The dynamics of glacial systems and glacier regimes

Glacial systems operate on a balance of inputs, storage, and outputs, which determine whether a glacier grows, shrinks, or remains stable.

Components of a glacial system

• Inputs - These include snowfall accumulation, avalanches, debris from surrounding areas, heat energy, and meltwater that feed into the glacier.
• Storage - The primary store is ice, but glaciers also hold debris, moraine (accumulated rock and soil), and temporary meltwater within their structure.
• Outputs - Losses occur through ablation (which includes melting of snow and ice), sublimation (ice turning directly into vapour), and the deposition of sediment as the glacier moves.

Understanding glacier regimes

A glacier's regime describes its state of growth or retreat based on the balance between accumulation and ablation:

• Positive regime - When accumulation (snow and ice gain) is greater than ablation (loss through melting, evaporation, calving, wind erosion, and avalanches), the glacier thickens and advances.
• Negative regime - When ablation exceeds accumulation, the glacier thins and retreats.
• Stable regime - When accumulation equals ablation, the glacier remains steady in size and position.

Zonal distribution within a glacier

• Accumulation zone - Found at higher altitudes where snowfall exceeds melting, leading to a net gain of ice.
• Ablation zone - Located at lower altitudes where melting and other losses exceed snowfall, resulting in a net loss of ice.

The past and present distribution of ice sheets and ice caps in the Northern Hemisphere

The distribution of ice sheets and ice caps has significantly changed over geological time.

Modern distribution of ice

• Greenland ice cap - The largest modern ice sheet in the Northern Hemisphere, covering much of Greenland.
• Alpine glaciers - Small, isolated glaciers found in high mountain ranges such as the Alps in Europe and other elevated regions.

Past extent during the Late Pleistocene

During the Late Pleistocene, ice coverage was far more extensive:

• Laurentide ice sheet - Covered vast areas of North America, stretching across modern-day Canada and parts of the northern United States.
• Cordilleran ice sheet - Dominated western North America, particularly over the mountainous regions of Canada and Alaska.
• Fennoscandinavian ice sheet - Extended over northern Europe, including Scandinavia and parts of the British Isles.
• East Siberian ice sheet - Spread across northern Russia and Siberia.
• Central Asian ice cap - Present in high-altitude regions of Central Asia, though smaller in comparison to other ice sheets.

Natural desertification and changes in desert distribution

Deserts, as extreme dry environments, also experience changes in their distribution over time.

Causes of changing desert distribution

• Increasing aridity - Modern desert areas are becoming drier due to long-term climate trends.
• Historical wetter periods - Known as "pluvials," these rainy periods coincided with glacial advances in temperate zones, leading to reduced desert extents in the past compared to today.

Evidence of past wetter climates in the Sahara region

The Sahara provides substantial evidence of past wetter climates.

Historical indicators of wetter conditions in the Sahara

• Ancient agricultural activity - The region was once a granary for ancient Rome.
• Fossil soils and water - Laterite soils and ancient water deposits, dating back approximately 25,000 years.
• Wadis in the Egyptian desert - Numerous dry riverbeds (wadis) suggest that water once flowed regularly.
• Lake Chad's past extent - Once covering an area of around 300,000 km² and large enough to drain into the sea, Lake Chad is now almost entirely dried up, with terraces up to 50 m high marking its former shoreline.
• Pollen evidence - Pollen from oak and cedar trees, along with findings of small crocodiles in the Tibesti Mountains.
• Cave paintings by the Garamantes - This ancient civilisation left behind artwork depicting pastoral lifestyles.
• Relict radial drainage in Ahaggar Mountains - Drainage patterns suggest past water flow.
• Fossil ergs - Sand dunes potentially shaped by water as well as wind.
• River and lake shrinkage - The headwaters of the River Volta have dried up due to droughts since 1974, causing Lake Volta to diminish in size, while the River Niger shows signs of reduced historical flow.
• Historical forests and farmland - Areas that supported forests and agriculture in the early 1900s have now turned to desert.