Changing Distribution of Extreme Environments (SL IB Geography)

• Glaciers and ice sheets advance (grow) in winter and retreat (shrink) in summer
• Glaciers are systems with inputs, outputs and stores
  • Output is through ablation (melting)
  • Input (accumulation) of snow
  • Stores of glacial ice form layer by layer each year over 20–30 years
  • Equilibrium of the glacier is achieved when input and output are balanced
    • There is no gain or loss of ice and the glacier remains the same size
• The regime of glaciers can be classified as follows: 
  • Steady: accumulation = ablation
  • Retreating: ablation is greater than accumulation and occurs at lower elevations
  • Advancing: accumulation is greater than ablation and occurs at higher elevations
  • A glacier will advance if temperatures are 0 °C or below

Past and present cold environments

• Evidence from current landforms shows the Earth has had interglacial (warm) and glacial (cold) periods
  • Glacial periods saw glacial advance and expansion and sea levels dropped
  • An interglacial saw glacial retreat, contraction and sea level rise
• About 21 000 years ago, there was the last glacial maximum (the ice age), and cold environments covered more than 30% of the Earth's surface.
• At this point, the Earth's average temperature was 6 °C (average now is 14–15 °C)
• The climate was drier because most of the water on Earth's surface was ice, resulting in less precipitation
• Sea levels dropped, and shorelines extended farther out, creating more land (water was trapped in ice sheets)
• The polar ice sheets covered much of the UK and most of southern Europe was periglacial
• Currently, the Earth is in an interglacial period, with glaciers retreating

Distribution of ice in the past

The extent of global ice during the last ice age

(Pleistocene epoch)

• The distribution of hot arid and semi-arid lands has changed over time
• Changing climates have changed the extent and distribution of hot deserts
• A large percentage of the world was wetter after the last ice age and this reduced the amount of land covered in desert
  • Canyons formed when the climate was wetter (interglacial period ≈8 000 years ago) and present-day landforms such as pedestals
• Until about 5 000 years ago, there were only narrow strips of hot deserts to be found on the west coasts of South America and Africa
• Gradually, these areas of desert increased in size, but new hot deserts also began to form
• The Sahara region was not always a desert
  
  • Between 11 000 and 5 000 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 influenced the extent and distribution of these drylands

Over time, the distribution of drylands has increased through human activity. Note that the most at-risk areas already fringe the driest areas. 

Pleistocene pluvials

• The end of the Pleistocene corresponds with the end of the last glacial period (approximately 11 700 years ago)
• A pluvial refers to the characteristic of rainfall (higher)
• Events that happened a long time ago frequently have an impact on the evolution of arid and semi-arid landforms