Why the Glacier Equilibrium Line Altitude is Important

Palaeoclimatic reconstructions based on the limits of former glaciers use estimates of the associated equilibrium line altitudes (Benn and Gemmell 1997). The equilibrium line altitude is defined as the elevation at which mass balance is equal, where accumulation of snow is exactly balanced by ablation over a period of a year (Hoinkes, 1970) meaning that mass balance and the equilibrium line altitude for individual glaciers are usually strongly correlated (Braithwaite and Raper 2010). Therefore there is a very close connection between the Equilibrium Line Altitude and local climate, providing an important indicator of glacier response to climate change and hence allowing for reconstructions of former climates and the prediction of future glacier behaviour (Benn and Lehmkuhl, 2000).

In order to truly understand the, often complicated, relationship between a glaciers equilibrium line altitude and the corresponding climate of the local area, definitions of mass balance, accumulation and ablation area and the equilibrium line altitude, must be initially discussed.

The ablation area is the lower region of a glacier where snow loss (ablation) exceeds snowfall. The melting of the glacier and the calving of icebergs is the major form of ablation, expressed quantitatively as units of water equivalent, given in meters (Braithwaite 2002). The accumulation area is the upper region of a glacier where snow accumulation exceeds melting. A simplified description in the identification of a cirque depicts the area being an ‘arm chair-shaped bowl’ (Flint 1971) with the headwall exceeding 35 degrees and the arcuate floor underachieving 20 degrees (Evans 1977). Accumulation occurs mostly through snowfall whether it is direct or blown from neighbouring plateaus and summits. The equilibrium line altitude (ELA) is the boundary between the ablation area and the accumulation area, the elevation at which mass balance is equal, where accumulation of snow is exactly balanced by ablation over a period of a year (Hoinkes 1970). Glacier mass balance is the total difference between the gains and losses over a given period of time, for example a calculation of positive mass, explains that the glacier is gaining mass overall and a negative calculation when the glacier is being seen as losing mass (Benn and Evans 1997). On many glaciers, the amount of annual ablation and accumulation varies systematically with altitude, although this simple pattern is often complicated by local influences. Glacier mass balance reflects the climate of the region in which the glacier is situated together with site specific glacier morphology and local topographic setting. The idea of mass balance is therefore an important link between climatic inputs and glacier behaviour allowing the advance and retreat of many glaciers to be understood in terms of region or global climatic change.

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Because the equilibrium line is the place where annual accumulation totals exactly balance ablation totals, the ELA is closely connected with local climate, particularly precipitation and air temperatures, being highly sensitive to perturbations in either of these 2 variables, with rises in response to decreasing snowfall and/or increasing frequency of positive air temperatures and vice versa (Benn and Evans 1997). Perhaps the best illustration of glacier-climate interaction is the relationship between the net balance and the ELA. When the annual mass balance of the glacier as a whole is negative the ELA rises, and when the balance is positive, the ELA falls. Variations in the altitude of the equilibrium line on a particular glacier, therefore, can be used as an indicator of climatic fluctuations (Kuhn, 1981). It is useful to specify the climate at the ELA as some unique combination of precipitation and temperature. (Benn and Evans)

If a climate change occurs that increases the mass balance the glacier will advance, in an attempt to reach a new equilibrium position. The surplus of accumulation that exists must be balanced by an increase in ablation, which is accomplished by expanding the low-elevation terminus zone of the ablation area. If a climate change occurs that overall reduces the mass balance, the glacier will retreat in an attempt to achieve equilibrium. The retreat will reduce the area of the glacier in the lowest elevation terminus area where ablation is highest. If by retreat mass balance equilibrium is reached the glacier will cease retreating. However, the definition of the ELA initially does not imply that the glacier is in equilibrium and therefore the glacier may be gaining or losing mass on an annual basis. The ELA value associated with zero annual mass balance for the whole glacier is known as the steady-state ELA. When the annual ELA coincides with the steady-state ELA, ice mass and geometry are in equilibrium with climate, and the glacier will neither grow nor shrink. (Benn and Lehmkuhl 2000) However, majority of individual glacier ELAs deviate significantly from local climate ideals due, for example, to patterns of shading and snow redistribution by wind and avalanching.

The main variables affecting mass balance at the ELA are winter precipitation (accumulation) and summer temperatures (ablation). A strong relationship exists between summer temperature and precipitation at the ELA of modern glaciers and this has been shown empirically by Ohmura et al. (1992) for 70 glaciers worldwide. Ohmura et al. found that winter accumulation plus summer precipitation (= annual precipitation) had a close relationship with summer temperature (Jun/July/Aug) However, Hughes and Braithwaite (2008) showed that the relationship between accumulation and summer temperature at the glacier ELA was more complicated – with annual temperature range playing an important role. They continued to show that because of the role of annual temperature range, there must be a relationship between annual mean temperature and accumulation on a glacier

