Fossil desert sediments as much as 500 million years old have been found in many parts of the world. Nearly 18,000 years ago, sand seas in two vast belts covered almost 50 percent of the land between 30 degrees north and south; today they cover about 10 percent. As of November 2026, about one-third of Earth's land surface is desert, and a desert is a dry place that gets very little rain. Some deserts are hot, but others have cold winters or are cold all year, and the sand itself is only part of the story, since sand dunes cover only about 10 percent of the world's deserts.
Scientists study the Atacama Desert as a Mars analog because it is extremely dry; Antarctica is a Mars analog because it is both cold and essentially dry. The Atacama Desert in Chile is one of the driest places on Earth, and as of November 2026 it is the driest desert outside the icy poles. In the Atacama, rain of 1 millimeter or more may fall only once every 5 to 20 years, and the driest parts have no vegetation and not even the cyanobacteria found under stones in most other deserts. As of November 2026, the largest desert in the world is Antarctica, a cold desert of ice.
Small animals gather water in ways that scientists have tested. In the Namib Desert, the beetle Onymacris unguicularis stands head-down on dune ridges to collect water from fog. For the fog-basking beetle, the head-standing behavior mattered more than the structure of its shell for collecting fog water, and fog was needed to trigger it. The thorny devil, a spiky lizard of Australia, collects water with grooves in its skin that lead to its mouth. Its spines are surrounded by grooves that connect to each other, so water moves along its body by capillary action, even uphill, to its mouth.
Heat is the other pressure on desert life. The hottest air temperature ever recorded in Death Valley was 134 degrees Fahrenheit, or 57 degrees Celsius. As of November 2026, that record was set on July 10, 1913. Death Valley is so hot partly because heated air rises, cools, and is recycled back down to the valley floor, where it is compressed and heated even more. In the desert heat, people should drink plenty of water and stay in the shade during the hottest part of the day. Many desert animals sleep in the heat of the day and come out at night, when it is cooler.
You may have heard that the Sahara has always been a desert. In fact, during the African Humid Period, roughly 11,000 to 5,000 years ago, the Sahara was likely far wetter and greener, with vegetation, wetlands and perhaps even large lakes. Winter rain from the Mediterranean was as important as the summer monsoon for greening the Sahara during the African humid period. In September 2024 rain partly filled Sebkha el Melah, a normally dry salt lake in Algeria; NASA satellites watched it fill, and scientists use such events to study the Sahara's past greening.
Words to know
- analog: close match
- sediments: settled layers
- monsoon: seasonal rains
- capillary: thin-tube
- cyanobacteria: tiny bacteria
