Find out why deserts are so dry and how human activities and climate change are altering their extent.
The surprising reasons for dry deserts
Deserts are mysterious places, cold at night and hot during the day, where many secrets are hidden. They are characterized by their low rainfall, generally less than 25 centimeters per year. But why are some places on Earth so dry? The answer lies in global air circulation patterns.
Solar energy heats the air and evaporates moisture near the equator. This warm, dry air rises and moves toward the poles, then falls back down around 30 degrees latitude. This pattern, called the Hadley cell, influences the trade winds and explains why the great deserts, such as the Sahara, Gobi and Kalahari, are located at these latitudes.
The role of topography and ocean currents
Interactions between winds and topography also have an impact on the distribution of deserts. For example, air coming from the ocean and hitting a mountain range releases its moisture as rain or snow on the slopes. However, when it crosses the mountains and descends on the other side, it is dry. The Mojave Desert in California is an example of such a phenomenon, located in the rain shadow of the Sierra Nevada.
In some cases, inland regions are drier because of their distance from moisture sources. Cold ocean currents can also influence the distribution of precipitation. For example, the Atacama Desert, one of the driest places on Earth, is bordered by the ocean but sees little rainfall because of coastal fogs that trap moisture.
Cold deserts and climate change
It is important to note that not all deserts are hot. Parts of the Arctic and Antarctic are also considered deserts because of their low rainfall. The cold prevents the air from holding moisture, resulting in reduced precipitation.
Climate change is also changing the boundaries of deserts around the world. The Hadley cell is expected to expand as temperatures rise, accelerating water evaporation and further drying out the air. The balance between precipitation and evaporation determines the desert nature of a region. With global warming, it is likely that desert areas will expand.
The impact of human activities on the extension of deserts
Human pressure on landscapes also contributes to the modification of deserts. Deforestation for agriculture eliminates native vegetation, and some studies suggest that the reduction of tropical forests decreases rainfall. If water evaporates instead of being retained by plants in the soil, a feedback cycle causes landscapes to gradually dry out.
Semi-arid areas on the periphery of existing deserts are particularly vulnerable. Often, it is a combination of factors that drive desert expansion, such as human activity and climate change, that overlap and amplify each other.
By better understanding the mechanisms that influence desert drought and being aware of the impact of our actions on these fragile environments, we can act to preserve these unique ecosystems and combat the consequences of climate change.
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