glacier mass balance and climate The relationship between glaciers and climate has been studied for many years. Its basis lies in the observed correlation of glacier advances with climatic deterioration and glacier retreat with climatic amelioration. However, this relationship is not as straightforward as it at first seems.
It is useful to think of glaciers as systems, with zones of inputs and outputs of mass and energy and zones of storage (Fig. 1). Ice is obviously the most significant component of glacier mass. Mass is added to a glacier through a number of processes. The most important of these is through the accumulation of snowfall in the upper reaches of the glacier (called the accumulation zone). Over time and with the increased pressure as further snowfalls are added, this snow turns into ice and is incorporated into the glacier. Hence the glacier gains mass. Other accumulation processes include avalanches, the freezing of basal melt water and the addition of rime. Towards the snout of the glacier, mass is lost from the system mainly through melting but also by other processes such as calving (the loss of large ice masses from the glacier into bodies of water) deflation (removal of ice by wind), and sublimation (the phase change of ice directly to water vapour in cold, dry environments). These processes are collectively termed ablation. The difference between accumulation and ablation for a glacier over one year is termed the net mass balance. The mass balance thus describes the ‘health’ of a glacier.
When a glacier displays positive mass balance, accumulation of mass is greater than ablation. This can be achieved by an increase in snowfall, for instance, or by a decrease in ablation. The glacier will respond to such changes in many ways but will tend to thicken and advance its snout. Conversely, under conditions of negative mass balance (when ablation is greater than accumulation) the glacier will lose mass by retreat of its snout or by lowering and thinning of its surface. Negative mass balance may also have a climatic cause, such as an increase in temperature (which increases the melting component of ablation) or a decrease in total precipitation (which reduces accumulation).
Zones of accumulation and ablation can therefore be seen to have a spatial dimension, accumulation processes being most effective at higher altitudes and ablation becoming predominant near the snout of the glacier. In addition, these processes will also vary temporally; during the winter the glacier may accumulate mass over its whole surface, whereas during the summer ablation processes may take over. Ablation is usually greatest at the snout and falls to zero at higher altitudes. Accumulation is greatest at higher altitudes and falls to zero at lower altitudes. The zones of net accumulation and net ablation are thus separated by a point where accumulation and ablation are both zero; this is called the equilibrium line.
It is clear then that glaciers will respond to climatic variations by changes in input and output. The speed with which a glacier will respond to a climatic stimulus is called the response or relaxation time and is defined as the time taken for the glacier to reach equilibrium after a change in input. The response time tends to be longer for large glaciers than for small ones owing to the larger amount of ice flux through the system that is required to initiate changes in the terminus. As a result, if changes in inputs occur often, some large glaciers may never achieve equilibrium. The shape of a glacier may also affect the response time through its influence on the distribution the area of a glacier with altitude.
The position of a glacier snout is generally more responsive to climatic changes than the glacier surface; it may fluctuate significantly on a yearly timescale while the surface height remains relatively constant. In addition, the positions of glacier snouts respond more sensitively to periods of negative mass balance (by showing retreat) than periods of positive mass balance (when the snout may be stationary but the thickness of the glacier increases). This is because increased ice flux from the accumulation zone may take a number of years to affect the position of the snout (yet will lead to a thickening of the glacier), while increased ablation will make its effects felt first at the terminus of the glacier.
Many morphological variables must be analysed to explain the behaviour of glaciers; these include aspect and relief. Climatic controls include temperature, precipitation, and continentality (Fig. 2).
Temperature. Although glaciers receive heat from a number of sources, including geothermal heat and frictional heat at the glacier bed, solar radiation has the biggest impact on ablation. Melting of the ice takes place in two ways. Short-wave radiation from the Sun and long-wave radiation from the valley sides, from water vapour, and from material lying on the glacier is affected by the reflectivity of the glacier surface (albedo). This is greatest when the glacier is white (usually in early summer before the winter snow cover has melted) and least in late summer when ablation is therefore most significant. Secondly, conduction of heat to the glacier surface from the air and condensation of water vapour are important variables causing ablation and are magnified by high winds. As a result, there seems to be a close relationship between glacier ablation and mean summer temperature.
Precipitation. Knowledge of total yearly precipitation for an area in which glaciers are present does not necessarily give an accurate estimate of glacier cover or behaviour. On low-latitude glaciers, for instance, much, or all, of the precipitation may fall as rain during the summer months and run off the glacier and out of the system as melt water. This precipitation is glaciologically ineffective, contributing little to the mass balance of the glacier. As a result, glaciers in areas of very high precipitation (e.g. the west coast of Canada, the South Island of New Zealand, and the Patagonian icefields in South America) may have very high accumulation rates, but these are offset by equally high ablation. In consequence, these glaciers fluctuate wildly in response to changes in these variables.
On the other hand, in extremely arid and cold environments (northern Greenland and central Antarctica, for instance) glacier ice is very stable and long-lived, because most (if not all) of the precipitation falls as snow and, by contributing directly to glacier mass, is glaciologically effective.
Continentality. Distance to the nearest ocean (or continentality) has been shown to be a significant variable accounting for the amount of precipitation received by glaciers. Continentality also affects the temperature regime experienced by the glacier. Glaciers in coastal locations tend to have higher accumulation and ablation rates (if they are in mass balance) than glaciers in continental interiors. This is because air from the oceans is moist, having picked up water vapour through evaporation, resulting in high levels of snowfall over the glaciers. In extreme continental areas precipitation may be very low but, owing to the low temperatures, may be preserved as snow.
Although the glacier–climate relationship has been demonstrated from many mountainous regions of the world, there are some glaciers which behave asynchronously to climatic inputs. One of the most important factors which disrupt this glacier–climate signal is when calving makes up a significant component of the ablation process.
When ablation is dominated by glacier melting, retreat or advance of the snout can be seen to be related to climatic factors. Calving, on the other hand, involves the large-scale and often catastrophic release of ice blocks from the glacier snout into water. This process is very rarely climate-driven; it is affected more by factors such as water depth at the calving front and topographic factors such as valley width. In addition, it is now known that the calving speeds of glaciers vary, according to whether the glacier ends in fresh water or tide water; freshwater glaciers are more stable and calve more slowly. The reasons for this behaviour are, as yet, unknown.
Precipitation seems to be the forcing mechanisms which accounts for the climatic component in calving glacier dynamics. This is because ablation is dominated by calving and not by melting; hence this suppresses the importance of temperature. In areas where glacier snouts are both land-based and end in water they may display asynchronous responses to climatic change, with land-based glaciers responding to variations in temperature and calving glacier fluctuations being controlled by precipitation and topographic factors.
Although the links between climate and glacier mass balance are evident, the study of climate change through knowledge of glacier oscillations is fraught with difficulties. For instance, there are only a few glaciers for which we have detailed mass balance records for periods of more than a few years. In addition, in most cases the relative contributions to the mass balance equation of non-climatic processes such as avalanches and calving are not known. As a consequence, it is often extremely difficult to extract the climatic signal from the fluctuation behaviour of glaciers.
However, in the absence of long-term climatic records in many remote parts of the world, glacier fluctuations provide us with the only record of climate change. This ‘proxy record’ allows us to reconstruct past climates and to monitor present-day climatic fluctuations. Correct interpretation of the glacio-climatic record depends on a proper understanding of the response of glaciers to variations in climatic inputs.
Stephan Harrison
Bibliography
Sugden, D. E. and and John, B. S. (1976) Glaciers and landscape. Edward Arnold, London.
