2-2-1-glacial-dynamics-and-systems

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Exam code:9GE0

Mass Balance System

Large rivers (mass) of ice, moving downhill, under the influence of gravity

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 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 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

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

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

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

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

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

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