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Upon hearing the word "desert" most people conjure up images of lots of sand and heat. It's a common misconception. Some deserts are blazing hot and others happen to be the coldest, least sandy places on Earth, like Antarctica, for instance, or the frigid Gobi Desert of China.
About the only thing all deserts have in common is their lack of abundant and reliable water. They are dry. That, as the word implies, makes them some of the most open, treeless, deserted lands in the world. And because they are already living on the edge, the habitats of desert animals and plants also tend to be among the most easily disrupted, according to desert researchers.
Desert Recipe
In the simplest terms, a desert can be defined as anyplace where the rain- or snowfall is less than the rate that things dry up. As to why they exist at all, one need look no further than a satellite image of Earth, or a world map with all the major deserts highlighted, to see that there's a method to the planet's deserts: The majority of desert lands fall inside two bands north and south of the tropics, at midlatitudes.
The reason for this is atmospheric. It starts in the tropics where there the sun beats down ferociously and evaporates a lot of water, causing the thunderheads to pile up. As that warmed air reaches higher altitudes, it cools and rains out its moisture, then is pushed toward the poles by global air currents. This dried-out air tends to cycle back down over the midlatitudes where it creates high-pressure systems. High-pressure systems result in fair, dry weather, as any meteorologist can attest, whereas low-pressure systems spawn storms.
Another thing that grows deserts is the rain shadow effect, which is created by mountains blocking wet weather from inland areas. When a storm has to climb a mountain, its air cools with the higher altitude and produces more rain and snow — dumping the moisture on the mountains. By the time the air makes it to the desert, it's squeezed dry.
Some desert areas have multiple mountain ranges blocking moisture, like Death Valley, in California, which is cut off from Pacific Ocean storms by no less than three mountain ranges. Other deserts are behind only one range, but they are real doozies: the Atacama of Peru, which is behind the Andes, and the Tibetan Plateau and Gobi, which are behind the gigantic Himalayas.
A third ingredient for a desert is cool water offshore — if an ocean is nearby, that is. Cold currents, like those off the western United States, western and southern Australia, or southwestern Africa, don't tend to spawn summer rainstorms, and they keep coastal and interior areas dry. It's the opposite case along the Atlantic Coast of the U.S., which has very warm waters from the Gulf Stream. The warm, moist air makes for muggy summers and wetter winters as well.
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Wastelands
Finally, humans have long played a role in making deserts, but not the kind that are particularly good for wildlife or people. Many human activities can degrade marginal lands, i.e., those that are nearly deserts, and drive them over the edge.
"Good examples include land degradation as a result of vegetation loss due to grazing and/or drought," says Nicholas Lancaster, researcher and director of the Center for Arid Lands Environmental Management at the Desert Research Institute in Reno, Nev. His list of most desert-degrading human activities is topped by surface disturbance by off-road vehicles and animals, increasing salinity of agricultural lands due to poor irrigation practices, scarce water resources, and overuse of surface water and groundwater, and urbanization of growing populations.
Overgrazing of animals and poor agricultural practices, both of which cause massive soil loss and erosion, takes about 12 million hectares of marginal lands out of use every year, according to an online report by the American Association for the Advancement of Science (AAAS).
This is not only bad for desert wildlife, but it's a human tragedy. Some examples: Since 1965, degradation of already marginal lands on the edge of the Sahara Desert forced one-sixth of the people of Burkina Faso and Mali to flee to cities. Between 1965 and 1988, in Mauritania, the proportion of nomads who grazed animals on the land fell from 73 percent to 7 percent, while the population of the capital city Nouakchott shot up from 9 percent to 41 percent, according to AAAS.
Global warming is also making itself known in the American deserts. The mysterious die-off of vast stands of piñon trees — the source of pine nuts valued by humans and wildlife — may be from hotter summers in the higher elevation deserts of Arizona and New Mexico.
The warming climate is also tinkering with where, when and how much rain falls in deserts, although the specifics are still unclear. Some desert regions may get greener. Others may experience droughts that last for decades — potentially drying up entire cities.
Good News
But there is good news. Some marginal lands that were thought to be on the verge have been brought back from the brink. One example is the Machakos District in Kenya. In the 1930s, it was thought to be a lost cause. But over the decades, despite a population explosion, water and soil conservation measures have improved the land.
These measures include cutting hillside terraces to stop soil erosion and digging water-storage ponds, according to work by Mary Tiffin of Drylands Research in the United Kingdom. New farming methods have also helped in densely populated, semi-arid areas of northern Nigeria, according to Tiffin and her colleagues.
So, even though many deserts are becoming a lot less deserted, there's no reason they have to become wastelands as well.