Within the following pages the focus develops upon tropical glaciers in the Andes range, due to their particular degree of variance along latitude in relation to the zero degree isotherm. The difference between the ELA and 0°C isotherm is a good indicator of the sensitivity of tropical glaciers to climatic global warming. It rises significantly from below zero meters in the inner tropics to several hundred metres in the outer tropics. From below zero degrees: the 0°C isotherm is above the ELA (Kaser and George 1997). Thus, glaciers in the outer tropics may be more easily affected by changes in precipitation as it governs the albedo and radiation balance. The outer tropics and inner tropics vary significantly regarding this, illustrating the extent of variability of glacier-climate relationships. Within the Peruvian Andes, mass accumulation takes place only during the wet season and predominately in the upper parts of the glaciers, whereas ablation occurs throughout the whole year. Thus, the vertical budget gradient is much stronger on tropical tongues than on those in mid latitudes (Lliboutry, morales and Schneider, 1997). Consequently under equilibrium conditions, tropical ablation areas are markedly smaller and the accumulation area ratio (AAR) has to be considered larger than in mid latitudes (Kaser and George 1997).

(Benn et al 2005) Glaciers of the tropics and subtropics inhabit high altitudes and differ in important ways from mid-and high-latitude glaciers in lower topographic settings. Consequently the methods used to reconstruct and interpret former glacier equilibrium line altitudes in low altitude regions need to be tailored to local conditions, as methods and protocols developed for other settings may not be appropriate. Annual variations in mean daily temperatures are smaller than diurnal temperatures ranges. This constancy in the mean daily temperatures in the topics means that the 0 degree Celsius atmospheric isotherm maintains a fairly constant altitude and ablation occurs on the lower parts of glaciers all year.

Vertical mass balance profiles are also influenced by climatic setting. In the humid tropics ablation gradients tend to be steeper than in drier environments, due to altitudinal variations in the amount of snow, sleet and rain falling on the ablation zone during the wet months. Thus the mass balance profiles of tropical glaciers tend to exhibit a sharper inflection at the equilibrium line than those of mid latitudes glaciers.

The accurate reconstruction of past ELAs requires that the extent and morphology of the former glaciers can be accurately determined. Furthermore the age of the reconstructed glacier needs to be determined to enable researchers to use the ELAs as proxies for past climatic conditions (Benn 2005). The simplest assumption is that all ELA can be attributed to changes in temperature, which can be estimated by using an assumed average environmental lapse rate in the atmosphere. However, if there were associated changes in precipitation, the estimated temperature change would be different. The point applies even in humid tropics. For example, Kaser and Osmaston 2002 found that 20th century changes in the ELAs of glaciers in the Cordillera Blanca cannot be determined by temperature changes alone, but were also influenced by changes in humidity. However, the difficulty of separating out the temperature and precipitation signals need not negate the usefulness of ELA in providing palaeoclimatic information.

Glaciers of the Peruvian Cordillera Blanca region represent more than 25% of all tropical glaciers with the 260 glaciers stretching for 130km, reaching 6000m level at several summits. The climate is characterised by small seasonal but large daily temperature variations and the alteration of a pronounced dry season and wet season bringing 70-80% of the annual precipitation. ELA recordings were taken for the 1930s and 1950s on the massif of Santa Cruz, Alpamayo, Pucahirca, in the northern part of the cordillera Blanca by Kaser and George in 1997. The Accumulation Area method was utilised to determine the mean ELA, results highlighting a general reduction of precipitation amounts, mainly during the wet season and therefore a reduction in accumulation. A vertical shift of 35 to 58 meters was also observed – a significant rise showing glacier shrinkage and tongue retreat. It is suggested that a reduction in air humidity and its effect on the above mentioned atmospheric circulation system is the main reason for this determined retreat of the glaciers between 1930 and 1950 as it influences the mass balance in various respects. Furthermore, the rise of the ELA shows different values across the Santa Cruz – Pucahirca massif. Concluding, Kaser and George (1997) notify the reader that a combination of spatially uniform rising in air temperature and a decrease in air humidity with spatially different effects has to be taken into account as a cause for the glacier retreat between 1930 and 1950.

A second case study, located between 8.5 and 9°N, the Cordillera de Merida within the Venezuelan Andes, a region within the tropics, which possesses evidence suggesting glacial ice coverage was abundant in the past. Three geographic sub-regions were studied by Schubert (1984) with the view to reconstruct 9 palaeoglaciers. Two methods to determine ELA were used to develop paleoclimatic assumptions: Accumulation Area Ratio method and the Accumulation Area Balance Ratio. The above methods enabled the approximation of ELA during the LGM (last glacial maximum). Results acquired from the study were that the ELA of the individual glaciers lowered in response to decreased air temperatures and increased snow during the LGM. The in depth Investigation revealed the ELA of the 9 observed glaciers would have experienced a lowering of roughly 850-1420m throughout the LGM, coinciding with decreased air temperatures. The palaeoglacier reconstruction demonstrated the spatial extent to which the equilibrium line altitude is susceptible to change in response to climatic changes within the Cordillera de Merida. Conclusions implied a support towards a later proposition that a glaciers ELA is closely connected with the surrounding local climate (Benn and Lehmkuhl 2000).

