells as long as 7 million to 8 million years ago. What could have caused that aridification is the subject of a recent study published in Nature. Zhongshi Zhang of the Bjerknes Centre for Climate Research in Norway and colleagues have ruled out major mountain-forming episodes, changes in Earth’s orbit, and excessive atmospheric carbon dioxide. Instead, climate simulations indicate that the African summer monsoon, which brought moisture from the tropical Atlantic to northern Africa, was weakened by the shrinking of the ancient Tethys Sea . Critics say the theory remains speculative because too little is known about the ancient geology of the region.