What are ice sheets, and how do they work? What effects do glacial processes in Antarctica have upon the continent itself and on the rest of the world?
Worldwide, glaciers cover about 10% of the Earth's land area, and Antarctica accounts for about 85% of this total cover. By volume, Antarctica contains 90% of the world's glacier ice - enough ice to raise world sea level by over 60 metres if it were all to melt. The next largest volume of glacier ice, Greenland, is small in comparison containing a little over 7% of the world's total. The remaining 2 to 3% is found in other high latitude areas, such as parts of northern Canada and Alaska, and in high mountain ranges, such as the Himalayas, the Andes, and the Alps.
While the glacier ice of Antarctica, which covers over 99% of the continent, is often referred to as the Antarctic Ice Sheet, as pointed out in Section 1.1, there are two distinct areas of ice that have different characteristics and histories: the East and West Antarctic Ice Sheets. The main differences are:
These major differences are summarised in these plan view and cross-section diagrams of Antarctica:
While the term ice sheet might bring to mind the idea of something quite static - simply a cover of ice - this is far from the case. Ice sheets are constantly on the move: the processes which govern the behaviour of ice sheets are fundamentally the same, if on a much larger scale, as those governing other glaciers. Ice sheets come into existence as many smaller glaciers gradually enlarge and coalesce; and for this to happen, there needs to be a prolonged period over which accumulation (by snowfall) exceeds ablation (which can occur either by melting or by calving of icebergs into the sea). (The timing and reasons for ice sheet build up in Antarctica are discussed in Section 1.2.)
Ice sheets, however, are not monolithic. As described previously, the Antarctic Ice Sheet contains the large East Antarctic Ice Sheet (grounded above sea level) and the West Antarctic Ice Sheet (grounded mainly below sea level). Ice also extends along the Antarctic Peninsula. Nested within these areas of ice there are many individual glaciers that can be studied separately, even though they are connected to, and form part of, the larger ice mass. For example, around mountain ranges ice sheets can be fed by cirque glaciers and valley glaciers, and near their margins they shed ice through ice streams and ice shelves.
Glaciers in all of their different forms have profound effects on the landscape through processes of erosion and deposition as discussed further below. Ice sheets have the additional effect of depressing the elevation of the bedrock on which they sit due to their great weight. This is explained through the concept of isostasy: where the ice is thickest, Antarctic crust has been depressed by as much as 500 metres. For comparison, a similar quantity of ice was present over North America about 20 000 years ago centred around Hudson Bay. Since that time, as the ice sheet melted away to gradually remove the overburden, the continental crust in this area has risen by several hundred metres, and is still rising today at rates of around 10mm per year.
It is important to think of an ice sheet not as a giant mass of stationary ice but as a system with inputs and outputs of matter and energy. All glaciers, from the smallest cirque glaciers to the largest ice sheets, are conveyors of ice - transporting ice from areas of net input (the accumulation zone) to areas of net output (the ablation zone). In Antarctica, a polar desert, the rate of accumulation is very low (over much of the interior snowfall is less than 50mm per year); but with low temperatures year round, rates of melting are also low.
Accumulation occurs as snowfall and frost formation adds mass to the surface of the ice. Over time, the snow crystals are buried under more recent precipitation, eventually reaching a depth where they are compacted into glacier ice. Under pressure, the glacier ice deforms and flows like a viscous fluid. It is hard to imagine solid ice actually flowing; and the velocity of flowing ice is too slow to be perceived when standing on a glacier. However, glacier ice does flow, and evidence for this is best seen in patterns of crevasses and flowstripes shown on aerial photographs. In energy terms, there is an input of potential energy where the ice surface is high, and energy is dissipated through kinetic energy of motion as the glacier ice flows down slope from the accumulation zone. Some of this energy is lost as friction and some through the geomorphological work that is done by glaciers, e.g. by eroding, entraining, transporting, and depositing material.
As with other natural systems, the balance of inputs and outputs and the rates of ice flow in the glacier system vary over time due to both external and internal factors. External factors include changes in snowfall and air temperature: if, for example, there is a decline in snowfall and/or increase of temperature, then there will be a period of time when melting (or sublimation) exceeds the input of new ice and the glacier will lose mass until it reaches a new equilibrium between accumulation and ablation. (It is important to remember that even when a glacier is shrinking, ice still moves forward from accumulation zone to ablation zone.) An increase in snowfall relative to rates of melting has the opposite effect causing the glacier to gain mass. The relationship between a glacier's total accumulation and ablation at any one time is known as its mass balance. Averaged over a year, this gives the glacier's net balance which can be positive or negative, indicating whether it has increased or decreased in size.
A great deal of research is currently being focused on estimating how the mass balances of the WAIS and the EAIS are responding to recent warming and how they may respond to future warming caused by increasing levels of greenhouse gases in the atmosphere. Given the huge proportion of the world's glacier ice contained in Antarctica, any future changes in mass balance will have very significant effects on the rest of the world. If a warming world causes a period of negative mass balance averaged across Antarctica, then the net transfer of H2O from ice on land to meltwater entering the oceans will contribute to eustatic sea level rise.
Estimating changes in the mass balance of Antarctic ice as a whole is fraught with difficulties and requires that a combination of field surveys and remote sensing techniques are applied across a huge area over a lengthy period of time. Changes in ice elevation and thickness can be measured using satellite altimetry and ice-penetrating radar, and changes in ice velocity and aerial cover can be identified from study of satellite images. The relationship between mass balance and climate is also more complex than meets the eye. Higher average air temperatures do not necessarily promote a negative mass balance: warmer air holds more water vapour than colder air, and this could in fact lead to more snowfall in certain areas of the continent causing ice sheet thickening.
