Arid climates: distribution, characteristics, causes

Arid Environments (Cambridge IGCSE/A‑Level Geography 9696 – Topic 10)

Learning Objective

Explain the distribution, key physical characteristics and underlying causes of arid climates, and evaluate how these factors affect human activity and environmental management.

1. Distribution of Arid Climates

Arid (desert) and semi‑arid (steppe) climates occur in five principal settings. In the exam, you may be asked to interpret a world‑map showing the latitudinal belts of aridity (Köppen B‑climates) and to identify the setting from the map.

2. Physical Characteristics of Arid Climates

• Precipitation
  • Deserts: < 250 mm yr⁻¹; Steppes: 250–500 mm yr⁻¹.
  • Highly erratic – often one to three intense events per year.
• Potential Evapotranspiration (PET)
  • Typically 1 500–2 500 mm yr⁻¹ (2–3 × P).
  • Creates a chronic moisture deficit.
• Temperature
  • Hot deserts: Day > 45 °C, night < 0 °C; diurnal range 30–45 °C.
  • Cold deserts/high‑altitude deserts: Summer < 30 °C, winter < –5 °C; annual range > 30 °C.
• Vegetation
  • Widely spaced xerophytes (e.g., Acacia, Prosopis, succulents).
  • Adaptations: deep tap‑roots, reduced leaf area, waxy cuticles, CAM photosynthesis.
• Soils

  Soil Type (World Reference Base)	Key Features	Typical Example
  Aridisols	Low organic matter, high calcium carbonate (caliche) or gypsum; often have a hardpan.	Sahelian soils, Australian interior.
  Calcisols	Pronounced calcium carbonate accumulation, very low water‑holding capacity.	North‑African deserts.
  Gypsisols	Gypsum crusts, soluble mineral surface layers.	Atacama Desert.
  Loess deposits	Wind‑blown silt, fine‑grained, high erodibility.	Chinese Loess Plateau (semi‑arid).

• Wind & Erosion
  • Typical wind speeds: 10–30 m s⁻¹; gusts > 40 m s⁻¹ in major dust‑storm events.
  • Landforms: transverse dunes, longitudinal dunes, desert pavements, yardangs.
  • Dust‑storm frequency: 5–30 storms yr⁻¹ in the Sahara; up to 100 yr⁻¹ in the Gobi during drought years.
• Hydrology
  • Intermittent streams (wadis, arroyos) that flow only after rain.
  • Runoff coefficient often > 0.6 during events – rapid surface flow.
  • Groundwater is deep (> 30 m) and limited; oases occur where the water table intersects the surface.

3. Causes of Aridity

Aridity is the result of a persistent water‑budget deficit (P < PET). The main drivers are:

1. Global Atmospheric Circulation – Subtropical Highs
   • Descending branches of the Hadley cell at ~30° N and 30° S create high‑pressure belts.
   • Air descends adiabatically (≈ 9.8 °C km⁻¹), warming and reducing relative humidity → suppression of cloud formation.
   • Surface pressure in these belts: 1 015–1 030 hPa.
   • IPCC (AR6) projects a poleward shift of the subtropical highs under all warming scenarios, potentially expanding arid zones by 5–15 % by 2100.
2. Rain‑shadow Effect
   • Moist air forced up windward slopes cools at the moist adiabatic lapse rate (~6 °C km⁻¹) and precipitates.
   • Leeward side experiences dry, descending air; precipitation can fall below 50 mm yr⁻¹.
   • Key examples: Atacama (Andes), Great Basin (Sierra Nevada), Tibetan Plateau (Himalayan rain‑shadow).
3. Cold Ocean Currents
   • Cool currents (Benguela, Canary, Humboldt, Leeuwin) lower sea‑surface temperatures, stabilising the lower troposphere.
   • Stability suppresses convection, so coastal deserts receive very little rain.
4. Continentality
   • Interior locations far from maritime moisture sources receive air masses that have lost most of their water vapour.
   • Resulting precipitation often < 100 mm yr⁻¹ (e.g., Central Asian deserts, Australian interior).
5. Long‑term Orbital (Milankovitch) Variations
   • Changes in Earth’s axial tilt and precession modify insolation patterns, altering the intensity and latitude of subtropical highs over 20 000–100 000 yr cycles.
   • Evidence from paleoclimate records shows desert expansion during periods of high summer insolation in the Northern Hemisphere.
6. Human Influences (Secondary Causes)
   • Over‑grazing, deforestation and inappropriate irrigation accelerate desertification by reducing surface cover and increasing soil salinity.
   • Climate change may shift the position of subtropical highs and intensify cold‑current upwelling, further expanding arid zones.

