Ice sheets and glaciation
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 Key physical features, there are two distinct areas of ice that have different characteristics and histories: the East and West Antarctic Ice Sheets.
The main differences are:
- The East Antarctic Ice Sheet (EAIS) has about 9 times the volume of the West Antarctic Ice Sheet (WAIS).
- The EAIS has an average thickness of 2226m compared with the WAIS maximum of 1306m.
- The EAIS reaches a higher elevation (over 4000m at Dome Argus) than the WAIS, and the EAIS reaches a maximum thickness of 4776m (near Dome Circe, 69°56’S, 135°12’E).
- In contrast with the EAIS, the WAIS sits on bedrock that is mostly below sea level (lowest bed elevation of 2496m below sea level).
These major differences are summarised in these plan view and cross-section diagrams of Antarctica:
Response of Antarctic glaciers to climate change
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:
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 The Coast And Adjacent Ocean.
Student activity 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?