At present, the jury remains out on how the EAIS will respond to global warming this century; but there is good evidence that many glaciers in the Antarctic Peninsula have already shifted to a negative mass balance and are in retreat. This is the situation for almost all other glaciers outside of Antarctica. The WAIS is also losing mass in some areas and is considered more vulnerable than the EAIS. This is because its average elevation is lower, much of it is grounded below sea level, and in places it is buttressed by ice shelves that themselves could be vulnerable to warming. If there were a major collapse of the WAIS (an unlikely but not impossible scenario over the next couple hundred years or so) this could contribute up to 6 metres of sea level rise. The link below shows land areas that would be flooded under this scenario.
A full understanding of the dynamics of Antarctic ice sheets and how they will respond to climate change requires awareness of the variation that exists within the ice sheets themselves. In the cold, interior regions of Antarctica ice flow is very slow, on the order of tens of metres per year, and the ice is predominantly cold-based. However, around the margins of the ice sheets there are areas where the glacier ice becomes warm-based and the discharge is much faster, sometimes exceeding 1000m per year. These ice streams only account for about 10% of Antarctica's area but they are the major conduits for the transfer of ice from the accumulation zones to the ablation zones. Therefore long-term studies of the volume and flow rates of ice moving through ice streams are essential for estimating changes in the ablation rate for the ice sheets as a whole.
This link shows how the velocity of ice flow varies across the continent.
This link contains an animation of ice flow through an ice stream.
Also important for the transfer of ice through the ice sheet system are the processes occurring at ice shelves. These are areas where part of an ice sheet extends into the sea and floats because ice is less dense than water. The lack of frictional resistance with bedrock causes the ice in ice shelves to move at a high velocity, up to 3km per year, and glacier ice is discharged to the sea as the edges of the ice shelf break off and float away as icebergs (a process termed 'calving'). Ablation of the Antarctic Ice Sheet occurs primarily through this process rather than by melting of the ice surface. There is, however, melting along the base of floating ice shelves. The Ross Ice Shelf and the Ronne-Filchner Ice Shelf each cover an area larger than the British Isles. Smaller ice shelves along the Antarctic Peninsula are losing mass as a result of climate change as described further in Section 1.5.
In addition to transporting ice, glaciers shape the land through erosion and deposition. There are many factors that control the rates of both of these processes; but in general, areas of faster moving, warm-based ice shape the land more rapidly than areas of slow, cold-based ice. In Antarctica, ice streams are particularly important in this regard. Through glacial plucking and glacial abrasion they deepen and widen their valleys to create the classic parabolic shape ('U-shape') associated with present and former glaciated valleys. As a major valley is widened, the spurs of tributary valleys become worn back to form truncated spurs. Glacial erosion that is concentrated on two or more sides of an area of high relief leads to the formation of arêtes and pyramidal peaks. These large-scale features of glacial erosion can be seen in Antarctica where rock protrudes above the surface of the ice sheet.
Since much of Antarctica's glacier ice reaches the coastline and beyond, much of the material carried in Antarctic ice is taken to the sea and deposited as ice rafted debris; and, given the limited area of land that is ice free, there are not many places where depositional landforms caused by glaciation are easily viewed. However, moraines can be seen around the margins of glaciers that terminate inland, such as glaciers that terminate in the Dry Valleys.
1. On the base map of Antarctica locate and label the following places and areas of ice:
You can use atlases and/or any of the links shown below:
2. Study this aerial photo of part of the Antarctic Peninsula and attempt the tasks and questions which follow.
2a. Trace a copy or make a sketch map of the area covered in the photo.
2b. Identify and label the following features: arêtes, pyramidal peak, corrie glaciers, valley glaciers, crevasses, area of calving, icebergs
2c. Add arrows to indicate the apparent flow directions of the ice.
2d. Write a paragraph explaining the locations of the crevasses.
3. An important way of measuring the surface velocity of Antarctic ice is the method of 'crevasse tracking'. Crevasses are easy to identify on the ice surface, and they last long enough to be carried along with the ice flow. By identifying individual crevasses, and measuring how far they have moved during the time between two satellite pictures it is possible to work out the ice velocity.
3a. Why do you think many crevasses need to be tracked on a single glacier to get a good estimate of the surface velocity of the ice?
3b. What advantages does crevasse tracking using satellite imagery have over ground-based field surveys for estimating ice velocities across Antarctica?
3c. The attached Excel spreadsheet contains surface ice velocity data calculated for many points along the Byrd Glacier which cuts through the Transantarctic Mountains and feeds into the Ross Ice Shelf. Click here to view the location of the Byrd Glacier on the Shackleton Coast:
Download the spreadsheet:
Copy the data into your own spreadsheet and using the functions in Excel conduct the following descriptive statistical analyses:
4. One way of assessing the impact of climate change on Antarctic ice is to see whether or not ice flow is speeding up in the ice streams that feed ice from the interior towards the coast. In places where ice shelves are breaking up, there is concern that the grounded ice flow will speed up causing a net loss of glacier mass (negative mass balance) and a contribution to sea level rise.
Considering the type of data that you worked with for question 3, write a couple of paragraphs to describe the data you would need to test the idea described above. What descriptive statistics would you need and what type of 'inferential' statistical test would help you to decide whether the collapse of ice shelves has a statistically significant effect on the velocity of the ice streams that feed them?