Drylands represent vast global ecosystems, characterized by inherent water scarcity. Covering over 40% of the Earth’s terrestrial surface, these regions are found on every continent. They are home to a substantial portion of the world’s population, impacting human well-being and global ecological balance. Understanding these environments is important for addressing environmental and societal challenges.
What Are Drylands?
Drylands are defined by an aridity index (AI), a ratio of average annual precipitation (P) to potential evapotranspiration (PET). An AI of less than 0.65 classifies an area as a dryland, indicating that average rainfall is less than the potential moisture lost through evaporation and transpiration.
Drylands are categorized into four types based on their aridity index. Hyper-arid regions have an AI of less than 0.05, experiencing extremely low rainfall, typically below 200 mm annually. Arid regions fall within an AI range of 0.05 to 0.20, while semi-arid areas have an AI between 0.20 and 0.50. The least dry category, dry sub-humid, is characterized by an AI from 0.50 to 0.65. These classifications help in understanding the varying degrees of water deficit experienced across dryland environments.
Globally, drylands cover approximately 47% of the Earth’s land surface. They are found on all continents, with a notable presence in Africa and Asia, including regions like the Sahel and much of the Middle East. This widespread distribution highlights their significant role in global climate patterns and ecosystems.
Ecosystems and Human Life in Drylands
Dryland ecosystems support a variety of plant and animal life that have developed adaptations to cope with water scarcity and variable climates. Plants often exhibit features like deep root systems to access groundwater, succulent leaves for water storage, or small, waxy leaves to minimize water loss through transpiration. Animals in these environments have also evolved strategies such as nocturnal activity to avoid daytime heat, efficient water use, and specialized diets.
These ecosystems encompass diverse landscapes, including rangelands, grasslands, shrublands, and woodlands, with forests covering over a quarter of the world’s total forest area. Drylands also hold substantial carbon reserves, with their soils containing 27% of global organic carbon and 97% of global inorganic carbon. They are significant in global carbon cycles and climate regulation.
Human populations have long inhabited drylands, developing traditional livelihoods and cultural practices tailored to these environments. Pastoralism and rain-fed cultivation are common agricultural strategies, with drier areas often focusing on livestock and less arid areas on crops. These communities have developed indigenous strategies to manage water scarcity and cope with droughts. Many dryland populations, particularly in developing countries, depend on these ecosystems for food, forage, fuel, and water, with 90% of dryland-supported people living in these regions.
Major Threats to Drylands
Drylands face environmental pressures, with desertification being a major concern. Desertification refers to the degradation of land in arid, semi-arid, and dry sub-humid areas, resulting from factors including climatic variations and human activities. This process leads to loss of biological or economic productivity, impacting agricultural output and increasing vulnerability to climate change. Overgrazing, deforestation, and inappropriate agricultural practices are human-induced causes of desertification.
Water scarcity is another threat in drylands, stemming from natural climatic conditions and human actions. Drylands naturally experience low and erratic rainfall, coupled with high temperatures that lead to water loss through evaporation and transpiration. Human activities, such as unsustainable water extraction for irrigation and other uses, exacerbate this deficit, depleting surface and groundwater reservoirs.
Climate change intensifies these vulnerabilities. Rising global temperatures increase evapotranspiration rates and alter precipitation patterns, leading to more frequent and intense droughts. This reduces soil moisture retention, compromises soil structure, and amplifies the risk of wind and water erosion, accelerating land degradation and desertification. The expansion of arid biomes due to climate change is projected, with up to 80% of this expansion occurring in developing countries by the end of the 21st century.
Protecting Dryland Environments
Addressing the challenges in drylands involves implementing sustainable management and conservation strategies. Sustainable land management (SLM) practices are important to these efforts, aiming to prevent and reduce land degradation while enhancing resource productivity. These practices focus on maintaining soil health, conserving water, and preserving biodiversity.
Techniques within SLM include conservation agriculture, which minimizes soil disturbance through practices like reduced tillage and the use of cover crops. Water harvesting techniques, like collecting rainwater and directing it to crops or storage, are also employed to maximize available moisture. Afforestation and reforestation efforts also restore degraded lands, improve soil stability, and increase water infiltration.
Promoting drought-resistant crop varieties helps ensure food security in unpredictable climates. Community-based conservation programs are also important, integrating traditional knowledge with scientific approaches to empower local populations in managing their resources sustainably. These integrated approaches build resilience in dryland social-ecological systems, fostering ecological restoration and improved human well-being.