Glaciers as systems

Glaciers function as open systems that receive inputs of energy and matter, store them temporarily, and release outputs. This involves continuous flows between different parts of the system, helping to maintain the glacier's structure and movement.

Key components of glacial systems

• Inputs - Materials and energy entering the glacier from external sources.
• Stores - Locations within the glacier where materials are held.
• Flows - Movements of ice, water, sediment, or energy between stores.
• Outputs - Materials and energy leaving the glacier.

These components interact dynamically, with flows transferring elements like ice and debris downhill under gravity.

Inputs into glacial systems

Inputs primarily add mass and energy to the glacier, often from atmospheric or geological sources.

Examples include:

• Snow - Falls as precipitation or arrives via avalanches, which are large masses of snow, ice, and rock sliding quickly down a mountainside.
• Condensation - Water vapour from the air condenses and freezes onto the glacier.
• Deposition - Water vapour turns directly into ice crystals without becoming liquid first.
• Rock material - Fragments collected as the glacier erodes the landscape or from rocks falling onto the glacier from surrounding slopes.

Stores within glacial systems

Stores act as reservoirs for the glacier's mass, holding ice, water, and debris in various forms.

Examples include:

• Ice - The primary store, forming the main body of the glacier.
• Meltwater - Stored on the surface in supraglacial lakes (lakes on top of the glacier) or within the glacier's structure.
• Debris - Rock fragments from processes like freeze-thaw weathering.

Flows in glacial systems

Flows transfer materials and energy, driving the glacier's movement and shaping its form.

Examples include:

• Meltwater flows - Water moves from surface stores, such as supraglacial lakes, to channels at the glacier's base.
• Debris flows - Rock material shifts from surface storage to form landforms or is transported within the ice.

These flows often follow gravity, moving downhill and contributing to the glacier's overall advance.

Outputs from glacial systems

Outputs remove mass and energy, often at the glacier's lower end.

Examples include:

• Meltwater - Ice melts and flows out as liquid water.
• Evaporation - Surface snow melts and turns to vapour.
• Sublimation - Ice or snow changes directly to water vapour without becoming liquid.
• Wind removal - Strong winds blow away snow.
• Calving - Blocks of ice break off from the snout (front) into water, forming icebergs, especially for glaciers ending in seas or lakes.

Glacial mass balance

The glacial mass balance compares the glacier's gains and losses over a year, determining whether it grows, shrinks, or remains stable. This balance influences whether the glacier's front advances (moves forward) or retreats (moves back).

Key terms in mass balance

• Accumulation - The addition of snow and ice to the glacier.
• Ablation - The loss of ice through melting, sublimation, or other processes.
• Mass balance - The net difference between accumulation and ablation, also known as the glacial budget.

Zones within a glacier

Glaciers have distinct areas based on the balance of accumulation and ablation.

These areas are:

• Zone of accumulation - Upper part where accumulation exceeds ablation, leading to net gain.
• Zone of ablation - Lower part where ablation exceeds accumulation, leading to net loss.
• Equilibrium point - Middle area where accumulation equals ablation, with no net change.

These zones shift with seasonal and climatic variations, affecting the glacier's overall behaviour.

The annual mass balance determines the glacier's regime

• Positive regime - More accumulation than ablation, causing the glacier to grow and advance due to excess mass in the upper zone.
• Negative regime - More ablation than accumulation, causing the glacier to shrink and retreat due to insufficient inputs.

How glaciers gain mass

Glaciers build mass through processes that add snow and ice, often over long periods. This begins with snowfall and involves transformation under pressure.

Formation of glacial ice through compression

1. Fresh snow, with low density (about 90% air), falls and settles.
2. It compresses into granular snow, with reduced air spaces.
3. Further pressure creates firn, which is compressed snow surviving at least one melt season; this can take 100–300 years to become glacier ice.
4. Final compression produces glacial ice, dense with less than 20% air, able to withstand a year's ablation.

In temperate areas, ice forms more quickly because snow melts faster, removing air and refreezing as denser material.

Other ways glaciers gain mass

• Avalanches - Infrequent but add large volumes of snow rapidly.
• Wind deposition - Winds blow powder snow onto the glacier, increasing accumulation.

How glaciers lose mass

Ablation reduces glacier mass through various mechanisms, often intensified in warmer periods or at lower elevations.

Processes of ablation

• Melting - Ice turns to meltwater, creating outflows from the glacier.
• Sublimation - Ice or snow converts directly to water vapour.
• Calving - Ice breaks off at the snout due to cracks from wind or water erosion, forming icebergs.
• Wind erosion - Strong winds remove snow, sometimes triggering avalanches that relocate mass elsewhere.

These processes are more pronounced in the zone of ablation, contributing to negative regimes.

Dynamic equilibrium and changes over time

Glaciers can maintain a stable state despite internal changes, but they respond to seasonal and long-term variations in inputs and outputs.

Dynamic equilibrium in glaciers

Dynamic equilibrium occurs when accumulation equals ablation over a year, keeping the glacier's size and snout position unchanged, even with short-term fluctuations.

Seasonal and yearly variations

• In colder months, accumulation dominates, causing temporary advances.
• In warmer months, ablation increases, leading to retreats.
• Over a year, these may balance, but yearly variations (e.g., unusually high snowfall) can cause temporary advances or retreats.
• Long-term records assess overall health, with benchmark glaciers like Gulkana Glacier in Alaska used to predict wider trends.

Long-term climatic influences

• Historical periods show how temperature affects mass balance.

• During the Little Ice Age (cooler temperatures), many glaciers advanced due to positive regimes; for example, the Mer de Glace in the French Alps advanced approximately 1.1 km.

• Post-1850, rising global temperatures led to negative regimes and retreats, with the Mer de Glace retreating by nearly 2.4 km since then.

Feedback mechanisms and climate change

Changes in inputs or outputs trigger feedbacks that can stabilise or amplify effects on glaciers, providing insights into climate change.

Types of feedback

• Negative feedback - Counteracts changes to maintain balance; for example, increased ice input speeds up movement, outputting more meltwater and ice to keep mass constant.
• Positive feedback - Amplifies changes; for example, retreating glaciers reduce albedo (the reflective ability of ice), absorbing more solar energy, raising temperatures, and causing further retreat.

Links to climate change

• Calculating mass balance over years reveals climate impacts, such as the Greenland Ice Sheet's rapid mass loss due to positive feedback.
• Monitoring shows accelerating retreat, with cumulative mass changes indicating broader environmental shifts.