4. Water‑Balance Equation for Arid Regions

P – ET_p = ΔS

• P = precipitation (mm yr⁻¹)
• ET_p = potential evapotranspiration (mm yr⁻¹)
• ΔS = change in soil‑moisture storage (mm yr⁻¹). In most deserts ΔS is negative, typically –200 to –400 mm yr⁻¹.

Implications for management

• If P < ET_p, a chronic moisture deficit limits agriculture without supplemental water.
• Increasing ΔS (e.g., through water‑harvesting structures such as zai pits, contour bunds, or stone‑lines) can raise effective soil moisture, improving crop yields.
• Quantifying the deficit helps evaluate the feasibility of irrigation schemes and the sustainability of groundwater extraction.

5. Case‑Study Box – Sahel Drought (1970‑2000)

Use this example to demonstrate the link between causes, impacts and management.

• Causes of aridity in the Sahel
  • Weakening of the West African monsoon caused by a northward shift of the subtropical high.
  • Land‑use change: extensive over‑grazing and deforestation reduced surface moisture and albedo.
  • Feedback loop: reduced vegetation → lower evapotranspiration → further decline in monsoonal rainfall.
• Impacts on people & environment
  • Crop failures and livestock loss; > 2 million people migrated southward.
  • Desert‑front advanced up to 100 km into former semi‑arid zones.
  • Dust‑storm frequency doubled, affecting air quality across the Atlantic.
• Evaluation of mitigation strategies
  • Great Green Wall (afforestation & agroforestry): mixed outcomes; where tree survival is high, soil moisture and crop yields improve, but poor seedling survival in extremely arid sections limits overall success.
  • Water‑harvesting (zai pits, stone‑contour lines): locally raises yields by 30–70 % and reduces runoff, but labour‑intensive and difficult to scale without external funding.
  • Livestock management (controlled grazing, herd‑size reduction): reduces pressure on vegetation, yet cultural resistance can hinder adoption.
  • Overall assessment: pilot projects show tangible benefits, but long‑term sustainability depends on continued climate monitoring, community participation, and integration with national policies.

6. Summary Points (Assessment‑Ready)

• Arid climates cluster in subtropical high‑pressure belts but also arise in rain‑shadow, cold‑current, high‑altitude, and interior continental settings.
• Physical traits: very low, erratic rainfall; PET 2–3 × P; extreme temperature ranges; sparse xerophytic vegetation; shallow, caliche‑rich Aridisols/Calcisols; strong winds (10–30 m s⁻¹) and frequent dust storms; rapid runoff and deep, scarce groundwater.
• Primary causes: descending air in the Hadley‑cell subtropical highs, topographic rain‑shadows, cold ocean currents, continentality, and Milankovitch‑driven insolation changes; human activities can intensify aridity, and climate‑change projections indicate possible expansion of subtropical deserts.
• The water‑balance equation (P – ET_p = ΔS) quantifies the moisture deficit that underpins desert conditions and guides the selection of water‑harvesting and irrigation strategies.
• Effective case‑study analysis (e.g., Sahel drought) should link the physical causes of aridity, the resulting human and environmental impacts, and a critical evaluation of mitigation measures, fulfilling the syllabus requirement for a detailed specific example.