Changing Distribution of Extreme Environments (DP IB Geography)
Revision Note
Changing Distribution of Cold Environments
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
(Pleistocene epoch)
Changing Distribution of Hot Arid Environments
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
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
The Changes to Hot Desert Climate Over Time
5 000 years ago to the present day | Over Asia and Africa, the monsoon rains began to lessen, and so began arid conditions. 3000 years ago, conditions were similar to today. |
About 8 000 years ago | During this interglacial period, conditions were warmer and more humid. Forests thrived and were widespread in warm and wet conditions. Aridity fell dramatically, with many of the present-day deserts being grasslands. |
Last glacial maximum 20 000 years ago | During the last glacial maximum, aridity was widespread, with cold deserts in the north. In the south, deserts existed in similar locations to the present day but were more extensive. |
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