The planet’s land surface contains vast stretches of land known collectively as drylands, characterized by a distinct lack of moisture. These expansive biomes, which include both deserts and steppes, play a substantial role in global ecology. Determining the precise extent of these dry regions is complex, as different classification methods yield varying measurements of their total area. Understanding these arid and semi-arid landscapes requires examining their scale and the differences in their underlying climatic definitions.
The Estimated Global Coverage
The amount of Earth’s landmass classified as dryland is significant, often cited in ranges that reflect different scientific criteria. Using the broadest definition, which includes hyper-arid, arid, semi-arid, and dry sub-humid zones, drylands cover approximately 40% to 41% of the world’s terrestrial area. This figure is based on the Aridity Index, a metric comparing average annual precipitation to potential evapotranspiration (the total water that could evaporate or be transpired by plants). For a region to be classified as dryland, this ratio must be less than 0.65.
A more conservative figure suggests that deserts and steppes together account for about 30% to 33% of the land surface. This lower percentage typically excludes the less extreme “dry sub-humid” regions from the calculation. The inclusion of immense cold deserts, such as Antarctica, which is technically a desert due to its extremely low precipitation, also influences the total percentage. The specific percentage used depends heavily on whether scientists employ a strict precipitation-based classification or the comprehensive Aridity Index favored by organizations like the United Nations Environment Programme (UNEP).
Distinguishing Arid Deserts from Semi-Arid Steppes
The two major components of this vast dryland area, deserts and steppes, are differentiated by the severity of their water deficit and the resulting vegetation. True deserts are categorized as arid or hyper-arid zones, defined by receiving less than 250 millimeters of precipitation annually. In terms of the Aridity Index, these areas fall below a ratio of 0.20, indicating that the potential water loss through evaporation is five times greater than the actual precipitation.
The extreme lack of moisture in deserts supports minimal plant life, typically restricted to highly adapted, drought-resistant species like xerophytic shrubs and succulents. Examples include the Sahara and the Atacama, which demonstrate the planet’s driest conditions. These regions often experience dramatic temperature swings between day and night due to the lack of atmospheric moisture to trap heat.
Steppes are classified as semi-arid regions, representing the transition zone between true deserts and more humid environments. They receive a higher, though still limited, amount of rainfall, usually ranging between 250 and 500 millimeters annually. This precipitation range is sufficient to sustain a continuous cover of grasses and small shrubs, forming the expansive grasslands known as steppes.
Steppes correspond to an Aridity Index ratio between 0.20 and 0.50, meaning they still experience a significant water deficit, but it is less severe than in arid zones. The Great Plains of North America and the Eurasian Steppe are prime examples, illustrating how this slightly increased moisture allows for productive grazing ecosystems.
Climatic and Geographic Drivers
The existence of massive dryland regions is largely dictated by global atmospheric circulation and the distribution of continents and mountain ranges. Many of the world’s hot deserts, such as the Sahara and the Arabian Desert, are formed by the subtropical high-pressure belts located around 20 to 30 degrees latitude north and south of the equator. In these zones, air sinks, warms, and dries out, suppressing cloud formation and precipitation.
Mid-latitude deserts and steppes, like the Gobi in Asia, are often created through a phenomenon called continentality. These areas are situated deep within continental landmasses, far from the moisture-laden winds originating from the oceans. This geographical remoteness results in extremely low humidity and limited rainfall, leading to arid conditions.
Another significant mechanism is the rain shadow effect, which is responsible for the formation of many cold deserts and steppes. When moist air is forced up and over high mountain ranges, it cools and drops its moisture on the windward side. By the time the air descends on the leeward side, it is dry and warm, creating a “shadow” of aridity. The Patagonian Desert in South America, which lies east of the Andes Mountains, is a clear example.