Deserts are among the oldest topographical features/regions on Earth. According to geologists, most deserts have existed for tens of millions of years. For context, the oldest desert is the Namib in Southern Africa, which dates back to ~50-80 million years ago, followed by the Kalahari, also in Africa (~60 mya), the Gobi in Central Asia (~10-45 mya), and the Atacama in South America (~10-15 mya). The Antarctic Ice Sheet, a cold desert, began forming 34-35 mya.
A desert is defined as a very large area of land that has witnessed little to no precipitation over millions of years, thus altering living conditions which in turn create unique biomes and ecosystems, on which thrive uniquely-adapted flora and fauna. This process is called ‘desertification’, which is caused by several factors. One of the most prominent causes is the blocking of precipitation by nearby mountain ranges, which can happen if the mountains are perpendicular to the predominant wind flow. Precipitation refers to the process by which frozen water that forms in the atmosphere, falls back to Earth i.e. as rain and snow. For instance, though Antarctica is a really chilly place with a lot of ice and snow, due to a lack of precipitation, it remains dry and is classified as a desert.
The lack of vegetation exposes the unprotected surface of the ground to denudation (the geological process in which moving water, ice, wind, and waves erode the Earth’s surface, leading to a reduction in elevation and in relief of landforms and landscapes). There are two types of deserts, based on temperature: Hot Desert and Cold Desert. Although both deserts are dry and inhospitable, they are different from each other in terms of their climate and the life they support. The Polar regions (the Arctic and the Antarctic) notice little precipitation year-round, and are thus called Polar deserts or cold deserts. Deserts are categorised by the amount of precipitation they endure, the prevailing temperature, the causes of desertification, or geographical location. To be defined as a desert, an area must receive less than 12.5 cm of rainfall a year.
Most people picture deserts as large landmasses with towering sand dunes, dotted by dromedary (single-humped) camels. These hot deserts are formed by a weathering process. Large day-night temperature variations put strain on rocks, which consequently break apart. Although rain seldom occurs in deserts, occasional downpours result in flash floods. Rainwater falling on hot rocks shatter them, with the resulting rubble strewn over the desert floor and being eroded by the wind. These fine sand particles remain airborne for extended periods, moving around in the form of sand or dust storms. Overtime, the grains end up as level sheets of sand or are piled high in billowing sand dunes, which build up over millions of years. Meanwhile, other factors such as a lack of rain and extreme humidity ensures that these regions demain dry.
Cold deserts are formed due to similar atmospheric conditions that prevent rain. They are largely formed in the temperate regions, at higher altitudes and are located at the centre of a continent. Because of the low temperatures, pressure, moisture, distance from the sea, and poor vegetation, precipitation is very low in these regions. For eg. The Tibetan Plateau is one the highest deserts in the world.
Deserts can also be flat, stony, rocky, and host temporary lakes. When water bodies form and evaporate frequently, they leave behind salt pans. For eg. The Rann of Kutch in Gujarat. There may also be underground sources of water, in the form of springs and aquifers, creating oases.
Today, deserts make up about 33% of the Earth’s total land area. Polar deserts make up around 14.2 million sq km. In short, deserts occupy a third of the world’s total land area -- these parts are arid or semi-arid. Deserts are getting bigger, owing to a variety of reasons, including overgrazing and deforestation.
Climate change, however, is the biggest factor. A lack of periodic precipitation has brought dry spells of desert-like weather patterns closer to where we live.