The ice caps of Antarctica and Greenland represent the world’s largest reserves of frozen water, defined by sheets of permanent ice. It seems paradoxical that these frigid, icy expanses are scientifically categorized as deserts. This classification stems not from a lack of water, but from the low annual moisture input they receive. The definition of a desert is based on aridity, meaning the region experiences low precipitation, making the frozen poles a type of cold desert.
Defining a Desert by Precipitation
The scientific classification of a desert is determined almost entirely by the amount of precipitation a region receives, not by its temperature. Any area on Earth that receives less than 250 millimeters (about 10 inches) of precipitation annually is defined as a desert. This technical criterion is the reason polar regions, despite their icy appearance, share the same designation as the hot, sandy Sahara Desert. Cold deserts experience the same moisture deficit at freezing temperatures. Antarctica, the driest continent on Earth, receives an average of only 166 millimeters of precipitation each year across its interior.
Atmospheric Mechanisms Causing Polar Dryness
The low precipitation found in the polar regions is a direct result of two atmospheric physics principles.
Cold Air and Moisture Capacity
The first is the relationship between air temperature and its capacity to hold water vapor. Extremely cold air, such as that found over the ice caps, holds significantly less moisture than warm air. The maximum amount of water vapor air can contain decreases exponentially as the temperature drops. Because the air over the poles is intensely cold, the absolute humidity is low, limiting the moisture available to form snow or ice crystals. Any moisture that does exist quickly condenses and precipitates out as light snowfall, often appearing as diamond dust.
Polar High-Pressure System
The second factor is the pattern of atmospheric circulation known as the polar high-pressure system. Cold temperatures cause the air to become dense and heavy, leading it to sink toward the surface in a process called subsidence. This persistent descending air creates a stable, high-pressure zone, generally associated with clear skies. As the air sinks, it warms slightly, suppressing the vertical movement required for cloud formation and precipitation. This atmospheric stability prevents weather fronts carrying moisture from penetrating deep into the polar interiors, maintaining the hyper-arid conditions of the ice caps.
Ecological Consequences of Locked Water
The atmospheric dryness translates directly into a landscape that is ecologically comparable to a hot desert. While a massive amount of water is physically present in the form of ice sheets and glaciers, this water is biologically unavailable for use by organisms. The water is locked in a solid, frozen state, preventing the liquid water necessary for most biological processes.
This prevents the cycling of nutrients and limits the ability of plants to grow and establish biomass. The polar landscape is essentially barren, supporting only specialized, sparse life forms adapted to survive under hyper-arid and frigid conditions.
The environment lacks the continuous supply of liquid water that would sustain a complex ecosystem, which is the ultimate ecological definition of a desert. The lack of liquid water, combined with the extreme cold, creates an environmental barrier to life as effective as the intense heat and lack of rainfall in a tropical desert.