Glacial Dynamics & Systems (Edexcel A Level Geography)

Mass Balance System

Formation of ice

• Glaciers are defined as:

• Glaciers are open systems with direct inputs of snow and ice from precipitation, blown in on the wind or with avalanches

• Over 2 years, snow and ice settle and compact to form firn or névé 

• Each subsequent snowfall adds to these layers and further compacts the firn into glacial ice

  • Compaction squeezes air out of the firn, and the resulting glacial ice absorbs longwave light but scatters short-waved blue light, making the ice appear blue

  • The formation of glacial ice usually takes approximately 30 years, but in polar areas, such as Greenland, it can take up to 150 years

  • In temperate regions, transformation to ice takes as little as 100 years

  • However, in places such as Antarctica, ice has taken up to 4000 years to form; due, in part, to the lack of precipitation, which slows down the rate of compaction into ice

Glaciers as a system

• Glaciers are open systems with inputs and outputs to external systems, including fluvial and atmospheric systems

• There are flows of energy, ice, water and sediments between stores

Mass balance

• Mass balance is the gains and losses of ice within the glacier

• More accumulation over a year and the glacier has a positive regime or positive mass balance

• The glacier will gain mass and advance in response to high accumulation in the upper zone

• A negative mass balance or regime is when there is less accumulation than ablation (usually during the summer months)

• The glacier will lose mass and retreat in response to low accumulation in the upper zone

• Dynamic equilibrium is when the overall amount of ablation and accumulation balances over a year

• The glacier remains the same size and the position of the glacier front does not change

Case Study - Greenland Ice Sheet

Location

• Ice sheets record Earth's climate history through annual layers of trapped air bubbles

• Ice sheets contain huge quantities of frozen fresh water and have the potential to impact other earth systems, particularly the atmosphere and oceans if they melt

  • Ice sheet meltwater changes the ocean's density by decreasing salinity and temperature, impacting ocean circulation

• One of two remaining continent-sized ice masses, the Greenland Ice Sheet is the largest ice mass in the Northern Hemisphere

• Found between the Arctic and North Atlantic Oceans, northeast of Canada and northwest of Iceland, Greenland is part of the Realm of Denmark 

• The ice sheet covers roughly 80% of Greenland’s landmass - an area of over 1.7 million km2, containing more than 2.5 million km³ of stored ice

• At its thickest, it is over 3km and it weighs enough to depress the earth's crust by approx. 1km

• With a series of drainage networks, ice flows outwards from the centre, via outlet glaciers and ice streams to Greenland's coastline

Historical data

• The Greenland Ice Sheet was part of a series of ice sheets covering large parts of the Northern Hemisphere during the last ice age

• It included the Laurentide Ice Sheet over North America and the Eurasian Ice Sheet over Europe

• During the glacial maximum, the Greenland Ice Sheet held an extra 4.1m of ice (sea level equivalent) and is the only one remaining in the current interglacial period

• Past glacial data shows that Greenland's Ice Shelf was extensive, but, data shows that there was significantly less ice during past interglacial periods than today

Current data

• Data shows the mass loss of ice over recent decades

• Mainly due to increased air and ocean temperatures 

• Iceberg calving, meltwater runoff, and ocean-driven melting have all increased and contributed to a negative surface mass balance

• Over recent years, Greenland’s melt season has dropped well below the 1981-2020 average

• As a result, global mean sea levels have risen by approx. 0.7mm - which is greater than the Antarctic ice sheet contribution

• Continued melting and Greenland could contribute 5 to 33 cm to sea level by 2100

• If the Greenland Ice Sheet were to melt completely, scientists estimate that sea levels could rise 7.4m globally

Future

• Continued global warming will increase the rate of ice sheet melting as a positive feedback mechanism

• Exposed ground reduces the albedo effect on the surface, increasing ground warming and therefore, snow melt

• Increased melting leads to the release of stored carbon and methane into the atmosphere, adding to the greenhouse effect and increased warming

• The height of the land would be lower, however, with the release of weight, the isostatic rebound would eventually counteract this, as Greenland rose

• Large amounts of freshwater could affect the thermohaline circulation and cut off equatorial warm waters arriving with the Gulf Stream along the coast of the UK

Accumulation & Ablation

• Inputs are known as accumulations 

  • Accumulation is from direct and indirect snowfall

  • Avalanches from one area onto the ice mass

  • Windblown debris from another area onto an ice mass

  • Any accumulation is transferred down hill by gravity

• Outputs (called ablation) 

  • Ablation is accumulation losing mass through:

  • Melting at the margins of the ice mass

  • Evaporation 

  • Sublimation

  • Calving from the front margin where it meets the sea

  • Avalanches 

• The balance between the accumulation and ablation over a year is called the glacial budget

• It determines if the mass of the glacier has increased or decreased

• There are two zones:

  • Accumulation zone

    • Found in the upper part of the glacier

    • Inputs are usually more than the outputs

    • There is a net gain of ice during the year

    • Glacier front advances

  • Ablation zone

    • Found in the lower part of a glacier

    • Output exceeds inputs

    • Net loss of ice during the year

    • Glacier front retreats

• Where gains and losses balance on the glacier, the area is called the equilibrium line or point

• Over time, variations in the glacial budget will move the line up or down the glacier

• Linked to the advance and retreat of the glacier front

Variations in Accumulation & Ablation

• A glacial system has positive and negative feedback loops to keep it in dynamic equilibrium

  • E.g. sediment on the glacier absorbs insolation and begins heating

  • This leads to melting of the ice 

  • Exposing more sediment and increasing the rate of insolation absorption 

  • This is a positive feedback loop 

• Output through ablation (melting) is balanced by glacial input of accumulation (usually snow)

  • Accumulation increases with:

    • Increased rates of input - more snowfall = more ice mass in the longer term

    • Lower temperatures - this lowers rates of melting

    • Lower wind speeds - slows rates of transfer out of the system

    • Lower rates of insolation - albedo effect is increased

  • Ablation increases with:

    • Lower rates of input - less snowfall reduces future ice formation

    • Increase in global temperatures - reduces snow formation, and increases rate of melting

    • Higher wind speeds - transfer out of the system increases and prevents snow from settling

    • Increased insolation - lowered albedo effect 

• Equilibrium of the glacier is maintained when input and output is balanced - neither a gain or loss of ice and the glacier remains the same size

• Glacial health is assessed over a 10 year period and 75% of current glaciated areas are in negative mass balance (retreating)

• Caused through increased global temperature of 0.6°C over the last decade