Hot semi-arid regions & desertification (WJEC Eduqas GCSE Geography B): Revision Note

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The notes on this page cover part 3.4.1 of the WJEC Eduqas GCSE B Geography.

3.4.1 - What are the physical processes operating in hot semi-arid regions that make them vulnerable to desertification?

• An overview of the location and global distribution of environments vulnerable to desertification.

• How this distribution pattern is changing over time.

• The relationship of this distribution to the global circulation of the atmosphere and to the dominance of high pressure systems.

• Changing climatic patterns over periods of decades to include unreliable rainfall patterns and higher rates of evapotranspiration in hot semi-arid regions.

• Smaller scale processes related to changing patterns of vegetation, evapotranspiration and micro-climate.

Location and global distribution of vulnerable environments

• The distribution of hot semi-arid lands has changed over time

• They are areas of water scarcity, where rainfall is seasonal (semi-arid)

• Temperatures are high, leading to high rates of evaporation and transpiration

• These areas have high levels of climatic uncertainty, and many areas can experience drought for several years 

Changing location of semi-arid areas

• Changing climates have changed the extent and distribution of semi-arid areas

• A large percentage of the world was wetter after the last ice age, and this reduced the amount of semi-arid land

• Until about 5000 years ago, there were only narrow strips of hot deserts to be found on the west coasts of South America and Africa

• Gradually, these semi-arid areas increased in size, and new hot deserts began to form

  • The Sahara region was not always a desert

  • Between 11000 and 5000 years ago, the area was known as the Green Sahara with lakes, lush vegetation and animals of the savanna biome such as elephants and lions

• These are natural climatic variations forming hot deserts; however, human activity has also influenced the extent and distribution of these drylands through desertification

Global circulation and hot semi-arid regions

• Aridity is affected by the global circulation of air

• Semi-arid deserts are found in the tropics and subtropics due to two large convective cells called Hadley Cells

  • These atmospheric cells control the air circulating between the equator and the tropics in the northern and southern hemispheres

  • The area where the dry air descends is a zone of high pressure known as the subtropical high

• Note that as the cool, dense, dry air descends, it is being warmed by high levels of insolation 

Changing climatic patterns

• Areas which are vulnerable to desertification have experienced changing climatic patterns over recent decades

• These changing climatic patterns have increased the risk of desertification

• The main climate changes include:

  • Unreliable rainfall patterns

  • Higher rates of evapotranspiration

• Climate change leads to less reliable precipitation, resulting in both droughts and intense rain events

  • Droughts cause vegetation to die off due to a lack of water, leaving the soil exposed

  • Intense rain events on bare, baked soil lead to increased surface runoff rather than infiltration

    • This leads to soil erosion, washing away nutrient-rich topsoil

• Higher average temperatures lead to increased rates of evapotranspiration

  • Higher temperatures increase the rate at which water evaporates from the land surface and transpires from plants.

  • This rapid moisture loss means plants struggle to hold on to water and the soil dries out quickly, losing its moisture content and structure

Small-scale climatic patterns

• Desertification and the removal of vegetation lead to small-scale climatic patterns

• Shade

  • The tree canopy shades the ground beneath it

  • This lowers ground temperature, affecting the local micro-climate and the types of plants that can grow beneath it

  • If the vegetation is removed, the lack of shade leads to increased temperatures

• Evapotranspiration

  • Plants absorb water through their roots and release it as water vapour through their leaves

  • This process:

    • Cools the surrounding air

    • Increases humidity

    • Can lead to cloud formation

  • This process creates a microclimate around the vegetation

• Wind reduction

  • Vegetation acts as a natural windbreak, reducing wind speed in the immediate area

  • This creates a more sheltered, warmer, and less humid environment directly behind the vegetation

  • Removal of the vegetation increases soil erosion by wind