When looking for how past and present changes in glacier dynamics might be able to inform us of future behaviour, it is important to understand the processes that affect the movement of individual glacial terminus. Whilst the most obvious and widely-attributed influence on global glacial retreat is an increase in air temperature, a study by Scherler et al. (2011) suggests that this is not the only influence on the response of glaciers to climate change. The authors look at the effect of debris cover on terminus dynamics of a number of Himalayan glaciers - which had not yet been established in the region - and look at the regional differences of glaciers at the mountain-belt scale.
As was mentioned before in this blog, glacial meltwaters and snow are an important resource for people around the world. This is particularly true in Central and South Asia, where as the paper states:
“Snow and glacial meltwaters make an important contribution to the drinking water, agriculture, and hydropower supply of densely populated regions...”
This is a very immediate concern for the people of this region, as whilst mountain-river discharge will increase in the short term (perhaps leading to flooding in low-lying areas), in the long term the discharge will decrease; which may well have widespread humanitarian impacts as a result of water scarcity in the region.
Several glaciers in the central Himalaya were found to have stagnant reaches up to several kilometres. Despite evidence of glacial shrinking, such as increased meltwater ponds and surface lowering, the fronts of these studied glaciers have remained stable. All were debris-covered, and thus as the authors of the study note; debris cover can modify a glacier’s response to climate change. However, despite this growing body of evidence, no study to date has looked into the impact that debris cover can have regionally on glaciers.
A number of aims of the study can be identified. The first was to determine if glaciers in the greater Himalaya region display evidence of spatial patterns relating to altered mass balance and frontal dynamics and whether these patterns are related to climatic variations or other factors. Secondly, the study aimed to see if spatial variation of debris covered glaciers can be directly linked to regional differences in frontal dynamics.
The study looked at 286 mountain glaciers and indentified 6 regions that differ in climate and topography. For the climate forcing hypothesis presented later in this post, it is important to note that as the study moves over from the western most sites (Karakoram and Western Himalaya) towards the central Himalaya, there is a notable decrease in the influence of mid-latitude westerlies and the increasing influence of the Indian monsoon.
Across all moraines, rates of frontal change were observed between -80 and +40 m yr-1. In Karakoram however, 58% were stable or slowly advancing with a mean rate of approximately +8 m yr-1. This greatly contrasted with all of the other regions. The study thus observed the highest concentration of retreating glaciers in the areas where debris-covered glaciers were lowest.
The simplest models of glacier dynamics denote that the time scale of a glacier’s response to climate change is inversely proportionate to the surface slope, as well as being affected by local climate and glacier size. However, these factors do not alone explain the differences observed between debris covered and debris free glacial fronts. The summary at to this point is made clear in the paper:
“...widespread debris cover on many Himalayan glaciers reduces their retreat rates, which are therefore unsuitable as indicators of recent climate change.”
What is made clear here is that whilst the differing movements of these glaciers are indicative of varying debris cover and topography (increased hillslope angle leading to increased flux of rocky debris), they do not represent the effects of differing climate.
Figure from Scherler et al. (2011)
However, the paper goes on to suggest that further regional climatic differences can be seen to have an effect on other glaciers in the region. It is suggested that the approximate 50% stable or advancing glaciers in the westerly Karakoram are not related to stagnant terminus regions (as a result of topographic factors) but are a consequence of different mass-balance regimes associated with their climatic setting. The paper proposes that historical changes in the westerly-derived winter precipitation may cause the shifts towards positive mass balances. Three points of evidence are given to support this theory:
1) That the westerly jet stream, which provides the highest moisture transport during the winter, has strengthened and shifted to lower elevations. This therefore increases the potential for snow and ice accumulation in the study region.
2) Karakoram tree ring records support the theory of an increase in 20th century winter precipitation.
3) Summer temperatures in the areas showing less frontal retreat have decreased by a small amount, linked to higher precipitation and increased cloudiness (reduced insolation).
The importance of understanding glacial sensitivities across many different regions is very important. In all but one of the six regions studied in this paper, most of the stored ice is found in glaciers with more than 20% debris cover. Thus, knowing how debris cover affects the response of a glacier to climate change is significant for our predictions of future water resources in the region. What we currently know is that debris cover does slow melt rates and therefore the response of a glacier to temperature rises is reduced. Thick debris cover can also have other limiting effects, including containing the effects of decadal to centennial variations in solar radiation and the effect of anthropogenic forcing from atmospheric dust and soot deposition so that only minor changes in mass balance are observed.
The wider applicability of this study is promising, as debris-covered glaciers are common in other mountain ranges globally. To give truly representative estimates of future water availability and sea level rise, then the models that calculate mass balance estimates must include debris cover. At present they do not account for this.
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