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Scientists [believe / belief] the Sahara desert is twice as old as we [previous / previously] thought. Science books generally say the Sahara is around three million years old. However, a new study from a centre for [primate / climate] research in Norway says it could be around seven million years old. Researchers [used / using] computers to try and calculate when [largish / large] parts of North Africa became [dessert / desert] . Their tests showed that global warming seven million years ago [dried / drying] a lot of the land in what is today the African [nation / national] of Chad. A sea called the Tethys Sea started shrinking. This made the African summer monsoons [less / fewer] frequent, which helped form sand [dunes / domes] in Chad. The scientists say this is how the Sahara first started.

The Sahara is one of the world's [best-known / best-knowing] and largest deserts. It covers about 10 per cent of the whole African [incontinent / continent] and forms large parts of Algeria, Chad, Egypt, Sudan and other nations. The [sands / sand] stretch from the Atlantic Ocean in the west [to / by] the Red Sea in the east. Not everyone [agreeing / agrees] that the Sahara is as old as the Norway research says it is. Stefan Kröpelin, a [geology / geologist] in Germany, says [real / really] geological evidence is needed to be [sure / shore] . He said the Norway research is based [on / one] numbers and not evidence, saying: "Nothing you can find in the Sahara is older than 500,000 years old". He added that our knowledge of the Saharan climate is only from 10,000 years ago and that our knowledge is "full of [gap / gaps] ".