Edited sentence: As a result of drought in the Sahel, the southern boundary moved south  overall during that period..    Deforestation has also caused the Sahara to advance south in recent years, as trees and bushes continue to be used as fuel source.But in southern Sahara, the drying trend was soon counteracted by the monsoon, which brought rain further north than it does today.By around 3400 BC, the monsoon retreated south to approximately where it is today,    leading to the gradual desertification of the Sahara.During the last glacial period, the Sahara was even bigger than it is today, extending south beyond its current boundaries.Half of the Sahara receives less than  of rain per year, and the rest receives up to  per year.Once the ice sheets were gone, northern Sahara dried out.The Sahara has one of the harshest climates in the world.The Sahara is now as dry as it was about 13,000 years ago.The climate of the Sahara has undergone enormous variation between wet and dry over the last few hundred thousand years.The end of the glacial period brought more rain to the Sahara, from about 8000 BC to 6000 BC, perhaps due to low pressure areas over the collapsing ice sheets to the north.The hot air rises and pulls in cool, wet air from the ocean, which causes rain.The monsoon is due to heating of air over the land during summer.The prevailing north-easterly wind often causes the sand to form sand storms and dust devils.The rainfall happens very rarely, but when it does it is usually torrential when it occurs after long dry periods, which can last for years.The southern boundary of the Sahara, as measured by rainfall, was observed to both advance and retreat between 1980 and 1990.These conditions are responsible for what has been called the Sahara pump theory.This was caused by a stronger tilt in Earth's axis of orbit than today, and perihelion occurred at the end of July.Thus, though it seems counterintuitive, the Sahara was wetter when it received more solar insolation in the summer