• By Elizabeth C, December 14, 2009 @ 6:55 am From deforestation and desertfication. Erosion of either man made material or just weather probably broke down the area, and caused the head to swell and expand the desert. Hope this helps

• By Michel Verheughe, December 17, 2009 @ 3:44 am When the Sahara was a forest, the axis of the earth was different and so were the seasons. Here is why the Sahara is a desert today: Air rises over the equator because it is warm. Air sinks over the poles because it is cold. When air rises, it creates a low pressure and the rising moisture condenses in clouds that fall back as rain. This is why we have the equatorial rain forests! Over the poles, the air sinks, moisture dissolves as the air warms up and the sky is clear. The two poles are desert regions with very little precipitations. If the earth wasn’t spinning and the Coriolis effect not working, the air would then move from equator to the poles and back. But the Coriolis effect makes any air mass to turn to the right hand side in the northern hemisphere and the opposite in the southern. Because of that, the air rising from the equator never gets a chance to reach the pole and falls back at roughly latitudes 30 north and south, creating high pressure zones with dry climate. Between those two high pressure zones, at roughly latitude 60 north and south, a zone of low pressure exist along the front between cold and dry polar air and, mild and moist temperate air. It works like this: Latitude 0 (equator): low pressure Latitude 30: high pressure Latitude 60: low pressure Latitude 90 (Pole): high pressure. Now, go and look at a world map and notice how, everywhere around the world, places that are at the same latitude as the Sahara are deserts. Of course, deforestation has increased the size of the sahara to some extend. But it is its latitude that makes it a desert.

• By answer boy, December 19, 2009 @ 7:51 pm Climate history An oasis in the Ahaggar MountainsThe climate of the Sahara has undergone enormous variation between wet and dry over the last few hundred thousand years.[15] During the last glacial period, the Sahara was bigger than it is today, extending south beyond its current boundaries.[16] The end of the glacial period brought better times to the Sahara, from about 8000 BC to 6000 BC, perhaps due to low pressure areas over the collapsing ice sheets to the north.[17] Once the ice sheets were gone, the northern part of the Sahara dried out. However, not long after the end of the ice sheets, the monsoon, which currently brings rain only as far as the Sahel, came further north and counteracted the drying trend in the southern Sahara. The monsoon in Africa (and elsewhere) is due to heating during the summer. Air over land becomes warmer and rises, pulling in cool wet air from the ocean, which causes rain. Paradoxically, the Sahara was wetter when it received more solar insolation in the summer. Changes in solar insolation are caused by changes in the Earth’s orbital parameters (9,000 years ago the Earth’s axis had a stronger tilt than it does presently, and perihelion occurred at the end of July).[18] By around 3400 BC, the monsoon retreated south to approximately where it is today,[19] leading to the gradual rather than abrupt desertification of the Sahara.[20] The Sahara is currently as dry as it was about 13,000 years ago.[15] These conditions are responsible for what has been called the Sahara Pump Theory. The Sahara has one of the harshest climates in the world. The prevailing north-easterly wind often causes the sand to form sand storms and dust devils.[21] Precipitation, while rare, is not unknown. Half of the Sahara receives less than 2 centimetres (0.79 in) of rain a year, with the rest receiving up to 10 cm (3.9 in) a year.[22] 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. As a result of drought in the Sahel, the southern boundary showed an overall southward movement of 130 kilometres (81 mi) during that period. [23]. Deforestation has also caused the Sahara to advance southward in recent years, as trees and bushes continue to be used a fuel source.

• By mrsg, December 22, 2009 @ 8:26 pm The water ran out!

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