The transformation of the Sahara from a green landscape into a desert lasted for several thousand years. 5000 years ago, a change in monsoon winds and a decrease in precipitation was reflected in the landscape.
At first, the tropical plantings disappeared, then the grass, until the picture became clear that the Sahara would become the largest desert on earth. The Sahara’s ecosystem has been changing very slowly, and not just for several centuries, as scientists had assumed before.
When restoring the climatic and vegetative history of the Sahara, scientists encountered a problem: many signs of the historical development of the Sahara have long been hidden by sand. One of the few places that are still available is in northern Chad, whose sedimentary layers each year increase by an average of 1.3 mm. Scientists succeeded in taking a sample of the earth at a depth of 7 meters, and using the obtained particles of sand, they found out the climatic and vegetative history of this region.
Period from 6 tis. 4 tis. BC. was the green and fertile phase of the Sahara. In the period from 4200 BC to 3900 BC The main lake Joa Sahara ceased to receive a supply of fresh waters. Flowing rivers separated or dried up. Lake Joa became a salt lake that was supplied only by groundwater.
With the disappearance of the growing season, part of the sand in the sediments has increased. Pollen grains, which can only be collected from remote areas, contributed to the formation of savanna or desert around the lake. The constant northeast wind brought them from the Mediterranean, scientists explain. Thus, close to 2700 BC the situation that made the Sahara the largest desert on earth was formed.