Many of the world's largest and most famous deserts are located in two belts between 15° and 30° North and south latitude. Included in this group of deserts are the sahara, the world's largest desert, and the Libyan Desert of North Africa. Other members of this group include the Syrian Desert, Rub' al-Khali (Empty Quarter), and Great Sandy Desert of Arabia; the Dasht-e-Kavir, Lut, and Sind of southwest Asia; the Thar Desert of Pakistan; and the Mojave and Sonoran Deserts of the united States. In the Southern Hemisphere deserts that fall into this group include the Kalahari Desert of Africa and the Great Sandy Desert of Australia. The formation of the Atacama Desert in South America, the world's driest place, can be partly attributed to its location between these latitudes.
The location of these deserts is controlled by a large-scale atmospheric circulation pattern driven by energy from the Sun. The Sun heats equatorial regions more than high-latitude areas, which causes large-scale atmospheric upwelling near the equator. As this air rises, it becomes less dense and can hold less moisture, a condition leading to the formation of large thunderstorms in equatorial areas. This drier air then moves away from the equator at high altitudes, cooling and drying more as it moves, until it eventually forms two circumglobal downwelling belts between 15-30°N and S latitude. This cold downwelling air is dry and can hold much more water than it has brought with it on its circuit from the equator. These belts of circulating air, known as Hadley cells, are responsible for the formation of many of the world's largest, driest deserts. As this air completes its circuit back to the equator, it forms dry winds that heat up as they move toward the equa tor. The dry winds dissipate existing cloud cover and allow more sunlight to reach the surface, which consequently warms even more.
Deserts formed by global circulation patterns are particularly sensitive to changes in global climate; seemingly small changes in the global circulation can lead to catastrophic expansion or contraction of some of the world's largest deserts. For instance, the sub-Saharan Sahel has experienced several episodes of expansion and contraction, displacing or killing millions of people in this vicious cycle. When deserts expand, croplands dry up and livestock and humans cannot find enough water to survive. Desert expansion is the underlying cause of some of the world's most severe famines.
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