Desertification: Causes and Management

Analysing the Physical Causes

• Why does less rain lead to even less rain? The answer lies in the intense relationship between the climate and the soil.
• Climate change causes rainfall to become less predictable. When rain does fall, it often happens in short, intense bursts that cannot soak into the ground, leading to high surface runoff rather than infiltration.
• Rising global temperatures increase rates of evapotranspiration, drying out the soil and preventing underground aquifers from recharging.
• High evaporation rates also draw salts to the surface, causing salinisation, which leaves a toxic crust that kills plant roots.
• As vegetation dies, the bare, light-coloured soil reflects more sunlight (the albedo effect), which disrupts local atmospheric convection and further reduces rainfall.

Analysing the Human Causes

• Demographic pressure is a massive driver of land degradation. Kenya's population, for instance, is predicted to double between 2010 and 2030, drastically increasing the demand for food and fuel.
• Conflict also forces migration; in the Darfur region of Sudan, 3 million people became refugees, moving to fragile marginal land and stripping it of resources.
• In the Sahel, $80\%$ of domestic energy comes from burning firewood, leading to rapid deforestation.
• Overgrazing occurs when nomadic farmers (like the Masai in Kenya) are restricted to smaller areas, causing livestock to strip vegetation and trample the soil until it is densely packed.
• Over-cultivation exhausts the soil when farmers grow intensive monocultures or thirsty cash crops without leaving the land fallow (unplanted) to recover its nutrients.

The Mechanism of Soil Erosion

• When vegetation is removed by overgrazing or deforestation, the soil loses the root systems that bind it together.
• Exposed to high temperatures, the fragile soil bakes hard and cracks, making it completely impermeable to water.
• Without a leaf canopy to intercept intense rainfall, the water washes away the nutrient-rich topsoil in a process called soil erosion. Over 24 billion tonnes of fertile soil vanish globally each year.
• Water flowing rapidly over bare, sloping land cuts deep channels into the surface, an extreme form of degradation known as gully erosion.

The Vicious Circle of Desertification

The interdependent factors above create a positive feedback loop:

1. Climate change leads to lower rainfall and higher temperatures.
2. Vegetation dies from drought or is removed by humans.
3. The lack of roots causes the soil to bake hard and become impermeable.
4. Intense rain washes away the unanchored topsoil (erosion).
5. Soil exhaustion makes the land completely infertile, stopping new plants from growing.
6. Less transpiration from plants means less atmospheric moisture $\rightarrow$ further reduced rainfall.

Evaluating Management: Water and Soil

• Every time you build a wall, you stop something from moving; in Burkina Faso, local communities build walls to stop their livelihoods from washing away.
• Bunds (or "Magic Stones") are low stone walls ($0.5\,\text{m}$ to $1.5\,\text{m}$ high) built along the natural contour lines of a slope.
• They slow down surface runoff, force water to infiltrate the soil, trap sediment, and can increase crop yields by $50\%$.
• In Kenya, sand dams are reinforced concrete walls built across an ephemeral river.
• They trap sand behind them, storing water in the pores (up to $40\%$ of the sand's volume), providing clean water for over 60 years and reducing waterborne diseases by $40\%$.
• Farms can also use drip irrigation, which drips water directly onto plant roots, reducing water loss by over $90\%$ compared to traditional flood irrigation.

Evaluating Management: Tree Planting

• The Great Green Wall is an international initiative launched in 2010 to plant an $8000\,\text{km}$ long and $15\,\text{km}$ wide strip of trees across the Sahel.
• Afforestation and agroforestry (growing trees among crops) work through multiple mechanisms: roots bind the soil, the canopy provides shade to reduce ground temperatures, and leaf litter boosts nutrient cycling.
• By 2020, the Great Green Wall had restored 18 million hectares and created 350,000 jobs.
• However, the project was only between $4\%$ and $15\%$ complete by 2020 due to funding issues, regional conflict, and a lack of maintenance.
• In contrast, the Senegal Acacia Project focuses on drought-resistant native trees that produce gum arabic, ensuring the local community profits from the trees and protects them.

Evaluating Appropriate Technology

• Why use a multimillion-dollar tractor when a £5 cardboard box can save a forest?
• Appropriate technology involves tools that are cheap, easy to maintain, and use local materials, ensuring long-term sustainability.
• In Nigeria, locals use solar cookers made from cardboard and foil. They cost just £5 and use free energy, reducing the need to cut down trees for fuelwood.
• Fuel-efficient stoves are also used; their improved insulation reduces wood use by up to $50\%$.
• When evaluating these strategies, we must weigh their benefits against their limitations:

Strategy	Advantages	Disadvantages
Bunds (Magic Stones)	Low cost; uses local labour; traps nutrients; increases yield by $50\%$.	Heavy manual labour required; stones must be available locally.
Solar Cookers	Smoke-free (health benefit); uses free solar energy; very cheap (£5).	Only works when the sun is shining; slow cooking times; low durability (lasts 2-3 years).
Sand Dams	No evaporation or mosquito breeding; filters water; lasts 60+ years.	Dependent on local geology; requires high community cooperation to build.
Drip Irrigation	Minimal water loss ($90\%$ reduction); delivers moisture straight to roots.	Requires initial investment in piping; pipes can be easily damaged by animals or sun.