奇速英语

    The Sahara desert（撒哈拉沙漠）is twice as old as we thought. Science books say the Sahara is around three million years old. However, a new study from a centre for climate research says it could be seven million years old. Researchers used computers to calculate when parts of North Africa became desert. Their tests showed that global warming seven million years ago dried a lot of the land in what is today's Chad（乍得湖）. A sea called the Tethys Sea（特提斯海） started shrinking. This cut the number of summer monsoons, which helped make sand hills in Chad. The scientists say this is how the Sahara started. 

    The Sahara is one of the best-known and largest deserts. It covers about 10 per cent of Africa. It is very big in Algeria, Chad, Egypt, Sudan and other nations. It goes from the Atlantic Ocean in the west to the Red Sea in the east. Not everyone thinks the Sahara is seven million years old. A geologist in Germany says more evidence is needed to be sure. He said the new research is just based on numbers. He said: "Nothing you can find in the Sahara is older than 500,000 years old". Our knowledge of the Saharan climate starts from 10,000 years ago. That knowledge is "full of gaps". 

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