The following final case study locates within the Cordillera Real and Cordillera de Quimsa Cruz, in the Bolivian Andes, due to the low latitude, glaciation within the area is reduced and glaciers are small (80% cover less than 0.5 km²). Within the study Rabatel et al (2008) 15 proglacial margins were investigated, leading to a further reduction of 10 principle moraines being identified that mark the successive positions of glaciers over the last four centuries. The ELA was determined on each glacier using the Accumulation Area Ratio method. The reconstruction of the glacier ELA and observations of any changes in mass balance, Rabatel et al (2998) expressed that glacier maximum may be due to a 20 to 30% increase in precipitation and a 1.1 to 1.2 degrees Celsius decrease in temperature compared with present conditions. Increasing accumulation of snow above the ELA suggests snowfalls have a strong influence on the net radiative balance in the ablation zone via the albedo. The researchers continue to explain that within the early 18th century, glaciers began to retreat at varying rates until the late 19th to early 20th century; this trend was generally associated with decreasing accumulation rates. By contrast, glacier recession in the 20th century was mainly the consequence of an increase in temperature and humidity.

Although the ELA of a glacier, past or present can be a first-rate proxy for the glacier-climate relationship, the methodology chosen can greatly effect the interpretation of the results. Different reconstruction methods of changes in ELA may produce a range of results for the same glacier and therefore inconsistent and unreliable conclusions will be inferred. Ramage et al (2005) compared ELA reconstruction methods within the Junín Plain in the central Peruvian Andes. Descriptions of each method were given and limitations were highlighted. The Toe-to-Headwall Altitude Ration method, a commonly used method due to its relative ease of determining ELAs using map data, expressed that low errors were still evident. The Accumulation Area Ratio method: based on the empirically derived ratio of accumulation area to total area of glacier, however, AARs are likely to be highly variable between glaciers even within small regions, depending on the extent and distribution of debris cover in the ablation area, and the relative importance of direct snowfall and avalanching as mechanisms of accumulation (Benn and Lehmkuhl 2000) A more accurate method of palaeo ELA calculation is the Furbish and Andrews (1984) Accumulation Area Balance Ratio a method developed due to the inconsistencies within the AAR not accounting for altitudinal distribution of a glaciers surface area. Therefore, this method can be used to derive ELA under assumed steady-state conditions and uses an idealised linear mass balance curve to calculate ELA. Ramage et al (2005) concluded that the methods did not greatly differ for this region and morphology, yet each method possessed different correlation values.

The relationships between climate, glaciers and topography are, however, not those of simple cause and effect but are characterized by interdependence. They are also scale and time-dependent in that as a glacier grows it increasingly modifies its climate and the topography (Sutherland 1984). In addition Hodge et al (1998) suggests that although the ELA is a phenomenal proxy for determining glacier climate relationships, it can unfortunately be influenced by other non-climatic factors; for example avalanche, topographic variances and debris fall (Hodge et al 1998). Avalanche can equal increased accumulation, thus a positive mass balance equating to an ELA fall. This can occur regardless of climatic influence; therefore the resulting ELA may not give a 100% accurate representation of the glacier-climate relationship. Smith et al (2005) argues further, that reconstructed tropical LGM ELA lowering may be due to ‘local enhancing factors’ not solely climate. Additionally, glacier melting, coupled with avalanches, climate and local geographic topography may distort the observed ELA with reference to climatic impact therefore not 100% accurate for indicating glacier-climate relationship.

In addition to the ELA other variables have been measured that allow inferences to be made about the relationship between glaciers and the associated past climate. Radiation is a major component in the ablation of ice from glaciers and the effectiveness of radiation on a glacier is a function of the aspect, slope and nature of the snow or ice cover of the glacier surface as well as latitude, date and time of day (Sutherland 1984). Various energy-balance models (Williams, 1975; 1979; Kuhn, 1981) have been devised to model these factors and explain the distribution of present-day and former glaciers as well as investigate general glacier/climate relationships. In the South East Grampians, Sissons and Sutherland (1976) established that the deviation of one unit of the isolation factor from the local value for a horizontal glacier was equivalent to raising or lowering the ELA by 25 m, thus providing a physical link between the two variables.

Studies of modern glaciers, particularly of the relationships between their mass balance and the local climate, have provided a sound physical basis on which past climates can be inferred from former glaciers. Studies of the mass balance of modern glaciers have indicated that the equilibrium line altitude is the most critical parameter in the link between glaciers and climate (Sutherland 1984). In relation to tropical glacier regions, sharp changes in the area of the 0°C isotherm level, highlights the sensitivity of the ELA to climatic changes, posing the notion that to truly understand the complex association between the ELA and the glacier-climate relationship, knowledge on its relative position to the 0°c level is required. Furthermore, it is safe to assume that the ELA to some extent is a good indicator of climatic change especially in relation to tropical glaciers which have a greater degree of sensitivity to climate. However, it must be noted that there are non-climatic influences upon the calculated ELA for any glacier, past or present and therefore the glacier-climate relationship is a far more complex issue to observe and understand.