The Atacama Desert is a desert plateau in South America covering a 1,600 km strip of land on the Pacific coast, west of the Andes Mountains. The Atacama Desert is the driest nonpolar desert in the world, as well as the only true desert to receive less precipitation than the polar deserts and the largest fog desert in the world. Both regions have been used as experimentation sites on Earth for Mars expedition simulations. The Atacama Desert occupies 105,000 km2 (41,000 sq mi), or 128,000 km2 (49,000 sq mi) if the barren lower slopes of the Andes are included. Most of the desert is composed of stony terrain, salt lakes (salares), sand, and felsic lava that flows towards the Andes.
The desert owes its extreme aridity to a constant temperature inversion due to the cool north-flowing Humboldt ocean current and to the presence of the strong Pacific anticyclone. The most arid region of the Atacama Desert is situated between two mountain chains (the Andes and the Chilean Coast Range) of sufficient height to prevent moisture advection from either the Pacific or the Atlantic Ocean, a two-sided rain shadow.
Despite modern views of Atacama Desert as fully devoid of vegetation, in pre-Columbian and Colonial times a large flatland area known as Pampa del Tamarugal was a woodland but demand for firewood associated with silver and saltpeter mining in the 18th and 19th centuries resulted in widespread deforestation.
In spite of the geographic and climatic conditions of the desert, a rich variety of flora has evolved there. Over 500 species have been gathered within the border of this desert. These species are characterized by their extraordinary ability to adapt to this extreme environment. Most common species are the herbs and flowers such as thyme, llareta, and saltgrass (Distichlis spicata), and where humidity is sufficient, trees such as the chañar (Geoffroea decorticans), the pimiento tree, and the leafy algarrobo (Prosopis chilensis).
The llareta is one of the highest-growing wood species in the world. It is found at altitudes between 3,000 and 5,000 m (9,800 and 16,400 ft). Its dense form is similar to a pillow some 3 to 4 m (9.8 to 13.1 ft) thick. It concentrates and retains the heat from the day to cope with low evening temperatures. The growth rate of the llareta has been recently estimated at about 1.5 cm/year (0.59 in/year), making many llaretas over 3,000 years old. It produces a much-prized resin, which the mining industry once harvested indiscriminately as fuel, making this plant endangered.
The desert is also home to cacti, succulents, and other plants that thrive in a dry climate. Cactus species here include the candelabro (Browningia candelaris) and cardon (Echinopsis atacamensis), which can reach a height of 7 m (23 ft) and a diameter of 70 cm (28 in).
The Atacama Desert flowering (Spanish: desierto florido) can be seen from September to November in years with sufficient precipitation, as happened in 2015.
The climate of the Atacama Desert limits the number of animals living permanently in this extreme ecosystem. Some parts of the desert are so arid, no plant or animal life can survive. Outside of these extreme areas, sand-colored grasshoppers blend with pebbles on the desert floor, and beetles and their larvae provide a valuable food source in the lomas (hills). Desert wasps and butterflies can be found during the warm and humid season, especially on the lomas. Red scorpions also live in the desert.
A unique environment is provided by some lomas, where the fog from the ocean provides enough moisture for seasonal plants and a few animal species. Surprisingly few reptile species inhabit the desert and even fewer amphibian species. Chaunus atacamensis, the Vallenar toad or Atacama toad, lives on the lomas, where it lays eggs in permanent ponds or streams. Iguanians and lava lizards inhabit parts of the desert, while salt flat lizards, Liolaemus, live in the dry areas bordering the ocean. One species, Liolaemus fabiani, is endemic to the Salar de Atacama, the Atacama salt flat.
Birds are one of the most diverse animal groups in the Atacama. Humboldt penguins live year-round along the coast, nesting in desert cliffs overlooking the ocean. Inland, high-altitude salt flats are inhabited by Andean flamingos, while Chilean flamingos can be seen along the coast. Other birds (including species of hummingbirds and rufous-collared sparrow) visit the lomas seasonally to feed on insects, nectar, seeds, and flowers. The lomas help sustain several threatened species, such as the endangered Chilean woodstar.
Because of the desert's extreme aridity, only a few specially adapted mammal species live in the Atacama, such as Darwin's leaf-eared mouse. The less arid parts of the desert are inhabited by the South American gray fox and the viscacha (a relative of the chinchilla). Larger animals, such as guanacos and vicuñas, graze in areas where grass grows, mainly because it is seasonally irrigated by melted snow. Vicuñas need to remain near a steady water supply, while guanacos can roam into more arid areas and survive longer without fresh water. South American fur seals and South American sea lions often gather along the coast.